Synthesis of variable bandgap conjugated polyelectrolytes via metal catalyzed cross-coupling reactions

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Title:
Synthesis of variable bandgap conjugated polyelectrolytes via metal catalyzed cross-coupling reactions
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xii, 140 leaves : ill. ; 29 cm.
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Ramey, Michael Brian, 1973-
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Polyelectrolytes   ( lcsh )
Polymers -- Synthesis   ( lcsh )
Chemistry thesis, Ph. D   ( lcsh )
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bibliography   ( marcgt )
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Thesis:
Thesis (Ph. D.)--University of Florida, 2001.
Bibliography:
Includes bibliographical references (leaves 132-139).
Statement of Responsibility:
by Michael Brian Ramey.
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Printout.
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Vita.

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SYNTHESIS OF VARIABLE BANDGAP CONJUGATED POLYELECTROLYTES
VIA METAL CATALYZED CROSS-COUPLING REACTIONS




















By

MICHAEL BRIAN RAMEY


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF 4WE JN.VESITY OF FLORIDA IN PARTIAL FULFILLMENT
.OP TE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

U .' NIVEiRS IT OF FLORIDA

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This dissertation is dedicated to James B. and Jewell Q. Ramey, and Ralph and Georgia

Quails whose lifelong work, encouragement, and love have made the construction of this

dissertation possible.


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ACKNOWLEDGMENTS

The greatest acknowledgment goes to my Lord and Savior, Jesus Christ, whose

loving sacrifice wipes clean the imperfections of us all. My wife, Jennifer Ramey, has

walked hand in hand with me throughout my growth as a Christian and scientist and

without her support and love my life would not be complete. Family members mentioned

in the dedication essentially gave up a large portion of their lives in order to make mine a

success and that debt can never be repaid. The investment in my life was paid with the

sweat of their brow and with intellectual and emotional guidance. Now that I am

expecting the arrival of my first child in September 2001, I can only hope to reflect the

same attitude to my son or daughter.

I would also like to thank those around me in the professional arena. Dr. John

Reynolds has helped guide me through the process of becoming a Ph.D. scientist and has

set an excellent example of the life of a Christian man. Dr. Kenneth Wagener has almost

been like a second research advisor to me as he is available and helpful for all students

who come to him in search of advice on research, professionalism, or life. The students,

post-docs, and visiting scientists make the George and Josephine Polymer Research

Laboratory an outstanding environment in which to work. Special thanks go out to my

closest friends on the polymer floor: Jason Smith and Cameron Church who have had the

pleasure of jumping through the same hoops and sharing the same experiences as

members of the same entering graduate class; Dean Welsh who is a fellow NASCAR









fan, workout partner, and scientific consultant; and C.J. Dubois who is one entertaining


Cajun, but respectful and competent lab co-worker.


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TABLE OF CONTENTS

page

ACKNOW LEDGMENTS ................................................................................................... iii

LIST OF TABLES .............................................. ........................................................ vii

LIST OF FIGURES......................................................................................................... viii

ABSTRACT...................................................................................................................... xi

INTRODUCTION....................................................... ................................................ 1

The Origination of Polymer Chemistry......................................... ............................... 1
Background and Theory of Conjugated Polymers .......................................................... 2
Bandgap: From Dienes to Extended Conjugation Systems ....................................... 3
Luminescence: Photo- and Electro- ............................................ ................................ 8
Conjugated Polymers for Electroactive Applications............................................ 11
Palladium(0) Coupling Reactions ................................................................................. 13
General Catalytic Cycles and M echanism ........................................ .......... ........... ... 14
Conjugated Polyelectrolytes............................................. ......................................... 17
Scope of the Dissertation......................................................................................... 18

CATIONIC POLY(p-PHENYLENE)'S ......................................................................... 20

Introduction ............................................................................................................. 20
Early Synthetic Attempts .................................................................................... 20
Suzuki Couplings ................................................................................................ 22
Results and Discussion............................................................................................ 25
M onomer and M odel Compound Syntheses................................................... 25
Neutral Polymer Syntheses ................................................................................. 30
Polymer Quaternization....................................................................................... 39
Physical Properties of PPP Type Polymers............................................................... 41
Conclusions .......................................................................................................... 44

CATIONIC POLY(p-PHENYLENE-co-THIOPHENE)'s .............................................. 47

Introduction ............................................................................................................. 47
Early Synthetic Attempts .................................................................................... 48
Optimization of the Stille Coupling Polymerization................................. .......... ... 50
Results and Discussion............................................................................................ 54






Monomer Syntheses and Suzuki Coupling Test Reactions................................... 54
Neutral Polym er Syntheses ................................................................................. 59
Polym er Quaternization....................................................................................... 65
Physical Properties of PPT Type Polym ers............................................................ 66
Conclusions ....... .................................................................................................. 69

CATIONIC POLY(p-PHENYLENE-ETHYNYLENE)'s........................................... 71

Introduction ............................................................................................................. 71
Early Synthetic Attem pts ............................................................. .....................71
Palladium (0) Coupling Reactions ............................................................................ 72
Dialkoxy-Poly(p-phenyleneethynylene)'s ........................................ ........... ............. 75
Results and Discussion.................................................................................................. 78
M onomer Syntheses ............................................................................................ 78
Neutral Polym er Syntheses ....................................................................................... 87
Polymer Quaternization......................................................................................... 94
Physical Properties of PPE Type Polym ers................................... .............. ............ 95
Conclusions ......... ....................................................................................... ........... 99

CONCLUSIONS ....................................................................................................... 101

EXPERIM ENTAL .......................................................................................................... 107

Chapter 2 ............................................................................................................... 108
Chapter 3 ............................................................................................................... 116
Chapter 4 ............................................................................................................... 122

REFERENCES.......................................................................................................... 132

BIOGRAPHICAL SKETCH .......................................................................................... 140




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LIST OF TABLES


Table Page

1-1. Brief Summary of Emission Wavelength for Differing Conjugated Polymer
Structures.................................................................................................... 12

2-1. Catalyst effect on the molecular weight properties of PPP-NEt2 polymers................ 36

2-2. Elemental Analysis results for PPP monomers and polymers.................................. 37

2-3. Effect of DBNEt or DINEt on the molecular weight of PPP-NEt2 polymers ............ 38

3-1. Structures of the organohalides and triflates for the Stille reactions .......................... 51

3-2. Structures of the organotin monomers for the Stille reactions. .................................. 51

3-3. GC/MS results of Suzuki coupling of 2,5-thiophene diboronate ester and 4-
brom otoluene................................................................................................. 58

3-4. Gel permeation chromatography results for Stille coupling of PPT-NEt2.................... 61

3-5. Elemental Analysis results for PPT monomers and polymers.................................... 62

3-6. Summary of optical data for PPT-NEt type polymers............................................. 68

4-1. Elemental analysis results for PPE monomers and polymers..................................... 81

4-2. Summary of optical data for PPE-OC9(20) type polymers........................................ 97











LIST OF FIGURES


Figure Page

1-1. Structures of poly(p-phenyleneterephthalamide) (1), poly(benzobisthiazole) (2), and
poly(p-phenylene) (3).......................................... ................ .......................... 3

1-2. Application of Frost's circle to illustrate the energies of molecular orbitals within
cyclic system s..................................................... ............................................. 5

1-3. Band structure and density of states (DOS) diagram of a simple one dimensional
metal (polyacetylene) prior to and after a Peierls distortion........................... 7

1-4. Geometrical relaxation of a PPV chain in response to photo- or electo- excitation..... 10

1-5. Polaron, bipolaron, and singlet exciton energy levels in a non-degenerate ground-
state polym er. ...................... ............ ............................................................ 10

1-6. Electronic transitions in a conjugated polymer (i.e. PPV) showing both singlet and
triplet states. ................................................................................................... 11

1-7. General catalytic cycle for Pd(0) cross coupling reactions......................................... 15

2-1. Synthetic methods to poly(p-phenylene) ............................................................. 22

2-2. Suzuki coupling approaches to substituted poly(p-phenylene). %............................... 23

2-3. Anionic poly(p-phenylene)'s reported in the literature....... ............................ 24

2-4. Cationic poly(p-phenylene)'s reported in the literature (R = hexyl).;:.........:. ......... 25

2-5. Conversion of 1,4-dimethoxybenzene to 2,5-diiodohydroquinone. ......................... 26

2-6. Bromination of hydroquinone in the 2,5 positions ................................ ....... 27

2-7. Williamson etherification of DIHQ or DBHQ. ......................................... ........... 28

2-8. Synthesis of di-boronic phenylene reagents for use in Suzuki couplingS.:... ....- ........ 29

2-9. Synthesis of neutral and cationic PPP model compounds. .:: ........:.... ......30


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2-10. Suzuki polymerizations for neutral alkoxy-amine containing PPP's ..................... 31

2-11. Cl-Pd-Cl bond angle for PdCl2(dppe), PdCl2(dppp), and PdCl2(dppf) catalysts........ 34

2-12. Gel permeation chromatogram for PPP-NEt2(dppf)[12].................... ........... ... 36

2-13. Envisioned boronic reagents for a more substituted PPP-NEt2 polymer................ 38

2-14. Pd catalyzed coupling to diboronic reagents for Suzuki couplings........................ 38

2-15. Q uaternization of PPP-NEt2.......................................... ........................................... 40

2-16. UV-Vis / Emission behavior of neutral and water soluble PPP-NEt.......................... 43

2-17. Photoluminescent spectrum of PPP-NEt2[12] in THF with normalized and linear
em mission scale................................................................................................ 44

2-18. TGA thermograms for neutral and water soluble PPP-NEt under N2......................45

3-1. Literature examples of phenylene-co-thiophene type polymers................................. 49

3-2. General scheme for the Stille polymerization....................................................... 49

3-3. Representative polymer repeat units of Stille polymerizations ................................ 52

3-4. Synthesis of 2,5-bis(trimethylstannyl)thiophene ..................................................... 55

3-5. Synthesis of 2,5-thiophene diboronate ester ............................................................ 56

3-6. Test coupling reaction of 2,5-thiophene diboronate ester and 4-bromotoluene.......... 57

3-7. Stille coupling polymerization scheme for PPT-NEt2................................................ 60

3-8. 'H and L"C NMR spectra of PPT-NEt2[28]............................................... .......... .. 63

3-9. Synthesis of PPT-NEt2[29] via Suzuki coupling polymerization............................. 64

3-10. Quaternization of PPT-NEt2 to form PPT-NEt3+ .................................... ........ .. 66

3-11. Normalized UV-Vis absorption and solution photoluminescence for PPT-NEt type
polym ers. ........................................................................................................ 67

3-12. TGA thermograms for neutral and water soluble PPT-NEt under N2...................... 69

4-1. Early synthetic methodologies toward poly(p-phenyleneethynylene)'s [PPE]............ 72

4-2. General reaction scheme for the Heck-Cassar-Sonogashira-Hagihara reaction........... 73

4-3. Activation of Pd(II) compound to active Pd(0) catalyst................................... 74



S .ix






4-4. Synthesis of dialkoxy poly(p-phenyleneethynylene)'s via the Sonogashira reaction.. 75

4-5. Representative structures of synthetic modifications to poly(p-
phenyleneethynylene)'s.................................................................................. 77

4-6. Williamson etherification to synthesize various 1,4-dialkoxyphenylene's. ................78

4-7. lodination of various 1,4-dialkoxybenzene's............................................................. 79

4-8. Synthesis of various 1,4-diethynylphenylene monomers ........................................... 80

4-9. Synthesis of 2,5-bis(6-bromohexyl)-1,4-diiodobenzene............................................ 83

4-10. Gas chromatography analysis of purification of 6-bromohexylmethylether (43) by
vacuum distillation using (a) simple vigreux column and (b) spinning band
column n........................................................................................................... 85

4-11. Rehahn's route to cationic PPP's........................................................................... 86

4-12. Williamson etherification to "protect" bromo endgroups........................... ....... .. 86

4-13. Envisioned application of Rehahn's strategy to PPE's.............................................. 86

4-14. General synthesis for alkoxy-amine containing PPE's.............................................. 87

4-15. 'H and '3C NMR of PPE-NEtz/OC9(20)[54] in CDC3 ............................................ 92

4-16. Expansion of the aromatic region of the 'H NMR of PPE-NEt2/OC9(20)[54] in
C D C 13......................................................................................................... 93

4-17. Conversion of PPE-NEt2/OC9(20)[54] to cationic polyelectrolytes........................ 94

4-18. Normalized UV-Vis absorption and solution photolumninescence for PPE-OC9(20)
type polym ers. .................................................................................................... 97

4-19. TGA thermograms for neutral and protonated PPE-OC9(20) under N2.................. 98








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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 VARIABLE BANDGAP CONJUGATED POLYELECTROLYTES
VIA METAL CATALYZED CROSS-COUPLING REACTIONS

By

Michael Brian Ramey

May 2001

Chairman: Dr. John R. Reynolds
Major Department: Chemistry

Metal catalyzed coupling reactions such as the Stille, Suzuki, and Sonagashira

(Heck) have become useful tools for the organic chemist over the past two decades for

the formation of carbon-carbon bonds. Tolerance of functional groups, reasonable

reaction temperatures, and high yields have allowed these techniques to be applied to the

synthesis of conjugated polymers. These syntheses offer access to a wide variety of

conjugated backbone structures that have previously been difficult to reach using

traditional polymerization techniques.

Poly(p-phenylene) [PPP], poly(p-phenylene-co-thiophene) [PPT], and poly(p-

phenylene-co-ethynylene) [PPE] electrolytes have been prepared by using one of the

aforementioned coupling techniques. A methodology was applied whereby charge neutral

polymers were first synthesized and then converted to the corresponding cationic

polyelectrolyte. This "pre-polymer" technique allows for studies comparing neutral

polymer properties (i.e., absorption, luminescence, solubility) to those of the






polyelectrolyte. Significant changes in the polymers' visible absorption and emission

wavelengths occur between the differing backbone structures. The polyelectrolytes'

optical transitions are shifted to higher energies (blue-shifted) versus the absorption and

emission of the neutral version within the same polymer backbone type.

