Synthesis of functionalized poly(p-phenylene)s via palladium catalyzed suzuki cross-coupling polymerization

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Synthesis of functionalized poly(p-phenylene)s via palladium catalyzed suzuki cross-coupling polymerization
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Table of Contents
    Title Page
        Page i
    Acknowledgement
        Page ii
    Table of Contents
        Page iii
        Page iv
    Abstract
        Page v
        Page vi
    Chapter 1. Introduction
        Page 1
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    Chapter 2. Monomer synthesis and model studies
        Page 17
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    Chapter 3. Polymer synthesis and characterization
        Page 63
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    Chapter 4. Conclusions
        Page 154
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    Chapter 5. Experimental
        Page 160
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    Appendix A. NMR analysis
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    Appendix B. Miscellaneous
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    References
        Page 233
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    Biographical sketch
        Page 243
        Page 244
Full Text









SYNTHESIS OF FUNCTIONALIZED POLY(P-PHENYLENE)S VIA PALLADIUM
ACETATE CATALYZED SUZUKI CROSS-COUPLING POLYMERIZATION













By

PETER BALYS BALANDA


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY


UNIVERSITY OF FLORIDA


1997













ACKNOWLEDGMENTS


I wish to express my gratitude the faculty and staff of the Chemistry Department at

the University of Florida. Foremost, I wish to thank Dr. John R. Reynolds for his patient

support and guidance, and my committee members, Dr. Merle Battiste, Dr. Daniel Talham,

Dr. Kenneth Wagener, and Dr. Henk Monkhorst, for the same. I also thank Dr. David

Powell, Dr. Dean Schoenfeld, Ms. Maria Ospina, and Ms. Lidia Matveeva for supplying

the mass spectral results discussed herein. I wish to thank Dr. Kitty Williams for

assistance with thermal analysis, Dr. Schanze, Dr. YiBing Shen and Mr. Kevin Ley for

assistance with photoluminescence experiments, and Mr. Donald Cameron for making

routine GPC analysis so painless. I thank Ms. Lorraine Williams for her incredible

secretarial skills, and Ms. Jennifer Irvin for her careful reading of this text.

I would like to thank the members of the Butler Polymer Research Laboratory, past

and present, for the support and friendship they have provided. I wish to thank my wife

Catherine for all of her love and support. I thank my parents, Mindaugas and Virginia

Balanda, and all of my brothers, family and friends for their continuing efforts on my

behalf. Finally, I would like to thank my children for the special love they give.

This work was supported by grants from The Moltech Corporation and the Air

Force Office of Scientific Research.














TABLE OF CONTENTS

ACKNOWLEDGEMENTS......................................................................... ii

A B S T R A C T ........................ ................. .......................... ........................ v

CHAPTERS

1 INTRODUCTION

Evolution of Mechanical Properties within a Polymer System.............................. 1
Evolution of Electronic Properties within a Conducting Polymer System................. 6
Poly(p-phenylene): The Quintessential Rigid-Rod Polymer............................... 10

2 MONOMER SYNTHESIS AND MODEL STUDIES

Introduction ...................................................................................... 17
The Suzuki Cross-Coupling Reaction................................................... 17
Nickel Mediated Coupling of Aryl Halides.............................................. 21
Alkoxy-Substituted Poly(p-Phenylene)s by Suzuki Polymerization................. 23
Results and Discussion......................................................................... 26
General..................... ... ...... ..................... ............ 26
Monomer Synthesis........................................................................ 26
M odel System s................................................................ ............. 40

3 POLYMER SYNTHESIS AND CHARACTERIZATION

Introduction ...................................................................................... 63
G general M ethods................................................................................ 63
Polymerization with Stoichiometric Balance................................................. 67
Polymerization with Stoichiometric Imbalance............................................ 120
Synthesis of Dialkoxy/Dialkyl Copolymers................................................ 136
Synthesis of Dicationic Polyelectrolytes.................................................... 138

4 CONCLUSIONS

P oly m erization ............................................. ......... ...... ....... ............ .. 154
Properties ....................................................................................... 155

5 EXPERIMENTAL

General Materials and Instrumentation...................................................... 160
M onom er Synthesis........................................................................... 162
T osylate Synthesis........................................................................ 162
Boronic Acid and Ester Synthesis...................................................... 163
Halogenations ................................................................................. 168








Etherification of c, a-Dibromo-p-Xylenes............................................. 170
Etherification of Hydroquinones........................................................ 172
Quaternization of Monomeric Diamines................................................ 175
Model Compound Synthesis................................................................. 176
Methoxyethoxy- and Triethoxy- Substituted Poly(p-Phenylene)s....................... 178
Suzuki Polymerizations with Mass Balance........................................... 178
Suzuki Polymerizations with Mass Imbalance......................................... 182
Poly[2,5-Bis( 1,3,5-Trioxanonyl)- 1,4-Phenylene].................................... 193
Tertiary and Quaternary Amine Functionalized Poly(p-Phenylene)s.................... 196
Synthesis of Amine Functionalized Poly(p-Phenylene)s............................. 196
Quaternization of Amine Functionalized Poly(p-Phenylene)s....................... 200
M o del S tu dies............................................................ ...................... 202

APPENDICES

A NMR ANALYSIS............................................................................. 205

B MISCELLANEOUS.......................................................................... 224

R E F E R E N C E S .................................................................................... 233

BIOGRAPHICAL SKETCH..................................................................... 243














Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

SYNTHESIS OF FUNCTIONALIZED POLY(p-PHENYLENE)S VIA PALLADIUM
CATALYZED SUZUKI CROSS-COUPLING POLYMERIZATION

By

Peter Balys Balanda

August 1997



Chairman: Professor John R. Reynolds
Major Department: Chemistry


Poly(p-phenylene (PPP) is a rigid, highly conjugated polymer composed entirely of

aromatic rings. As such, it is electroactive, as well as thermally stable. PPP itself is

neither soluble nor melt processable. Functionalizing the polymer with appropriate

substituents allows PPP to be processed for structural and device applications. One

successful route to PPPs is by palladium [0] catalyzed coupling of a dihaloaryl monomer

with a phenylenediboronic acid or ester. Early studies of this and other metal catalyzed

PPP syntheses centered on the polymerization of phenylene monomers with alkyl

substituents, or with electron withdrawing groups. The coupling reaction is less favorable

when electron donating groups are present. Electron rich systems offer interesting

electronic properties, such as blue light emission and stable p-doping. One of the most

active, but least stable catalysts for Suzuki cross-coupling is palladium acetate. Through

interaction with the reaction medium, the palladium [II] is reduced to Pd[0], which can then

initiate the coupling reaction. In this work, the reaction process was explored and related to

the structural properties of the polymers formed. The products of various side reactions








were found to be incorporated into the polymer chain. Intrachain biphenylene units were

identified as products of boronic acid self-coupling, and the effect of this reaction on the

polymerization was discussed. Solvent polarity was discussed in terms of its effects on

reaction rate and degree of polymerization. Polymerizations were found to proceed more

rapidly in higher dielectric solvents. Both charge neutral and cationic PPPs were formed.

The molecular and electronic structures of the polymers were examined by standard

characterization methods. Polymers containing side-chains attached via ether linkage were

found to absorb in the near UV and displayed photoluminescence in the visible blue region.

The charge neutral materials, containing two triethoxy groups on alternating phenylene

moieties, formed tough films. The cationic PPPs, which have quaternary alkyl ammonium

ions attached to the PPP backbone by an ether linkage, showed a four order of magnitude

increase in fluorescence intensity relative to their tertiary amino PPP counterparts, and so

are well suited for use in pH sensing devices.














CHAPTER 1
INTRODUCTION

Evolution of Mechanical Properties within a Polymer System


Naturally occurring polymers have helped clothe and house man for thousands of

years. Only this past century did we begin to replace or to supplement naturally

occurring fibers and gums with synthetic materials. Many of the early synthetic materials

were developed through attempts to mimic natural products. Of the early

accomplishments, two stand out in this regard. The first successful synthetic fiber, nylon

6,6, was developed as an alternative to natural silk; synthetic cis- 1,4-polybutadiene was

produced as an alternative to natural rubber. In both of these cases, motivating factors

were largely economic. Nylon 6,6 promised to create and capture a new corner in the

textile market, while cis -1,4-polybutadiene severed the United States tire industry from

its complete dependence on foreign rubber.1 Synthetic polymeric materials have now

been incorporated into virtually every aspect of our lives, forming the cornerstone of the

industrial chemical world.

Yet, as vast and diversified as the field of polymer science is today, it grew from

the seemingly simple, hard fought for idea that linear polymers exist as very long chains

of covalently bonded atoms.2 The macroscopic properties of molecular systems, it

follows, depend on the structural and chemical nature of the polymer chain. Repetitive

intra- and interchain interactions, which together generate the bulk properties of the

physical system, arise from the basic chemical elements of the individual repeat units.

Nowhere is this more confounding than in the artful design of proteins, wherein linear


1 Ironically this same industry is now largely dependent upon foreign oil.
2Staudinger, H. Die Hochmolecularen; Springer-Verlag: Berlin, 1932.








polymers comprised of a veritable handful of amino acids form the delicately folded

arrays so intimately tied to life.

It is not necessary, however, to invoke the most complex materials in nature to

illustrate this effect. One might instead begin with the two carbon molecule ethane. As

the two-carbon units -(CH2CH2)- are covalently linked together, end on end, the physical

properties of the material dramatically change. Figure 1-1 illustrates the effect of

molecular weight on the melting point within a homologous series of n-alkanes

containing an even number of carbon atoms. As the number of carbon atoms in the


150 -

100 t II
10 tough plastic
-.- 50 i
0 brittle plastic
0 -___
0

-50 wax

S-100
liquid
-150
gas
-2 0 0 . I i . . I i I
0 100 200 300 400 500
Number of C2H4 repeat units

Figure 1-1. Dependence of melting point on the degree of polymerization of high density
polyethylene (HDPE, 1), and the approximate physical character of the material in a
given size range.3


linear chain increases, the vapor pressure drops and the number of van der Waals

interactions4 between chains increases. Gas condenses to liquid; liquid viscosity

increases; molecules crystallize; chains entangle; chains become long enough to bridge

the gap between crystallites and tough materials are produced.

3Data points were obtained from: a) Aldrich Handbook Catalog of Fine Chemicals, 1996-1997; b) Hogan,
J.P. In Kink-Othmer Encyclopedia of Chemical Technology, Vol. 16, 3rd ed. ; Grayson, M., Exec. Ed.;
John Wiley & Sons: New York, 1984; pp 421-433.
4Attractive forces resulting from dipole-dipole interactions, induced or otherwise.








Similar to the melting point increase observed in the homologous series of

alkanes, the mechanical properties of most polymers improve with increasing molecular

weight until some critical threshold weight is attained. Properties then level off or

increase less rapidly, eventually becoming asymptotic. Increases in molecular weight are

generally accompanied by decreased solubility and increased melt viscosity (provided the

materials melt at all). Thus, a polymer may be obtained which displays wonderful

mechanical properties, yet cannot be processed. The optimization of molecular weight is

a crucial component of industrial polymer chemistry. If the molecular weight is too high,

the polymer is unprocessable; if it is too low, the polymer is nonfunctional.

As might be expected, different types of polymers achieve these conditions in

different molecular weight regimes. High density polyethylene, with a high degree of

flexibility and regularity, easily orders into a crystal lattice. Low density polyethylene,

which exhibits a high degree of branching, does not order well, and so displays a lower

degree of crystallinity. Figure 1-2 illustrates the effect of side group frequency on density

in polyethylene, a factor which bears directly on solubility and processability. For this

reason, the introduction of side groups onto polymer chains has become a standard

method of disrupting crystallinity for the purpose of increasing polymer solubility.

A reduction in rotation around a chemical bond due to unsaturation or to

resonance delocalization causes chain stiffening. The resultant loss in freedom of motion

typically translates to a decrease in the crystalline content of the polymer system. Even

so, this decrease need not be accompanied by a loss in mechanical properties, provided

intrachain interactions remain high. Polyamides, for example, typically acquire sufficient

mechanical strength at lower molecular weights than does polyethylene, yet exhibit lower

levels of crystallinity than PE due to increased chain stiffness and a reduction in

symmetry. Hydrogen bonding and dipole-dipole interactions present in the polyamide

systems can account for this decrease in critical threshold molecular weight.








0.85 130
--e-Melting point
0.8 - Crystallinity 125

S- 0.75
S- 120
*S 0.7
S- 115 M)
S0.65

0.6 110
0.55-
0.55 .... t-7 ti-.. i -t-r-i-i- I ....ti--,-- 105
0 0.5 1 1.5 2 2.5 3 3.5
Degree of branching (CH3/10OC)


Figure 1-2. Effect of branching on the crystalline nature of polyethylene.5


Polymers lacking crystalline regions are termed amorphous, as are the noncrystalline

regions of semicrystalline polymers. Since it is the crystallites within the bulk polymer

which scatter light, amorphous polymers are transparent. Stretching a polymer affords

newly oriented polymer strands the opportunity to crystallize, such that severely stressed

amorphous polymers become opaque.

When aromatic rings are present, the chains become stiffer still, and rE-7r

intrachain interactions can occur. While it can be more difficult for such a system to

obtain a high degree of crystallinity, it also becomes more difficult to disrupt that

crystalline order once it is achieved. An example of such a system is KevlarTM 2, shown

in Figure 1-3 wherein strong intrachain hydrogen bonding, in combination with a rigid

polymer backbone, allow the generation of tough insoluble fibers. Thus, within various

polymer systems, a wide variety of microscopic properties contribute, in fine balance, to

the macroscopic properties of the bulk.




5Data represents PE samples of Mw = 5.0-5.4 x 104. Quirk, R.P.; Alsamarraie, M.A.A. In Polymer
Handbook, 3rd Ed.; Brandrup, J. Immergut, E.H., Eds.; John Wiley & Sons: New York, 1989; p. v/15-26.





5


0

N /NH 1
H O }n
0 0
H =KevlarTm
HN- \N4_ 2

0 n

Figure 1-3. Structure of poly(p-phenyleneterephthalamide) (KevlarTM) showing
hydrogen bonding interactions.


KevlarTM, or poly(p-phenyleneterephthalamide), is one of an interesting class of

materials known collectively as rigid rod polymers. Rather than establishing a random

coil structure in solution, rigid rod polymers maintain an extended, ribbon-like

conformation. As a result, they are subject to shear orientation and may display liquid

crystalline behavior. Other rigid polymer systems include polyheterocycles such as

poly(benzobisthiazole) 3 and the all aromatic hydrocarbon poly(p-phenylene) (PPP, 3).


H

NXHN Sn G n
H
3 4
Figure 1-4. Structures of poly(benzobisthiazole) (3) and poly(p-phenylene) (4).


These systems have the added property of being entirely conjugated. As such, they are

exceptionally rigid and thermally stable. As an added bonus, these materials are

electroactive. They are redox active materials whose dielectric and spectral properties are

variable and intimately tied with their redox state.








Evolution of Electronic Properties within a Conducting Polymer System


The unsaturated analog of polyethylene is polyacetylene (5).6 Both materials

have been produced by Ziegler-Natta polymerization of the corresponding monomeric

gases. Polyacetylene so produced is a low density, fibrous material with few structural

defect sights.7 The material is extremely rigid relative to polyethylene, and is intractable.

While the chemical and electronic properties of ethane and polyethylene are similar, the

chemical and electronic properties of ethene and polyacetylene are not.

As mentioned above, the physical properties of discrete small molecules build up

as the molecular weight increases within a homologous series. In a similar sense, the

electronic properties of tc-conjugated polymer systems evolve as well. In the

qualitatively useful Htickel Molecular Orbital Theory, Pz orbitals of planar conjugated

systems are linearly coadded under the assumption that the 7C and (Y systems may be

treated independently of each other.8 The total number of molecular orbitals thus created

must be equal to the total number of atomic orbitals consumed. This method has been

utilized to describe aromaticity in benzene, and likewise to account for the relative

instability of cyclobutadiene (see Figure 1-5).9

An extended linear conjugated system can be qualitatively described in the same

fashion by application of cyclic boundary conditions, wherein the molecule is essentially

treated as if it were a giant macrocycle with equally spaced carbon atoms. The orbital

energies are given by the expression


E = a + mjp (1-1)




6Ito, T.; Shirakawa, H.; Ikeda, S. J. Polym. Sci., Polym Chem. Ed. 1974, 12, 11.
7Lieser, G.; Wegner, G.; MUiller, W.; Enkelmann, V.; Meyer, W.H. Makromol. Chem. Rap. Commun. 1980,
1,627.
8In HMO Theory any orbitals which are orthogonal to each other are considered to be noninteractive.
9For further explanation see Carey, F.A.; Sundberg, R.J. Advanced Organic Chemistry. Part A: Structure
and Mechanisms, 3rd ed.; Plenum Press: New York, 1990; pp 37-46.








where


mj = 2 cos jz7 for j = 1,2, ...,n (1-2)
n+1


The Coulomb integral x is related to the binding energy of an electron to the 2p orbital.

The resonance integral 13 is related to the energy of an electron in the field of two nuclei.

If the energy difference between the lowest and the highest molecular orbitals thus
generated remains an arbitrarily constant value 413, then it becomes clear that the energy

molecule Frost's circle molecular orbitals relative orbital orbital type
energies

SCa 213 antibonding
cc- -- aC nonbonding
cyclo- 4 4
butadiene ,o.
butadiene a + 213 bonding



S_2 13 antibonding


benzene + bonding
C o + 2Pnding
Figure 1-5. The application of the Frost's circle mnemonic to illustrate the relative
energies of molecular orbitals within the two cyclic systems as predicted by HMO
Theory.

differences between molecular orbitals becomes increasingly small as the number of

linearly combined atomic orbitals increases.10 Ultimately this value would be

insignificant relative to the thermal energy of an electron. Several consequences arise

from this simple first approximation. First, the molecular orbitals, although remaining

quantized, would effectively coalesce into a one-dimensional band, similar to the three-

10LCAO-MO Theory of the organic chemist, as applied here, is similar in concept to Tight-Binding Band
Theory of physics. For a more complete explanation see Hoffman, R. Angew. Chem. Int. Ed. Engl. 1987,
26, 846.








dimensional conduction bands of metals. The relation of the orbital energy to the number

of orbitals with that energy can then be described by a density of states diagram (see

Figure 1-6). Secondly, a one electron contribution from each carbon atom in the

electrically neutral system would provide a half-filled band. Because the orbitals are so

close together, electrons in the highest energy occupied orbitals at the Fermi level1 are

free to roam into neighboring unoccupied orbitals where they would experience high

mobility. On this basis, a completely conjugated high molecular weight linear polymer

with the chemical repeat unit -(CH=CH)- should be metallic. The third consequence of

this model, however, is one which ultimately precludes metallic behavior in this system.

The molecular orbitals at the Fermi level are close enough together in energy to behave as

if degenerate. By the Jahn-Teller theorem,12 when degenerate orbitals are unevenly filled

with electrons, the energy of these orbitals change as a consequence of a symmetry

lowering vibration, to become nondegenerate so that total energy of the system is

lowered. This change in orbital energies, referred to in solid state systems as a Peierls

distortion,13 opens a gap in the Pz band, and physically distorts the polymer chain to

lower symmetry such that adjacent carbon atoms dimerize, forming alternately single and

double bonds. The Pz band is thus broken into an empty conduction band and a full

valence band. The resultant material, polyacetylene, is a semiconductor (trans-

polyacetylene: Cy < 10-5 S cm-1) with a band-gap (Eg) of 1.4 eV.14

Polyacetylene can be made conductive (ca. 103 S cm-1) by the addition of

electrons into the conduction band, or by removal of electrons from the valence band.15,16

These redox processes, known as n-doping and p-doping, respectively, result in yet


11The Fermi level (EF) is the energy level which has a 50% chance of being occupied by an electron, and
so represents the midpoint in energy of a symmetric half-filled band.
12Jahn, H.A.; Teller, E. Proc. Roy. Soc. 1937, A161, 220.
13Peierls, R.E. In Quantum Theory of Solids; Oxford Univ. Press: London, 1955; p 108.
14Williams, J.M. In Adv. Inorg. Chem. Radiochem. 1983, 26, 235.
15Shirakawa, H.; Louis, E.J.; MacDiarmid, A.G.; Chiang, C.K.; Heeger, A.J. J. Chem. Soc., Chem.
Commun. 1977, 578.
16Chiang, C.K.; Druy, M.A.; Gau, S.C.; Heeger, A.J.; Louis, E.J.; MacDiarmid, A.G.; Park, Y.W.;
Shirakawa, H. J. Am. Chem. Soc. 1978, 100, 1013.


