The effects of halogenation of the monomer, solvent type, and palladium catalyst

on the molecular weight were determined for each set of neutral polymers by monitoring

chain extension by gel permeation chromatography. In the case of the PPP derivatives, it

was found that the Suzuki polymerization proceeds the fastest to maximum molecular

weight in a DMF / aqueous media using PdCl2(dppf) catalyst with di-iodinated

monomer. Polymerizations using di-brominated monomers reached similar molecular

weight values but only after longer reaction times. Polymer chain growth in this system

was limited by the precipitation of polymer from the reaction solution and not the

reactivity of the halogenated monomer. PPT polymers synthesized using the Stille

reaction proceeded to highest molecular weight values in anhydrous DMF using

PdC12(PPh3)2 catalyst and di-iodinated monomer. Triethylamine / THF solvent systems

using PdCl2(PPh3)2 catalyst with a small amount of Cul co-catalyst and di-iodinated

monomer were the best conditions for the PPE Sonagashira polymerizations. Di-

brominated monomers were ineffective in reaching polymeric materials when used in

either Stille or Sonagashira polymerizations. The conversion procedure to the

polyelectrolyte was determined to be sufficiently mild not to induce breakages of the

backbone, thus allowing the molecular weight characteristics foihe neutral species to be

roughly applied to the polyelectrolyte.






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CHAPTER 1
INTRODUCTION


The Origination of Polymer Chemistry

Over the past 100 or so years, polymer science and chemistry have evolved from

early rubber and Bakelite type chemistries to extensively characterized and

commercialized materials. Looking back on the early evolution of this branch of science,

today's observer would find vigorous debates on the exact nature of polymers: were they

linear polymers held together in long chains by covalent bonds or merely

"agglomerations" of smaller molecules held together by ionic forces? Today, we know

that they are indeed based on the first principle as proposed and defended by Staudinger.'

Necessity proved to be the mother of invention as the need to replace natural

items such as silk (Nylon 6,6) and rubber (cis-1,4-polybutadiene), imported from foreign

countries to the United States, was of utmost importance during World War I as the

conflict threatened to cut off supplies. From these beginnings, the study and everyday

use of man-made polymers has exploded (possibly best exemplified by the whispering of

the line "Just one word: plastics" in the 1967 Hollywood movie The Graduate).

Synthetic polymers are a major cornerstone of the entire industrial chemical world and

basic research on these materials has enabled scientists to understand natural polymers,

such as proteins, on deeper levels than ever before. With such broad and sweeping

applications and variations throughout polymer chemistry, a complete overview of the

science would be impossible; therefore, a "guided tour" will be presented herein outlining



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the buildup of repeat units and properties within conjugated polymer systems (materials

with alternating single and double/triple bonds). These polymers have exciting new

applications for optical display markets, which could never have been envisioned during

the early days of the science.


Background and Theory of Conjugated Polymers

One of the simplest organic molecules is the two carbon molecule ethene,

CH2CH2, which exists as a gas under standard temperature and pressure. Polymerization

of the molecule leads to long chains of covalently bound two carbon units, -(CH2CH2)-,

termed polyethylene. As the number of covalent bonds increases, the material moves

through liquid (20 repeats), waxy (100 repeats), brittle plastic (200 repeats), and tough

plastic (>200-300 repeats) stages of mechanical properties. In the 100-200 repeat unit

regime as chain lengths become long enough to entangle, a material with plastic qualities

emerges that bridges the gap between crystallites to form tougher materials.

Most polymers have a minimum molecular weight threshold where mechanical

properties do not change greatly with additional coupling. Higher molecular weights

may produce polymers that are more difficult to process due to poor solubility or very

high melting temperatures. A balance must be achieved for each particular polymer

system to make good materials that can be molded for use. As more complicated

polymer systems are envisioned, several factors control the molecular weight to solubility

ratio. Side chain branching from the backbone of linear polymers reduces ordering and

lowers the degree of crystallinity and has become a standard method of increasing

polymer solubility. Incorporation of unsaturated bonds or aromatic rings in the backbone

reduces rotational freedom of the polymer chain, thereby stiffening the sain.

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Functionalized polymers capable of hydrogen bonding interactions can have lowered

solubility as well.

Polymers that form extended, ribbon-like structures in solution rather than a

random coil conformation are termed rigid-rod polymers. Such polymers are exemplified

by poly(p-phenyleneterephthalamide) 1, poly(benzobisthiazole) 2, and poly(p-phenylene)

3, shown in Figure 1-1. Polymer 1 maintains its rigid-rod nature by hydrogen bonding

between chains and polymers 2 and 3 maintain the same nature by being entirely

conjugated. The conjugated polymers have the unique property of being electroactive,

meaning they have dielectric and spectral properties (such as luminescence) that depend

on applied voltages. Because of the electroactive nature of conjugated polymers, they

have become a major focus of research over the last 20 or so years. It is easy for one to

focus solely on the optical properties due to the visual nature of humans; however, it is

important not to forget mechanical property considerations, because solubility and

processing difficulties must always be dealt with in these systems.







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1 2 3

Figure 1-1. Structures of poly(p-phenyleneterephthalamide) (1), poly(benzobisthiazole)
(2), and poly(p-phenylene) (3).


Bandgap: From Dienes to Extended Conjugation Systems

The simplest of the conjugated polymers is polyacetylene, -(CH-CH)-, which was

synthesized by Ziegler-Natta polymerization of the monomeric gas.2 The material is of








low density, fibrous, and has few defect sites.3 Polyacetylene is intractable due to its

extensive conjugation and rigid nature. Just as mechanical properties build up with

additional coupling of monomer to polymer chains, electronic properties of n-conjugated

polymers grow analogously. Hickel Molecular Orbital Theory provides a qualitative

description of the behavior of n electrons of planar conjugated systems and can be used

to explain the electronic behavior of such systems. The overlap integrals (Sy) for orbitals

perpendicular to one another are considered to be zero.

This method can be utilized to describe the aromaticity in benzene and can be

extended to linear conjugated systems by treating them as giant cyclic structures with

equally spaced carbons. Orbital energies are given by the expression


E= c+2fcos( J) J= 1,2,...,N (1-1)
N+1

where J is the orbital number, counting upward from the lowest-energy orbital J = 1, and

N is the number of carbon atoms (also the number of basis orbitals) in the chain. The

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

resonance integral 1 is related to the energy of an electron in the field of two nuclei. The

maximum energy between the lowest and highest molecular orbitals is arbitrarily set at a

constant value of 43. Figure 1-2 shows the application of the Frost's circle4 mnemonic to

illustrate to energy levels for cyclobutadiene and benzene.

As the number of linearly combined atomic orbitals increase (corresponding to

larger ring size in the Frost circle), it becomes clear that the energy difference between

molecular orbitals becomes increasingly small. The energy to excite an electron from the

HOMO to LUMO level would be insignificant relative to the thermal eergy of an' ,:,



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electron. The orbitals would merge into a one-dimensional band, similar to the

conduction bands of metals. Electrons in the highest energy occupied orbitals would be

free to move into the unoccupied orbitals where they would have a high mobility. This

simple model would allow for polyacetylene to be metallic with no barrier to the free

movement of electrons in the system.



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

a 21 antibonding


S} -- a nonbonding


4[ a + 20 bonding


Sa 2 antibonding



a + 2 bonding


Figure 1-2. Application of Frost's circle to illustrate the energies of molecular orbitals
within cyclic systems.



Experiments have proven that polyacetylene is not a metallic conductor in its

neutral state. This is accounted for by analyzing the orbitals at the Fermi level. The

Fermi level is the energy level which has a 50% chance of being occupied by an electron

and represents the midpoint in energy of a symmetric half-filled band. The molecular

orbitals at the Fermi level are close enough in energy to behave as if degenerate. The

Jahn-Teller theorem5 predicts that when degenerate orbitals are unevenly filled with


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

vibration. The orbitals become nondegenerate and the total energy of the system is

lowered.

In solid state physics terminology, the Jahn-Teller effect is known as a Peierls

distortion6 and opens a gap in the pz band which physically distorts the polymer chain to

achieve a lower energy. Figure 1-3 graphically represents the effect of the Peierls

distortion on the band structure and density of states (DOS) of polyacetylene. Figure 1-

3a,b would exemplify a metallic conductor with no energy difference for electrons to

migrate into the unfilled conduction band. Figure l-3c,d demonstrates that as adjacent

carbons along the polymer chain dimerize, alternate single and double bonds are formed

as a discrete energy/band gap develops (Eg). The p, band is broken into an empty

conduction and a full valence band. Polyacetylene is a semiconductor (a < 10-5 S cm'1)

with a band-gap (Eg) of 1.4 eV.7

S Polyacetylene can reach conductivities on the order of 104 S cm' by the addition

of electrons into the conduction band or removal of electrons from the valence band,

termed n-doping and p-doping, respectively.8 These processes result in further structural

changes in the system and defects known as solitons are formed.9 A negative soliton

corresponds to a resonance stabilized carbanion and a positive soliton corresponds to a

resonance stabilized carbocation. These charged solitons move under the influence of an

applied electric field.10

From the initial discoveries concerning the conductive and redox chemistry of

polyacetylene, the field of electroactive polymers has exploded into one of the most



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active research areas of polymer chemistry. The importance of the early work on
polyacetylene was confirmed by the awarding of the 2000 Nobel Prize in Chemistry to


EEg


D(E)
d


Figure 1-3. Band structure and density of states (DOS) diagram of a simple one
dimensional metal (polyacetylene) prior to and after a Peierls distortion.
a) Band structure prior
b) DOS prior
c) Band structure after Peierls distortion
d) DOS after Peierls distortion.
Eg is the bandgap, which for a semiconductor such as polyacetylene is twice the
activation energy for conduction.


1








those most responsible for the early work, Alan J. Heeger, Alan G. MacDiarmid, and

Hideki Shirakawa. The door for a myriad of creative syntheses to incorporate aromatic

hydrocarbons, heterocycles, vinyl, and ethynyl groups into the backbone of it-conjugated

polymers was opened by the initial work on polyacetylene. Important properties not

envisioned with the discovery of polyacetylene, such as electrochromism and

electroluminescence, have evolved with these newer materials. The properties and

syntheses of these materials are much too varied and exciting to sufficiently cover in this

dissertation, but an excellent starting reference for investigating these materials is The

Handbook of Conducting Polymers." Focus will be placed, herein, on the property of

electro/photo-luminescence and the synthetic application of palladium (0) catalyzed

coupling reactions to the preparation of conjugated polymers.

Luminescence: Photo- and Electro-

Much of the discussion and graphical representations presented in this

introduction to the electroluminescence of conjugated polymers is based on the review of

the topic by Richard H. Friend and Neil C. Greenham in "Electroluminescence in

Conjugated Polymer" in The Handbook of Conducting Polymers (see Ref. 11). Please

refer to this reference for a more complete discussion of the technical specifics for

construction and properties of light emitting diodes (LED's).

Electroluminescence is the generation of light by electrical excitation and was

first reported for an organic semiconductor in 1963 by the observed emission of light

from single crystals of anthracene.12 Studies on these simple electroluminescent organic

semiconductors established that the process responsible for the emission of light requires

the injection of electrons from one electrode and holes from the other, the capture of one




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by the other recombinationn), and the radiative decay of the excited state (exciton). The

first example of electroluminescence from a conjugated polymer was first reported in

1990 using poly(p-phenylenevinylene)[PPV] as the semiconductor between metallic

electrodes.'3 In LED's, a voltage bias is placed across the electrodes at a sufficient level

to achieve injection of positive and negative charge carriers from opposite electrodes and

upon migration, the positive and negative charges combine to form an exciton which

subsequently releases energy as light.

The excitation to form an exciton may also be achieved by exposing the polymer

to light of a wavelength that matches its absorption maximum and is termed

photoluminescence. A singlet exciton is generated by photoexcitation across the

polymer's 7 n* bandgap, and radiatively decays to emit light. Emission spectra for the

same polymer excited either electrically or photolytically are usually very similar,

indicating that the excited state responsible for light generation is identical for both

methods of excitation.

"Polaronic" excited states are formed along the polymer due to the ability of

polymers to rearrange chain geometry to reduce the strain that can be produced by the

charged excitations (excitons). Polyacetylene has a degenerate ground state allowing

formation of soliton-like chain excitations with a nonbonding n level in the middle of the

n n* semiconductor gap.14 In polymers with nondegenerate ground states, the two

senses of bond alternation do not have equivalent energies. The charged excitations of a

nondegenerate ground-state polymer are termed polarons or bipolarons and represent

localized charges on the polymer chain. Figure 1-4 shows the nondegeneracy of PPV

along with a schematic representation of an intrachain exciton. Two nonbonding midgap




10


"soliton" states form bonding and antibonding combinations, producing two gap states

symmetrically displaced about the midgap (see Figure 1-5). The levels can be occupied

by 0 to 4 electrons giving a positive bipolaron (bp2+), positive polaron (p'), polaron

exciton, negative polaron (p), or negative bipolaron (bp2).






ground state excited state
~--



exciton

Figure 1-4. Geometrical relaxation of a PPV chain in response to photo- or electo-
excitation.





it conduction band


-mn 'e-t l-e
lu, minescence .


bp2 pl I- potaron- m* P" bp2"
n valence band exoiton

Figure 1-5. Polaron, bipolaron, and singlet exciton energy levels in a non-degenerate
ground-state polymer.'



-* Si let and triplet excitons have been shown to exist in conjugae polymers.

Taking into account both Coulonbic and electron-lattice interactions, the tplet exciton
s o ttVI'.- ";citon..

and singlet exciton are no longer of the same energy nor of thee se sife. e triplet

exciton becomes more localized than the singlet excitonwhic my extend :over several
..... .. .4.... .. .

"AF
..' ... ..* ... JI: :.
.... .. ? ::.. .. : ,.::. ,.: =;...:i a. ,,. .





11


polymer repeat units. Calculations have shown that the triplet exciton is stabilized by

0.65 eV with respect to the singlet exciton and is localized over not much more than a

single polymer repeat unit for PPV.15 Figure 1-6 shows the relative arrangement of

ground and excited state energies for a conjugated polymer including the experimentally

measured higher energy triplet (T*). Typically, excitation occurs to a singlet exciton that

undergoes some vibrational release of energy and then returns to the ground state via the

release of light energy. The relaxation before emission of light results in the energy of

emitted light being of slightly lower energy than the energy of the 7 to 7T* level (Stoke's

shift).


singlet


triplet
T


inter-
1 -system induced absorption
crossing



T
absorption luminescence

S _

Figure 1-6. Electronic transitions in a conjugated polymer (i.e. PPV) showing both
singlet and triplet states.


Conjugated Polymers for Electroactive Applications

Control of the n to t* energy gap of a conjugated polymer is of utmost

importance in order to tune the wavelength of emitted light through the visible light

region. The energy gap can be modified by directly changing the type of conjugation


;4. .-.... .
A===. .X="..
,!'!,L*, ,,
E.. ..:.. ._* "C











Table 1-1. Brief Summary of Emission Wavelength for Differing Conjugated Polymer
Structures.

Structure Name Emission peak (nm)


otn


Poly(p-phenylene)
PPP


Polyalkylfluorene


" n


R R








P n





n




n
R


Poly(p-phenylenevinylene)
MEH-PPV





Poly(p-phenylenevinylene)
PPPV



Poly(3-alkylthiophene)
P3AT


along the backbone, addition of side-chain groups with electron donating or electron

withdrawing substituents, or disruption of conjugation length by insertion of non-

conjugated segments. The more electron rich a system is, the farther into the lower

energy, red emission portion of the visible spectrum it will be. Electron deficient

polymers will emit in the higher, blue emission portion of the spectrum.


420-465


605






570




460-560


I








Blue emission is found in poly(p-phenylene) (PPP),16 polyalkylfluorene,17

fluorinated polyquinoline,'8 PPP-based ladder copolymers,19 and lower gap polymers

with interrupted conjugation.20 PPV gives emission in the yellow-green light region and

the emission color can be moved toward the red by substitution with electron-donating

groups such as alkoxy chains at the 2- and 5- positions on the phenyl ring.21 The very

electron rich heterocycle containing polyalkylthiophenes emit in the red region of the

spectrum.22 Table 1-1 lists the polymers mentioned above with their corresponding

emission peak in nanometers. It should be noted for exact LED configuration the

reference for each type of polymer should be referred to as the negative electrode and

transport layer material in solid state emitting devices can affect emission characteristics.

The references listed represent the initial pioneering studies done on each material in the

early 1990's.

Many variations and methods of device construction have been attempted over the

last decade with the above polymer types and others to improve device output.

Discussion of all the variations in LED construction will not be presented here due to the

focus of this research being aimed at the syntheses of new conjugated polymers. In

particular, a focused discussion of the palladium(0) catalyzed Suzuki, Stille, and

Sonagashira coupling reactions will be conducted.


Palladium(0) Coupling Reactions

An important component was added to the toolbox of the synthetic organic

chemist in the early 1970's, by the development of cross coupling reactions involving

metal catalysis of organometallic species. Equation 1-3 illustrates the simple principles

involved in a cross coupling reaction. R and R' are typically sp2 hybridized carbon


.. ;i" .." "








R-M + R'-X -- R-R' [with Pd(0) catalyst] (1-3)

species, M is a metal (tin, boron, etc.), and X is a halogen or triflate. Palladium catalyzed

reactions of Grignard reagents was first reported by Yamamura et al.23 and then expanded

into a synthetically versatile method by Negishi et al. to include organoaluminum,24

zinc,25 and zirconium reagents.26 Many other organometallic reagents have been used as

nucleophiles for the cross coupling reactions including organolithiums,27

organostannanes (Stille),28 organosilicon,29 and organoboron (Suzuki)30 compounds.

Terminal alkenyl (Heck)3" and alkynyl (Sonogashira)32 carbons are also effective for the

reaction, though not organometallic species. Several good reviews are present in the

literature dealing with the reactions, mechanisms, and synthetic utilities.33


General Catalytic Cycles and Mechanism

All of these cross coupling reactions are mechanically and synthetically similar

and the general catalytic cycle will be described in this chapter. More focus on reaction

specifics for the differing coupling reactions will be provided in subsequent dissertation

chapters in which the chemistry involved utilizes the particular method. All of these

coupling reactions proceed through a three step cycle involving 1) oxidative addition of

an aryl halide (or other sp2 C-X species) to Pd[O]; 2) transmetallation, wherein a second

aryl group is transferred from the metallated species to Pd; and 3) reductive elimination

of a biaryl species (see Figure 1-7). If difunctional metallated and aryl halide reagents

are used, oligomeric and polymeric materials may result. Electron withdrawing groups

facilitate the oxidative addition step, while the nature of the halide or leaving group

affects the reaction rate following the trend I>OTf>Br>>C. The transmetallation step

may be rate limiting if the metallated species is sterically hindered.


: ;..': :? :: .."' .
::' : : :*.f. ** ::- -: ? ^
:. : .. ...... ,
* *'< "1' .. : ... .. .:. <1,
.:


; ;. f
I*
''"











Ar-Ar rU 4 -s ArX

reductive oxidative
elimination addition

L Ar L Ar
Pd P
L Ar' L/ X





transmetallation



M-X Ar'-M
X = I, Br, etc.

M = B(OR)2, SnR3, etc.

Figure 1-7. General catalytic cycle for Pd(0) cross coupling reactions.



Reactions are conducted under anaerobic conditions in a variety of solvents such

as THF, DMF, and toluene. For the Suzuki reaction, water and base are added to

accelerate the formation of a more active boronate anion for the transmetallation step;

otherwise the other methods are performed in dry solvent. Pd(II) catalysts such as

PdCI2(PPh3)2 are usually employed in the reactions due to their general storage and

handling advantages over Pd(0) catalysts, such as Pd(PPh3)4, which are air and moisture

sensitive. When using a Pd(II) compound, the reduction of the Pd(II) species to Pd(0)

must occur before the cycle can begin. The exact nature of this conversion is debated but

may include the homo-coupling of the Metallated species. Ligands present on the Pd aid

. .-









in solubility and activity of the catalyst, but can undergo ligand transfer instead of the

desired aryl moiety, particularly in the case of triphenylphosphonium ligands.

Often, Stille and Suzuki polymerizations can be applied to the same desired

polymers and general guidelines should be followed in making the best choice. The

chemistry behind the synthesis of aryltin compounds used in Stille reactions involves the

use of chlorinated alkyl tin reagents which are toxic and re-generated during the course of

the reaction. Therefore, if possible the Suzuki reaction should be used. Boronic Suzuki

reagents and the salts formed during the catalytic cycle are relatively "harmless." The

drawback to the Suzuki reaction is that for many electron rich aryl groups, boronic esters

or acids are much too unstable to withstand the numerous couplings needed for

polymerization. Obviously, the Heck and Sonogashira reactions are applied specifically

to the formation of vinylene and ethynylene linkages and are not alternatives to many

Suzuki or Stille routes. The reagents for each are fairly stable organic and the by-

products of the catalytic cycle are mineral acids.

The palladium (0) reactions hold several advantages over other polymerization

techniques when used to make conjugated polymers. Many free radical polymerizations

convert activated double bonds to single bonds in order to achieve the couplings, while

step growth polymerizations often involve the coupling of carbon atoms to heteroatoms

with an associated release of a small molecule such as H20. The Pd catalysts are

generally stable and tolerate most functional groups, allowing a wide range of

polymerization possibilities. Many complex repeat unit structures can be constructed by

mixing different ratios of metal and halogenated reagents. Of course, ovieal a j

stoichiometric balance (1:1) of total halogen to metal ftnc~aityo us be as
" ,. "" .. h. .. ... i : .
.. .... .... .. *.
~ j ~ ~ .E -, JIi









these are step growth polymerizations. Catalyst residues and by-products are easily

removed from the polymer product.

In the synthesis of conjugated polymers, several factors influence ultimate

molecular weight properties. A balance must be maintained in a polymerization between

a solvent that will keep the polymer chain in solution, so that additional couplings can be

performed, and that will also solubilize the Pd catalyst so that it remains active. Chain

growth is terminated by premature polymer precipitation. In general, DMF, THF, and

toluene are effective solvents for the coupling reactions, with DMF able to stabilize the

catalysts the most due to its coordination ability. The polarity of the solvent should

match the polarity of the polymer to best keep it in solution. Temperatures must be

carefully monitored as excess thermal energy can degrade the catalyst and promote the

degradation of the active functionalities at the end of the polymer chains, thereby

terminating chain extension. Temperatures at or below 80 C are commonly used.


Conjugated Polyelectrolytes

As mentioned earlier, one important issue in the conjugated polymer field is that

of processability. Traditionally, branched or long alkyl side chains are added to

conjugated polymers to increase solubility. Although this is effective, chlorinated and

high boiling solvents are often necessary to dissolve the polymers. One approach to

overcoming this difficulty is to create polar conjugated polymers that are water soluble.

The interesting luminescent properties mentioned previously will still be present, but now

the polymers may be processed from the more environmentally and industrially friendly

solvents, such as ethanol, water, etc. Side chains can be functionalized with carboxylate,

sulfonate, and quatemized ammonium groups to achieve the water solubility.


.:.
-. s '*:' ...








There are also numerous chain extension and folding effects to be studied that are

unique to polyelectrolytes. Flexible polyelectrolytes have been the focus of a

considerable amount of research for many decades.34 Decreasing the ionic strength of a

dilute aqueous solution of polyelectrolyte leads to an expansion of the polymer coils and

an increase of solution viscosity due to strong intra- and inter-molecular forces.

Separation of the intra- versus inter- molecular forces is a difficult experimental task and

only recently has the understanding of the single chain behavior of flexible

polyelectrolytes been achieved by Monte Carlo simulations.35

Conjugated, stiff-chain polyelectrolyes remain in an extended conformation

regardless of the ionic strength of the solution. Effects observed from lowering the ionic

strength of the system must therefore be due to intermolecular forces. Conjugated

polyelectrolytes represent interesting models for studying the screened coulombic

interactions in polymeric systems. Interesting applications may also be available for

these materials in membrane manufacturing.36 Specifics and references for literature

examples of conjugated polyelectrolytes synthesized by palladium (0) catalysis will be

given in the introduction to Chapter 2.


Scope of the Dissertation

This body of work focuses on incorporation of quaternized 2,5-dialkoxyamine-

phenylene or quaternized 2,5-dialkylamine-phenylene salt moieties into the backbone of

conjugated polymers. Suzuki, Stille, Sonagashira (Heck), and ADIMET polymerization

techniques will be used to synthesize neutral polymers of the following types: poly(p-

phenylene)[PPP], itily(p-phenylene-co-thiophone.)[PPT], and poly(p-phenylene-co-

ethynylene)[PPE], whereby the phenylene portion of the repeat u it is initially

*,L ...... :: .. .
:- : .. i : ... *,:''.: ;( i : :.:: :\ i
4 : E.., .
.. .;. ., .. ..:: :.::. '. :. ::::..








synthesized with neutral alkoxy-triethylamine or alkylbromide side chains. These neutral

polymers can be analyzed using traditional techniques (GPC, NMR, etc.) and will possess

absorption and luminescence wavelengths that vary over the visible wavelength range

based on the electronic makeup of the backbone.

Alkoxy-triethylamine containing polymers were treated with bromoethane to form

the cationic dibromide salt of the original polymer. Likewise, the alkylbromide

containing polymers were treated with triethylamine to achieve the desired

polyelectrolytes. Molecular weight characteristics of the neutral polymer can be

approximately applied to the polyelectrolytes, since the treatments of the neutral polymer

do not break backbone linkages. Optical properties will be extensively investigated

focusing on the emission, absorption, and electrochromic responses from both the neutral

and water soluble polymers in solution and as prepared films


I .. .












CHAPTER 2
CATIONIC POLY(p-PHENYLENE)'S


Introduction


Early Synthetic Attempts

Poly(p-phenylene) (PPP) has long been a synthetic target for polymer chemists

due to theoretical calculations and observations on ill-defined materials37 that show PPP

to possess good mechanical strength and high chemical resistivity.38 Possibly, the most

important property of PPP is its ability to be used as a blue emitter in electroluminescent

devices.39 The advantages of using a polymer to emit light in the consumer electronics

industry are immense, as the more numerous polymer processing techniques allow for the

creation of "flat panel" computer and high definition television screens unavailable with

traditional materials and techniques. A thin film of PPP is placed between a high work

function anode (indium tin oxide coated glass) and a low work function cathode

(calcium). Under appropriate forward bias, holes and electrons are injected into the

polymer film, resulting in the formation of positive polarons onone side of the film and

negative polarons on the opposite side. The polarons migrate toward each other and a

singlet exiton is formed resulting in the emission of blue light.

Two major factors have hindered the synthesis of PP. As the.-piber of rings in

an unsubstituted, linear "pure PPP" typ polymer increase, solulit optyef resitik .,

chain diminishes quickly, leading to an insoluble, intractable. pd-- j t 6 little r

no use. However, the methodology applied to solubiwling PPP ani utin


91, +w !:. T N A Hs....








that do not reflect the characteristics of "pure PPP". Thermal, mechanical, and chemical

stability are reduced and the optical absorption and emission wavelengths are shifted

from the expected values. Nevertheless, the molecular weight enhancements and

solubility of resulting substituted PPP's often outweigh the property differences between

themselves and "pure PPP".

A second hindrance to PPP synthesis is that traditional polymerization techniques

are not designed to grow a chain via carbon-carbon bond formation, but typically via

carbon-heteroatom (oxygen or nitrogen) coupling. Often the somewhat "exotic" methods

used to create PPP actually enhance side reactions leading to structurally poor polymers.