* *.y "






(a)











EG








(c)


Figure 1-6. Band structure and density of states (DOS) diagram of a simple one
dimensional metal prior to and after undergoing a Peierls distortion. (a) Band structure
prior (b) DOS prior; (c) band structure after; (d) DOS after. EG is the bandgap, which,
for a semiconductor, is twice the activation energy for conduction. The D(E) relates to
the relative number of energy levels at energy E.








another structural change within the system. A defect in the polymer chain termed a

soliton is formed.17 A negative soliton corresponds to a resonance stabilized carbanion; a

positive soliton corresponds to a resonance stabilized carbocation. Charged solitons

move under the influence of an applied electric field,18 interconverting equivalent

asymmetric ground states. While doped polyacetylene systems with conductivities

greater than 105 S cm-1 have been produced,19 these materials are environmentally

unstable and cannot be readily processed.

The discovery of conductivity in doped polyacetylene began a flurry of research

that has not subsided. A tremendous number of 7t-conjugated polymers have been

synthesized. Such obstacles as environmental stability and processability have been

seriously challenged. A mere survey of this field is today a topic for multivolume sets.

Suffice it to say, aromatic hydrocarbon, heterocycles, vinyl and ethynyl groups have been

used in a myriad of combinations, with and without secondary functionalization to

produce a tremendous catalog of materials. The properties and uses of these materials are

as variable as their structures. New discoveries are continually being made as researchers

seek to optimize and fine tune the properties of their materials. And, while the discovery

of nt-conductivity launched a quest for new materials, it launched a reexamination of old.

One such material is poly(p-phenylene) (PPP).


Poly(p-phenylene): The Quintessential Rigid-Rod Polymer


Poly(p-phenylene)s (PPP's) form an interesting class of electroactive, thermally

stable rigid-rod polymers.20 Unlike polyacetylene, wherein two degenerate ground state


17Su, W.P.; Schrieffer, J.R.; Heeger, A.J. J. Phys. Rev. Lett., 1979, 42, 1698.
18Neutral solitons can exist, but do not serve as charge carriers.
19Naarman, H.; Theophilou, N. Synth. Met., 1987, 22, 1.
20For reviews on PPP, see: (a) Percec, V.; Tomazos, D. In Comprehensive Polymer Science; Aggarwal,
S.L, Russo, S., Eds.; Pergamon Press: Oxford, 1992; 1st supply p 318. (b) Jones, M.B.; Kovacic, P. In
Comprehensive Polymer Science; Eastmond, G.C., Ledwith, A., Russo, S., Sigwalt, P., Eds.; Pergamon
Press: Oxford, 1989; Vo. 5, p. 465. (c) Kovacic, P.; Jones, M.B. Chem. Rev. 1987, 87, 357. (d)
Elsenbaumer, R.I.; Shacklette, L.W. In Handbook of Conducting Polymers; Skotheim, T.A., Ed.; Marcel
Dekker: New York, 1986; Vol. 1, Chapter 7, p 213. (e) Speight, J.G.; Kovacic, P.; Koch, F.W. J.








forms exist, poly(p-phenylene) has one low energy aromatized form, and a second, higher

energy quinoidalized form.21 Transition 1 in Figure 1-7 represents this energy

difference-the band gap of the polymer prior to doping, which is typically recorded as

the high energy edge of the UV-Vis absorption spectrum. When an electron is removed

from PPP, a radical cation species, termed a positive polaron, is formed. The formation

of the polaron is accompanied by a structural distortion at the region of the defect, the

quinoidalization, which generates new molecular orbitals within the band gap. Theory

predicts the polaron to be paramagnetic, and this has been confirmed by esr

spectroscopy.22,23 At the onset of doping, an esr signal develops. With continued







Polaron Bipolaron





3 4 3
1:

-i









Figure 1-7. Charge associated defect states in poly(p-phenylene). Adapted from Bredas, J.L.;
Street, G.B. Acc. Chem. Res. 1985, 18, 309.


Macromol Sci., Rev. Macromol. Chem. 1971, C5 (2), 295. (f) Noren, G.K.; Stille, J.K. Macromol. Rev.
1971,5,385.
21Bredas, J.L.; Street, G.B. Acc. Chem. Res. 1985, 18, 309.
22Scott, J.C.; Krounbi, M.; Pfluger, P.; Street, G.B. Phys. Rev. B: Condens. Matter 1983, 28, 2140.
23Kaufman, J.H.; Colaneri, N.; Scott, J.C.; Street, G.B. Phys. Rev. Lett. 1984, 53, 1005.








oxidation, the esr signal disappears, signaling removal of electrons from the polaron

band. The resultant species, known as a cationic bipolaron, is believed to be the main

charge carrier in p-doped PPP.24 As the number of bipolarons increases, molecular

orbitals associated with them develop into midgap bipolaron bands, which can be probed

spectroscopically.25 Bipolarons exist as a charge associated state. Coulombic repulsions

push the charges apart, while aromatic stabilization energy and steric repulsions contain

them. Negative polarons and bipolarons are formed in the case of n-doping.26

PPP is photoluminescent (Fig. 1-8). Irradiation of PPP at the band gap results in

electron excitation from the valence to the conduction band, generating a spin separated

excited state. A rapid lattice relaxation follows, resulting in the formation of a species

known as a neutral bipolaron exiton, or a singlet exciton. The structural form of the

singlet exciton is similar to that of the bipolaron; mid gap molecular orbital formation

likewise occurs. Radiative decay proceeds from the singlet exiton state, providing a

photon of light blue light appropriate to the energy difference between the mid gap states.

The lattice then relaxes to its ground state structure. Because of the structural changes

which accompany the excitation/decay processes, the electronic absorption/emission

spectra of PPP display a significant Stokes shift. Thus, similar to other conducting

polymers, PPP is not self-absorbing.27

PPP is electroluminescent. Under appropriate conditions PPP emits blue light

upon application of an electric field.28 In the simplest case, a thin polymer is sandwiched

between a high work function anode, such as indium doped tin oxide coated glass, and a

ow work function cathode, such as calcium. A forward bias is applied until such time as

holes and electrons are injected into the polymer film from the anode and cathode,


24Peo, M.; Roth, S.; Dransfeld, K.; Tieke, B.; Hocker, J.; Gross, H.; Grupp, A.; Sixl, H. Solid State
Commun. 1980, 35, 119.
25Crecelius, G.; Stamm, M.; Fink, J.; Ritsko, J.J. Phys. Rev. Lett. 1983, 50, 1498.
26Bredas, J.L. In In Handbook of Conducting Polymers; Skotheim, T.A., Ed.; Marcel Dekker: New York,
1986; Vol. 2, Chapter 25, p 859.
27Feast, J.W.; Friend, R.H. J. Mater. Sci. 1990, 25, 3796.
28Gremrn, G.; Leditzky, G.; Ullrich, B.; Leising, G. Adv. Mater. 1992, 4, 3621.













singlet exciton




hv





///////Photoluminescence
Photoluminescence


negati


100-500 nm


,- Electroluminescence



-+O


ive polaron positive p




singlet exiton


olaron


Figure 1-8. Luminescence in poly(p-phenylene). Adapted from Feast, J.W.; Friend, R.H. J.
Mater. Sci. 1990, 25, 3796.

respectively. Hole injection into the HOMO results in positive polaron formation on one

side of the film; electron injection into the LUMO results in negative polaron formation

on the other. The two polarons then migrate toward each under the influence of the

applied bias. When they meet, the charges annihilate each other, and a singlet exiton is

formed. As in the case of photoluminescence, radiative decay and structural relaxation

follow.29

29Holmes, A.B.; Bradley, D.D.C.; Brown, A.R.; Burn, P.L.; Burroughes, J.H.; Friend, R.H.; Greenham,
N.C.; Gymer, R.W.; Halliday, D.A.; Jackson, R.W.; Kraft, A.; Martens, J.H.F.; Pichler, K.; Samuel, I.D.W.
Synth. Met. 1993,55-57,4031.








Synthetic approaches to production of PPPs have been varied. Early attempts to

produce and characterize well defined high molecular weight materials were plagued by

structural irregularities and hindered by solubility difficulties. Oxidative cationic

polymerization with cupric chloride (Fig. l-9a),30 oxidative electrochemical

polymerization of 1,4-dialkoxybenzenes,31 Grignard coupling of 1,4-dibromobenzene in

the presence of a nickel catalyst (Fig. l-9b),32 and the electrochemical reduction of 1,4-

dihalobenzenes in the presence of a nickel catalyst,33 have been used towards PPP

syntheses. Free radical34 and transition metal35 catalyzed polymerization of protected

5,6-dihydroxy-l,3-cyclohexadiene gave soluble precursor polymers which could be

thermally converted to unsubstituted PPP. Reaction of the diGrignard reagent of 1,4-

dibromo-2,5-dialkylbenzenes with an equimolar amount of the untreated 1,4-dibromo-

2,5-dialkylbenzene gave structurally homologous, readily soluble PPP's with an average

degree of polymerization of ca. 13.36

A major improvement in methodology came with the utilization of Suzuki

coupling for the A-B polymerization of 4-bromo-2,5-dialkylbenzeneboronic acids, and

for the AA/BB polymerization of 1,4-dibromo-2,5-dialkylbenzenes with benzene

diboronic acid (Figure l-9c, Figure 1-10).36,37 Placement of alkyl substituents on the

benzene moiety, in combination with transition metal mediated cross coupling chemistry,

resulted in soluble, processable PPP's with chain lengths on the order of 100 rings.38 A

variety of other functional groups have been introduced, including carboxylate,39




30(a) Kovacic, P.; Wu, C. J. Polym. Sci. 1960, 47, 448. (b) Brown, C.E.; Kovacic, P.; Wilkie, C.A.;
Kinsinger, J.A.; Hein, R.E.; Yaniger, S.I.; cody, R.B. J. Polym. Sci., Polym. Chem. Ed. 1986, 24, 255.
31Yamamoto, K.; Nishide, H.; Tsuchida, E. Polym. Bull. 1987, 17, 163.
32Yamamoto, T.; Hayashi, Y.; Yamamoto, Y. Bull. Chem. Soc. Jpn. 1978, 51, 2091.
33Stille, J.K.; Gilliams, Y. Macromolecules 1971, 4, 515.
34Ballard, D.G.H.; Courtis, A.; Shirley, I.M.; Taylor, S.C. Macromolecules, 1988, 21, 294.
35Gin, D.L.; Conticello, V.P.; Grubbs, R.H. J. Am. Chem. Soc. 1992, 114, 3167.
36Rehahn, M.; Schluter, A.-D.; Wegner, G.; Feast, W.J. Polymer, 1989, 30, 1054.
37Rehahn, M.; Schluter, A.-D.; Wegner, G.; Feast, W.J. Polymer, 1989, 30, 1060.
38Rehahn, M.; Schltiter, A.-D.; Wegner, G. Makromol. Chem. 1990, 191,1991.
39Wallow, TI.; Novak, B.M. J. Am Chem. Soc. 1991, 113, 7411.









1) CuC12 / AIC13 / 02
2) MeOH / HCI


Mg


1. n-BuLi
-Br
2. (CH3)3B
3. H30+


Ni (0) cat.
-Cl -
Zn


R
-/ OH
Br-\ / -B
OH
R


R

CH3SO2 \ /- OSO2CH3


R R'

V-/


Ni (0) cat.
Zn


R


<:n


Pd (0)
cat. _
LiCI
dioxane


R R'
A

n


Figure 1-9. Synthetic routes to poly(p-phenylene). R is typically H, alkyl or an electron
withdrawing group.


0


Ni (0)
cat.


Pd (0)
cat.
aq. base


n







RO Pd(PPh3)4 R
SK2CO3 -
Br / Br + (HO)2B / B(OH)2 water
^_y i_'Jwater _' _y n
OR benzene OR

Figure 1-10. Suzuki coupling reaction for the synthesis of poly(p-phenylenes).

sulfonate,40 and alkoxy sulfonate groups.41 Recently, a variety of functionalized PPP's

have been synthesized via homocoupling of dichloro- ,42,43,44 di(methanesulfonyl)-,45 and

di(trifluoromethanesulfonyl)benzenes42 with a Ni[0] catalyst in the presence of excess

zinc (Fig. 1-9d,e). Stille coupling has been used for the synthesis of PPP's (Fig. 1-9f).46

In addition, the thermal cyclization of enediynes and o-phenyldiynes gave PPPs (Fig. 1-

9g) and poly(1,4-naphthylenes), respectively.47

This dissertation describes the synthesis and characterization of poly(p-
phenylene)s substituted with various alkyl and alkoxy sidechains. The appropriate

monomers were prepared and polymerized via Suzuki cross coupling methods. The

properties of the polymers were examined as a function of molecular weight and repeat

unit structure. This study focuses on the synthesis of poly(p-phenylene)s by the AA/BB

polymerization of 1,4-dibromobenzenes substituted with cationic and polar nonionic

groups.










40Rulkens, R.; Schuize, M.; Wegner, G. Macromol. Rapid Commun. 1994, 15, 669.
41Child, A.D.; Reynolds, J.R. Macromolecules 1994, 27, 1975.
42Percec, V.; Okita, S.; Weiss, R. Macromolecules, 1992, 25, 1816.
43Wang, Y.; Quirk, R.P. Macromolecules, 1995, 28, 3495.
44Kaeriyama, K.; Mehta, M.A.; Masuda, H. Synth. Met. 1995, 507.
45percec, V; Bae, J.-Y.; Zhao, M.; Hill, D.H. Macromolecules 1995, 28, 6726.
46Qian, X.; Pena, M. Macromolecules, 1995, 28, 4415.
47John, J.A.; Tour, J.M. J. Am. Chem. Soc. 1994, 116, 5011.














CHAPTER 2
MONOMER SYNTHESIS AND MODEL STUDIES


Introduction


The Suzuki Cross-Coupling Reaction


The selective cross coupling of sp2 hybridized organic halides and triflates with

organoboron compounds in the presence of a negatively charged base1 or fluoride ion2 is

known as the Suzuki cross coupling reaction. Base is required for formation of a reactive

boronate species, and for the destruction of boric acid which is produced in the reaction

process. The general Suzuki cycle involves (i) oxidative addition of an aryl halide (or

other sp2 C-X species) to Pd[0]; (ii) transmetallation, wherein a second aryl group is

transferred from boron to Pd; and (iii) reductive elimination of a biaryl species.

Oxidative addition is often the rate determining step, with electron withdrawing groups

facilitating the reaction. The reaction rate is also dependent upon the nature of the

leaving group, with I > OTf > Br >> Cl.3 The transmetallation step may be rate limiting,

particularly if the arylboronate is sterically hindered,4 and may involve more than one

pathway. The most straightforward vision of the transmetallation step is that shown in

Figure 2-1, wherein an anionic "ate" complex of the arylboronate reacts with the

arylpalladium(II) halide.5 ArB(OH)3- is considerably more reactive in terms of the

nucleophilicity of the Ar group than ArB(OH)2,6 and is present in aqueous solution at pH

I(a) Miyaura, M.; Yamada, K.; Suzuki Tetrahedron Lett. 1979, 20, 3437. (b) Miyaura, N.; Yamada, K.;
Suginome, H.; Suzuki, A. J. Am. Chem. Soc. 1985, 107, 972.
2Wright, S.W.; Hageman, D.L.; McClure, L.D. J. Org. Chem. 1994, 59, 6095.
3For a review of the Suzuki reaction see: Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
4Watanabe, T.; Miyaura, N.; Suzuki, A. Synlett 1992, 207.
5Negishi, E. Aspects of Mechanism and Organometallic Chemistry; Brewster, J.H., Ed.; Plenum Press:
New York, 1978; p 285.
6Onat, T. Organoborane Chemistry; Academic Press: New York, 1975.









11-12.7 Alternatively, the halide can be displaced by base, particularly alkoxide, to form

an intermediate organopalladium alkoxide which may then react with a neutral arylboron

compound.8 When transmetallation is slow relative to oxidative addition, self-coupling

between two arylboronate species can occur.9 The carbonylation of aryl and alkenyl

boronates by Pd[0] coupling,10 and the Pd[0] catalyzed cross-coupling of aryl and alkenyl

boronates with alkenes have also been reported. 1


Ar-Ar'


reductive
elimination
iii


Ar\

ArPdL2
Ar,/


PdL4


ArX


oxidative
addition
i


T /Ar
L2Pd
X


transmetallation
,_ ii


K2C03
+
H2o


B(OH)3
KX
f KX


Ar'B(OH)3K+
+
KHCO3


Ar'B(OH)2
+
K2CO3
+
H20


B(OH)4 K+


KHCO3

Figure 2-1. Suzuki cycle. (i) Oxidative addition; (ii) transmetallation; and (iii) reductive
elimination. From: (a) Suzuki, A.; Miyaura, N. Chem. Rev. 1995, 95, 2457. (b) Smith, G.B.; Dezeny,
G.C.; Hughes, D.L.; King, A.O.; Verhoeven, T.R. J. Org. Chem. 1994, 59, 8151.



7Norrild, J.C.; Eggert, H. J. Am. Chem. Soc. 1995, 117, 1479.
8(a) Maitlis, P.M. The Organic Chemistry of Palladium; Academic Press: New York, 1971; Vol. 2, pp 119-
120. (b) Anderson, C.B.; Burreson, B.J.; Michalowski, T.J. J. Org. Chem. 1976, 41, 1990.
9(a) Campi, E.M.; Jackson, W.R.; Marcuccio, S.M.; Naeslund, C.G.M. J. Chem. Soc., Chem. Commun.
1994, 2395. (b) Gillmann, T.; Weeber, T. Synlett 1994, 649. (c) Song, Z.Z.; Wong, H.N.C. J. Org. Chem.
1994,59, 33.
100he, T.; Ohe, K.; Uemura, S.; Sugita, N. J. Organomet. Chem. 1988, 344, C5.
11Cho, C.S.; Uemura, S. J. Organomet. Chem., 1994, 465, 85.








Other difficulties involving coupling reactions of electron rich species with Pd[0]

apparently arise as a result of exchange reactions involving catalyst stabilizing ligands,12

and competitive hydrolysis of the carbon-boron bond.13 Base catalyzed hydrolysis of the

C-B bond is more prevalent when electron donating groups are present on the phenyl

ring.14 Certain transition metal salts have been shown to accelerate the rate of base

catalyzed hydrolysis,15 limiting the use of catalysts based on those metals, nickel and

copper included, to reactions involving anhydrous conditions. Organopalladium

complexes are typically less reactive than the corresponding nickel complexes, due

primarily to their reduced tendency to readily interconvert between square planar and

tetrahedral forms.16'17 Loss of functionality through hydrolysis of the arylpalladium(II)

halide intermediate is apparently negligible relative to losses due to hydrolysis of the

ArB(OH)3- species.

The Stille cycle is similar to the Suzuki cycle, but arylstannanes are used rather

than boronates. Negishi coupling utilizes arylzinc complexes.18 Stille and Negishi

reactions are typically run under anhydrous, anaerobic conditions. Suzuki coupling

reactions are most commonly employed under anaerobic heterogeneous organic/aqueous

conditions, although the reaction proceeds more quickly in homogeneous media. 19,20

Recently, a mechanism was suggested for a Pd[0] catalyzed self-coupling of arylboronic

acids which proceeds under aerobic, base free conditions.21 Yields reported were

significantly lower than those obtained by Suzuki coupling, but apparently boronic acid

coupling occurs in the presence of aryl halides.