Electrochemical polymerizations have been attempted both oxidatively with 1,4-

dialkoxybenzenes40 and reductively with 1,4-dihalobenzenes in the presence of a nickel

catalyst.41 Chemical oxidation polymerizations have been conducted with cupric chloride

(Figure 2-la).42 Thermal conversion of radically43 or transition metal polymerized4

protected 5,6-dihydroxy-l,3-cyclohexadiene to unsubstituted PPP overcame solubility

difficulties with soluble "pre-polymer" intermediates that can be processed and

subsequently converted to "pure PPP" (Figure 2-lb). Thermal cyclization of enediynes

and o-phenyldiynes gave PPP's and poly(l,4-naphthylenes), respectively (Figure 2-lc).45

Nickel catalyzed Grignard couplings of 1,4-dibromobenzene have also been performed

by Yamamoto et al. (Figure 2-1d).46 The Grignard coupling route provided structurally

pure PPP oligomers. This mild route was promising, but termination by inherent

chemistry or precipitation of the growing polymer negated higher molecular weights.

Attachment of alkyl side chains led to a more homogeneous polymerization and higher

degrees of polymerization. Nickel catalyzed homocoupling of dichloro-,47







di(methanesulfonyl)-,48 and di(trifluoromethanesulfonyl) benzenes49 in the presence of

excess zinc have afforded functionalized PPP's (Figure 2-le,f ).


1. CuCI2 I AICIa / 02
2. MeOH / HCI


R*t .heat
S-2HOR
RO OR


R R

" f46h


RO OR
RO OR


R R

\/~


R
Br- -Br M.
R


R
Br- /-MgBr
R


Ni(O)
cat.


R
Cl CI
R


Ni (0) cat.
Zn


R1 R
RO2SO- OS02R N ct.
R = CH3, CF3

Figure 2-1. Synthetic methods to poly(p-phenylene).


Suzuki Couplings

A major improvement in PPP synthesis came in 1989.when Rehahn and

coworkers applied the more reactive Suzuki coupling reaction methodology to the:



L M.. .... .. .::.. .::
.. ~ : .. ::J :; ._ ; :,,411








polymerization. 5051 A-B polymerization of 4-bromo-2,5-dialkylbenzeneboronic acids

and AA/BB polymerization of 1,4-dibromo-2,5-dialkylbenzeneboronic acids was

performed (Figure 2-2). Chain lengths of 100 rings were achieved leading to

processable, substituted PPP's.52

The success of the Suzuki reaction with its use of less electropositive boron

reagents, high yield couplings, and tolerance for mixed aqueous / organic solvent systems

opened the door to a variety of functionalized PPP's hitherto unreachable. One of the

most interesting sub-fields to arise from this methodology was the synthesis of

conjugated, rigid polyelectrolytes. The first rod-like polyelectrolytes were reported in the

early 1980's and were based upon poly(l,4-phenylenebenzobisoxazole) and poly(l,4-

phenylenebenzobisthiazole).53 Careful incorporation of anionic or cationic functionality

into a PPP yields a material that possesses the beneficial properties of a conjugated

polymer with the aqueous solubility and processability of a polyelectrolyte. The

environmental utility of aqueous processing techniques applicable to polyelectrolytes is a

potential advantage of these materials for use in an industrial setting. Carboxylate

(Figure 2-3a,b),54 sulfonate (Figure 2-3c),55 and sulfonatopropoxy groups (Figure 2-3d)56

have been used to create anionic PPP polyelectrolytes.

R R R
Br 1. n-BuLi Br H Pd (0) cat. (a)
Br Br Br &a- -d (a)



R A
S- Pd ) Rcat.
Br Br + H }0 -c (b)
y_0HO H O aq.b|e n
R R
a.. .
Figure 2-2. Suzuki coupling approaches to substituted poly(p-phenylene).


I ... 4
..O

..'> :.::;i... ii : : .""'47 .* ""r:













COOH (CH2) n R


n
HOOC (CH2)6 R
(a) \ (b)



HOOC SO3-


R S03'Na+

n n
R 0
(c) (d)


'038

Figure 2-3. Anionic poly(p-phenylene)'s reported in the literature.



Highly charged cationic ammonium and pyridinium PPP polyelectrolytes were

reported in the mid 1990's by Rehahn and co-workers (Figure 2-4a,b).57 Dr. Peter B.

Balanda of the Reynolds' research group used an alternate methodology to include

cationic quaternary ammonium salt side chains into a PPP backbone (Figure 2-4c).58

Poly[2,5-bis(2- {N,N,N-triethylammonium -1-oxapropyl)-1,4-phenylene-alt-1,4-

phenylene] dibromide (PPP-NEt3+) was synthesized via a Suzuki protocol. The polymer

was used in the assembly of blue emitting solid state devices via layer-by-layer

polyelectrolyte self-assembly with sulfonatopropoxy PPP. 59 The material also proved

very useful as a buffer layer for hybrid ink jet printed LED's using sulfonatopropoxy

substituted poly(phenylene-vinylenes).60




S, i : .















\ /- -' / n \ --' /- / n \ ^- v- / n
(CH2) R (a) (CH2)6 R (b) 0 (c) PPP-NEt3+
\ (a) r (b)

SB
Figure 2-4. Cationic poly(p-phenylene)'s reported in the literature (R = hexyl).



Due to the important applications available for PPP-NEt3+, it was evident that a

closer inspection of the synthesis, along with scale-up procedures was needed. In

particular, a focused look at a new palladium catalyst with stabilizing ligands and higher

reactivity was a primary concern. Other synthetic investigations to be accomplished were

the effect of the halogenated monomer on the molecular weight of the polymer and the

effect of more unsubstituted phenylene rings in the polymer backbone. With more

unsubstituted phenylene rings, a system that resembles "pure PPP" better might be

created, but problems of solubility could arise also. The results and discussion following

will address these aspects in greater detail.


Results and Discussion


Monomer and Model Compound Syntheses

Previous work had shown that the most promising route for the cationic water

soluble PPP synthesis was to first create a neutral PPP analog and then quaternize the

amine sites post polymerization. Figures 2-5 and 2-6 show the syntheses of 2,5-

diiodohydroquinone and 2,5-dibromohydroquinone, respectively. 1,4-dimethoxybenzene



I.
. V. ....








(4) was iodonated under acidic conditions using potassium periodate, iodine, and a mixed

solvent system consisting of 90:7:3 HOAc/ HzO/ H2S04 by volume with heating to yield

2,5-dimethoxy-1,4-diiodobenzene (5).61 Compound 5 was reacted with boron tribromide

in methylene chloride at -78 oC, producing 2,5-diiodohydroquinone (DIHQ).62 It

should be noted that boron tribromide is a very reactive reagent with large amounts of

HBr gas liberated during the aqueous workup of the reaction. DIHQ is recovered as a

crude brown solid. Recrystallization from THF and hexane affords colorless crystals of

pure product. Both steps are high yielding (81% and 76%) with an overall 62% yield

based on starting material 4. Analysis of the crude material by 'H NMR shows the only

organic product was the desired compound 4. It was later found that use of this brown

material was sufficient for the Williamson etherifications to follow. 90 % yield of the

crude material was obtained.


H3CO H3C I H I
K104,12 BBr3
AcOH/ H20 / H2SO4 MeCl2
OCH3 70 C /12h I OCH3 -78 OC RT OH
4 5 12h DIHQ
81% 76%/

Figure 2-5. Conversion of 1,4-dimethoxybenzene to 2,5-diiodohydroquinone.



2,5-dibromohydroquinone (DBHQ) was synthesized in 40% yield from the direct

bromination of hydroquinone (6) in methylene chloride and acetic acid. The reaction

proceeds through three stages. The initial setup involves the suspension of hydroquinone

in the solvent system. As the first equivalent of bromine is added, the resulting

monobrominated species enters solution and as the second bromine adds to the phenyl

ring, the desired product precipitates out of solution maklia piduct redor la i..A ,


A. ,. .. ; il l H .. .









matter of filtration. The reaction's lower yield is probably a result of some DBHQ

remaining dissolved in the solvent. No attempts were made to recover this "lost"

material. Recrystallization of the slightly pink crude product from a hot 4:1 (v/v) water

to isopropanol solvent solution removed the undesired impurities.



H 2.1 eq. Br2 OH

S MeC2/ AcOH /
HO 6 HO
DBHQ
40%
Figure 2-6. Bromination of hydroquinone in the 2,5 positions.



DIHQ and DBHQ were subjected to Williamson etherification conditions in

refluxing acetone with 2.1 equivalents of 2-chlorotriethylamine hydrochloride (7) and 4.0

equivalents of K2C03 for three days, as outlined in Figure 2-7, to produce the desired 1,4-

dihalo-2,5-dialkoxyamine phenylene monomers, DINEt and DBNEt. Four equivalents

of K2C03 were necessary for deprotonation of the hydroquinone and the hydrochloride

salt of the 2-chlorotriethylamine reagent which was deprotonated in situ. Isolation of the

organic chlorinated amine would be difficult as cyclization to the aziridinium ion would

likely occur. Grinding of the K2CO3 by mortar and pestle, followed by drying in an oven

overnight, generally increased yields by 10%. The monomers were isolated and

recrystallized twice from methanol / water to achieve maximum purity and dried over

CaSO4 under vacuum to ensure dryness for the accurate mass measurements necessary

for step growth polymerizations. DBNEt was recovered in lower yield due to larger

amounts of material being lost during the recrystallization steps.




... .. .
..' ."iii N .. .* ,"* "s i.'"'1 *'. r ."" "







Figure 2-8 outlines the preparation of various boronic reagents to be used in

conjunction with DBNEt and DINEt in the Suzuki polymerizations to follow. The

general reaction for all boronic species proceeds via the formation of the di-Grignard

reagent of dibromo-benzene or dibromo-biphenyl,63'6 followed by quenching with

trimethyl borate. The boronate intermediate can be treated with aqueous acid to form the

diboronic acid or with neopentyl glycol in a transesterification manner to produce the

diboronic ester. Drying of the hydroscopic boronic acid is troublesome, and with the

exact mass balance requirements necessary for polymerizations, the easily stored and

purified boronic ester was preferred. The reactions are carried out in one pot with overall

yields ranging from 30-40 %. Isolating the boronic acid, followed by transesterification

using benzene to azeotropically distill off the H20 by-product did not improve yields

substantially (5% gain).



N

OH
-1 /2 N 4 eq. K2CO
+ 2.1 eq.N HCI x
acetone/ reflux
HO 7 3 days
C'


0
X Product %Yield
I DINEt 75
Br DBNEt 38

Figure 2-7. Williamson etherification of DIHQ or DBHQ.









BrBr HO > B B
2.2 eq THF B(OCH3)3 HO 0 0
or + Mg reflux 30% 8
Br -rBreflux


35% 9

Figure 2-8. Synthesis of di-boronic phenylene reagents for use in Suzuki couplings.


Figure 2-9 outlines the preparation of a three ring model compound that was used

as a guide for assigning peaks in the 'H and '3C NMR of subsequent polymers and also as

a standard for luminescence sensing studies conducted with Dr. Kirk Schanze and

Benjamin Harrison at the University of Florida.6 Phenylboronic acid and Pd(OAc)2

were used as purchased from Aldrich Chemical Company. Contamination of the product

with Pd(0) does occur when using Pd(OAc)2, as it lacks solubilizing ligands to keep the

catalyst from precipitating. This will be a more difficult issue to address in the polymer

syntheses to follow, but the contamination could easily be removed from the low

molecular weight compound, 10, by the addition of decolorizing carbon and filtration

through sebaceous earth (Celite). Quaternization of compound 10 was achieved by

stirring in THF and bromoethane at 40 oC for 3 days. During the course of the reaction,

the desired product, 11, precipitated out of solution. NMR peak values for both can be

found in Chapter 5 (Experimental) of the dissertation. As expected, compounds 10 and

11 display extreme solubility differences. Compound 10 is soluble in relatively non-

polar solvents such as halogenated organic (CHCl3 and CH2CI2) and the more polar

THF, while compound 11 is soluble in very polar solvents such as acetonitrile and water.


.. I.







Both 'H NMR integration and elemental analysis (22.54 % Br) indicate a nearly

quantitative level of quaternization.


Neutral Polymer Syntheses

The general Suzuki polymerization is outlined in Figure 2-10. The

dialkoxyamine-dihalogenated benzene monomer, boronic reagent, Pd catalyst of choice,

and mild base such as K2C03, Na2CO3, or NaHCO3 are stirred in a mixed aqueous /

organic (THF, DMF, acetone) solvent system with heating to 70 oC. Special care is taken

to ensure that the reaction vessel and solvents are fully degassed with Ar prior to addition

of the catalyst and the reaction conducted under a blanket of the inert gas. Atmospheric

02 in the reaction may contribute to oxidation of the Pd catalyst and decrease its catalytic

activity and/or increase the rate of homocoupling of the boronic reagents.66





0 o
HO Pd(OAc)2
/ \ + 2.2 eq
SHO DMF/H20
K2CO3 10
DINEt K 000
S70 70






EtBr
10 }
THF : ,
40C







F: .". 2-9- p k. .t .cto;"P P:.: :
..i .. .. .. ."...



t..g ) .: v..
.. 5.







The original polymerizations were conducted by Dr. Peter Balanda and focused

on the synthesis of PPP-NEt2 These initial synthetic investigations used DBNEt,

Pd(OAc)2 as the catalyst, with DMF, THF, and acetone as solvents. Usable polymeric

materials were synthesized, with DMF polymerizations giving the highest molecular

weights by GPC. Several obstacles remained. Using Pd(OAc)2 as catalyst resulted in the

precipitation of black, metallic Pd into solution and contamination of the polymer.

Removal of this impurity often proved difficult, if not impossible, and some loss of the

polymer was inevitable.
i*


N


+ B
or

x C-0^C


K2CO3
H20
organic solvent

N)


[12]


Pd catalyst
70 C


/N

0



q PPPBP-NEta [13]


N


Figure 2-10. Suzuki polymerizations for neutral alkoxy-amine containing PPP's.


Reagent

DINEt
DBNEt


':""`"""" ` ` --- iZL~:-




32


Kowitz and Wegner published results from Suzuki polymerizations using the

more active dichloro[l,l'-bis(diphenylphosphino)ferrocene] palladium (II) [PdCl2(dppf)]

as catalyst in a THF based solution at room temperature with very high molecular

weights and percent conversion to polymer.67 The methodologies presented in reference

30 were applied to the synthesis of the amine substituted PPP-NEt2.

The synthesis of PdCl2(dppf) was first reported in 1984 by Hayashi et al.68 The

PdCI2(dppf) has two advantages over Pd(OAc)2. The dppf [diphenylphosphino ferrocene]

ligand provides solubility to the catalyst as the polymerization proceeds, thus preventing

contamination of the polymer with black Pd(0). With one objective to increase scale of

the reaction, contamination must be avoided to prevent loss of product during "cleaning"

steps. A second advantage is that palladium catalysts with bidentate phosphine ligands

are more efficient catalysts than those with unidentate phosphines. The bidentate

phosphine ligands create a unique geometry of the catalyst, minimizing the angle

between the chlorine ligands and somewhat lengthening the palladium to phosphine bond

distance. The bond lengthening reduces steric crowding between the phosphines and the

palladium center. The Cl-Pd-Cl bond angle for two common palladium catalysts with

bidentate ligands, dichloro[l,2-bis(diphenylphosphino)-ethane] palladium (II)

[PdCl2(dppe)] and dichloro[ 1,3-bis(diphenylphosphino)-propane] palladium (II)

[PdCl2(dppp)]69, along with PdCl2(dppf) are shown in Figure 2-11. PdCl2(dppf) has the

smallest Cl-Pd-Cl bond angle of the three catalysts (87.80). Experiments by Hiyashi and

coworkers revealed a direct relationship between the Cl-Pd-Cl bond angle and catalyst

efficiency.32 The two chlorine ligands occupy the sites where the species to be coupled

will eventually reside before reductive elimination. The reduced angle leads to a rate




... .
---------i---- I ____."- 11:' 1* IIII 111 I -.. .* ^ ^.- u***= '..








increase in the reductive elimination step, which is often the rate limiting step in Suzuki

couplings, thus increasing the overall rate of the reaction.

Polymerizations of bisneopentylglycol-l,4-phenylenediboronate and DBNEt

with PdCl2(dppf) in THF / aq. NaHCO3 at 75 oC for 3 days yielded improvements over

previous work [PPP-NEtz(dppf)[12]]. A polymer with higher molecular weight,

Mn=18,700 g/mol [compared to 15,900 g/mol for Pd(OAc)2 in DMF polymer PPP-

NEtz(Br-72)[14]], and lower polydispersity index of 1.18 was synthesized (see Table 2-

1). Elemental analysis for the polymers and other compounds discussed throughout this

chapter are shown in Table 2-2. Figure 2-12 shows the GPC trace for PPP-

NEt2(dppf)[12]. GPC traces for the other PPP-NEt2 polymers are similar with retention

time and peak width varying for molecular weight and polydispersity, respectively. Scale

up by a factor of 2 to 3 times the original scale was successful using the PdCl2(dppf)

catalyst as well as prevention of Pd(0) contamination. Data shown in Table 2-1 for the

Pd(OAc)2 polymers was taken from the dissertation of Dr. Peter Balanda. Subsequent

polymerizations conducted using Pd(OAc)z reproduced this data within experimental

errors. It should be noted that the low polydispersity found for the PdCl2(dppf) is an

effect of the polymer isolation procedures, which in all likelihood fractionated off some

lower molecular weight species. Suzuki polymerizations should behave like traditional

condensation polymerizations with statistically governed polydispersities of 2.

It was further believed that the use of the more active iodinated species, DINEt,

would lead to an increase in molecular weight. Using identical protocols, reactions were

conducted to couple DINEt or DBNEt with bisneopentylglycol-1,4-phenylene

diboronate (8) via Suzuki protocol. Both reactions were quenched by precipitation into




r'':i' ;: i";t






P Ph C Ph C Ph
Pd)Ph P CI mGs 0 Ph

Ckd P Fe Pd
P\Ph CI p Ph ph

PdCl2(dppe) PdCl2(dppp) PdCl2(dppf)

Cl-Pd-Cl Bond Angle 94.20 90.80 87.80

Figure 2-11. Cl-Pd-CI bond angle for PdCl2(dppe), PdCl2(dppp), and PdCl2(dppf)
catalysts.


MeOH after 3 hours. GPC results in chloroform (vs. PS standards) revealed low

molecular weight oligomers ( M < 1,500 g mol"', multi-modal ) for the reaction

usingthe dibromonated species [PPP-NEt2(Br-3)]. The reaction using the diiodonated

reagent [PPP-NEtz(I-3)[15]] reached a Mn = 10,900 g mol-1 with a continuous

polymeric distribution. Published results22b using DBNEt in the reaction for 72 hours

showed a Mn = 15,900 g moll' for the resulting polymer [PPP-NEt2(Br-72)[14]]. The

elemental analysis and GPC results are summarized in Tables 2-2 and 2-3, respectively.

Longer reaction times (complete polymerization stopped after 24 hours) with DINEt

[PPP-NEt2 (1-24)[16] ; Mn = 15,300 g moll' ] approached the molecular weight values

reported for PPP- NEtz(Br-72)[14]. The use of DINEt leads to the formation of a

polymer with similar molecular weight properties to PPP-NEtz(Br-72)[14] in a shorter

amount of time. Once the polymer has reached a certain molecular weight, it begins to

precipitate out of solution, stopping polymer growth, and negating the advantages of the

more reactive iodine reagent at longer reaction times.






". :.; :; i: i ; ., ..". .







In order to gain insight into a polymer that mimics "true" PPP more accurately,

PPPBP-NEt2[13] was synthesized (see Figure 2-10). The boronic ester of biphenyl was

coupled with DBNEt using the polymerization conditions determined for PPP-

NEt2(dppf)[12] [PdCl2(dppf), DMF/H20, and NaHCO3]. During the course of the

polymerization, it was noted that the polymeric / oligomeric materials being formed were

precipitating out of solution much earlier than for the PPP-NEt2 reactions. Subsequent

workup revealed only low molecular weight components ( M < 5000 g/mol) for the

isolated polymer as determined by GPC versus polystyrene standards and a high level

(1.21%) of bromine endgroups as detected by elemental analysis. Only 51% of a tan

material was recovered indicating that the precipitating oligomers are causing an

imbalance in the functional groups present in solution. Without a proper balance of

reactive endgroups in step growth polymerizations, chains will be prevented from

growing into high molecular weight polymers. Insolubility was quickly reached in the

growing PPPBP-NEt2[13] system, as evidenced by early polymer precipitation from the

reaction media.

The low molecular weight and solubility problems of the PPPBP-NEt2[13]

system naturally led to the swinging of the experimental pendulum back to more

substituted phenylene systems. A polymer with every phenylene ring substituted with an

alkoxy-amine side chain should be more soluble during the polymerization and the

subsequent quatemized polymer more water soluble. Figure 2-13 shows two reagents

that could be used in a Suzuki polymerization to achieve the maximum substituted PPP.

The synthesis of compounds 17 and 18 was attempted by the reaction of DBNEt with

magnesium turnings or n-butyllithium followed by quenching with trimethyl borate











500


400


300

E
200


100


5 10 15 20
Retention Time (min.)


Figure 2-12. Gel permeation chromatogram for PPP-NEt2(dppf)[12].


Table 2-1. Catalyst effect on the molecular weight properties of PPP-NEt2 polymers.
reaction Calibration M. MP M,
catalyst solvent yield method kgmol-1 kgmol-1 kgmol-1 Mw/Mn


Pd(OAc)2 THF 38% PS 5.0 4.8 19.5 3.91
PPPa 3.9 3.8 12.9 3.29
Pd(OAc)2 DMF 76% PS 15.9 24.3 35.0 2.20
PPP 10.8 15.6 21.5 1.99

PdCl2(dppf) THF 95% PS 18.7 19.4 22.1 1.18
PPP 12.4 12.8 14.4 1.16
Pd(OAc)2 acetone 92% PS 12.6 21.4 28.6 2.27
PPP 8.8 14.0 18.0 2.05


GPC results in CHC13 vs. polystyrene standards.
Pd(OAc)2 data taken from Dr. Peter Balanda dissertation U. of Florida.
Universal calibration using values derived for PPP in THF.






.. .. .,
I.


, ;. ,'.


. .


36




37





Table 2-2. Elemental Analysis results for PPP monomers and polymers.
Species %C %H %N %I %Br Anal. Calcd. for

Theo. 38.59 5.40 5.00 45.30 C18H3oN20212
DINEt
Exp. 38.81 5.53 4.90

Theo. 41.00 5.41 6.83 34.33 C14H22N202Br2
DBNEt
Exp. 41.13 5.44 6.71
Theo. 78.21 8.76 6.08 -C3oH4oN202
PPPmodel
(10) Exp. 78.41 9.56 5.90

PPPmodel+ Theo. 60.34 7.45 4.14 23.34 C34H5oN202Br2
(11) Exp. 61.27 7.67 4.29 22.54

Theo 74.95 8.91 7.28 C24H34N202Bro.026
PPP-NEt2
(dppf)[12] Exp. 75.21 8.94 8.01 0.45

PPP-NEt2 Theo. 74.95 8.91 7.28 0.53 C24 H34N202Br0.026
(Br-72)[14]
(Br-72)[14] Exp. 75.09 8.92 8.05 0.54

PPP-NEt2 Theo. 74.28 8.77 7.22 1.47 C24H34N202Io.045
(1-3)[15] Exp. 67.32 8.42 6.01 1.48

PPP-NEt2 Theo. 74.82 8.83 7.27 0.76 C24H34N202 o.023
(I-24)[16] Exp. 71.85 8.45 6.78 0.77

PPPBP- Theo. 78.60 8.30 6.11 C30H38N202
NEt2[13] Exp. 70.25 8.22 5.75 1.21

C24H34N202
PPP-NE3 Theo. 54.02 7.85 4.63 21.48 1.6 C2H5Br
PPP-NEt3+
[19] 2.54 H20
Exp. 52.35 7.61 4.31 21.40

PPPBP- Theo. 60.27 7.09 8.05 23.63 C34H4aN202Br2
NEt3+ [20] Exp. 65.68 8.05 5.35 11.98










Table 2-3. Effect of DBNEt or DINEt on the molecular weight of PPP-NEt2 polymers.
reaction reaction reaction
polymer solvent type time M, MP M M/Mn
(hours)

PPP-NEt2 DMF/H20 Suzuki 72 15.9 24.3 35.0 2.20
(Br-72)[14]
PPP-NEt2 DMF/H20 Suzuki 3 10.9 13.3 16.6 1.52
(I-3)[15]
PPP-NEt2 DMF/H20 Suzuki 24 15.3 19.5 27.5 1.80
(1-24)[16]


Molecular weight values are expressed in
GPC relative to polystyrene standards.


units of kg mol'.


Figure 2-13. Envisioned boronic reagents for a more substituted PPP-NEt2 polymer.


R

Br- -Br
R


+ B' -B
CB Ke0


Figure 2-14. Pd catalyzed coupling to diboronic reagents for Suzuki couplings.




~ ~ : .' : 5 : !. ..::. [! :* : ....:!: l:


Pd(OAc)2
DMF, heat


R
R







and transesterification with neopentyl glycol. Both attempts were unsuccessful, possibly

caused by an interaction of the amine groups to the trimethyl borate hindering formation

of the new phenyl-boron bond. Grignard and lithiation procedures were effective as

evidenced by a substantial amount of dehalogenated material in the crude isolated

material. Future work could explore using a palladium catalyzed reaction between di-

halogenated phenylene's and diboron pinacol ester (see Figure 2-14) that has been shown

to effectively produce boronic reagents for Suzuki reactions70 as an alternative route to

the desired compound 18.


Polymer Quaternization

Quaternization of the amine sites followed preparation of the neutral polymers.

Synthesis of poly[2,5-bis(2- { N,N,N-triethylammonium }-l-oxapropyl)- ,4-phenylene-alt-

1,4-phenylene] dibromide (PPP-NEt3+[19]) is accomplished by heating the neutral

polymer in a DMSO / THF solution with bromoethane for 3 days (Figure 2-15). 'H-

NMR indicates that a high degree of the amine sites are quaternized (-90%). Elemental

analysis for bromine content also reflects 90% quaternization (see Table 2-2). The beauty

of synthesizing the neutral polymer first is in the ease of traditional polymer analyses that

can be performed. Analysis of charged polyelectrolytes can be a rigorous and difficult

undertaking,especially with GPC due to aggregation and charge interaction with the

column material. Assuming the methods used to quaternize the amine sites are gentle

enough not to break bonds along the PPP backbone or cleave side chains, molecular

weight data corresponding to the neutral polymer should be a good reflection of the

molecular weight of the water soluble version. PPP-NEt3+[19] displayed excellent

solubility in both acidic and neutral aqueous media. Solutions were stable over the time
L. *








frame of days with only minimal precipitation of polymer from solution observed on

samples stored over a month.


EtBr
THF/DMSO
heat


PPP-NEt,


Figure 2-15. Quaterization of PPP-NEt2.


PPPBP-NEtz[13] was subjected to the same quaternization conditions as shown

in Figure 2-15. Complete quaternization of this material was not achieved as the neutral

material was difficult to dissolve in the quaternization media. 'H NMR analysis of the

material was unsuccessful due to the poor solubility in common deuterated solvents.

Elemental analysis (Table 2-2) of the oligomers revealed a 11.98 weight percent of

bromine. Of this amount 10.77% of bromine is due to quaternized ammonium sites and

1.21% is inherent from the parent PPPBP-NEt2[13]. Full quaternization of all amine

sites would require 23.63% bromine. Overall, this indicates that approximately half of

the amine sites were quaternized. The resulting quaternized oligomers were no longer

soluble in CHC13 or THF, but had reasonable solubility on the order of 5 x 10-3 M (based

on repeat unit MW) in warm acetonitrile ot DMSO. Cloudy "suspensions" in neutral

H20 were formed in the 10'3 M concentration range. The polymer was soluble in water

only if the pH was lowered to around 2 or 3.