120'Keef, D.F.; Dannock, M.C.; Marcuccio, S.M. Tetrahedron Lett. 1992, 33, 6679.
13Muller, D.; Fleury, J.-P. Tetrahedron Lett., 1991, 32, 2229.
14Kuivial, H.G.; Reuwer, J.F., Jr.; Mangravite, J.A. Can. J. Chem. 1963,41, 3081.
15Kuivial, H.G.; Reuwer, J.F., Jr.; Mangravite, J.A. J. Am. Chem. Soc. 1964, 86, 2666.
16Gillie, A.; Stille, J.K. J. Am. Chem. Soc. 1980, 102, 4933.
17Ozawa, F.; Ito, T.; Nakamura, Y.; Yamamoto, A. Bull. Chem. Soc. Jpn 1981, 54, 1868.
18(a) Negishi, E.; Baba, S. J. Chem. Soc., Chem. Commun. 1976, 596. (b) Negishi, E. Acc. Chem. Res.
1982,15, 340.
19(a) Suzuki, A. Pure Appl Chem. 1985, 57, 1749. (b) See reference 13.
20For a review of Stille coupling see: Stille, J.K. Angew. Chem. Int. Ed. Engl. 1986, 25, 508.
21Moreno-Mafias, M.; P6rez, M.; Pleizats, R. J. Org. Chem. 1996, 61, 2351.








Phosphine stabilized palladium catalysts are typically employed in Suzuki, Stille

and Negishi coupling reactions. Such catalysts as Pd(PPh3)4 are thermally stable and

easily prepared, but they suffer an order of magnitude reduction in reaction rate relative

to phosphine free catalysts such as Pd(OAc)2, [43-C3H5)PdCl]2, and Pd2(dba)3-C6H6.22

A second difficulty with Pd(PPh3)4 catalysis is aryl scrambling (see Figure 2-2). Phenyl

groups originating in the triphenylphosphine moiety of the catalyst can migrate to the

palladium center and become incorporated into the coupling process. The extent of


Me

Me3Sn z N N .- N

o::)OMe DMF OMe
+ (Ph3P)4Pd 21.9%
5 mol % "\ +
-- 10C5 o C
Br-& /OMe

OMe
54.8 %
Figure 2-2. Aryl scrambling in the Stille coupling reaction. From: Segelstein, B.E.; Butler,
T.W.; Chenard, B.L. J. Org. Chem., 1995, 60, 12.


scrambling has been found to increase dramatically when electron donating groups are in

a resonance favorable ortho or para position on the aryl halide. In addition, phenyl

groups were found to enter the cycle from added tetraphenylphosphonium ion. This

process was also facilitated by the presence of an electron donating methoxy group on the

aryl halide.23 Similarly, methyl/phenyl exchange has been shown to compete with

transmetallation in the related reaction of methylpalladium(II) halides, although the

formation of free phosphonium ion was not observed.24 A modified Stille cycle, which

accounts for aryl scrambling and free phosphonium ion equilibrium, has been proposed

22(a) Beletskaya, I.P. Bull. Acad. Sci. USSR, Div. Chem. Sci. (Engl. Transl.) 1990, 39, 2013. (b) Wallow,
T.I.; Novak, B.M. J. Org. Chem. 1994, 59, 5034.
23Kong, K.-C.; Cheng, C.-H. J. Am. Chem. Soc. 1991, 113, 6313.
24Morita, D.K.; Stille, J.K.; Norton, J.R. J. Am. Chem. Soc. 1995, 117, 8576.








(Figure 2-3), with the stipulation that the Suzuki cycle is similar enough so as to be

affected by the same factors.


Ar+PPh3 + Pd
vi


ArPh3P /PPh3 ArPh2P\ /PPh3 {Ar'B(OH)3K+}
ArX Pd Pd
P/ \ P/ \ Ar'SnMe3
Ar X iv Ph X

iU
PdLn

\ ii /
Ii Ph3P\ /PPh3 ArPh2P\ /PPh3 Me3SnX

Ar-Ar' Pd Pd B(OH)3
and/or Ar/ Ar' v Ph/ \Ar'
PhAr'

Figure 2-3. Modified Stille cycle. (i) Oxidative addition; (ii) transmetallation; and (iii)
reductive elimination; (iv, v) aryl exchange; (vi) free phosphonium ion formation. From:
Segelstein, B.E.; Butler, T.W.; Chenard, B.L. J. Org. Chem., 1995, 60, 12.


Nickel Mediated Coupling of Aryl Halides


The first use of transition metal catalysis for the synthesis of poly(p-phenylene)

marked the beginning of a new era in conducting polymer chemistry, as this methodology

has since been applied to a great variety of aromatic hydrocarbon and heterocyclic

systems. The original coupling methodology utilized nickel salts to catalyze the

homocoupling of dibromobenzenes in the presence of one equivalent of magnesium

metal.25 The catalytic cycle is similar to the Suzuki cycle (refer to Figure 2-1), except

that an intermediate Grignard reagent is formed which participates in the transmetallation

step. Successful polymerization requires a stoichiometric addition of magnesium.



25(a) Yamamoto, T.; Hayashi, Y.; Yamamoto, Y. Bull. Chem. Soc. Jpn 1978, 51, 2091. (b) Rehahn, M.;
Schltiter, A.-D.; Wegener, G.; Feast, W.J. Polymer 1989, 30, 1054.








Alternatively, it was found that homocoupling of aryl bromides and iodides could

be achieved in good yields when zero-valent nickel complexes were employed.26 Such

complexes can be generated in situ,27 and if a stoichiometric amount of zinc dust is

employed, the reaction becomes catalytic in nickel.28 The use of excess zinc in DMF

allows the rapid formation of biaryls from aryl chlorides.29. This methodology has been

extended to include the homocoupling of aryl mesylates30 and triflates,31 and has been

utilized for the synthesis of functionalized PPPs.32,33,34,35 A catalytic cycle for this Ni

mediated coupling in polar solvents has been proposed (see Figure 2-4),29 and has been

supported by electrochemical studies.36 In this mechanism, the aryl halide can enter the

catalytic cycle by oxidative addition to Ni[0] or Ni[I] species. Biaryl product formation

occurs by reductive elimination from the Ni[III] complex. Zinc serves a reducing agent

for Ni[III] and Ni[II] species, supplying the electrons ultimately needed for carbon-

carbon bond formation.













26Semmelhack, M.F.; Helquist, P.M.; Jones, L.D. J. Am. Chem. Soc. 1971, 93, 5908.
27Kende, A.S.; Liebeskind, L.S.; Braitsch, D.M. Tetrahedron Lett. 1975, 3375.
28Zembayashi, M.; Tamao, K.; Yoshida, J.; Kumada, M. Tetrahedron Lett. 1977, 4089.
29Colon, I.; Kelsey, D.R. J. Org. Chem. 1986, 51, 2627.
30Percec, V.; Bae, J.; Zhao, M.; Hill, D.H. J. Org. Chem. 1995, 60, 176.
31Yamashita, J.; Inoue, Y. Kondo, T.; Hashimoto, H. Chem. Lett. 1986, 407.
32(a) Percec, V.; Okita, S.; Weiss, R. Macromolecules, 1992, 25, 1816. (b) Percec, V.; Okita, S.; Bae, J.
Polym. Bull. 1992, 29, 271. (c) Percec, V; Bae, J.-Y.; Zhao, M.; Hill, D.H. Macromolecules 1995, 28,
6726. (d) Percec, V.; Pugh, C.; Cramer, E.; Okita, S.; Weiss, R. Makromol. Chem. Macromol. Symp. 1992,
54/55, 113. (e) Grob, M.C.; Feiring, A.E.; Auman, B.C.; Percec, V.; Zhao, M.; Hill, D.H. Macromolecules
1996,29,7284.
33(a) Phillips, R.W.; Sheares, V.V.; Samulski, E.T.; DeSimone, J.M. Macromolecules, 1994, 27, 2354. (b)
Wang, Y.; Quirk, R.P. Macromolecules, 1995, 28, 3495.
34Kaeriyama, K.; Mehta, M.A.; Masuda, H. Synth. Met. 1995, 507.
35Yamoto, T.; Sugiyama, K; Kushida, T.; Inoue, T.; Kanbara, T. J. Am. Chem. Soc. 1996, 118, 3930.
36Amatore, C.; Jutand, A. Organometallics 1988, 7, 2203.








Ni[II]C12L2

,- Zn, L


1/2 ZnX2

1/2 Zn



L
I
L -Ni[I]- X

L


Ni[0]L3


L L L
\/
Ar- Ni[III]-- Ar
XI


L L 1/2Zn -
Ar -Ni[III]- Ar 1/2 ZnX2 /

X


L
I
Ar- Ni[II]- X
I
L

1/2 Zn


1/2 ZnX2
L
I
-Ni[I]- L
L


L ArX

Figure 2-4. Proposed catalytic cycle for nickel catalyzed homocoupling of aryl halides,
mesylates, and triflates in polar solvents with excess zinc. Taken from: Percec, V.; Bae, J.;
Zhao, M.; Hill, D.H. J. Org. Chem. 1995, 60, 176.


Alkoxy-Substituted Poly(p-Phenylene)s by Suzuki Polymerization


Given that transition metal mediated homo- and heterocoupling reactions proceed

most efficiently with electron withdrawing groups on the benzene ring,29'37 it comes as

no surprise that the majority of substituted PPP's reported have sidechains which are

attached to the benzene ring via aryl/alkyl or aryl/carbonyl linkage. When Suzuki

coupling was employed to couple 2,5-dialkoxy-4-bromophenylboronic acids (AB

methodology), significantly lower degrees of polymerization (10-32 rings) were achieved


37Segelstein, B.E.; Butler, T.W.; Chenard, B.L. J. Org. Chem. 1995, 60,12.


Ar-Ar


L








over longer reaction times than were achieved with the dialkyl analogs (up to 100 rings)

(see Figure 2-5).38 The polymerization showed solvent dependence, with


Structure R or R/R' X a

SC4H9 32 (30b)
OR
OR C8H17 21

C12H25 10
RO i-C5Hjc 24



OR OR'
/ C4H9/C8H17d 22
x y
SC4H9/C12H25d 22
RO R'O

aBy NMR end-group analysis C3-Methylbutyl (isopentyl)
bBy VPO dRandom composition assumed: x/y = 1

Figure 2-5. Dialkoxy PPPs by Suzuki coupling polymerization of AB type monomers to
produce homopolymers and copolymers.


polymerization enhancement in aqueous basic DMF relative to the strictly two phase

toluene/aqueous Na2CO3 system.38 Also, it should be noted that the boronic acid group

present in the AB monomer is itself an electron withdrawing group which can enhance

the rate determining oxidative addition step.

AA/BB polymerization methodology was used for the polymerization of 2,5-di(3-

sulfonatopropyloxy)-l,4-dibromobenzene with 1,4-benzenediboronic acid in aqueous

basic DMF.39 High quality film forming polymers were obtained. Initially, a water

soluble Pd[0] catalyst based on sulfonated triphenylphosphine was used.40 Later, this

catalyst was replaced by Pd(OAc)2. The negative electronic effects of electron donating


38Vahlenkamp, T.; Wegner, G. Macromol. Chem. Phys. 1994, 195, 1933.
39(a) Child, A.D.; Reynolds, J.R. Macromolecules 1994, 27, 1975. (b) Kim, S.; Child, A.D.; Reynolds,
J.R. Unpublished results.
40Casalnuovo, A.L.; Calabrese, J.C. J. Am. Chem. Soc. 1990, 112, 4324.








groups towards polymerization were apparently offset by the maintenance of a

homogeneous, or nearly homogeneous, polar reaction medium. These materials were

photo- and electroluminescent, giving off blue light.

In this dissertation, work has been extended to include two dicationic homologues

of PPP, and to further explore the effectiveness of Suzuki coupling for the formation of

neutral, alkoxy substituted poly(p-phenylene)s. Towards the latter end, new dialkoxy

monomers with a high degree of polar character were synthesized. It was proposed that

monomers containing oligoethylene oxide sidechains would allow the utilization of

nearly homogeneous aqueous/organic reaction media for the polymerization reactions.

Triethoxy groups, as well as other oligoethyleneoxy sidechains, have been used to

solubilize rigid polyesters,41 poly(p-phenylene vinylene)42 and polyphosphazenes.43

Besides solubilizing the polymers, these cation coordinating sidechains impart a range of

interesting properties, such as ionic conductivity, to an already interesting class of

materials.44 The monomers synthesized in this work were polymerized under various

conditions, using Pd(OAc)2 as catalyst. Terphenyl model compounds were synthesized

under the same conditions to provide assistance in polymer analysis, and for the purpose

of gaining better insight into the polymerization reaction. To compare the reactivities and

to contrast the physical properties of alkyl and alkoxy substituted PPPs, two similar alkyl

monomers were synthesized and polymerized. Aryl dichloride monomers were likewise

prepared for nickel catalyzed homopolymerization.








41(a) Bhowmik, P.K.; Garay, P.O.; Lenz, R.W. Makromol. Chem. 1991, 192, 415. (b) Lenz, R.W.;
Furukawa, A.; Bhowmik, P.K.; Garay, R.O.; Majnusz, J. Polymer, 1991, 32, 1703.
42Garay, R.O.; Mayer, B.; Karasz, F.E.; Lenz, R.W. J. Polym. Sci., PartA.:Polym. Chem. 1995, 33, 525.
43(a) Blonsky, P.M.; Shriver, D.F.; Austin, P.E.; Allcock, H.R. J. Am. Chem. Soc. 1984, 106, 6854. (b)
Allcock, H.R.; Napierala, M.E.; Cameron, C.G.; O'Connor, S.J.M. Macromolecules, 1996, 29, 1951. (c)
Allcock, H.R.; Kuharcik, S.E.; Reed, C.S.; Napierala, M.E. Macromolecules, 1996, 29, 3384.
4Lauter, U.; Meyer, W.H.; Wegner, G. Macromolecules, 1997, 30, 2092-2101.








Results and Discussion

General

All products were characterized using 1H and 13C NMR spectroscopy, and

elemental analysis. Melting points were taken for all solids. Selected FT-IR spectra were

recorded. Polymers were further characterized by gel permeation chromatography (GPC)

and UV-Vis spectroscopy. Selected polymers were examined by differential scanning

calorimetry (DSC), thermogravimetric analysis (TGA), cyclic voltammetry, and

fluorescence spectrophotometry. All data are consistent with the proposed structures.

Yields, unless otherwise noted, represent yields after purification. These data are listed in

Chapter 5: EXPERIMENTAL, and are selectively discussed herein.

Monomer Synthesis

Dialkoxy (glyme) monomers. The syntheses of dialkoxy monomers, 2,5-

dibromo- 1,4-bis(1,4-dioxapentyl)benzene (6) { or: 2,5-dibromo- 1,4-bis(methoxyethoxy)-

benzene; M-OR5; ArBr2(OCH2CH2OCH3)2} and 2,5-dibromo-l,4-bis(l1,4,7-

trioxanonyl)-benzene (7) {or: 2,5-dibromo-l,4-bistriethoxybenzene; M-OR9;

ArBr2[(OCH2CH2)3H]2} are illustrated in Figure 2-6. In the initial step, hydroquinone

(8) was dibrominated in 48% yield at room temperature in a 50/50 (V/V) mixture of

glacial acetic acid and methylene chloride. Bromine (2.1 eq.) was added dropwise over

four hours to a vigorously stirred suspension of hydroquinone in the solvent mixture.

Part way through the addition, the solids dissolved, presumably with the formation of

monobromohydroquinone. As the reaction proceeded to completion, relatively pure 2,5-

dibromohydroquinone (9) precipitated from solution.

Tosylates (10, 11) of 2-methoxyethanol (12) and 2-(2-ethoxyethoxy)ethanol (13)
were prepared by dropwise addition of an excess of triethyl amine into a stirred solution

of the appropriate alcohol and p-toluenesulfonyl chloride in acetone or DMF. Product









OH OH K2CO3
Br2 acetone
S CH2C12 /HOAc Br Br A 0
OH OH 11
8 11
9 [48%] (68%)
0
K2CO3 0
K acetone
O 10 Br -Br

Br --Br (72%)


M-OR5 M-OR9 0

o 7



@ 0 ~ TsC1 T
HO ^-- -' ------- IsO ^
12 Et3N / acetone / r.t. 10
(67%)

TsCl
HO- O'-- 0-- O--,-TsC1- TsO- /O @ 0 ,
Et3N / acetone / r.t.
13 11
(81%)

Figure 2-6. Synthesis of methoxyethoxy- and triethoxy-substituted dibromobenzene
monomers suitable for the generation of alkoxy-substituted poly(p-phenylene)s via
Suzuki cross coupling polymerization.


was obtained by extraction from cold aqueous HC1 solution with methylene chloride,

which was then removed, after washing with 10% aq. NaOH and water, by reduced

pressure thin film rotary evaporation (rotovap). Extraction with diethyl ether instead of

with methylene chloride resulted in greater product contamination with starting alcohol.

By carefully controlling the addition of base so as to preclude excessive warming, the

tosylates were obtained in good yield as colorless oils, without the need for further

purification.45 When p-toluenesulfonyl chloride was used in excess, or when a

45Based on GC, NMR and elemental analysis results.








significant exotherm was observed, it was necessary to flash the product mixture through

a silica gel column to remove residual p-toluenesulfonyl chloride, p-toluenesulfonic acid,

and a green colored byproduct. Similar results were obtained using pyridine in place of

triethyl amine.

Monomers M-OR5 and M-OR9 were obtained by Williamson ether synthesis in

72% and 68% yields, respectively. Dibromohydroquinone was refluxed for several days

in deaerated acetone with an excess of the appropriate tosylate and an excess of

anhydrous K2C03. Similar treatment of 2,5-dichlorohydroquinone with 11 gave 2,5-

dichloro-l,4-bis(1,4,7-triozanonyl)benzene (14) in 77% yield. When dibromo-

hydroquinone was reacted with 2-methoxybromoethane, M-OR5 was obtained in 12%

yield. No significant product was obtained when 2-methoxychloroethane was used as the

alkylating agent. Representative NMR spectra of M-OR5 and M-OR9 are shown in

Figures 2-7 and 2-8.

Dialkoxy (amine) monomers. The synthesis of 2,5-bis(3-[N,N-diethyl-amino]-l-

oxapropyl)- 1,4-dibromobenzene (M-NMe2, 15) and 2,5-bis(3-[NN-dimethyl-amino]-1-

oxapropyl)-l,4-dibromobenzene (M-NEt2, 16) is illustrated in Figure 2-9. The route to

15 and 16 is similar to that taken for the synthesis of M-OR5 and M-OR9. 2,5-

Dibromohydroquinone (9) was treated with an appropriate alkylchloride in refluxing

acetone in the presence of anhydrous potassium carbonate. M-NMe2 and M-NEt2 were

synthesized using 2-chloroethyldimethyl amine hydrochloride (17a) and 2-chlorotriethyl

amine hydrochloride (17b) in 29% and 38%, respectively. These were unoptimized

yields of the purified products. Recall that this Williamson ether synthesis route failed

for M-OR5 production when an alkylchloride was used in place of an alkylbromide.

Reactivity differences can be explained by in situ aziridinium ion (18) formation.

M-NEt2 was quatemrnized in 93% yield by stirring 30 days at room temperature in

bromoethane. Elemental analysis showed the product, M-NEt3+ (19) to be ca. 98%


















b


*CDC13 ,


j


M-OR5
6


6 5 4 3


(b) Tentative Shift Assignments
Peak Observed (ppm)

b 7.16(s, 2 H)
i 4.14 (t, J =4.8 Hz, 4 H)
j 3.88 (t J =4.6 Hz, 4 H)
m 3.76 (t, J =4.6 Hz, 4 H)
n 3.62 (t, J =4.6 Hz, 4 H)
o 3.55 (q, J =6.9 Hz, 4 H)
p 1.22 (t, J =6.9 Hz, 6 H) o

b n







J I I
it____JLii


7 6


ppm


o~p

Sm


j

0


Br-


5 4 3 2


M-OR9
7
P


1 ppm


Figure 2-7. 300 MHz proton NMR spectra of dialkoxy monomers M-OR5 and M-OR9.
(a) M-OR5. (b) M-OR9.