"-E:L.t. .::
... :" J 'i!'., '. ... ;. ,:.'.:' ... ..


PL-. J n

PPP-NEt3+[1 9]










Physical Properties of PPP Type Polymers

For optical display uses, such as organic light emitting devices (OLED's)

envisioned for PPP-NEt3+[19], the two most important physical properties for the

polymer are absorption and emission wavelengths and thermal stability. The absorption

and emission wavelengths will obviously control the color of the display device and

LED's operating under a high bias are limited in lifetime by thermal and electric field

induced degradations. Materials with low barriers to thermal degradation are of limited

use.

The solution absorbance and emission behavior of the newest PPP-NEt2[12] and

PPP-NEt3+[19] samples match the data reported for the initial polymer samples prepared

by Dr. Peter Balanda. Absorption spectra for the neutral polymer in THF (plot c), neutral

polymer in IM HC1 (plot b) and quatemized polymer in H20 (plot a) are represented on

the left half of the graph in Figure 2-16. Interestingly, a significant blue shifting of the

solution absorption maximum occurs from the neutral (4max = 350 nm) to quaternized

(rnax = 330 nm) polymer. In theory, the charges formed on the amine sites along the

backbone of the PPP polymer should repel other polymer chains and each other on the

same chain, stiffening each chain. This new state of the polymer should reduce steric

interactions and red shift the absorbance to lower energy. If this red shifting is occurring,

it is overcomefJy the additional effect of creating very specific point charges in space

along the backbone which in turn have their own effect on the absorption pushing it into

higher energy levels.

The solution emission results are plotted on the right half of Figure 2-16 and are

shown on a log scale to allow a comparison of the large differences in intensity of


I ... ..
,,.. .. ... W,. ,.. .':. :.:.
U 4-i..,. "." i :*B | *i ::







emission between the neutral polymer in THF (plot f), neutral polymer in IM HCI (plot

d), and the quaternized polymer in H20 (plot e). Each polymer has a brilliant blue

solution and thin film luminescence with an emission maximum wavelength of ca. 410

nm in THF and water. Intensity of luminescence increases 4 orders of magnitude in the

quaternized PPP-NEt3+[19] over the neutral polymer. This is attributed to the quenching

of the excited state by the lone pair of electrons on the nitrogen sites in the neutral

polymer. Quaternization prevents this quenching mechanism. Figure 2-17 shows the

emission results on a linear intensity scale normalized to 1 for the neutral polymer, PPP-

NEt2[12]. When the plot is viewed in this scaling, it is easily seen that the line shape of

emission is typical of photoluminescent polymers in solution with a broad peak and small

shoulder that tails into higher wavelength regions.

It was theorized that the PPPBP-NEt2[13] would have fewer side chain to

backbone interactions than PPP-NEt2[12] and thus a ,max at higher wavelength. Optical

absorbance and emission in solution was identical to that of the higher molecular weight

PPP-NEt2[12] samples. UV-Vis absorption measurements were taken on thin films cast

from THF of both PPPBP-NEt2[13] and PPP-NEt2[12]. The plots were nearly identical

with an absorption maximum just above 350 nm. If we assume that the oligomeric

PPPBP-NEt2[13] has reached a degree of polymerization such that its maximum

absorption wavelength has been achieved (typically this would be 12-15 rings or only a

degree of polymerization of 4 in this case), the similarity in thin film absorption data

indicates that the alkoxyamine side groups along the backbone of PPP-NEt2[12] are

disturbing the conjugated backbone planarity very little, allowing the conjugated

backbone to maintain a very rigid conformation.





43




2.5- 1

d
2- -- 0.1

o '
S-
t-4
S 1.5- 0.0




f O





0- 10

200 250 300 350 400 450 500 550 600
Wavelength


Figure 2-16. UV-Vis / Emission behavior of neutral and water soluble PPP-NEt.
a) PPP-NEt2[12] in THF: plots c and f
b) PPP-NEt2[12] in 1 M HCI: plots b and d
c) PPP-NEt3+[19] in HzO: plots a and e
Figure taken from Balanda, P.B.; Ramey, M.B.; Reynolds, J.R.
Macromolecules 1999, 32, 3970.


Thermal analysis by TGA (under nitrogen atmosphere) (Figure 2-18) indicated an

onset for decomposition over 300C for PPP-NEt2[12] and at ca. 230C for PPP-

NEts+[19] (with a small amount of water loss at lower temperatures). From the

perspective of device applications, the most important degradation event is the one which

occurs first. The first degradation event for both polymers was determined to be side

chain cleavage, including the loss of ethyl bromide for the quaternized sample. The


.' .. .. .. .










1.0


0.8





*- 0.4,
0.2-




0.0

350 400 450 500 550 600
Wavelength (nm)


Figure 2-17. Photoluminescent spectrum of PPP-NEt2[12] in THF with normalized and
linear emission scale.



fact that the thermal degradation of these alkoxy substituted PPP's is a relatively clean

process may provide a route to hydroxylated PPP's. Samples of PPP-NEt2[12] heated to

3000 for 10 min were no longer soluble in CHCI3 or THF, but did possess blue

photoluminescence when exposed to ultraviolet light. Treatment of PPP-NEt2[12] with

BBr3 (a reagent known for its ability to cleave aryl ethers) also resulted in a material

insoluble in CHCl3 or THF with the above mentioned emission characteristic.



Conclusions

An interesting water soluble poly(p-phenylene) (PPP-NEt3+[19]) has been

synthesized by a variety of modifications of Suzuki polymerization techniques. The use
..6 *





45

110(
(a)
90

U 70

^ .50-

"z 30

10

-10-*
0 200 400 600 800 100
Temperature


110 (b)
90 -

70


I)



-cO
-10 -.
0 do0 400 660 800 1000
Temperature
Figure 2-18. TGA thermograms for neutral and water soluble PPP-NEt under N2.
a) PPP-NEt2[12]
b) PPP-NEt3+[19]


of PdCl2(dppf) as catalyst has increased synthetic yield to the point whereby a relatively

high molecular weight polymer with low polydispersity can be made without the extra

steps of "cleaning" precipitated Pd out of the polymer. The PdCl2(dppf) catalyst was also

successful in allowing polymerization scale-up to the multi-gram level. For this system,

maximum chain growth is limited by the precipitation of longer polymer chains during

the coarse of the reaction.




i" t





46


Increasing the number of unsubstituted phenyl rings in the polymer backbone,

PPPBP-NEt2[13], lowers molecular weight due to precipitation of the polymer out of the

reaction prior to high conversions. Subsequent optical absorption data on thin film

castings of less substituted samples to the more substituted PPP-NEt2[12] helps support

the theory that the alkoxyamine side groups on PPP-NEt2[12] have minimal interactions

that affect backbone planarity.










































i Aq 'T4












CHAPTER 3
CATIONIC POLY(p-PHENYLENE-co-THIOPHENE)'s


Introduction

While poly(p-phenylene)'s such as PPP-NEt3+[19] are typically strong blue-

emitting polymers, it is desirable to have structurally similar materials with a range of

emission wavelengths. One approach to "tune" the emission wavelength of a polymer is

to chemically change the makeup of the backbone structure. By incorporation of more

electron rich moieties into the repeat unit structure, the highest occupied molecular

orbital (HOMO) to lowest occupied molecular orbital (LUMO) electronic bandgap is

lowered. Specifically, electron rich species raise the HOMO and have little effect on the

LUMO, thereby decreasing bandgap overall. As the bandgap is lowered, the energy

needed to excite an electron into the LUMO is reduced and therefore the energy

(wavelength) of light emitted upon relaxation will be of lower energy. For a more

complete description of light emission consult chapter 1 of this dissertation.

Inclusion of heterocycles, such as thiophene, furan, and pyrrole into co-polymers

with PPP attract much attention due to the substitution possibilities on phenylene rings

and the bandgap reduction due to a greater tendency towards planarity and electron

richness of the heterocycle. The original work to be discussed in this chapter will use the

incorporation of thiophene into the backbone of an alternating substituted phenylene-co-

thiophene polymer to create a water soluble polymer that emits at higher wavelengths

than the "parent" PPP-NEt3+[19] polymer discussed in Chapter 2 of this dissertation.







Early Synthetic Attempts

An early approach to incorporate thiophene units into a poly(p-phenylene-co-

thiophene) backbone was based on a poly(l,4-diketone) prepared by a Stetter reaction

that was treated with Lawesson's reagent to incorporate sulfur into the backbone (Figure

3-la).71 The harsh conditions required was a major flaw in this approach, as crosslinking

was promoted. Czerwinski et al. used a Grignard coupling between p-dibromobenzene

and 2,5-dibromothiophene in various feed ratios to incorporate thiophene and phenylene

units into the backbone (Figure 3-1b).72 Alternating copolymers containing arylene and

bithiophene repeat units have been synthesized via electrochemical polymerization of

1,4-di-2-thienylarylenes (Figure 3-1 ).73 The electrochemical polymerizations form

insoluble films on conductive substrates limiting polymer processing to the initial

deposition. Pelter et al. used zinc chloride to metalate the 5 and 5' positions of 1,4-di-(2-

thienyl)phenylene and reacted the intermediate with 1,4-dibromo-2,5-disubstituted

benzenes via a Grignard coupling (Figure 3-1d).74 The polymers could be doped by

ferric chloride or iodine to conductivities between 10-5 and 10-3 C-' cm .

Dr. Luping Yu and co-workers first report the synthesis of an alternating poly(p-

phenylene-co-thiophene) by a Stille coupling polymerization in 1993.75 The Stille

reaction offers much more flexibility in the selection. of monomers and reaction

conditions than many of the pathways shown in Figure 3-1. Figure 3-2 shows a general

polymerization scheme for a Stille type polymerization. In this case, 1,4-diiodo-2,5-

dialkoxybenzenes were reacted with 2,5-bis(tributylstannyl)thiophene. The polymers

were analyzed via gel permeation chromatography revealing a number average molecular






'. ; ... : .'.. ... :.....









+-00 -N N NaCN -


0' n


Lawesson's
Reagent


SBr Br + y Br Ni aca. SMg
2. Ni(acac)2


R

2 eq. M + Br Br Pd(0)

R


M = ZnCI, SnMe3

R = H, alkyl, alkoxy, functional group


Electrochemical
Oxidative Polymerization


R

S \

R


+ Br Pd(O) S


ZnCI R R

R = alkyl, alkoxy, nitro

Figure 3-1. Literature examples of phenylene-co-thiophene type polymers.



X-R-X + BusSn-R'-SnBus3 d(o) R-R'
n
X = I, Br, OS02CF3, COCI

R, R' = aromatic, vinyl, heterocyclic, etc.

Figure 3-2. General scheme for the'Stille polymerization.


(c)







n (d)


H 0


S .




50


weight of ca. 14,000 g mol-' versus polystyrene standards. This class of polymer

possesses a bandgap of ca. 2.4 eV (520 nm), falling between that of poly(p-phenylene),

3.0 eV (413 nm), and polythiophene, 2.1 eV (590 nm).76 An emission at 525 nm was

present in photoluminescence studies conducted in THF when the polymer solution was

excited with a wavelength of light corresponding to its absorption maximum when the

polymer solution was excited with a wavelength of light corresponding to its absorption

maximum.


Optimization of the Stille Coupling Polymerization

In order to maximize the efficacy of the Stille reaction for polymerizations, a

more detailed study by Yu et al. was conducted in 1995 to examine monomer, catalyst,

and solvent effects on the molecular weight of a variety of conjugated polymers.77 The

organohalides and triflates shown in Table 3-1 and the organotin monomers shown in

Table 3-2 were combined under a variety of conditions in the presence of palladium

catalysts. Polymer repeat unit structures are given in Figure 3-3.

Several general conclusions could be made from the polymers synthesized from

the different combinations of monomers and reaction conditions. Diiodo-substituted

monomers are more reactive than dibromo-substituted monomers. Dialkyl-substituted

phenylene monomers gave higher molecular weights than the more electron rich

dialkoxy-substituted phenylene monomers. The oxidative addition step in a palladium

catalyzed reaction is usually facilitated by electron withdrawing or less electron donating

groups. PPP type polymerizations were found to be poor in all cases with dimeric or

trimeric species formed. In general, the organotin monomer prefers to be electron rich

and the organohalide (or triflate) to be electron deficient.




: ".: .i ",:' t ""t, : .. .. '




51




Table 3-1. Structures of the organohalides and triflates for the Stille reactions.



R
1-8

Compound R X

1 OC8HI7
2 OC8HI7 Br
3 OC8HI7 OTf
i~ ~ ~ II. @ *ii iii i;liii








ii4 C CI








5 C81117 Br
-6 C8H17 OTf
7 OCIIH23 OTf
==== ; rl!i .. ..iiE I i ii



















S8 no substitution OTf
*Taken from Bao, Z.; akin, C.; Yu, J. Am. Chem. Soc. 1995, 117, 12426.


Table, 3-2. Structures of the organotin monomers for the Stille reactions.
upon! aIr susan suu















R
Me38n SnMe 3 R'3Sn -O SnR' 3

12 -14

Compound. R R$
12 OCH3 CH3
13 OCH3 n-49
14 OC7HI5 CH3
*Taken from Bao, Z.; Waikin, C.; Yu, L. J. Am. Chem. Soc. "1995, 117, 12426.



Typically 2 mol% catalyst was used as higher loadings of catalyst lowered

molecular weight. Adjustment of the 1: 1 organchalide to organtifioreactanit equivalent
i iir i i = ; i iiiiiillllii'liiiiiiiii i i iiiiiii == =
ii iiiliiiiiiiiiiii iii il~~ii iii =li i i iiii iiii~ i~ @ i ii 1= iiiiii ii~iiiiiiiii iii= = i i~ iiiiii! iii i ii ii iiii~iiiiii!iiiiiiiii ,= iiiiii i ~ iiiiiii iiiii ii iiii ili i iii iiiiiiiii! liiiili iiliiii i iili iiiiiiii
Ii .= iii~iiiiiiiiiii~ i~i ii =========" = ; iiiiiiiii i" liiiliiiiili~ i ii1 ii i 'iiiii iii iiiiiii iii
iiiiiiiliili iiii iiiii 1 1 iiliiliiiiii i "I i i i i ii iii ii i i i i L iiiii ii i ii iiii i i i i i i i= i i ii i ii iii~ iii iii = i = = iiiii iii iii iiiii iiiii ii= i iii i i i i i i i iii 1 ==i i i i i = = iiiii = iiii iii ii iii
.. i ii(1~ ii iiiiii
i i ililii ii iiii i ....................................... ... ;i;~ i iiiiii ii ii i i ii i i il i i i i i i i i i i l ii i i i i i i i i ii i i i i i i i i
= =;i iii iiiiiiii iiii iii iiiii i. c ;ii il ~ i lliiiiiiiiiiiiiiiiiiiiiii i i i ii.............== iiiiiiiiiiii i i ii~iiiiiiiii ii, ii i iliiiiiiiiiiiiiiii~l iii
iii iil i ii i i iii i ii iiiiiiiiiili~ i =il ii! iii il~ i ;iii i iiiii !i i iii ilii i ii i i iii iiii i ii il i iiii ii i iiili il lini i i i i i i ~ iiiiii i ii i iiiiii i iiii iiiiil i il i iiii i in
xiiiii riii i i a i =Ei~i i iiiiii= = i iii i = =iii iii i!= iiiiii iii l i i = !li i = == .. iii= = = .. iiiiiiiiiiiiii i ii ii~~lIi=iiili iil~ii~ iii~lii iiiiiiiiiiiiliiilii i ilIiIiillllii iii iiiiii iii ii i i iii i iiii iii iii~iilliiiiiiiiiiiiiiiii iii liiiiiii~ ~~iiiliiiiii= ii~
;;1;RJiiiiiii iiiiiiiiiiiiiii f l iiiiiiiiliiiiiiiii iiiiiliiiiiiiiii iiii iiiiiiiiiiiii iiiii iiiiliiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii iii.iiiili~i
'""; ; i iiiiiiiiiiiiiiiiii iiii ii iiiili iiiiii a iliiiiiiii i i ,i iiiiii!iiiiiiiiiii iiiiiiiii
i i iiiiiiiiiiiiiiiiii i i = = =i iiiiciiii =iiii ``
... .. .. ...... ... ................ ..... .. .= .... ... .. .... .... .. .. .... ........ .... =.... ............. ............... r = ........ ...... .......... .. .. .. .. .... ........ ;- I .. .. ........ ......... ........................ .....
:::::::::::::::::::::::: : :::::::::::::::::: :::: ::: : :::: : : : : :::::::::::::::::::::: : : : ::::::: :::::::::: :: : : : : ::::::: :::: :::::::: ::::::: ::::::::::: :::: ::::: : :::: ::: :: ::: : ::: ::::::::: :: : :: :::::::::: :::: ::::::::::::::::::: : ::::::: : ::::: .. ........ .:::::::: :::
!iiiiiii ii iii iii iii iliii~ iii = i i i~ iiii i ili i~ ii!! iiiii = i ii iii~ iii i~ ilii = =ii =i ii ili= ii i li !i i iiiiiiii ii ii i i !!!i = ii iiii !!iliii iii!!!!!!ii!iiiiii !!i ii
;;N-.~i. n ~;~ ;;;I ~


;s;;;'': o,


ii
I !
i~~li~i ....; ;Ea:;il

':':i;i;
;;; ,,,;0@,i..Pr !Ii;








rl,,, o ii iii s
ii il ..p "8I,
@: 8~












sa"NiOi.,r,,







ratio to account for reduction of the palladium (II) catalyst to the active palladium (0)

species by the organotin species increased molecular weights. The organotin compound

would be used in a 1.02 equivalent amount compared to 1.00 equivalent of organohalide.



R R


RR n
PPT PPV






Rn R R
n
PPP PPV

Figure 3-3. Representative polymer repeat units of Stille polymerizations.
R = CnH2zn+ or OCnH2n+i
Taken from Bao, Z.; Waikin, C.; Yu, L. J. Am. Chem. Soc. 1995, 117, 12426.



Ability of the solvent to both solubilize the coupled species and stabilize the

catalyst is of utmost importance in the Stille reaction and becomes an even more

important issue when addressing conjugated polymers and their inherent solubility

problems." Common solvents for the Stille reaction include THF, toluene, and DMF.

DMF is known to accelerate the palladium-catalyzed reactions by acting as a ligand to the

palladium center.7 In the case of the PPT polymers synthesized above, it was found that

DMF did accelerate the polymerization, however, the growing polymers were not

sufficiently soluble to remain in solution sufficiently long enough to achieve higher

molecular weights. THF was able to solubilize the polymer and stabilize the catalyst for

reaction times up to 7 days.



S ...: !
: '. :T ,.:.,,,r;.;,,,.:.".







Number average molecular weights of up to 22,000 g mol' were achieved for the

PPT polymers using 1.00 equivalent of 1,4-diiodo-2,5-dioctylbenzene, 1.02 equivalents

of 2,5-bis(tributylstannyl)thiophene, and 2 mol% PdC12(PPh3)2 catalyst in 80 oC DMF for

one week. Of all organotin monomers studied, the 2,5-bis(tributylstannyl)thiophene (see

Table 3-2) was the most reactive, as the electron donating property of the sulfur atom

may accelerate the transmetalation step which has been proposed to be the rate

determining step for palladium-catalyzed cross coupling reactions.80 An alternate tin

reagent not used in Yu's study is 2,5-bis(trimethylstannyl)thiophene, which is a more

"friendly" reagent in that it is a solid at room temperature allowing for purification by

recrystallization. The distillation techniques needed to purify 2,5-

bis(tributylstannyl)thiophene are difficult and workers may be exposed to alkyltin vapors,

which are very toxic. One drawback to the use of 2,5-bis(trimethylstannyl)thiophene is

the transfer of a methyl group during coupling instead of the desired thiophene group and

limiting chain lengths in polymerizations.

This methodology appeared attractive for coupling distannylated thiophene with

2,5-bis(3-[N,N-diethylamino]-l-oxapropyl)-1,4-diiodobenzene (DINEt)[see Chapter 2].

If successful, a neutral poly(p-phenylene-co-thiophene) with alkoxyamine side chains on

the phenylene units would be created that should become water soluble upon treatment

with ethyl bromide. The solution emission of this polymer should fall in the green to

yellow visible wavelength range by comparison to literature values. Using the insights

into the Stille coupling found by Yu and coworkers, and taking into account the specific

differences between the proposed system and the literature examples, a systematic

approach was designed to maximize the molecular weight of poly({2,5-bis[2-(N,N-




....... '. .... ." : h







diethylamino)- -oxapropyl]-1,4-phenylene }-alt-2,5-thienylene) (PPT-NEt2) which is

easily converted to the quaternary ammonium salt, poly(2,5-bis[2-(N,N,N-

triethylammonium)- -oxapropyl]- 1,4-phenylene-alt-2,5-thienylene I dibromide (PPT-

NEt3+). It was also desired to determine if a Suzuki type polymerization would work

using thiophene diboronic reagents. A Suzuki approach would allow for the use of much

less toxic boronic reagents than the tin reagents used in Stille couplings. The results and

discussion following will address these aspects in greater detail.


Results and Discussion


Monomer Syntheses and Suzuki Coupling Test Reactions

In order to effectively synthesize PPT-NEt3+ of high molecular weight via a Stille

or Suzuki polymerization, 2,5-bis(3-[N,N-diethylamino]- 1-oxapropyl)- 1,4-diiodobenzene

(DINEt) (Figure 2-7) and 2,5-bis(trimethylstannyl)thiophene (21) or 2,5-thiophene

diboronic acid (22) as co-monomers were selected. Literature precedent for Suzuki

polymerizations involving thiophene were not present, but success of this type of

polymerization was desired because the aryl tin reagents used in the Stille reaction are

somewhat less reactive as compared to the aryl boronic acids used in the Suzuki

coupling. A diiodobenzene monomer was chosen over a dibromo reagent due to its

higher reactivity in Pd coupling reactions. Particular attention was paid to the stringent

monomer purification requirements needed for complete conversion of functional groups.

The synthesis of 2,5-bis(trimethylstannyl)thiophene (21) by literature

methodology is outlined in Figure 3-4.81 Thiophene was treated with 2.05 equivalents of

n-butyllithium and refluxed in a hexane / TMEDA solution for 30 minutes, cooled to 0 C

in an ice bath, and quenched with 2.05 equivalents of trimethylstannyl chloride. After








stirring overnight, aqueous extraction, followed by removal of hexane under reduced

pressure revealed a slightly brown solid. The stannylated compound was distilled under

vacuum, and recrystallized twice from pentane to yield white crystals in 69% yield.

The corresponding Suzuki reagent, 2,5-thiophene diboronic acid (22) was

prepared by treating 2,5-dibromothiophene with 2.2 equivalents of Mg, followed by

quenching with an excess of dry trimethylborate. The reaction was stirred overnight and

IM HCI was added to protonate the di-acid and dissolve all magnesium salts. After an

aqueous/ Et20 extraction, the crude product was precipitated into IM HCI, collected, and

recrystallized from hot H20. A 40% yield of white crystals was recovered and dried in

vacuo at 100 oC for 3 hours. As is the case with boronic acids, purification and drying

were simplified by reacting the di-acid with neopentyl glycol in refluxing benzene in a

transesterification manner to produce the 2,5-thiophene diboronate ester (23) as white

crystals in 70% yield as outlined in Figure 3-5.



2.05 eq. n-BuLi
TMEDA LiLi 2.05 eq. Me3SnCI -oSn Sn,
0hexane RT, overnight
ref lux, 3 h. 2

69%

Figure 3-4. Synthesis of 2,5-bis(trimethylstannyl)thiophene.



Due to the lack of literature attempts at polymerizing a thiophene di-boronic acid

or ester, a test coupling procedure was carried out to determine if the reagent would

couple before degradation. A simple three component ring system was chosen instead of

test polymerizations for the study, since too many factors are present in polymerizations

: : ..
.,. .*.. ,

I: .: .. r., "::' ". "ijlj"." !i *!. ..
W
0 J i




56


that may deter the reaction. Figure 3-6 shows the general Suzuki coupling reaction

employed to check reaction parameters. A variety of conditions were used to couple

compound 23 and 4-bromotoluene. 4-bromotoluene was chosen over 4-iodotoluene

because of its lowered reactivity. In essence, if conditions are found to allow the

coupling to occur with bromo-reagents, iodo-reagents should perform better under the

same conditions and oftentimes the reactivity of bromo-compounds is sufficient for use in

Suzuki couplings eliminating the need for the more expensive iodine compounds.

Sodium bicarbonate (NaHCO3) and PdCl2(dppf) (1 mol%) were used as base and

catalyst, respectively, in all test reactions. Solvents were varied between THF, DMF, and

toluene, along with adjusting the reaction temperature from reflux to room temperature.

In all cases a mixed 5:1 ratio of the organic solvent to water was used to promote

formation of the more reactive boronate anion. To account for the high reactivity the

thiophene diboronate ester, reactions were conducted via one pot or the diboronate ester

was added slowly in a solution of solvent from an addition funnel.




Br Br 2.2 eq. Mg BrMg MgBr B(OCH3)3 H ~
t1 THF C r H* HO'- S OH
22
40%


HO OH
HO'B B 'OH
/S- Or-


+ excess K
A(


Figure 3-5. Synthesis of 2,5-thiophene dib


OH benzene 0 0
OH H20 BY
OH 'H\OY
23
70%

oronate ester. ''


.. ," .
.. **:;' .. -'. .


ii "." ,* .'" .':"'.:i. : .. i*,' .
i* ~" .; .....:. .i .
.p ,' ..i








B0B + -BrH3 PdCl2dppf H3C CH
or 10- NaHCO3
solvent, temp.,
23 method
24

Figure 3-6. Test coupling reaction of 2,5-thiophene diboronate ester and 4-bromotoluene.



Test reactions were monitored by thin layer chromatography to check for the

consumption of molecule 23 and 4-bromotoluene along with the appearance of new

compound spots. Reactions which displayed positive TLC results were worked up

isolating the organic products. Subsequently, the material was analyzed by gas

chromatography/mass spectrometry (GC/MS) to determine composition. Table 3-3 lists

the various reaction combinations and results.

Surprisingly, coupling to any significant level does not occur in THF. Typically,

even if THF proves to be a poor solvent for a Suzuki coupling, the reaction will proceed

to the 20-30% range. GC/MS peaks were assignable only to the starting materials with a

very small percent (<2%) of mono-substituted thiophene present upon slow addition of

the boronate ester. It was hypothesized that THF may be degrading the reactive thiophene

boronate ester. To help determine if this was the case, a small sample of compound 23

was placed in THF with catalyst and THF or DMF (and the correct ratio of water)

without 4-bromotoluene and heated to 50 oC for 3 hours. The "reactions" were stopped

and analyzed by GC/MS. Both revealed the peak assignable to compound 23 with no

degradation products evident. From these results, it is evident that THF and DMF do not

have any degradation effects on the boronate ester. The use of toluene as a higher boiling

solvent did not promote the reaction and GC/MS revealed higher levels of degradation





:,:"! .








products such as thiophene and its mono-boronate ester as a result of the higher

temperatures.