Tentative Shift Assignments
Peak Observed (ppm)
b 7.14(s, 2 H)
i 4.11 (t, J = 4.7Hz, 4 H)
j 3.76 (t, J = 4.7 Hz, 4 H)
k 3.46 (s, 6 H)


Ji J


__L












j M-OR5
6


80 60


40 20 ppm


Tentative Peak Assignments
_____(ppm)_____
Peak Calcd Observed
a 157.8 150.25
b 120.7 119.08
c 108.7 111.30
i 72 71.11
j 72 70.17
m 72 69.88
n 72 69.52
o 64 66.65
p 14 15.14
*CDC13 b


160 140


80 60 40 20


Figure 2-8. Carbon 13 NMR spectra of dialkoxy monomers M-OR5 and M-OR9. (a)
M-ORS. (b) M-OR9. For chemical shift calculations see APPENDIX A.


Tentative Peak Assignments
(ppm)
Peak Calcd Observed
a 157.8 150.24
b 120.7 119.10
c 108.7 111.36
i 72 70.75
j 72 69.97
k 55 59.31


M-OR9
7



Br-
j m


a b


ppm


' . i . i . i . . . . i .... ...... .... . . . . . . . . . .











H
I +
R' R


Cl


K2C03
acetone


C1


R'/ R


Cl


17a,b


R+/R

or A HCO3
or Cl

24a,b


OH OH .
Br2 Br -Br 15 -
S CH2C12 / AcOH K2C03 N 16 -
HO HO 9 acetone R R

Figure 2-9. Synthesis of 2,5-bis(N,N-dialkylaminoethoxy)- 1,4-dibromobenzenes.


R
CH3
CH2CH3


quatemized. When M-NEt2 was stirred 5 days in acetonitrile with excess bromoethane,

M-NEt3+ was obtained in 93% yield. By elemental analysis, nearly quantitative

alkylation was achieved. Proton NMR, on the other hand (see Figure 2-10), reveals

unreacted tertiary amine sites. Comparison of the weighted integrals observed in the

NMR spectrum reveals that only 94% conversion was achieved. Proton NMR spectra of

M-NMe2, M-NEt2, and N-Et3+ are shown in Figures 2-10 to 2-12. Carbon 13 NMR

spectra can be found in Appendix A.


\ m
Br b i N-

0- 0

- Br M-NME2
\ 15


8 7 6 5 4 3

Figure 2-10. Proton NMR spectrum of M-NMe2 in CDC13.


* CHC13


2 1


ppm


bi


2









~ m
b i N
0Q n
Br M-NEt2
16


8 7 6 5 4 3
Figure 2-11. Proton NMR spectrum of M-NEt2 in CDC13.


m
Br- Br b i N
Br + \ n


*H20
** DMSO


2 1 ppm


k-=Br Br- mn
VN Br(^___ -

M-NEt3+
b 19

M**n
r jl.Jv11 A '
8 7 6 5 4 3 2 1 ppm
Figure 2-12. Proton NMR spectrum of M-NEt3+ in DMSO-d6/D20.

Dialkyl (glyme) monomers. The synthesis of dialkyl monomers, 2,5-dibromo-
1,4-bis(2,5-dioxahexyl)benzene (20) { or: 2,5-dibromo- 1,4-bis[(methoxyethoxy)methyl]-
benzene; M-R6; ArBr2(CH2OCH2OCH3)2} and 2,5-dibromo-l,4-bis(2,5,8-trioxadecyl)-
benzene (21) {or: M-R10; ArBr2[CH2(OCH2CH2)3H]2; 2,5-dibromo-l,4-
bis[(triethoxy)-methyl]benzene} is illustrated in Figure 2-13. Initially, 2,5-dibromo-p-
xylene (22) was brominated under free radical conditions using N-bromosuccinimide








(NBS) as the bromine source, and azobisisobutyronitrile (AIBN) as the free radical

initiator. The reaction was run in refluxing carbon tetrachloride and in refluxing benzene.

Yields were similar in the two solvents, ca. 26%. In both cases, the reaction mixture was

refluxed for 4-6 hours, allowed to stand overnight, and filtered to remove succinimide.

The resulting primary filtrate was washed, dried, and diluted with pentane. Upon


CH3

Br /--Br

CH3


CH2Br
NBS/AIBN Br
benzene / A -Br Br
(26%) CH2Br


NaOH


12
(72%)


M-R6

20


Figure 2-13. Synthesis of (methoxyethoxy)methyl- and (triethoxy)methyl substituted
dibromo-benzene monomers suitable for the generation of alkyl-substituted poly(p-
phenylene)s via Suzuki cross coupling polymerization.


standing in the freezer, white crystals of oa,c',2,5-tetrabromo-p-xylene (23) formed which

were sufficiently pure for further reaction. Attempts to further isolate product from the

secondary filtrate met with failure; crystalline mixtures of mono-, di-, tri-, and

unbrominated p-xylenes were obtained. NBS bromination of 2,5-dichloro-p-xylene in

carbon tetrachloride gave 2,5-dibromo-ca,oa'-dichloro-p-xylene (24) in 35% yield.

Separate treatment of 23 with 12 and 13 in the presence of NaOH provided 20 and

21 in 80% and 52% yields, respectively. A suspension of 23 in the appropriate alcohol


NaOH


13
(66%)


M-R10

21








was stirred vigorously with an excess amount of NaOH pellets. As the pellets dissolved,

23 disappeared. The reaction proceeded quickly with the lower weight, higher polarity

alcohol 12. Gas/liquid chromatographic (GC) analysis of a micromolar scale reaction

using 12 showed the reaction to be complete within 10 minutes. On a 20 g scale (of 23),

4 h of stirring was allowed to assure completion. Reaction of 23 with alcohol 13 required

overnight stirring. Product mixtures were diluted with water and extracted with a suitable

solvent, washed and crystallized over dry ice until pure by GC. Similarly, treatment of

2,5-dibromo-c,cL'-dichloro-p-xylene (24) with 13 gave 2,5-dichloro-l,4-di(2,5,8-

trioxadecyl)benzene (25) in 39% yield. Representative NMR spectra of M-R6 and M-

R10 are shown in Figures 2-14 and 2-15.

Synthesis of arylboronates and arylbisboronates. Arylboronic acids can be

synthesized from Grignard and lithium reagents by reaction of the reagent with boronates

or boranes, followed by hydrolysis.46 Herein, both methods have been applied, p-

Phenylenediboronic acid (26) was synthesized according to a literature47 Grignard route

(see Figure 2-16). 1,4-Dibromobenzene (28) was treated overnight with magnesium

metal in refluxing tetrahydrofuran (THF). Trimethyl borate was added to the chilled

reaction mixture, and the subsequent product was hydrolyzed with dilute aqueous acid.

Following extensive extraction with ether, the product was recrystallized from water.

Difficulties with this method are numerous. Incomplete formation of the di-Grignard

reagent, coupling to biphenyl and oligophenylenes, diarylation of a boronate moiety,

formation of polymeric THF, hydrolysis of the carbon-boron bond, and the presence of

large amounts of boric acid and magnesium salts all complicate product isolation and

purification. p-Phenylenediboronic acid was obtained in 50% yield by this route, but

difficulties increased with scale up, and results were inconsistent.




46Gerrard, W. The Chemistry of Boron; Academic Press: New York, 1961.
47Coutts, I.G.C.; Goldschmid, H.R.; Musgrave, O.C. J. Chem. Soc. (C), 1970, 488.













*CDC13
**H20



r


M-R6
20


3 2 1 ppm


Tentative Shift Assignments
Peak Observed (ppm)
b 7.67 (s, 2 H)
i, j 3.74 (s, 8 H)
1 4.58 (s, 4 H)
m 3.68 (m, 4 H)
n 3.63 (m, 4 H)
o 3.56 (q, J =7.1 Hz, 4 H)
p 1.22 (t, J =7.1 Hz, 6 H)





b


*


0

n


M-R10
i 21
21


-_Br


m
n
0O


7 6 5 4


3 2


1 ppm


Figure 2-14. 300 MHz proton NMR spectra of M-R6 and M-R10. (a) M-R6.
(b) M-R10.


Tentative Shift Assignments
Peak Observed (ppm)
b 7.68 (s, 2 H)
i 3.72 (t, J =4.6 Hz, 4 H)
j 3.62 (t, J =4.2 Hz, 4 H
k 3.42 (s, 6 H)
1 4.58 (s, 4 H)


8 7


6 5


^Mf--,---------------------------A------J.--------------------4---


F


. I I I I I I I Il I I I "I . I n I I I


. . . . . . . . . . . .. . .


I-






















*CDC13


160 140 120 100 80 60 40 20 ppm


M-R10
S21


i,j, m, n


80 60 40 20 ppm


Figure 2-15. Carbon 13 NMR spectra of M-R6 and M-R10.
chemical shift calculations see Appendix A.


(a) M-R6. (b) M-R10. For


M-R6
20


Tentative Peak Assignments
(ppm)______
Peak Calcd Observed
a 147.0 138.32
b 132.7 132.26
c 120.7 121.13
i 72 71.71
j 72 70.19
k 55 59.08
1 79 71.80


Tentative Peak Assignments
____(ppm)______
Peak Calcd Observed
a 147.0 138.37
b 132.7 132.15
c 120.7 121.01
i 72 70.75
j 72 70.50
1 79 71.61
m 72 70.26
n 72 69.81
o 64 66.62
p 14 15.13


140


nw~wvrv~w W OI! 4^^y ..^^^^^^^n'^


' I . . [ . I . i . . .I . . . . . . .1 . . . . . . . . . .








27 26
27 ~1) Mg /Et20 2
Br / Br 2) B(OCH3)3 Hq P: /H

(50%)
1) M)Mgg/EEt2t20
2) 2)B(OCH3)3 H HB(OCH3)3
Br3)Br OH OH
'-'*-'"1''HO OH
(50%)
1) Mg /Et20OX
2) B(OCH3)3 I
3) \ OH OH
28 DMF / toluene
OH OH \fk B ( A
B-- -_ H20
(39%) dv--/ 0 0O (66%)

Figure 2-16. Synthesis of bisneopentyl phenylenediboronate.


p-Phenylenediboronic (26) acid is hygroscopic. Attempts to generate anhydrous

diboronic acids via elevated temperature vacuum desiccation result in varying degrees of

polymeric anhydride formation. As a result, 26 presents difficulties when used in step

growth polymerizations, wherein exacting mass balance is required. These obstacles can

be overcome via careful 1H NMR determination of the water content.48 Alternatively,

the diacid can be converted into a nonhygroscopic cyclic diester (refer to Figure 2-16).46

Cyclic diesters of ethylene glycol,49 propylene glycol,50 and pinacol (2,3-dimethyl-2,3-

butanediol)51 have been used in Suzuki coupling reactions.

Esterification of 26 with neopentyl glycol by azeotropic distillation of water from

an NN-dimethylformamide (DMF) solution with toluene gave bisneopentyl p-phenylene-

diboronate (27) in 66% yield, based on 26. A more convenient synthetic approach is to

generate the cyclic ester directly from the dimethyl ester, and so avoid isolation and

subsequent purification of the diboronic acid.50 Thus, following the formation of

bisdimethyl phenylenediboronate, neopentyl glycol was added to the reaction mixture,

and the mixture was stirred overnight to affect transesterification. The subsequently

48Karakaya, B.; Claussen, W.; Schafer, A.; Lehmann, A.; Schlifter, A.-D. Acta Polymer. 1996, 47, 79.
49Wallow, T.I.; Novak, B.M. J. Am. Chem. Soc. 1991, 113, 7411.
50(a) Goldfinger, M.B.; Swager, T.M. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1993, 34(2),
755. (b) Goldfinger, M.B.; Swager, T.M. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1994, 35(1),
273.
51Lamba, J.J.S.; Tour, J.M. J. Am. Chem. Soc. 1994, 116,11723.








formed dicyclic diester 27 was separated from insoluble salts by filtration through celite.

Following further purification, 27 was obtained in 39% yield, based on 28.

Monofunctional arylboronic acids and esters were similarly produced for the

purpose of model compound studies, and for use as end capping agents in polymer

syntheses (see Figure 2-17). 4-Methylphenylboronic acid (29) was prepared by treatment

of 4-bromotoluene (30) with n-butyl lithium in THF at -77C, followed by the addition of

trimethylborate and hydrolysis with aqueous HC1. Esterification of 29 with neopentyl

glycol proceeded smoothly using toluene as an azeotropic agent for the removal of water.

Neopentyl 4-methylphenylboronate (31) was obtained in 47% yield. Esters of ethylene

glycol and propylene glycol were obtained without use of an azeotropic solvent. A


1. n-BuLi / THF
C- 2. B(OCH3)3 P- OH
H3C Br ~ H3 \ B\
3. H30+ OH

30 (40%) 29


PH HO"ROH A=.
H3C- B OH H3C R B\R

29

R yield solvent product
-CH2C(CH3)2CH2- 47% toluene 31
-CH2CH2- 51,82% neat 32
-CH2CH2CH2- 77% neat 33

Figure 2-17. Synthesis of cyclic esters of 4-methylphenylboronic acid.


suspension of 29 in the appropriate diol was heated overnight at 130C under an inert

atmosphere. After cooling to room temperature the diols were extracted with pentane.

The pentane was treated with MgSO4, filtered and chilled over dry ice. White crystalline

products formed which required no further purification. Ethylene 4-

methylphenylboronate (32) and propylene 4-methylphenylboronate (33) were formed in








81% and 77% yields, respectively. Similar treatment of 4-t-butylphenylboronic acid52
(34) with ethylene glycol gave the cyclic ester 35 in 63% yield. Neopentyl
phenylboronate (36) was isolated in low yield from a direct conversion of 4-
bromotoluene via lithiation with n-butyl lithium. The aryl lithiate was treated
successively with trimethylborate and neopentyl glycol. Enough product was obtained
for characterization, and to proceed with model reactions, but yields were not optimized.

In a related synthesis (see Figure 2-18), monomer M-OR9 in ether was treated
with 2.5 equivalents of n-butyl lithium at reduced temperature in ether to generate the
aryl dilithiate, which was then treated with 5 eq. of trimethyl borate. Following

formation of the bis(dimethyl) aryl diboronate, the product was stirred overnight with

(OCH2CH2)3H (OCH2CH2)3H (OCH2CH2)3H
Sa,b,c PDc d H PH
BrT -- B --- L O B-O /O" J H' B /O
0 HO OH
H(CH2CH20)3 H(CH2CH20)3 H(CH2CH20)3
M-OR9 37 38
(a) 2.5 eq. n-BuLi; (b) 5 eq. B(OCH3)3; (c) ethylene glycol; (d) water. Overall yield: 26%

Figure 2-18. Synthesis of 2,5-bistriethoxy-l,4-benzenediboronic acid 35. The triethoxy
group is conveniently represented as -(OEt)3

120 eq of ethylene glycol to form the dicyclic diester 37. Ether was removed, and an
attempt was made to extract the product from the ethylene glycol with hexane. The
extraction failed due to the preferential solubility of the monomer in ethylene glycol
relative to hexane. Cold water was added to allow a favorable partitioning. Hydrolysis

of 37 to the diboronic acid 38 ensued, and 38 was extracted into hexane. After drying the
solution with MgSO4 and filtering, fine white crystals of 38 were obtained in 26% yield.





52Prepared by S. Kim according to methods in: Kim, S. Aromatic Polyelectrolytes Based on Sulfonated
Polybenzobisthiazoles and Sulfonatopropoxy-Substituted Poly(p-phenylenes), Ph.D. Dissertation,
University of Florida, 1995.







Model Systems

Model compound synthesis. Suzuki coupling polymerization may be classified as
step-growth condensation polymerization, since it involves stepwise reaction between
bifunctional monomers wherein components of those monomers, the functionality, are
lost. The single most important consideration in step condensation polymerization is
monomer reactivity, since polymer of reasonably high molecular weight cannot be
achieved with less than 98 to 99 percent functional group conversion (see Figure 2-19).53
A general rule for applying a reaction methodology to a polymer system is


2000 -
kn= l+r
1600 1 +r-2rp
where r is the stoichiometric ratio
1200 and p is the extent of the reaction
xn
800 1x for r= 1.000
1 -p
800 :- 1-pr

400 X =- for p= 1.000
400 i, =1 + forp =I.000
1-r

0 -- ----- \-
0.9 0.92 0.94 0.96 0.98 1
p or r

Figure 2-19. Average degree of polymerization (Xn) as a function of stoichiometric ratio
r and extent of functional group conversion p.

that the reaction between two monofunctional molecules be essentially quantitative.
While the Suzuki reaction has been used for the formation of phenylene polymers,
reaction yields are highly substrate dependent. Model reactions were carried out, and
model compounds were synthesized to examine the utility of the method for the

53Odian, G. Principles of Polymerization, 2nd ed. John Wiley & Sons: New York, 1981.








polymerization of monomers M-OR5, M-OR9, M-R6, and M-R10, and to aid in the

NMR characterization of subsequently synthesized polymers.

Monomers M-0R5 and M-R6 were coupled with 32 and with 36 in aqueous DMF

to give terphenyls in moderate yields. The couplings were run under argon at 60C using

Pd(OAc)2 and IM Na2CO3 (aq.). In each case the reaction medium was deaerated prior

to the addition of the catalyst. The formation of the active Pd[0] species was noted by the

immediate darkening of the reaction media. The couplings with M-OR5 were

accompanied by an almost immediate display of blue luminescence, observed with a hand

held UV lamp. Following a 24 h reaction period, the terphenyls so formed were collected

by precipitation in water. After washing with water and drying, the products were taken

up in methylene chloride, washed with water, dried with MgSO4 (anh.), treated with



R1 RB /\ R1
R20
Br Br (aq" X \/X\
Pd(OAc)2 / 1 M Na2CO3 (aq)
R1 DMF / 24 h / 60C RI

reactant RI X R2, R2 product yield
6 M-OR5 -OCH2CH2OCH3 -H -CH2C(CH3)2CH2- 39 T1-OR5 25%
6 M-OR5 -OCH2CH2OCH3 -CH3 -H,-H 40 T2-OR5 63%a
20 M-R6 -CH2OCH2CH2OCH3 -H -CH2C(CH3)2CH2- 41 T1-R6 70%
20 M-R6 -CH2OCH2CH2OCH3 -CH3 -H,-H 42 T2-R6 84%a

Figure 2-20. Synthesis of terphenyl model compounds via Suzuki cross coupling, aCrude
yield.


carbon to remove palladium, and filtered. Solvent was removed and the trimers

recrystallized. The reaction scheme and product yields are shown in Figure 2-20.

Terphenyls T1-R6 and T1-OR5 carried approximately 0.3 water molecules per trimer, as

shown by 1H NMR. This value was incorporated into the elemental analysis calculations

for these molecules.








Nuclear magnetic resonance (NMR) analysis of terphenyl model compounds. 1H

NMR analysis of the model compounds T1-OR5, T2-OR5, and T1-OR5 was straight

forward. All protons were assigned based on comparison with calculated values, which

corresponded reasonably well with the observed chemical shifts.54 Only the outer ring

protons of T1-R6 were poorly resolved. The chemical shift assignments of monomers

and model compounds are listed in Table 2-1. In the 300 MHz 1H spectrum of T1-OR5,

the 2',2" protons, designated He, and the 3',3" protons, Hf, appear at 7.62 and 7.46 ppm as

ortho coupled doublets and triplets, respectively, integrating to the expected 4 H (see

Figure 2-21a). Hg appears at 7.32 ppm as a 2 H triplet, Hb at 7.03 ppm as a 2 H singlet,

shifted upfield 0.11 ppm relative to its position (7.14 ppm) in M-OR5 (see also Figure 2-

7a). Similarly, in T2-OR5 Hb appears as a singlet at 7.01 ppm (see Figure 2-22). He

and Hf occur as 4 H doublets at 7.52 and 7.23 ppm, respectively. Notably, the side-chain

protons and T2-OR5 are shifted upfield 0.06


Table 2-1. 1H NMR shifts of monomers and model compounds.
compound and associated chemical shifts (ppm)
Protona M-OR5 T1-OR5 T2-OR5 M-R6 T1-R6 T2-R6
Hb 7.14 7.03 7.01 7.68 7.50 7.48
He 7.62 7.52 7.43 7.33
Hf 7.41 7.23 7.42 7.23
Hg 7.32 7.38 -
Hh 2.40 2.41
Hi 4.11 4.05 4.04 3.72 3.55 3.55
Hj 3.76 3.62 3.63 3.62 3.52 3.52
Hk 3.46 3.34 3.36 3.42 3.34 3.35
H1 4.58 4.48 4.48
aproton assignments are according to the labeling in Figures 2-7 to 2-10 and 2-18 to 2-21.


54For NMR peak calculations see Appendix A.








to 0.14 ppm relative to their positions in M-OR5. The magnitude of shift difference

follows the trend Hj > Hk >> Hi, indicating that the upfield shifts result from the through

space effect of diamagnetic ring currents in the outer rings, and are not the result of

inductive effects resulting from an interaction between the oxygen attached to Ca and He.

A similar trend is observed in the sidechains of the dialkyl system (see Figures 2-

10a, 2-24a, and 2-25a). In this case, the greatest upfield shift relative to the monomer

position occurs with Hi, which like Hj in the dialkoxy system, is 4 bonds removed from

the central aromatic ring. The magnetic environments of Hi and Hj in T1-R6 and T2-R6

are similar, such that they appear as two closely spaced leaning multiplets, resembling in

appearance a pair of triplets of doublets, but with variable coupling constants. As was the

case for the dialkoxy trimers, the chemical shift of the singlet Hb is shifted downfield in

T1-R6 and T2-R6 relative to M-R6. In T2-R6, He and Hf appear as distinct 4 H

doublets at 7.33 and 7.23 ppm, while in T1-R6 these peaks overlap with each other, and

partially overlap with Hg.

Peak assignments (Table 2-2) in the 13C NMR spectra are rather ambiguous,

despite the simplifications granted by molecular symmetry. Peak positions were

calculated by adding incremental shift values to a base value of 128.5 ppm (see

APPENDIX A). Initial assignments were made on a best fit basis to those preliminary

calculations. Peak height was taken into consideration (see Figures 2-21b and 2-22b);

the tallest aromatic peaks, for example, were assigned to Ce and Cf, which occur in

greatest abundance (4 C each). These carbons also benefit from the nuclear Overhauser

effect (NOE), wherein peak height in rapidly tumbling systems is increased through

enhanced carbon relaxation due to dipole-dipole interactions with attached protons.

Likewise, the smallest peaks were assigned to carbons lacking protons. On the basis of

these criteria, the carbons in T1-R6 were assigned with reasonable certainty. In T1-OR5,

the absolute identities of Ce and Cf were not directly confirmed. However, heteronuclear

correlation spectroscopy (HETCOR) allowed unambiguous assignment of Ce and Cf in








T2-OR5 (see Figure 2-23). Subtracting the incremental shift values of the methyl group

from the observed peak positions in T2-OR5 provided theoretical values for Ce and Cf in

T1-OR5 which matched observed values to within 0.1 ppm. Similarly, following

HETCOR analysis of T2-OR5, only Cd and Cg in that compound remained ambiguous.

By adding the incremental shift values of the methyl group to the assigned chemical

shifts of the corresponding carbons in T1-OR5, peak assignments were obtained.

Satisfactory HETCOR results were not obtained for the dialkyl model compounds

T1-R6 and T2-R6. Absolute assignments of Ce and Cf were not made (see Figures 2-24b

and 2-25b), and in general, peak assignments in T1-R6 and T2-R6 are somewhat

dubious. Nevertheless, tentative peak assignments were made. Based on anticipated

peak height and calculated chemical shifts, assignment of Ca and Cb in T2-R6, and Cc in

T1-R6 was straight forward. Since the peak positions of the central ring carbons must be

similar in the two molecules, assignment of Ca and Cb in T1-R6, and Cc in T2-R6 was

made by comparison, which in turn confirmed the position of Cg in T1-R6.


Table 2-2. 13C NMR shifts of monomers and model compounds.
compound and associated chemical shifts (ppm)
carbon M-OR5 T1-OR5 T2-OR5 M-R6 T1-R6 T2-R6
Ca 150.24 150.46 150.46 138.32 140.94 140.71
Cb 119.10 117.24 117.18 132.26 130.95 130.92
Cc 111.36 131.23 130.91 121.13 134.64 134.64
Cd 138.05 135.13 140.44 137.57
Ce 129.47 129.28 129.33 129.22
Cf 127.96 128.69 128.06 128.74
Cg 127.01 136.66 127.10 136.67
Ch 21.17 21.11
Ci 70.75 71.11 71.13 71.71 70.86 70.92
Cj 69.97 69.42 69.08 70.19 69.52 69.49
Ck 59.31 59.09 59.10 59.07 58.96 58.90
C1 71.80 71.84 71.86
aCarbon assignments are according to the labeling in Figures 2-7 to 2-10 and 2-22 to 2-25.










Chemical Shift Assignments (ppm)
Peak Calcd Observed
b 7.27 7.03 (s, 2 H)
e 7.63 7.62 (t, J =7.2 Hz, 4 H) k
f 7.46 7.41 (t, J = 7.3 Hz, 4 H)
g 7.36 7.32 (t, J = 7.2 Hz, 2 H)
i 4.33 4.05 (t, J = 4.9 Hz, 4 H)
j 3.98 3.62 (t, J = 4.8 Hz, 4 H)
k 3.30 3.34 (s, 6 H)
*CDC13 **H20 ***TMS


(a) b
ef J

I

_jtl__ _q


7 6 5


3 2 1 ppm


13C NMR


** g 126.9 127.01
k i 72 71.11
j 72 69.42
k 55 59.09
*Based on T2-OR5 HETCOR
**CHCl3


160 140


80 60 40 20 ppm


Figure 2-21. NMR spectra of T1-OR5 in CDC13. (a) Proton NMR spectrum. (b)
Carbon 13 NMR spectrum.


1HNMR


e f


T1-OR5
39


Chemical Shifts (ppm)
Peak Calcd Observed
a 150.3 150.46
b 113.2 117.24
c 125.6 131.23
d 140.6 138.05
e 129.4* 129.47
f 128.0* 127.96


a d c


.~~ I . II . . I . . .I. 1 . . .


r . I . I ~ ~ l l l l l l l l l l [ l l l i . . . . . . . . . . . . . . . . . . . . .









Chemical Shift Assignments (ppm)
Peak Calcd Observed

b 7.27 7.01 (s, 2H)
e 7.51 7.52 (d, 4H, J=8.1Hz)
f 7.26 7.23 (d, 4H, J=8.1 Hz)
h 2.27 2.40 (s, 6H)
i 4.34 4.04 (t, 4H, J=5.0 Hz)
j 3.98 3.63 (t, 4H, J=4.8 Hz)
k 3.30 3.36 (s, 6H)

b *H20 **TMS
e f


__ XJL^______JJ


'H NMR


e f


L--'/i C 9Y h
'ab g h
T2-OR5
) 40


*


7 6 5


"3C NMR


b Based on T1-OR5
a By HETCOR
*CHC13


g
a dc


-0-W--N*** ^i WIN ,MWw^i^ 0,01


4 3 2


k


80 60 40 20 ppm


Figure 2-22. NMR spectra of T2-OR5 in CDC13.
(b) Carbon 13 NMR spectrum.


(a) Proton NMR spectrum.


1 ppm


Chemical Shifts (ppm)
Peak Calcd Observed
a 151.5 150.46
b 114.4 117.18
c 127.0 130.91
d 135.2a 135.13
e 127.3 129.28b
f 129.6 128.69b
g 136.3a 136.66
h 21.4 21.17
i 71.7 71.13
j 71.7 69.08
k 55.7 59.10


..........










/

0 ef
CH CH3


( T2-OR5
0 39


F2
(ppm)
116

118

120

122

124

126

128

130

132


Connectivity


7.7 7.6 7.5 7.4 7.3 7.2 7.1 7.0 6.9


Fl (ppm)


Figure 2-23. Heteronuclear chemical shift correlation (HETCOR) spectrum of T2-OR5
at 300 MHz for 1H and 75 MHz for 13C.





















e,f




g
*


7 6


c
a,d


5 4 3 2 1 ppm


13C NMR


*CHC13


Based on a T1-OR5


"~V4 h%#A~WJ*i~WIA~ ~


160 140


80 60 40 20 ppm


Figure 2-24. NMR spectra of T1-R6 in CDC13: (a) Proton NMR spectrum.
13 NMR spectrum.


Chemical Shift Assignments (ppm)
Peak Calcd Observed
b 7.69 7.50 (s, 2 H)
e 7.63 7.43 (s, 4 H)
f 7.46 7.42 (d, J = 5.7 Hz, 4 H)
g 7.36 7.38 (m, 2H)
i 3.70 3.54 (m, 4 H)
j 3.70 3.52 (m, 4 H)
k 3.30 3.34 (s, 6 H)
1 5.00 4.48 (s, 4 H)


1H NMR


0


*CHC13

(a)


b


e f


T1-R6
41


Shift Assignments (ppm)
Peak Calculated Observed
a 139.5 140.94
b 125.2 130.95
c 137.6 134.64
d 140.5b 140.44
e 126.7 129.33a
f 128.4 128.06a
g 126.9 127.10
i 72 70.86
j 72 69.52
k 55 58.96
1 79 71.84


J k


b T2-R6


(b) Carbon


j








a) 1H NMR

Chemical Shift Assignments (ppm)
Peak Calcd Observed
b 7.59 7.48 (s, 2H)
e 7.51 7.33 (d, 4H, J=8.1Hz)
f 7.26 7.23 (d, 4H, J=8.4 Hz)
h 2.27 2.41 (s, 6H)
ij 3.70 3.52 (m, 8H)
k 3.30 3.35 (s, 6H)
1 5.00 4.48 (s, 4H)

b
e
if


k

) 1 e f
CH37- g CH3
(a b C h
0
h T2-R6
0\ 42

*TMS

*


7 6


13C NMR


5 4


e ll f


b




a
d C


a Based on T1-R6
b Based on T2-OR5 HETCOR
*CHC13


3 2


1 ppm


160 140 120 100 80 60 40 20 ppm


Figure 2-25. NMR spectra of T2-R6 in CDC13: (a) Proton NMR spectrum. (b) Carbon
13 NMR spectrum.


Chemical Shifts (ppm)
Peak Calcd Observed
a 139.5 140.71
b 125.2 130.92
c 137.6 134.64
d 138.7 137.57
e 127.3 129.22b
f 129.6 128.74b
g 136.4a 136.67
h 21.4 21.11
i 71.7 70.92
j 71.7 69.49
k 55.7 58.90
1 78.7 71.86








Adding the incremental chemical shift value of the c-methyl group to the chemical shift

of Cg in T1-R6 in turn confirmed the assignment of Cg in T2-R6. By process of

elimination, the tentative assignment of Cd was confirmed. Subtracting the p-methyl

incremental chemical shift value from the observed chemical shift of Cd in T2-R6

supported the initial assignment in T1-R6. If HETCOR results from the dialkoxy system

are assumed to be applicable to the dialkyl system, Ce and Cf can also be assigned. It

should be noted that the relative positions of Ce and Cf based on the previously

mentioned HETCOR results are opposite that suggested by calculation, but place them in

good agreement with the corresponding carbon positions in T1-OR5 and T2-OR5 (Appm

<0.14).

Model reactions. No correlation can be made regarding the yields in these

reactions and the efficacy of Suzuki coupling for the polymerization of M-OR5, M-R6,

or related monomers, since the yields in these reactions were significantly lowered by

losses in the precipitation phase of the work up. A more quantitative description of the

reaction is shown in Figure 2-26. Monomers M-R6 and M-OR5 are modeled by 2,5-

dibromo-p-xylene (22) and 2,5-dibromo-l,4-dimethoxybenzene (43), respectively. All

are very soluble in acetone, and the electronic nature of the alkoxy and alkyl pairs are

comparable. To study the relative rates of the coupling reactions of alkoxy and alkyl

substituted p-dibromobenzenes, a single reaction flask was charged with equimolar

amounts of 22 and 43, along with a 10 molar excess of monoboronic ester 32, solid

Na2CO3, and 10 mol % Pd(OAc)2. The flask was degassed by the application of vacuum

while an aqueous methanolic acetone solution was sparged with argon. The solution was

quickly transferred to the reaction flask, initiating the reaction. When the solvent transfer

was complete, ca. one minute, an aliquot was taken for GC analysis. Within one minute,

the majority of dialkoxy monomer 43 and dialkyl monomer 22 had been consumed.

Small peaks identified by GC MS to be those of the corresponding dimers 46 and 47 were

observed. The major peaks were those of the disubstitution products, 44 and 45. A third









CH3

Br -i Br

H3C 22

+
OCH3

Br /--Br

H3CO 43
+


/B CH3

32
10ox
excess


H3C \ / Ct

48


Na2CO3
acetone / MeOH / H20
rt
Pd(OAc)2
ca. 10 mol %

1 minute


48
22 43
4 i 46


OCH3

H3 C -/ /a CH

-- .H3CO 45
+

CH3

Br / / CH3

H3C 46
+

OCH3

Br / / CH3

H3CO A


GC retention time -

Figure 2-26. GC trace of the same pot Suzuki coupling reactions of 2,5-dibromo-p-
xylene (22) and 2,5-dibromo-l,4-dimethoxybenzene (43) with ethylene 4-
methylphenylboronate (32) taken after one minute.


peak, corresponding to 4,4'-dimethylbiphenyl, was observed. This undesirable side

product occurs by the self-coupling of boronate 32 in an amount approximately

proportional to the amount of pre-catalyst added.

Pd[0] and Pd[II] have both been implicated in initiating or catalyzing the

reductive self-coupling or arylboronic acids.49'55'56 Reductive coupling by Pd[II] is the

more rapid event, which begins with a transmetallation event (see Figure 2-23), wherein

the aryl group migrates from the arylboronate to the Pd[II] center. A second


55Davidson, J.M.; Triggs, C. J. Chem. Soc., A 1968,1324.
56Heck, R.F. Palladium Reagents in Organic Synthesis; Academic: London, 1985; p. 184.








ArB(OH)2 + Pd(OAc)2 -- ArPdOAc + AcOB(OH)2

ArPdOAc + ArB(OH)2 -- ArAr + AcOB(OH)2 + Pd[0]

Figure 2-27. Pd(OAc)2 mediated self-coupling of aryl boronic acids. Adapted from:
Moreno-Maeas, M.; PNrez, M.; Pleizats, R. J. Org. Chem. 1996, 61, 2351.

transmetallation occurs, followed by reductive elimination to form the biaryl species. In

the process, Pd[II] is reduced to Pd[0]. Thus, Pd(OAc)2 is capable of consuming up to 2

equivalents of boronate functionality, which serves as a reducing agent, prior to initiating

Suzuki coupling. In a second reaction, aryl boronate 32 was self-coupled with 0.5 eq.

Pd(OAc)2 in the absence of aryl halide. The results are shown in Figure 2-28. After 20

min. of reaction at room temperature approximately 61% of 32 was converted into



C--& P]

32 2eq. H3C / BIi 32
-J 0'
(a) (b)
Acetone (aq.) / Ar
1 eq. Pd(OAc)2 Na2CO3
20 min.


48 0
44% 48 32 56% (61)*
< 1 eq. Pd(OAc)2 CHC13 / air



H3 -C)J-a C3 H3 Br\

S59% 22 h 41% (74)*
S72% 96 h 28% (84)*
GC retention time
2 x [48]
% conversion 2 x [48]
2 x [48] + [32]


Figure 2-28. Palladium catalyzed self coupling reaction of ethylene 4-methylphenyl-
boronate. (a) GC trace after 20 minutes. (b) Reaction scheme.








biphenyl 48. The reaction mixture was diluted with chloroform, treated with MgSO4,

then filtered to give a light orange solution indicative of the presence of Pd(OAc)2. The

reaction mixture darkened over a period of several hours, as Pd black formation

continued in the absence of base. Conversion to 48 continued at a reduced rate. The

identity of the catalytic species was not determined.

Typically, when the kinetics of step growth polymerizations are examined, the

assumption is made that the reactivity of the monomers is similar to that of the newly

formed dimers, trimers, etc. Suzuki coupling, being dependent upon the electronic nature

of the aryl rings, cannot be viewed in this light. In a reaction between equimolar

concentrations of difunctional M-OR5 or M-R6 and 32, a 1:2:1 monomer:dimer:trimer

product ratio is predicted for the case of equal monomer/dimer reactivity. Deviation from

this statistical ratio implies unequal reactivity.

M-OR5 and boronate 32 were reacted in a 1:1 mole ratio under typical conditions

for 6 hours at 40C. The product mixture was directly assayed by GC, providing the GC

trace in Figure 2-29. The solvent was removed and the products taken up in diethyl

ether. Following careful workup, a slowly crystallizing colorless oil was obtained. The

1H NMR spectrum was obtained, with care being taken to completely dissolve the

product mixture so that a representative spectrum would be assured. By 1H NMR, a ca.

1:1 mixture of M-OR5 and T2-OR5 was obtained. Small peaks, presumably due to

dimer 49 were observed. Thus, the reactivity of dimer 49 is significantly greater than that

of monomer M-OR5. As a result, molecular weight build up might not be expected to

follow the curve shown in Figure 2-19. If the reactivity of the bromine terminus of the

growing chains is greater than that of dibromo-monomers, then a broad molecular weight

distribution of oligomers could be present during the course of the reaction. If such a

reaction were to proceed to completion, such that all functional groups were reacted, a

typical step growth polydispersity index (PDI) might be achieved. If, however, a reaction

were stopped short of completion, such as when the catalyst becomes inactive, or the














-Br + B /
10


M-OR5


Pd(OAC)2
1M Na2CO3 (aq.)
acetone / MeOH
6 h / 400C


J o/
0


0 0

Br-0 0 \/ \/


Minor major
<1% ca.50%
/0 49 T2-OR5


major
ca. 50%
M-OR5


T2-OR5


M-OR5









dimer
49

GC retention time


reaction product


T2-OR5


M-OR5




7.8 7.6 7.4 7.2 7.0 ppm


Figure 2-29. Equimolar reaction of a monofunctional arylboronate (30) with a
difunctional dialkoxy monomer (M-OR5). (a) Reaction scheme. (b) GC trace of the
crude reaction mixture after 6 h. Retention times of M-OR5 and T2-OR5 were
individually determined from representative samples. (c) Aromatic region of the 1H
NMR spectra of [top to bottom]: the product mixture after workup, T2-OR5, and M-
OR5.








polymer precipitates from solution, a large number of unreacted monomer units or low

mass oligomers might remain. Previous researchers, using Pd(PPh3)4 catalysis, have

found PDIs close to 2.0 for alkyl-substituted PPPs; near quantitative yields were likewise

obtained.57

Similar results were obtained for the reaction of M-R6, although the reactivity

difference between the monomer and dimer is less than that observed for the dialkoxy

monomer M-OR5. This makes sense in terms of electronic factors. The resonance

delocalization of charge afforded by the biphenyl group would become a greater factor in

a system already affected by excessive electron density, i.e., with alkoxy substitution,

than with one not so affected, i.e., the alkyl system.

The GC retention times for M-R6 and T2-R6 were known from authentic

samples. To confirm the identity of dimer 50, the mass spectra of the product mixture

were analyzed. As shown in Figure 2-30c, a molecular ion peak was not observed in the

electron ionization (El) mass spectrum of M-R6. The highest mass peak corresponds to

the tropylium ion 51 resulting from scission of the benzylic carbon-oxygen bond. A 1:2:1

(M):(M+2):(M+4) ratio confirms the presence of two bromine atoms. A small molecular

ion peak containing one bromine (1:1 ratio of [M]:[M+1] peaks) was observed for dimer

50, probably a result of the increased charge delocalization in the molecular ion of the

biphenyl system relative to the benzene system (Figure 2-31 a). Sidechain scission to a

tropylium ion 52 was again observed. The base peak in the mass spectrum of dimer 50 is

presumably a fluorene/tropylium ion 53. Alternatively, a fluorene ion could form

following loss of the first side chain, followed by tropylium ion formation. Trimer T2-

R6, with the capacity for increased charge delocalization, displayed a fairly significant

molecular ion peak, as well as peaks corresponding to the loss of sidechains by benzylic

C-O cleavage (Figure 2-3 ib).



57(a) Rulkens, R.; Schuize, M.; Wegner, G. Macromol. Rapid Commun. 1994, 15, 669. (b) Karakaya, B.;
Claussen, W.; Schifer, A.; Lehmann, A.; Schliter, A.-D. Acta Polymer. 1996, 47, 79.








A fourth peak, labeled 56, was observed in the GC and GC-MS chromatograms.

The mass spectrum of 56 and the FT-IR spectrum of the product mixture are shown in

Figure 2-32. Analysis of the mass spectrum suggests that 56is a terphenyl in which one

of the side-chains is either an aldehyde or a benzylic alcohol. A large peak due to M + 1

for an aldehyde or M 1 for an alcohol is present. Aromatic aldehydes typically display a

strong molecular ion peak, although a strong M 1 peak is typically observed, rather than

an M + 1 peak.58 Benzyl alcohols commonly display strong M 1 peaks. However, the

expected M 1 CO and M OH peaks are not observed in the mass spectrum of 56. An

aldehyde carbonyl stretch was observed at 1693 cm-1 in the FT-IR spectrum of the

product mixture. A second carbonyl stretch at 1734 cm-1 has been attributed to residual

acetone.59 The most simple explanation of this result is that conversion of 23to 21 was

not complete. A small amount of unreacted benzylic bromide would be expected to

hydrolyze under the basic aqueous conditions of the Suzuki coupling reaction. The

benzylic alcohol so formed could then be oxidized to aldehyde by some Pd mediated

route.60 While impurities in M-R6 were not observed (GC, NMR, EA), the possibility

cannot be discounted. When the experiment was repeated in the absence of Pd, no

hydrolysis of M-R6 was observed.

Eventually, in the absence of stabilizing ligands, Pd[0] precipitates from solution.

Despite this, catalytic activity continued. A solution of M-R6, 4-methylphenylboronic

acid (29) (0.08 mole %) and 0.03 mole % Pd(OAc)2 in aqueous methanolic acetone

containing partially dissolved Na2CO3 was stirred at room temperature for 24 hours,

during which time Pd precipitated as a black solid intermixed with Na2CO3, and coated

portions of the reaction flask. A slight amount of orange color remained in the solution,




58Silverstein, R.M.; Bassler, G.C.; Morrill, T.C. Spectrometric Identification of Organic Compounds, 5th
ed. John Wiley & Sons: New York, 1991; p. 26.
59The carbonyl stretch of acetone shows concentration dependence, and has been observed at 1739 ppm as
a very thin film on acetone on NaCl. See APPENDIX B.
60A series of possible mechanisms are outlined in APPENDIX B.













+ leq. B CH3


M-R6


Pd(OAc)2

acetone / methanol / H20
Na2CO3


T2-R6


dimer
50


400 800 1200 1600 2000
9:22 12:42 16:02 19:22 22:42


scan number
retention time (min.)





HO ---0


m/z 334


2S0 300 350 400 450 010 1SO 600


Figure 2-30. Mass spectral chromatogram and fragmentation patterns from the GC/MS
analysis of the reaction of one equivalent of M-R6 with 30. (a) Reactants. (b)
Chromatographic plot corresponding to the fragmentation patterns. (c) Electron
ionization fragmentation pattern of M-R6, average of scans 1044 to 1048 minus 1016 to
1020; (b) Dimer 50, average of scans 1299 to 1303 minus 1271 to 1275.


(a)


1 eq.


0-


M-R6


TOT


(c)



loo % s


m/z410

M-R6+"


336
1 3 I.I. 1 5


.02
15 oa.


*4131 454 4 91


S3S 582


100 5so 200


















CH3 ?'-- B


m/z 270 ,

53 ',


272 mTZ J40

52






*


54





55


105 133 179


50 100 150 200 250


m/z434 +1

T2-R6+


54
359


299
I 34G


479 542 585


300 350 400 450 500 550 600


Figure 2-31. Mass spectral fragmentation patterns of the main reaction products observed
by GC/MS analysis in the reaction of one equivalent of M-R6 with 32. (a) Dimer 50,
average of scans 1299 to 1303 minus 1271 to 1275. (b) Trimer T2-R6, average of scans
1609 to 1613 minus 1587 to 1591.


m/z 422

50+*


T2-R6+"


.92


346&


3_"78


165


300 350


500 550 60O


2B3


270





















1O0 %


SME








59
-r r-r~r+
BO


(b)


0.50




0.45-




0.40-




0.35-



0.30-


m/z 299
57


10 19
91 12 165


100 IIi l 0I
1.00 X5 ,0


256
241


m/z 374 +1
56


31630
I 3 So


H OH

Ar ?








415 4718 504 S65 592


200 250 300 350 400 4S0 500


9


-A
2820 H
'I/ S


2739 t


1734


2694


3200 2800 2400 2000 1600 1200 800

Wavenumbers

Figure 2-32. Evidence for the formation of benzaldehyde formation during the Pd[0]
catalyzed reaction of M-R6 with 32. (a) MS fragmentation pattern for possible aldehyde
product 56. (b) FT-IR spectrum of the product mixture obtained by the 1:1 reaction of
M-R6 with 32.


..". I i. T m. 1. .. -i V.


550 600


299


37H


^*71








suggesting that some Pd(OAc)2 was present. Traces of trimer T2-R6 and biphenyl 48

were observed by GC (see Figure 2-33). Approximately 2.5 equivalents of 29 was added.


(a) 1 (b) 8-
-7 R2= 0.986
0.8 --
6
~'0.6
4 0
? :o 2 4- o

0.2-
0
o 0
0 2 4 6 8 10 0 2 4 6 8 10
Time (h) Time (h)

Figure 2-33. Change in relative concentration of M-R6 following addition of 2
equivalents of 4-methylphenyl boronic acid (29) under colloidal/heterogeneous Pd[0]
catalysis. (a). Relative concentration of M-R6 over time. (b) Plot of 1/[M-R6] versus
time.


The remaining color disappeared from the solution. After two hours the solution was

assayed. A small increase in biphenyl 48 formation was observed. Trimer T2-R6 was,

by two hours the dominant species in solution. The reaction proceeded with 2nd order

kinetics, suggesting a constant level of active catalyst was maintained over the course of

the reaction. In this instance, a steady state amount of dimer 50 was observed until the

end stage of the reaction.

Acetone is a solvent of choice for Suzuki cross coupling.15b Its polar nature

affords good miscibility with water, although phase separation does occur when the ionic

strength of the water increases. Rapid stirring and the addition of methanol, and/or

isopropyl alcohol assist in phase union. Unfortunately, this solvent system is not ideal for

solubilizing growing PPP polymer chains. DMF, is a better solvent for many materials,

and can be used at elevated temperatures relative to acetone. However, even DMF is a

poor solvent for many systems. Aromatic solvents such as toluene and anisole have









provided poor results with dialkoxy monomers.61 A model reaction similar to that

described in Figure 2-26 was run with 22, 43, and 32 using m-cresol as the solvent, and

sodium methoxide as base. The reaction was initiated by the addition of Pd(OAc)2

against flowing argon. Aliquots were removed,treated immediately with 10 % (w/v)

NaOH (aq.) to remove the m-cresol, and extracted with chloroform. After one minute at

45C, dimers 46 and 47 were seen in the GC, as were terphenyls 44 and 45. The catalyst

precipitated from solution in black clumps within three minutes. Product evolution

continued at a slow rate over the course of several hours. Competing favorably with

coupling was a side reaction leading to hydrolysis of the aryl carbon-bromine bond.

Figure 2-34 shows structures identified by GC and GC/MS, as well a chromatographic

plot of GC/MS data of a fraction collected 33 minutes after the addition of Pd(OAc)2.

Products representing mono- and di- coupling, loss of one bromine, and loss of the

boronate functionality were observed. Products resulting from the loss of methyl or

methoxy groups were not observed.



100% 00 00
nilz
,-i cs m/z



C\


Cl
I i C C i 'l'
--






800 1200 1600 2000 2400 2800
9:42 13:02 16:22 19:42 23:02 26:22
scan number
retention time (min.)

Figure 2-34. Suzuki coupling of 22 and 43 with 32 in m-cresol with NaOCH3 as base,
and Pd(OAc)2 catalysis. Chromatographic plot of the reaction products after 33 minutes.

61Balanda, P.B; Reynolds, J.R. Unpublished results. An anisole / 1M Na2CO3 reaction medium gave no
polymer with M-OR9.












H3C H3C
22, m/z 262 46, m/z 274


CH3

H3C / / CH3

H3C
44, m/z 286


58, m/z 184

CH3


H3C
62, m/z 106


CH3
\/\ CH3

H3C
59, m/z 196


H3Cq /=
HC/B & CH3
3"J^ 0
H3CH3COO
64,13CO64
64, 164


/CH3


60, m/z 92


OCH3

/ 5 m /z C H32 2


65, m/z 228


OCH3


H3CO
61, m/z 138


66, m/z 216


t
OCH3 OCH3

H3 00/ CH3-Br / r/CH3--
H3CO H3CO


45, m/z 319


47, m/z 306


43, m/z 294


Figure 2-35. Coupling and hydrolysis products observed by GC and GC/MS from the
Suzuki reaction of 22 and 43 with 32 in m-cresol with NaOCH3 as base, and Pd(OAc)2
catalysis.














CHAPTER 3
POLYMER SYNTHESIS AND CHARACTERIZATION

Introduction


This chapter describes the synthesis and characterization of poly(p-phenylene)s by

Suzuki cross-coupling polymerization with Pd(OAc)2 catalysis. A general overview of the

polymerization of the oligoethyleneoxy- monomers is first provided. The method is

general, and similar techniques were applied throughout. Following the brief overview,

the synthesis of several homopolymers will be described, and a brief characterization

provided. More detailed descriptions of characterization techniques follow. Finally,

polymerizations which are best understood following some detailed description of

homopolymer properties are described.

General Methods


Poly(p-phenylene)s with alkyl and alkoxy side-chains were synthesized by Suzuki

cross-coupling. In a typical polymerization, such as that depicted in Figure 3-1, a

dibromophenylene monomer, M-OR5, M-OR9, M-R6, or M-R10, was reacted with

bisneopentyl 1,4-phenylenediboronate (27) in a biphasic aqueous/organic solution

containing excess sodium or potassium carbonate. Palladium acetate was employed as a

catalyst precursor, exclusively. Polar aprotic solvents were the solvents of choice. The

conditions and methods, although fairly standard, were subject to subtle variation with

regard to such factors as amount and timing of catalyst addition, method of degassing,

temperature and duration of reaction. In some instances a stoichiometric imbalance was




.: 64








)\ \\ \DK
0





n 24h 60h





n0 M-OR9 0 P1-OR9 P2-OR9
Br_ Br 8^"J .
)-/ ~~Pd(OAc)2 (3 mol%) -
)m)m aq. Na2CO3 ()m n
( DMF THF
) 800C 600C
Q 24 h 60 h

( M m DMF THF
S0 M-OR9 0 0 P1-OR9 P2-OR9
1 M-R10 1 P1-RO10 P2-RO10


Figure 3-1. Mass balanced polymerization of M-OR9 or M-R10 with 27.

achieved by supplying the bishaloaryl monomer in slight excess over 27. In these

instances, a small amount of monofunctional arylboronate 32 was added such that the total
number of functional groups initially present was equal.
In one scenario, a Schlenk or a round bottom flask with an airless adapter was

charged with the dibromoaryl monomer, 27, Na2CO3, and Pd(OAc)2. The flask was
degassed by the application of vacuum. In a separate vessel, a mixture of water and
organic solvents was deaerated by sparging with argon. The solvents, under positive

argon pressure, were added to the reaction mixture via cannula to initiate the reaction.
Alternatively, the monomers and base were dissolved in the solvent mixture, and the entire
system degassed prior to the addition of catalyst. Typically, the reaction mixture was
degassed by sparging with argon. When DMF was the solvent, the reaction mixture was
either sparged with argon to remove air, or an ice bath was introduced, and the reaction
mixture was degassed by extended or cyclic application of vacuum. Freeze pump thawing








offered no advantage to these methods. When catalyst was added to a degassed system,

the catalyst was first degassed in a separate Schlenk tube, then transferred to the reaction

flask against blowing argon.

Reaction mixtures were vigorously stirred; methanol and/or isopropyl alcohol were

sometimes added to increase solution homogeneity. It has been suggested that Suzuki

cross coupling functions optimally at ca. 45C.1 In this study, temperature was typically

elevated to increase polymer solubility. After 24 to 60 h, polymers were precipitated by

pouring the reaction mixture into a large excess of water or aqueous methanol, and washed

free of residual salts. The crude polymers so obtained were typically black, owing to the

presence of Pd residues. Several methods were used to remove these residues. The

harshest method was treatment with an oxidizing agent, such as aqueous Br2 or HN03.

These reagents quickly removed black coloration, but ring bromination (dialkoxy PPP)

and sidechain scission (dialkyl PPP) were observed. The most effective method for

removal of residues was treatment of a chloroform solution of the polymer with activated

charcoal at reflux for two hours. Treatment of polymers in chloroform with aqueous

ethylenediaminetetraacetic acid (EDTA) was sometimes effective in removing Pd; however,

aqueous treatments of the polymers while in chloroform resulted in some polymer

precipitation as an interfacial emulsion, or scum. Regardless of the method of catalyst

removal, polymers were reprecipitated in aqueous methanol from a filtered chloroform or

THF solution, washed with methanol, and then washed with diethyl ether. Polymers were

either air dried, and then dried in vacuo, or were freeze dried from benzene. Representative

structures of the polymers synthesized by these methods are shown in Figure 3-2.


1Wallow, T.I.; Novak, B.M. J. Org. Chem. 1994, 59, 5034.












P-R6-20L


P1-OR9
P2-OR9
P-OR9-5
P-OR9-10
P-OR9-15
P-OR9-20
P-OR9-10P


PP-OR9


P-OR5
P-ORS-10P


P1-RIO
P2-RO10
P-R10-5
P-R10-10
P-R10-15
n P-R10-20
P-R10-11L


P-OR9/R10


Figure 3-2. Representative structures of the methoxyethoxy- and triethoxy-substituted
polymers synthesized by Suzuki cross-coupling polymerization.








Polymerization with Stoichiometric Balance

Synthesis of P-OR9 and P-OR10. The relative effects of alkyl and alkoxy

substituents on Suzuki polymerization were examined. Monomers M-OR9 and M-R10

were polymerized according to A-A/B-B methodology with equimolar quantities of

bisneopentyl 1,4-phenylenediboronate (27). Polymerization results are given in Table 3-1.

Corresponding gel permeation chromatography data are provided in Tables 3-2 and 3-3.

GPC results are discussed in detail in the next section. As anticipated, when polymerized

under identical conditions, M-R10 polymerized to a higher degree than did M-OR9.

Poly[2,5-bis(2,5,8-trioxadecyl)-l 1,4-phenylene-alt- 1,4-phenylene] (P1-Ro10), synthesized

in DMF [e = 36.71 (25C)]2 with 2M aq. K2C03 at 80C for 24 h achieved a molecular

weight high enough to make it insoluble in all standard organic solvents. P1-RO10 failed to

dissolve in CHC13, THF, m-cresol, DMSO, and NMP, even after standing for three

months, but dissolved in CF3CO2H after standing overnight. MALDI spectra were
obtained which suggest the number average molecular weight (M!) of P1-RO10 to be on

the order of 60,000 (60 kg mol-1). Poly [2,5-bis(1,4,9-trioxanonyl)- 1,4-phenylene-a/t- 1,4-

phenylene] (P1-OR9) formed under the same polymerization conditions was chloroform

soluble. Vapor phase osmometry (VPO) results in CHC13 suggest that a molecular weight

(MW) of 7650 was attained. MALDI results suggest a higher molecular weight (MW) of

ca. 35,000 was attained. These values represent peak values in the MALDI spectra and are

representative averages of several samples. Molecular weight and molecular weight

distributions obtained by MALDI do not necessarily correspond to GPC results, although

the peak molecular weight (MP) for P1-OR9 in CHC13 was 37,000.3 P2-OR9 and

P2-RO10 synthesized in THF [e = 7.58 (25C)] at 60C obtained significantly lower

molecular weights, even after 60 h. By GPC in CHC13, a M, of 44,000 was obtained for

P2-R10 synthesized with 6 mol% Pd(OAc)2. P2-OR9 obtained a Mw of ca. 9000

2Dielectric constants were taken from Table 5.16 in Lange's Handbook of Chemistry, 14th ed. Dean, J.A.,
Ed. McGraw-Hill, Inc.:New York, 1992.
3AUI GPC results are relative to polystyrene standards, unless otherwise noted.

















Table 3-1. Polymerization results for P-OR9 and P-R10 synthesized with mass balance.
Xmaxi Xmax2 PLc,f
monomera rxn rxn UV-xl 2 PLCUV- max
solvent conc. temp. mol% time yield Mb Vis Vis Xmax
product (mM) (C) cat. (h) (%) Vis log C1 (Vis log 2 (nm)
pout(nm) (nm)
P1-RO10 DMF 77.6 80 3 24 54 51,000d 300g -
P1-OR9 DMF 75.7 80 3 24 60 19,700e 364g 4.65 296h 4.30 412J
P2-R10 THF 7.7 60 6 60 38 11,100 4.40 298e 372k
P2-OR9 THF 8.3 60 3 60 46 5640 352h 4.39 292 4.10 408/418(sh)k
P-OR9/RO10 acetone 10.5 50 4 72 65 25,100f 338h 4.20 294 4.34 409/416(sh)k
PP-OR9 acetone 13.0 55 8 48 51 10,900 338h 4.11i 410/417(sh)k
aMonomer concentration is based on the bishaloaryl monomer in the organic solvent. b M, from GPC vs. polystyrene in
chloroform. Cphotoluminescence. dMALDI peak m/z. eMn = 7650 from VPO in chloroform solution. fGPC: MP of higher
of two distributions. gThin film on quartz. hIn THF solution, iBased on 1 ring/repeat. JExcitation at 380 nm. kExcitation
at 320 nm.


._-jC














Table 3-2. GPC results vs. polystyrene (PS) for P-OR9 and P-RO10 synthesized with mass balance.
CHC13 THF
detector M MP M detector M MP M_
polymer X (nm) kgmol-1 kgmol-1 kgmol-1 Mw/MI 2 (nm) kgmol-1 kgmol-1 kgmol-1 Mw/Mn
P1-OR9 254 19.7 36.8 75.2 3.82 254 7.73 7.03 26.3 3.40
400 22.4 39.2 79.7 3.56 -
P2-OR9 330 5.64 6.63 9.35 1.66 254 3.7 3.9 5.4 1.45
345 5.85 7.07 9.79 1.67 825.1c -
P2-RO10 254 11.1 8.8 43.8 3.96 254 6.0d 6.2d 10.8d 1.82d
1.9e 6.2e 10.4e 5.50e
P-OR9/R10 290 28.8 24.3 48.6 1.68b 254 665.9c -
[6.03]a 14.8 -
345 28.5 25.1 48.5 1.70b 4.3 -
_- [5.66]a ...
360 27.0 25.9 47.0 1.74b .
[5.66]a -
PP-OR9 330 10.9 12,900 17.5 1.61 254 7.2 8.2 12.3 1.7
aBimodal distribution; PDI ca. 3-7. bConsiders only high mass peak. CHigh mass peak, presumably due to aggregation.
fConsiders main peak only. gIncludes aggregates and oligomers. dExcludes low molecular weight tail. elncludes low
molecular weight tail.








with 3 mol% Pd(OAc)2. While transition state charge development is not a necessary

component of the proposed mechanistic cycle, the involvement of charged species is clear.

In this regard, the observed solvent effects correspond to previously described literature

expectations. Decreasing the polarity of the solvent resulted in a relative decrease in

molecular weights.

Synthesis of PP-OR9. A homopolymer, poly[2,5-bis(1,4,7-trioxanonyl)- 1,4-

phenylene] was synthesized by AA/BB Suzuki coupling of the correspondingly substituted

dibromoaryl (M-OR9, 7) and aryldiboronate (38) monomers according to the scheme in

Figure 3-3.


OR OR Pd(OAc)2 OR OR
HQ PH Na2CO3 (aq.) -
Br Br + B -
Br B HO _-0 OH acetone / MeOH
OR OR 55C / 5 days OR ORn
M-OR9 38 PP-OR9

Figure 3-3. Synthesis of PP-OR9 by Suzuki polymerization.
R = -CH2CH2OCH2CH2OCH2CH3

PP-OR9 was obtained in 51% yield as a light pink, powdery flake. The MW of 12,300

was determined by GPC in THF. The material could be solution cast into homogeneous

thin films, although the films were not free standing. The proton NMR spectrum of PP-

OR9, shown in Figure 3-4, confirms the structure, but it also reveals the material's

relatively low molecular weight. End groups are clearly visible in the aromatic regions of

both the 1H and 13C spectra.

The same polymer was made as a white powder by Ni[0] catalysis in DMF by the

homopolymerization appropriate dichloroaryl monomer, 2,5-dichloro-l,4-bis(1,4,7-

triozanonyl)benzene (14). Under Ni[0] catalysis with excess Zn in anhydrous DMF, the

polymer PP-OR9a was obtained as an oligomer (5-15 rings) in less than 20% yield. This

molecular weight was achieved within two hours. Further increases in molecular weight














(a)
n



i



*CDC13


aas

a r



endgroups


6 5 4 3


2 1 ppm


160 140 120 100 80 60 40 20 ppm


Figure 3-4. NMR spectra of PP-OR9.
Carbon 13 NMR spectrum in CDC13.


(a) Proton NMR spectrum in CDC13. (b)


6








were not achieved with increased reaction times. Similar results were reported by Yang et

al. for the synthesis of alkoxy-substituted PPP by the same method.4 Figure 3-5a provides

the reaction scheme for the synthesis of PP-OR9a by the Ni route. Figure 3-5b shows a

theoretical isotopic distribution pattern for an oligomer comprised of eight monomer units

with a chlorine atom at one end (and a single sodium counter ion). This distribution pattern

is reproduced in the mass focused high resolution MALDI spectrum in Figure 3-5c, as the

cluster of peaks on the right. On the left side is a cluster of peaks defining the isotopic

distribution of an eight ring oligomer with a hydrogen at each terminus. These same two

clusters can be seen as pairs of peaks in the full MALDI spectra shown in Figure 3-10.

These results clearly illustrate the susceptibility of electron rich systems to hydrolysis, even

by trace amounts of water. It is interesting to note that only very low levels of

dichlorinated oligomers were observed.

Gel Permeation Chromatography (GPC). The premise of universal calibration in

size exclusion chromatography (i.e., SEC or GPC) is that the hydrodynamic volume of a

well behaved polymer in dilute solution is the sole determiner of elution time. A well

behaved polymer does not interact with itself, nor does it interact with the column, other

than to diffuse into and out of the column pores. The hydrodynamic volume of a polymer

in a given solvent is approximately the product of the intrinsic viscosity [77] of the polymer

in that solvent and the molar mass of the polymer M. The molar mass of a monodisperse

polymer in solution is related to the intrinsic viscosity of the polymer by the Mark-

Houwink-Sakurada (MHS) equation



[r1] = KMa (3-1)

where K and a are constants which vary with the polymer, solvent and solvent

temperature.5 Multiplying both sides of eq. 3-1 by M provides a relation which expresses

4Yang, Y.; Pei, Q.; Heeger, A.J. Synth. Met. 1996, 78, 263.
5For a discussion of universal calibration see a polymer text such as Rudin, A. The Elements of Polymer
Science and Engineering; Academic Press: New York, 1982.









(a)

C





Cl \
i










(c) 5000


4000


3000

2000

1000

0


100%


75%


50%



25%


Ni(bipy)(PPh3)2
catalyst
------ 0p -


excess Zn
DMF
A


3120 3122 3124 3126 3128
Calculated Mass (m/z)


3080 3090 3100 3110 3120 3130 3140
Mass (m/z)


Figure 3-5. Reaction scheme and high resolution partial MALDI spectrum of PP-OR9a.
(a) Reaction scheme. (b) Theoretical isotope pattern for an eight ring oligomer with one
chlorine chain end and one sodium counter ion. (c) High resolution spectrum of eight ring
segments without and with a chlorine attached (includes sodium counter ion).








molar mass in terms of hydrodynamic volume


Mi[]i = KiMiai+I


(3-2)


where the subscript refers to the polymer type. At a given elution volume, two polymers

can be related to each other according to


Ml[r77]1 = K1Mlal+l = M2[r77]2 = K2M2a2+1


(3-3)


The solution properties of PPP were recently studied by the group of Wegner.6

Following reports that a PPP with regularly spaced dodecyl side chains showed

aggregation in solution,7 Wegner's group synthesized a PPP (69, see Figure 3-6) which

contained two different bulky groups, such that aggregation phenomena might be avoided.


CH3 SO3R Pd[O]
B H20 / THF
/_ Bj + Br Br NaHCO3
0 reflux
S 67 RO3S 68


CH3 S03R

R=
n
C12H25 RO3S 69
69


Figure 3-6. A bulky PPP from which the solution properties of PPP were determined.


Various methodologies, including static light scattering in toluene and trichloroethylene,

membrane osmometry, size exclusion chromatography with universal calibration, and

viscometry were utilized for the determination of molecular weight. The Mark-Houwink-

6Vanhee, S.; Rulkens, R.; Lehmann, U.; Rosenauer, C.; Schuize, M.; K6hler, W.; Wegner, G.
Macromolecules, 1996, 29, 5136-5142.
7Petekidis, G.; Fytas, G.; Witteler, H.; McCarthy, T.F. Colloid Polym. Sci. 1994, 272, 1457.








Sakurada (MHS) equation (Kpp, app) and the worm-like chain model (Ar7, B7) were

applied to obtain the [r7]-M relation necessary for universal calibration. As a result, the

parameters of the Mark-Houwink-Sakurada equation in THF at room temperature (Kpp =

0.00267 mL/g and cpp = 0.961), the hydrodynamic diameter (1.3 nm), and the persistence

length of the polymer (12.6 nm) were determined.

With these results, and the Mark-Houwink-Sakurada parameters for polystyrene in
THF (Kps = 0.0136 mL/g and xpp = 0.714),8 universal calibration can be applied for

PPPs in THF using the logarithmic form of eq. 3-3


logMipp = ---1 log KpS + (I +aps) logMiPS (3-4)
a~pp + 1 A ppI


where Mi,pp represents the molar mass of the ith slice from the GPC chromatogram.9

Average molecular weights for P-OR9 and P-RI0 type polymers were determined

by universal calibration based on the above MHS parameters and eq. 3-3. These results are

listed in Table 3-3. Unfortunately, clear evidence of aggregation can be seen in the THF

GPC chromatograms of the P-RI0 and P-OR9 polymers (see Figure 3-7 thru 3-9). The

polymers required heat to dissolve, were difficult to filter, and often precipitated from

solution during or after filtration. The GPC line shapes are often distorted or multimodal.

Compare, for example, the chromatogram of P1-OR9 in THF (see Figure 3-7) with those

in CHC13 (see Figure 3-8). Peaks have been observed in the 600,000 mass range of the

THF traces, which could only result from aggregation. Interestingly, these high mass

peaks are observed in the P-OR9 materials, but not in the P-RO10 samples. The dialkoxy

substituted polymers are expected to adopt a more planar structure in solution than the

dialkyl-substituted materials due to the decreased steric bulk of the oxygen linkage of the



8Brandrup, J., Immergut, E.H., Eds.; Polymer Handbook, 3rd ed.; John Wiley & Sons: New York, 1989.
9Grubisic, Z.; Rempp, P.; Benoit, H. J. Polym. Sci. Part B 1967, 5, 753.














Table 3-3. GPC results with universal calibration for P-OR9 and P-RO10 synthesized with mass balance.
CHC13 TTHF
detector Ml MP M, detector n MP Aw
polymer X_ (nm) kg mol-1 kg mol-1 kg mol-1 ZMwRJM X (nm) kgmol-1 kgmol-1 kgmol-1 Mw/Mn
P1-OR9 254 13.0 22.5 41.0 3.22 254 5.7 5.3 16.7 2.92
400 14.5 23.7 44.1 3.03 -
P2-OR9 330 4.4 5.0 6.8 1.56 254 3.0 3.2 4.2 1.39
345 4.5 5.3 7.1 1.57 340.2C -
P2-RO10 254 7.9 6.4 26.1 3.32 254 4.6d 4.7d 7.7d 1.67d
1.7e 4.7e 7.4e 4.42e
P-OR9/R10 290 18.1 15.6 28.6 1.58b 254 282.1h -
[4.6]a 10.1 -
345 18.0 16.1 28.6 1.59 3.4 -
[4.4]a ..- -.
360 17.1 16.5 27.8 1.62b -
[4.37]a -
PP-OR9 330 7.7 9.0 11.7 1.51 254 5.4 6.0 8.6 1.60


aBimodal distribution; PDI ca. 3-7.


bConsiders only high mass peak. CHigh mass peak, presumably due to aggregation.


fConsiders main peak only. gIncludes aggregates and oligomers. dExcludes low molecular weight tail. eIncludes low
molecular weight tail.








sidechain to the ring relative to that of a methylene group. The increased planarity should

then allow for increased interchain interaction and, hence, increased aggregation.


S8 -
0
4 /
//\\


0.

II ' I I \ I / -
0 10 20 30 40 50
Retention Time (min.)

Figure 3-7. GPC chromatogram of P1-OR9 in THF with 254 nm detection.


The GPC results obtained in chloroform are apparently less affected by aggregation

effects, and by interaction with the column. Higher masses were observed in CHC13 than

in THF, and CHC13 solutions were significantly easier to filter. All solutions were

prepared at the 5-10 mg/mL level, yet the detector response for P1-OR9 in chloroform

solution is significantly higher than in THF. It is possible that the THF chromatogram of

P1-OR9 does not represent the entire polymer sample, but a sample which has been

fractionated according to its solubility profile. The MHS parameters determined for PPP in

THF are not valid for PPP eluted in CHC13. Nevertheless, molecular weight data obtained

in CHC13 and so corrected are more likely to approximate actual molar masses than if left

untreated. These data are likewise found in Table 3-3.

It is interesting to note, from Figure 3-8b, that the peak molecular weights MP of

P2-OR9 and P2-R10, synthesized in THF are essentially the same, possibly representing

a point at which both materials began to precipitate from solution. The chromatogram of

P2-RO10 shows a shoulder on the high mass side which may represent further

polymerization of the precipitated material. In this case, the low signal intensity observed

for P2-R10 relative to that of P2-OR9 results from having the detector set significantly









(a) 2200 -


I 1640


1080
C
0

S520


-40
0

(b)
254 nm
1400-
S1200

1000

S800-
S600

S400
S200


254 nm
----- 360 nm
- --400 nm


6 8 10 12 14
Retention Time (min.)


-----400 nm


0 2 4 6 8 10
Retention Time (min.)


--254 nm


25
20 2
0

15




0

12 14 16
12 14 16


- 400 nm
S28.5

28 |

27.5 >

27 .

26.5

S26


Retention Time (min.)


Figure 3-8. GPC Chromatograms of P1-OR9 in CHC13. (a) Relative signal intensities as
a function of detector wavelength. (b) Chromatograms at 400 nm and 254 nm with
normalized signal intensities. (c) Magnified view of signal peaks at 400 nm and 254 nm.








P2-OR9 -----P2-RO10
(a) 140 60

120 A 50
o 100 4
40 cIM
S80
60 30
I 20
40
Q 20 10


-20 -10
ITHF
-2 0 1 I I I I I . I . I . . I 1 0
0 10 20 30 40 50
Retention Time (min.)

P2-OR9 --- P2-RO10
(b) 2000- 22


5 1495- 16.25
I-

S990- 10.5 =
,'

r485- 48 4.75

CHC13
-20 . .. . .. -1
0 2 4 6 8 10 12 14 16
Retention Time (min.)

Figure 3-9. GPC chromatograms of P2-OR9 and P2-RO10. (a) In THF solution with 4
columns and 254 nm UV detection. (b) In chloroform solution with a linear mixed be
column, and 330 nm detection.


far from the Xmax of the dialkyl PPP. The effect of detector wavelength on apparent

molecular weight can be observed in Figure 3-8. When the detector is set to a longer

wavelength, an apparent increase in molecular weight is observed. Longer chains have, on

average, more opportunities to realize longer conjugation lengths; longer conjugation

lengths correspond to longer wavelength UV-Vis absorptions.








Matrix Assisted Laser Desorption Ionization Time of Flight Mass Spectrometry

(MALDI-TOF MS) Analysis. MALDI is a relatively new technique which has only

recently been applied to the analysis of synthetic polymer systems.10 In a basic MALDI

experiment, a polymer sample is captured in high dilution (1 to 1000-10,000) in an easily

ionizable matrix material, typically an aromatic carboxylic acid. The mixture is spotted on a

solid support and bombarded by a pulsed laser under high vacuum. The laser power is

adjusted to just above the threshold for matrix ionization. If the power is too high, sample

degradation and multiple ionization events, as well as the profuse liberation of charge

neutral species, are more likely to occur. Following matrix ionization, which might be

physically likened to a detonation, the ions which form are accelerated under high potential

through a long tunnel into a mass spectral detector. During their flight, the ions are

separated on the basis of their mass/charge ratios (m/z) according to velocity differences

which arise therefrom. A variety of instrumental factors affect the outcome of the

experiment, including the length of the acceleration tube, the strength and duration of the

laser pulse, the quality of vacuum, the quality and intensity of electromagnetic fields, the

quality of the ion optics, and the detector sensitivity. Experimental factors not related to the

instrumentation which can affect the quality of MALDI results include sample preparation,





10(a) Karas, M.; Hillenkamp, F. Anal. Chem. 1988, 60, 2299. (b) Hillenkamp, F.; Karas, M.; Beavis,
R.C.; Chait, B.T. Anal. Chem. 1991, 63, 1193A. (c) Bahr, U.; Deppe, A.; Karas, M.; Hillenkamp, F.;
Giessman, U. Anal. Chem. 1992, 64, 2866. (d) Ehring, H.; Karas, M.; Hillenkamp, F. Org. Mass
Spectrom. 1992, 27, 1992. (e) Beavis, R.C.; Chaudhary, T.; Bhait, B.T. Org. Mass Spectrom. 1992,
27, 156.








polymer/matrix compatibility, polymer molecular weight and polymer

polydispersity.11,12,13,14,15,16,17

A recent finding was that sample resolution could be dramatically improved by

allowing the rapidly ionized polymer/matrix system to equilibrate prior to acceleration.18

The two spectra in Figure 3-10 illustrate the effect of using a short extraction delay. The

spectra were obtained from the same sample with similar laser power. When a longer delay

time was inserted between matrix ionization and ion extraction, a higher resolution

spectrum was obtained. With a 50 ns extraction delay time, the baseline drifts upwards in

the low mass region. All other MALDI results presented here were obtained with a 0 ns

delay. In many of these spectra, discrete peaks are not observed above the baseline.

Rather, the "baseline" rises and falls similar to a typical chromatogram (refer to Figure 3-

3). This discrepancy, coupled with uncertainties regarding the ionization and detection

processes, raises questions regarding the validity or nature of relative peak intensities.

Does the relative number of counts (ions striking the detector) at a given mass represent the

number of oligomers of that mass (or m/z)? When should peak height be measured relative

to a baseline, given that a clear baseline is not always readily observed? Does the observed

spectrum represent the entire polymer sample? What is the nature of the misrepresentation,



11Cotter, R.A. Anal. Chem. 1992, 64, 1027A.
12Juhasz, R.; Costello, E.E.; Biemann, K. J. Am. Soc. Mass Spectrom. 1993, 4, 399.
13(a) Danis, P.O.; Karr, D.E.; Mayer, F.; Holle, A.; Watson, C.H. Org. Mass Spectrom. 1992, 27, 843.
(b) Danis, P.O.; Karr, D.E. Org. Mass Spectrom. 1993, 28, 923. (c) Danis, P.O.; Karr, D.E.;
Westmoreland, D.; Piton, M.C.; Christie, D.I.; Clay, P.A.; Kable, S.H.; Gilbert, R.G. Macromolecules,
1993, 26, 6684.
14Burger, H.M.; Muller, H.M.; Seebach, D.; Bornsen, K.O.; Schar, M.; Widmar, H.M. Macromolecules,
1993, 26, 4783.
15(a) Eggert, M.; Freitag, R. J. Polym. Sci. Part A: Polym. Chem. 1994, 32, 803. (b) Freitag, R.;
Baltes, T.; Eggert, M. J. Polym. Sci. Part A: Polym. Chem. 1994, 32, 3019.
16Visy, C.; Lukkari, J.; Kankare, J. Macromolecules, 1994, 27, 3322.
17(a) Abate, R.; Ballistreri, A.; Montaudo, G.; Garozzo, D.; Impallomeni, G.; Critchley, G. Tanaka, K.
Rapid Commun. Mass Spectrom. 1993, 7, 1033. (b) Garozzo, D.; Spina, E.; Sturiale, L.; Montaudo, G.
Rapid Commun. Mass Spectrom. 1994, 8, 358. (c) Montaudo, G.; Montaudo, M.S.; Puglisi, C.;
Samperi, F. Anal. Chem. 1994, 66, 4366. (d) Montaudo, G.; Montaudo, M.S.; Puglisi, C.; Samperi, F.
Rapid Commun. Mass Spectrom. 1994, 8, 981. (e) Montaudo, G.; Montaudo, M.S.; Puglisi, C.;
Samperi, F. Rapid Commun. Mass Spectrom. 1994, 8, 1011. (f) Montaudo, G.; Montaudo, M.S.;
Puglisi, C.; Samperi, F. Rapid Commun. Mass Spectrom. 1995, 9, 453.
18Bencsura, A.; Vertes, A. Chem. Phys. Lett. 1995, 247, 142.








should it exist? Can useful molecular weight and molecular weight distribution information

be routinely and reliably extracted from MALDI spectra which have not been intensively

studied?

Recently, molecular weight averages and distributions have been calculated from

MALDI spectra for a variety of polymer types, and these results were compared with

results from other polymer characterization methods. Low molecular weight species with

good matrix compatibility were found to give better correspondence with GPC results.

Molecular weight averages for polymers with very narrow molecular weight distributions
(Mw/Mn < 1.10) agreed with conventional results. For Mw/Mn 1.10, MW averages

from MALDI differed from conventionally obtained averages by as much as 20%. At

higher PDI the results obtained were considered unreliable.19,20 MALDI and GPC spectra

have been compared on a theoretical basis. Differences in the most probable MW (MP),

and in the overall shape of the spectrum, result from differences in the way the data is

displayed. In a MALDI spectrum, intensity usually represents a number fraction plotted as

a function of MW. In a GPC spectrum, intensity represents a weight fraction plotted as a

function of log MW. Even with a narrow molecular weight distribution, MP as determined

by MALDI will be two repeat units lower in mass than MP as determined by GPC, as a

consequence of the differences in mathematical nature of the modes in the two systems.21

Molecular weight modes were calculated for PP-OR9a based on the MALDI

spectra in Figure 3-10 according to the expressions


-n I niMi
n (3-5)





19Burger, H.M.; Muller, H.M.; Seebach, D.; Bornsen, K.O.; Schar, M.; Widmar, H.M. Macromolecules,
1993, 26, 4783.
20Montaudo, G.; Montaudo, M.S.; Puglisi, C.; Samperi, F. Rapid Commun. Mass Spectrom. 1995, 9,
453.
21Jackson, C.; Larsen, B.; McEwen, C. Anal. Chem. 1996, 68, 1303.









4000


3000


2000


1000


0 u I M
1500 2000 2500 3000 3500 4000 4500
Mass (m/z)


3500

3000

2500

2000

1500

1000

500


5000 5500


2000 2500 3000 3500 4000 4500 5000 5500 6000
Mass (m/z)


Figure 3-10. MALDI
extraction delay.


spectra of PP-OR9a. (a) 90 ns extraction delay. (b) 50 ns


X niMi2
Mw i-i


(3-6)


90 ns extraction delay



2

2








X niMi
HZ =IniMi' (3-7)
-- Z tMi4


z+ = niM,4 (3-8)



where ni is the number of molecules of mass Mi. The number of molecules ni was taken as
the peak height, which was measured from a baseline arbitrarily drawn through the center
of the nonlinear broad noise band representing the baseline. For comparative purposes,
calculations were repeated with the peak heights adjusted according to the relative areas of
the isotopic clusters shown in Figure 3-5. The results of these calculations are compared in
Table 3-4 with molecular weight averages determined by GPC in THF. The GPC
chromatogram is shown in Figure 3-11. Note that with a 90 ns delay, the shape of the

MALDI spectrum approximated the line shape found in the GPC spectrum. That this
occurs is actually quite deceiving, given the differences in the plot axes. What is striking
about the results presented in Table 3-4 is that the Mn determined from the MALDI spectra

agrees within 3% with that determined by GPC with universal calibration, regardless of the
method of data treatment. Data obtained with a 50 ns extraction delay generated the same
M, as that obtained with a 90 ns extraction delay. Also, the MP value obtained by MALDI

is less than one repeat unit smaller than that obtained by GPC. It is important to note that
Mw/M- is significantly smaller for the MALDI data than for the GPC results. As a result,
Mw is unreasonably low. Greater deviations occur as mode order increases (Mz, Mz+1,

etc.), since these modes are more critically affected by loss of species in the high mass
region than lower modes are by similar losses in the low mass region.
Despite the promising results for the low molecular weight polymer PP-OR9a,
MALDI results for higher mass polymers are expected to be less reliable. Instances can
arise, however, when MALDI offers the best hope for understanding the molecular weight,








Table 3-4. A comparison of MALDI and GPC molecular weight results for PP-OR9a.
area Xn -
delay baseline correction range Mp a Mn Mw Mw /Mn
50 ns no no 5-15 3088 2669 2895 1.08
50 ns no yes 5-15 3088 2665 2891 1.08
50 ns yes no 5-15 3088 2769 2947 1.06
50 ns yes yes 5-15 3088 2765 2943 1.06
90 ns yes no 4-14 3088 2781 2931 1.05
90 ns yes yes 4-14 3088 2779 2930 1.05
GPC (THF vs. polystyrene) 3936 3295 4439 1.35
GPC (THF with universal calibration) 3182 2724 3535 1.30
aObserved.


and perhaps the molecular weight distribution of a polymer system. For example, Pl-

RIO0 was not soluble in standard solvents. GPC and VPO data were therefore not

obtained. While viscosity measurement might be obtained in trifluoroacetic acid, the

method provides limited data in the absence of an appropriate reference set. In addition,

trifluoroacetic acid appears to cause sidechain degradation in the P-R10 series, which

would skew the viscosity data.

Low resolution MALDI spectra were obtained for P1-OR9 and P1-R10. The

spectra were plotted with a mass axis linear with time of flight. Points were taken at

regular time intervals and the corresponding masses obtained.22 The signal intensity

(counts) at these masses was interpreted as the relative number of species (ni) at that mass
(Mi). From these values, Mn, Mw, and Mw/Mn were calculated. Two assumptions

were then made. First, it was assumed that the relative intensity might be better represented

if some appropriate baseline were applied. Second, it was assumed that the high mass

region was not fully represented and could be better represented by extrapolation. A well

fitting concave-up 2nd order polynomial was found for the high mass data, and the data set


22Taking representative points in time appears to provide a better representation of the mass distribution
than taking points at regular mass intervals, since the intensity was determined by number of counts in
time, rather than mass. When regular intervals of mass were sampled the MW modes calculated were
unreasonably high.








expanded by extrapolation to the minimum (intensity) of the curve. This point was then

used to set a zero intensity baseline, and the MW modes were again calculated. The results

of these calculations are found in Table 3-5, and the resultant spectra are shown, along with

the original spectra, in Figures 3-11 and 3-12.


Table 3-5. Molecular weight averages from MALDI-TOF MS.
without baseline with baseline
polymer Mn Mw Mw/Mn MAn Mw Mw/Mn
P1-OR9 33341 71426 2.14 29694 68214 2.30
P1-RO10 65008 112243 1.73 58567 103271 1.76

aThe number of species ni of mass Mi was taken as the number of counts from the intensity
scale, bThe number of species was adjusted according to a baseline drawn such that the
highest mass species was set to zero intensity. In this case the highest mass species was
obtained by extrapolation to the minimum of a well fitting 2nd order polynomial (see
Appendix C).


The MW values generated for P1-OR9 suggest molecular weights significantly

higher than was determined by VPO or GPC. While VPO data can be dramatically lowered

by the presence of small molecule impurities, the Mn values obtained from MALDI are

more than twice that which was found by GPC with universal calibration, which could

result from kinetic energy differences provided the lower mass ions are less effective in

eliciting a response in the detector. Meanwhile, the MALDI results remain questionable,

and the molecular weight of P1-RO10 undetermined.

UV-Vis absorption. Various models have been used to calculate or to explain the

electronic spectra of biphenyl and oligomeric polyphenylenes.23'24'25,26,27,28,29 In 1932,

the first X-ray evidence for planarity and reduced co-annular carbon-carbon length in

23London, A.J. J. Chem. Phys. 1945, 13, 396.
24Davydov, A.S. Zhur. Eksptl. Teoret. Fiz. (J. Exptl. Theoret. Phys.) 1948, 18, 515; Chem. Abstr.
1949, 43, 3714a.
25Dewar, M.J.S. J. Chem. Soc. 1952, 3544.
26Nakajima, T. Science Pepts. Research Insts., Tohoku Univ. 1953, 5A, 98.
27Murrell, J.N.; Longuett-Higgins, H.C. J. Chem. Soc. 1955, 2552.
28Kuhn, W. Helv. Chim. Acta 1958, 31, 1780.
29(a) Suzuki, H. Bull. Chem. Soc. Jpn 1959, 32, 1340. (b) Suzuki, H. Bull. Chem. Soc. Jpn 1960, 33,
109.





























997 14312 43136 87469 147312
Mass (m/z)


6.667 104


1.333 10
1.333 10 5


2.000 10
2.000 10 5


-2.5


n. M


ni i _1


-4.5 -3.5
-logMi


Figure 3-11. MALDI spectra of P1-OR9. (a) Actual spectrum with a nonlinear mass
scale. (b) Spectrum generated with number fraction vs. linear mass scales. (c) Spectrum
generated with weight fraction vs. log mass scales.


I I I . . .


I I


I I
























997 14312 43136 87469 147312
Mass (m/z)


S1.000 105 2.000 I 105 3.000 105
1 .000 10' 2.000 0O 3.000 105


n. M.
1i1


-6 -5.5


-5 -4.5
-logMi


I I -
-3.5 -3


Figure 3-12. Partial MALDI spectra of P1-RO10. (a) Actual spectrum with a nonlinear
mass scale. (b) Spectrum generated with number fraction vs. linear mass scales. (c)
Spectrum generated with weight fraction vs. log mass scales.


(a)




=3

d





0
0


(b)




n.








biphenyl was made available by Dhar.30 In 1933, Pauling and Sherman postulated

resonance contribution from a planar quinoid structure in biphenyl.31'32 In 1935, Hausser,

Kuhn and Speitz put forth the generalization that the wavelength and intensity of the

electronic absorption tends to increase with an increase in conjugation length.33 In 1938,

F6rster discussed the UV-Vis spectrum of biphenyl in terms of resonance.34 In 1939, an

increase in Xmax in the p-phenylene series (1-6 benzene nuclei) was used by Gillam and

Hey to corroborate X-ray data which revealed the conjugative nature of p-linked benzene

rings.35 Although the conjugative nature of the p-phenylene series was established, it was

recognized that this effect was much reduced relative to the ethylene series of conjugated

double bonds, or relative to that found in the diphenylethylene series (carotenoids). A plot

of kmax versus number of phenylene rings is shown in Figure 3-13. Despite a continuous


320-

300-

280

$ 260

i 240 -e- chloroform
S240-s- hexane
220-

200. . . . . . . . .
0 1 2 3 4 5 6 7
Number of Phenylene Rings

Figure 3-13. Progressive bathochromic shift in absorption maxima of the so called
conjugation band in a series of unsubstituted p-phenylene oligomers. The data was taken from
Table II in Gillam, A.E.; Hey, D.H. J. Chem. Soc. 1939, 1170.




30Dhar, J. Indian J. Phys. 1932, 7, 43.
31Pauling, L.; Sherman, J. J. Chem. Physics 1933, 1, 633.
32While it has long been assumed that the phenylene rings in multi-ring conducting polymers assume a
coplanar structure, this view has only recently been supported by excited state geometry calculations. See:
Beljonne, D.; Shuai, Z.; Friend, R.H.; Br6das, J.L. J. Chem. Phys. 1995, 102, 2042.
33Hausser, K.W.; Kuhn, R.; Seitz, G. Z. physik. Chem. 1935, B29, 391,
34F6rster, G. Z physik. Chem. 1938, B41, 287.
35Gillam, A.E.; Hey, D.H. J. Chem. Soc. 1939, 1170.








increase in absorption wavelength, the plot appears asymptotic, and sexiphenyl is

colorless. The strongest absorption band in the p-phenylene oligomer series occurs at ca.

205 nm. In 1950, using a linear combination of atomic orbitals (LCAO) approach, Platt

identified this band, which occurs in benzene. This band is made up of the absorptions of

Tc-tc* transitions occurring within localized benzene nuclei, i.e., transitions between 7rc and

7r* orbitals containing longitudinal nodes at the ring junction carbons.36 Similarly, long

wavelength bands were assigned to transitions between molecular orbitals delocalized over

the polyannular molecules.37 Utilizing quantum mechanics, Davydov derived an

expression for determining the energy of this lowest energy transition in PPP:



AEr. =A- 21MIcos[7r/(7r+ 1)] (3-9)



where M is the matrix element of the interaction between the nth and (n+ 1)th benzene ring.

From the experimental data of Gillam and Hey (shown in Figure 3-13), Davydov

determined Achloroform = 50900 cm-1, Mchloroform = 10300 cm-1, Ahexane = 50900 cm-1,

and Mhexane = 10200 cm-1. Accordingly, limiting values of 339 nm (chloroform) and 328

nm (hexane) were calculated.38 The same data, plotted directly as the wavelength versus

the change in wavelength with successive additions of p-linked rings, provides empirical

upper limits of 329 nm (chloroform) and 323 nm (hexane) (see Figure 3-14). Experimental

values [362 nm,39 379 nm,40 380-390 nm,41.395 nm.42] for unsubstituted neutral PPP are

variable, and typically exceed the theoretical limits, which has been attributed to the

presence of chemical defect sites such as p-quinoid units.


36Platt, J.R. J. Chem. Phys. 1950, 18, 1168.
37Platt, J.R. J. Chem. Phys. 1951, 19, 101.
38Davydov, A.S. Zhur. Eksptl. Teoret. Fiz. (J. Exptl. Theoret. Phys.) 1948, 18, 515; Chem. Abstr.
1949, 43, 3714a.
39Elsenbaumer, R.L.; Shacklette, L.W. In Handbook of Conducting Polymers; Skotheim, T.A., Ed.,
Marcel Dekker: New York, 1986; p. 219.
40Tieke, B.; Bubeck, C.; Lieser, G. Makromol. Chem., Rapid Commun. 1982, 3, 261.
41McKean, D.R.; Stille, J.K. Macromolecules, 1987, 20, 1787.
42Kovacic, P.; Hsu, L.C. J. Polym. Sci. 1966, 4, 5.










o Chloroform y = 329.43 + -1.6723x R= 0.99521
o Hexane -- y = 322.7 + -1.7767x R= 0.98468
320 -

310 -7
300

290

S280

S270

S260

250

240 .. I .. ,-, ,I
0 10 20 30 40 50
Av/Arings (nm)

Figure 3-14. Relation of change in absorption wavelength with wavelength per ring of
unsubstituted PPP oligomers. Absorption maxima taken from: Gillam, A.E.; Hey, D.H. J. Chem.
Soc., 1939, 1170 (for data, see Appendix B).


Observations that the low energy absorption bands in trans-stilbene43,44 and

biphenyl45'46 shifted to higher energies and lower intensities with increasing steric

congestion led spectroscopists to speculate about the nature of spatial and electronic

interactions in polynuclear aromatic compounds.47 Remington tried to connect steric

effects with differences in ground state structure.48 He reasoned that steric constraints

would increase the energy of the excited state, which should be planar, to a greater extent

than they would the energy of the ground state, thus increasing the energy of the transition.

Low oligomers (biphenyl,49'50 terphenyl,51'52) of PPP are known to be planar, at least on


43Ley, H.; Rinke, F. Ber. 1923, 56, 771.
44jones, R.N. J. Am. Chem. Soc. 1943, 65, 1818.
45Pickett, L.W.; Walter, G.F.; France, H. J. Am. Chem. Soc. 1936, 58, 2296.
460'Shaughnessy, M.T.; Rodebush, W.H. J. Am. Chem. Soc. 1940, 62, 2906.
47Suzuki, H. Bull. Chem. Soc. Jpn 1959, 32, 1340.
48Remington, W.R. J. Am. Chem. Soc. 1945, 67, 1838.
49Trotter, J. Acta Crystallogr. 1961, 14, 1135.
50Hargreaves, A.; Rizui, S.H. Acta Crystallogr. 1962, 15, 365.
51Rietveld, H.M.; Maslen, E.N.; Clews, C.J.B. Acta Crystallogr. Sect B 1970, 26, 693.
52Baudour, J.L.; Cailleau, H.; Yelon, W.B. Acta Crystallogr. Sect B 1977, 33, 1773.








time average, in the solid state, and to contain some twist angle in solution and in the gas

phase. Higher oligomers have been shown to have a torsional angle of 22.7 in the solid

state.53 An increase in planarity undoubtedly assumes some role in shifting the absorption

maxima to lower energies in the solid state. However, some small increase in wavelength

is expected between the solution and the solid state, independent of changes in

conformation. Dale observed a "normal red-shift" in moving from solution to the solid

state in such fused planar molecules as naphthalene and anthracene, which he accounted for

on the basis of differences in intermolecular interactions in the two states.54 The red-shift

increased with wavelength in a linear fashion. Similar results were found in conjugated

polyphenyene systems. Figures 3-10. and 3-11 illustrate this effect. Note that the data

from naphthalene and anthracene are plotted on the same graph. More significantly, data

from various phenylene systems taken by the same methods fall on the same line, despite



a 380- y = 21.558 + 0.90047x R= 0.99991
I 360
S340
CO
320

300
X 280
ea 260-
> 240-
220
200 250 300 350 400
Wavelength (nm) in a KCl pellet

Figure 3-10. The "normal red-shift" of rigid molecules. UV-Vis absorption bands of
naphthalene and anthracene in the solid state are plotted against the corresponding band in
hexane solution illustrating the observations of Dale using data from: Dale, J. Acta Chem.
Scand. 1957, 11, 650. Wavelengths in nm (values for hexane solution are in parentheses): Naphthalene:
292(285), 281(275), 272(267), 223(221). Anthracene: 394(376), 373(357), 353(339), 336(324), 318(309),
256(252), 249(246).


53(a) Delugeard, Y.; Desuche, J.; Baudour, J.L. Acta Crystallogr. Sect. B. 1976, 32, 702. (b) Baudour,
J.L.; Caileau, H.; Yelon, W.B. Acta Crystallogr. Sect. B 1977, 33, 1773. (c) Baudour, J.L.; Delugeard,
Y.; Rivet, P. Acta Crystallogr. Sect. B 1978, 34, 625.
54Dale, J. Acta Chem. Scand. 1957, 11, 650.










--y = -5.9277 + 1.0509x R= 0.99984


310 -
300-
290
280
270
260
250-
240
230 -


280 290 300
280 290. 30 0


Figure 3-11. Red-shift of p- and o-phenylene oligomers. UV-Vis absorption bands of
biphenyl, p- and o-terphenyl, p,p'-, o,p'-, and o, o'-quaterphenyl in hexane solution are
plotted against the corresponding band in the solid state (KC1 pellet) according to the
method of Dale using data from: Dale, J. Acta Chem. Scand. 1957,11,650.


320-

310

300

290

280

270
31


o Chloroform -- y = -200 + 1.5x R= 1
o Hexane ---y = -133.6 + 1.28x R= 1


15 3:


20 325 330 335 340 345
Wavelength (nm) in the solid state by diffuse reflectance


Figure 3-12. Solution absorbance versus reflectance ultraviolet absorbance for p-terphenyl
and p-sexiphenyl. Reflectance data from Elsenbaumer, R.L.; Shacklette, L.W. In Handbook of
Conducting Polymers; Skotheim, T.A., Ed., Marcel Kekker: New York, 1986; p. 219. Terphenyl, 320
nm; sexiphenyl, 345 nm; PPP, 362 nm. Solution data from Gillam, A.E.; Hey, D.H. J. Chem. Soc.,
1939, 1170.


having significantly different solution and solid state torsional angles. The o-terphenylene

system, for example, has a torsional angle of 45-50 in the solid state.55 Using the

principle of the normal red-shift (see Figure 3-12), and the differences in solid state and


55Clews, C.J.B.; Lonsdale, K. Proc. Roy. Soc. 1937, A161, 493.


230 240 250 260 270
Wavelength (nm) in KC1 pellet


I







solution absorption frequencies of terphenyl and sexiphenyl, the solution absorption of
PPP can be established. Accordingly, a PPP sample displaying a Xmax of 362 nm in the
solid state diffuse reflectance UV-Vis spectra should have a Amax of 343 nm in chloroform,
and 330 nm in hexane. Given the dramatic differences in absorption maxima of terphenyl
in diffuse reflectance (320 nm) and transmission (284 nm) spectra, it is clear that sample
comparison can only be made when all spectroscopic samples have been identically
prepared and analyzed.
Semiempirical and ab initio calculations have been performed on PPP. Extended
Htickel calculations by Wangbo et al. provided the first theoretical band structure.56'57
CNDO/S3 calculations by Ford et al. predicts three it-bands derived from the eig(I), e2g,
a2u(7), and elu valence states.58 The resulting PPP valence states are depicted in Figure 3-
13. The singlet final state was predicted to be almost entirely b2g(7T) blu(z*) in
character, and to be relatively insensitive to low levels (<13) of inter-ring twisting. The
band gap predicted by this model was 3.70 eV. Br6das et al. utilized ab initio calculations
based on the valence effective Hamiltonian (VEH) model to predict a band gap of 3.29 eV,



ZC C C C CC C
/&Y E2 /(c--c
~C C ^ T C, CP __ c- c


eig(0r) eig(0)
I 4.
blu(7r) and b2g(Irc) b3g(f) and aiu(r)
(bonding) (nonbonding)
Figure 3-13. Bonding valence 7t molecular orbitals and nonbonding valence it molecular
orbitals in PPP. As illustrated in Ford, W.K.; Duke, C.B.; Paton, A. J. Chem. Phys. 1983, 78, 4734.



56Grant, P.M.; Batra, I.P. Synth. Met. 1979, 1, 193.
57Wangbo, M.H.; Hoffman, R.; Woodward, R.B. Proc. R. Soc. London Ser. A 1979, 336, 23.
58Ford, W.K.; Duke, C.B.; Paton, A. J. Chem. Phys. 1983, 78, 4734.