Table 3-3. GC/MS results of Suzuki coupling of 2,5-thiophene diboronate ester and 4-
bromotoluene.
Solvent Temp Catalyst Method Result

THF RT PdCl2dppf 1 pot SM


THF reflux PdClzdppf 1 pot SM + degradation


Toluene reflux PdCl2dppf 1 pot SM + large degradation


THF reflux PdCl2dppf dropwise low % mono


DMF RT PdCl2dppf 1 pot low % mono, SM


DMF 72 oC PdCl2dppf 1 pot Product 24,75%


DMF 72 oC PdCl2dppf dropwise Product 24 ,90%


DMF 72 oC Pd(OAc)2 dropwise Product 24,90%

RT = Room Temperature (-22 1C)
SM = Starting Materials (Compound 23 and 4-bromotoluene)
Mono = One tolyl unit coupled to thiophene


The coupling was successful in all cases in which DMF at elevated temperatures

was used. The ability of DMF to coordinate to the catalyst and increase catalytic activity

is believed to account for the success of using this solvent in the reaction. GC/MS

revealed good yields of product 24 for all DMF reactions at 72 OC, with excellent yields



..i"... ..








for cases in which the boronate ester was added dropwise. As an additional test, the less

reactive Pd(OAc)2 catalyst was used and yields for the reaction were as high as those for

the PdCl2(dppf). The ability of Pd(OAc)2 to be used in the coupling is important because

the catalyst is one of the least expensive Pd catalysts. For polymerizations using

compound 23, the test reactions indicate that using DMF at 72 oC with one pot or

dropwise addition of the boronate ester will be the best choice. These conditions will

provide the best possibility for polymerization success with only inherent polymerization

difficulties left to deter the reaction, such as solubility and complete conversion of

functional groups before hydrolysis of the boronate ester from the thiophene.


Neutral Polymer Syntheses

Initial polymerizations were conducted using the Stille coupling route because of

available guidelines in the literature for Stille couplings when used in thiophene

polymerizations. The Stille coupling polymerization used in the synthesis of poly({2,5-

bis[2-(N,N-diethylamino)-1 -oxapropyl]- 1,4-phenylene )-alt-2,5-thienylene) (PPT-NEt2)

is depicted in Figure 3-7. Gel permeation chromatography (GPC) and elemental analyses

for the subsequent experiments described below are presented in Tables 3-4 and 3-5,

respectively. It should be noted that the carbon analyses are significantly lower than what

is expected which may be due to the fact that these highly aromatic polymers are difficult

to combust and some carbonization may have occurred during the measurements. The

iodine elemental analyses provide a rough method for approximating the degrees of

polymerization. Not unexpectedly, attempts to synthesize PPT-NEt2 using the

dibromobenzene derivative, DBNEt, were unsuccessful with only low molecular weight

coupling products observed (results not shown).





60




N N


S S + I
DMF, 700C
21




DINEt
DINE Polymer Time Method
PPT-NEt2(48) [25] 48 h 1 pot
PPT-NEt2(96) [26] 96 h 1 pot
PPT-NEt2(240) [27] 240 h 1 pot
PPT-NEt2(96-drop) [28] 96 h dropwise

Figure 3-7. Stille coupling polymerization scheme for PPT-NEt2.



Reactions were conducted under varied conditions to determine the optimal

needed to achieve the highest molecular weight possible for PPT-NEt2. The first set of

reactions were carried out as one pot syntheses, where the stannylated compound,

DINEt, and DMF were mixed together and heated to 70 oC. PdCI2(PPh3)2 was then

added in one portion in a catalytic amount to the reaction flask. PPT-NEt2(48)[25] and

PPT-NEtz(96)[26] were synthesized with 48 and 96 hour reaction times followed by

precipitation into MeOH. During the polymerization, polymer was seen to precipitate out

and coat the reactor. PPT-NEt2(48)[25] was collected in75% yield with a Mn of 3,200

g mol',while PPT-NEtz(96)[26] was collected in 80% yield with a Mn of 4,100 g mol-

(GPC versus PS standards)(see Table 3-2). Polydispersities of 1.7 were found for both,

but with the extensive fractionation during purification this value is not indicative of the

initial polymerization. Doubling the reaction time lead to a modest improvements in both

a .... ....

S .. ." .. ... ..
.. .. ." .... ': ." .. .... .
.:i: ": ... ; :i







yield and M,, while increasing the reaction time to 10 days in PPT-NEtz(240)[27] led to

no appreciable molecular weight enhancement ( Mn of 4,200 g mol-').

Later experiments were fine tuned to account for the possible degradation of the

2,5-bis(trimethylstannyl)thiophene when exposed to elevated temperatures in the reaction

medium. PPT-NEt2(96-drop)[28] was synthesized by slow dropwise addition of 21 to a

solution of catalyst, DINEt, and DMF via an addition funnel over the course of 4 hours

and allowed to run for 96 hours. The reaction was precipitated into MeOH, the crude

polymer recovered by filtration, followed by extraction with MeOH and acetone for 24

hours each, and finally collected by extraction with chloroform (via Soxhlet extractor).




Table 3-4. Gel permeation chromatography results for Stille coupling of PPT-NEt2.
reaction reaction reaction M, MP M,,
polymer solvent type time kg mol-1 kg mol-1 kg mol-1 Mw/Mn
(hours)
PPT-NEt2 DMF Stille 48 3.2 4.3 5.2 1.70
(48)[25]
PPT-NEt2 DMF Stille 96 4.1 5.8 6.9 1.68
(96)[26]
PPT-NEt2 DMF Stille 240 4.2 5.4 7.2 1.71
(240)[27]
PPT-NEt2 DMF Stille 96 5.3 6.9 9.0 1.70
(96-drop)[28] (dropwise)
GPC results in THF vs. polystyrene standards.


The chloroform soluble fraction constituted an 84% yield and 'H and 3C NMR

analysis gave expected shift values with the proton peaks appearing as broad multiplets

without defined splitting for all polymeric materials recovered (see Figure 3-8). This

polymer exhibits a 'In, of 5,300 g mol' (GPC versus PS standards). This methodology




,..lI a. ,,,T' *: *SS..J ******...;., ;.. : .: i- .. ,





62




Table 3-5. Elemental Analysis results for PPT monomers and polymers.
Species %C %H %N %I %Br Anal. Calcd. for

Compound Theo. 29.32 4.92 CH2SSn2
21 Exp. 29.65 4.60

Compound Theo. 54.52 7.20 C4H22042S
23 Exp. 54.70 7.14 -

PPT-NEt2 Theo. 66.33 8.04 7.03 2.52 C22H32N202SIo.079
(48)[25] Exp. 65.23 7.84 6.65 2.50


PPT-NEtz Theo. 67.13 8.14 7.12 1.19 C22H32N202SIo.037
(96)[26]
(96)[26] Exp. 63.64 8.03 6.46 1.20 -


PPT-NEt2 Theo. 67.13 8.14 7.12 1.19 C22H32N202SIo.037
(240)[27] Exp. 63.99 7.99 6.60 1.22 -

PPT-NEt2 Theo. 67.38 8.17 7.15 0.97 C22H32N202SIo.030
(96-drop)[28] Exp. 63.99 8.02 6.51 0.98 -

C22H32N202SIo.037
PPT-NEt3+ Theo. 51.09 6.88 4.58 0.77 26.20 2 C2 037
P-NEt3 *2.0 C2H5Br
(96)[30]
Exp. 49.87 6.48 3.18 24.18

C22H32N202SI0.o030
Theo. 51.16 6.89 4.59 0.62 26.24 2 2HSB
PPT-NEt3+ "2.0 C2HsBr
(96-drop)[31]
] Exp. 49.73 6.52 3.29 23.62


PPT-NE2 Theo. 68.04 8.23 7.23 C22H32N202S
(Suz)[29]
Exp. 64.70 7.98 6.60 0.09



produced a polymer with the lowest percent of halogentated endgroups(weight %I = 0.98;

approximates a degree of polymerization = 27, corresponding to 54 rings) and highest



::: ... *** ::. .. .: .. : ::; ,, ,. : l l




63



molecular weight by GPC of all trials. Slow addition of the more reactive tin compound


allows for immediate coupling of the thiophene to DINEt. This limits the exposure of the


stannylated compound to the elevated temperatures and lowers the chance of destroying


the mass balance leading to end-capping of the polymer chains with thiophene or


completely de-stannylating the thiophene. This method led to the highest degree of


polymerization for the PPT's prepared.


rr
d
C
b
a


|^ LI


LII &LW 131 l III ID I I NI II II II 41 II I II pp


Figure 3-8. 'H and '3C NMR spectra of PPT-NEt2[28].


'. .:* ** **


"


* *







The next synthetic progression after confirmation that a base set of PPT-NEt2

polymers had been created was to try the Suzuki coupling methodology as outlined in

Figure 3-9. Compound 23 was added dropwise to a stirred solution of DINEt,

PdCl2(dpp), and NaHCO3 in a DMF / H20 solvent solution at 70 oC. After 3 days, the

reaction was precipitated into MeOH, the crude polymer recovered by filtration, followed

by extraction with MeOH and acetone for 24 hours each, and finally collected by

extraction with chloroform (via Soxhlet extractor). The material, PPT-NEt2(Suz)[29],

was recovered in 50% yield.





0
0 O / PdCl2dppf
0 DMF, 700C "N



H S H
HO / n=2-4
DINEt Hydrolysis of Boronate Groups PPT-NEt2(Suz)[29]
Low Molecular Weights Nc-




Figure 3-9. Synthesis of PPT-NEt2[29] via Suzuki coupling polymerization.



Elemental analysis of the polymer (see Table 3-5) initially indicated a possible

high molecular weight polymer with only 0.09 % by weight iodine found in the sample,

however, GPC trials indicated very low molecular weight oligomeric species. UV-Vis

absorption data showed a peak X. = 452 nm, some ten nanometers higher in wavelength

than the well analyzed materials from the Stille polymerization (see Physical Properties

4 1 0. ,







section). This evidence supports the conclusion that the growing chains in the Suzuki

reaction are being terminated by hydrolysis of the terminal boronate functionalities.

Lower molecular weight species and starting materials with iodine groups present were

removed by the extensive extractions performed, thus accounting for the low %I.

Unfortunately, the Suzuki reaction for thiophene boronates is not applicable to

polymerizations due to the ease of hydrolysis of the boronate. The Suzuki type reagents

and techniques would be good candidates for synthesis of smaller 3 to 4 ring compounds

as evidenced by the successful test reactions.


Polymer Ouaternization

Cationic, water soluble polymers are easily formed from the neutral PPT-NEt2 by

quaterization with bromoethane in THF as shown in Figure 3-10. The quaternized

polymer, PPT-NEt3+, was precipitated into acetone, collected, and dried at 50 oC under

vacuum. Table 3-5 shows the elemental analysis results for PPT-NEt3+(96)[30] and

PPT-NEt3+(96-drop)[31], which are the quaternized forms of PPT-NEt2(96)[26] and

PPT-NEt2(96-drop)[28], respectively. The resulting polymers are soluble in acidic

solution and pH = 7 water. The quaternization efficiency, as determined by 'H NMR

integration (comparison of the integral value of the terminal side chain protons,N-

CH2CH3*, to that of the O-CH2* protons [18:4 for 100% quaternization]), was on the

order of 80-90% per sample. This is also reflected in the elemental analysis [26.24% Br

by weight for complete alkylation compared to the 23.62% Br found for PPT-NEt3+(96-

drop)[31].







: "










N B0

Bromoethane
*07n "THF n
SRT/ 5d

N-\

PPT-NEt2 PPT-NEt3+

Figure 3-10. Quaternization of PPT-NEt2 to form PPT-NEt3+.



Physical Properties of PPT Type Polymers

Figure 3-11 shows the UV-Vis absorbance and photoluminescence spectra for

PPT-NEt2[28] in THF and PPT-NEt3+[31] in H20 (normalized for convenience). It is

interesting to note the dramatic shift in absorbance maximum between the neutral and

charged polymers. PPT-NEt2[28] exhibits a ,max at 460 nm with a corresponding molar

absorptivity of about 18,000 L mol'cm-', while PPT-NEt3+[31]'s Xa, is blue shifted 49

nm to 411 nm with a corresponding molar absorptivity of about 16,000 L mol'cm-'. The

blue shift of the 7t to I* transition for these polymers may be due to a solvatochromic

effect. Fine tuning of the Amax could be achieved by controlling the extent of

quaternization, as incomplete quatemization leads to a lower extent of hypsochromic

shift.

Solution photoluminescence experiments revealed peak emission wavelengths of

519 nm and 494 nm for PPT-NEt2[28] in THF and PPT-NEt3+[31] in H20, respectively.




67


The excitation wavelength corresponded to the ,max of each polymer's absorbance. The

spectra display the typical characteristics of conjugated polymers in solution with a

Stoke's shifted emission maximum and tailing broadly to higher wavelengths. Table 3-6

summarizes the optical properties for both the neutral and water soluble PPT polymers.


a bc d


/







vi


300 400 500 600 700
Wavelength (nm)


Figure 3-11. Normalized UV-Vis absorption and solution photoluminescence for PPT-
NEt type polymers.
a) PPT-NEt3+[31] UV-Vis absorption in H20.
b) PPT-NEt2[28] UV-Vis absorption in THF.
c) PPT-NEt3+[31] emission in H20.
d) PPT-NEt2[28] emission in THF.


It is interesting to note that the trend of increased absorbance and emission

intensity when moving from neutral to quaternized species for the PPP-NEt2 system was




..,E si'.E ., .


ai

E 0.8


0.6



E
.! 0.4

8

S0.2
0


0.0




S68


not observed for the PPT-NEtz polymers. In this case, quaternization led to a decrease in

the emission output of the polymer in solution. Initial data from collaboration work have

indicated that very little light is emitted from devices made from electrostatically

deposited thin solid films of PPT-NEt3+[31] excited by voltage application. Further

work is needed to explain this occurrence, since NMR did not reveal any unexpected

peaks for the polymer post quaternization.




Table 3-6. Summary of optical data for PPT-NEt type polymers.
Polymer Absorbance Film Emission Emission Color
max (nm) Color ,max (nm) (Solution)

PPT- 460 Red 519 Green
NEt2[28]


PPT- 411 Red 494 Green
NEt3+[31]



Thermal de-alkylation of the amine sites can occur if the polymer is exposed to

elevated temperatures, thus indicating a dynamic equilibrium at the amine sites. This de-

alkylation is evidenced in the TGA for PPT-NEt3+[31] shown in Figure 3-12 where an

initial degradation event starting at 200 oC is observed, followed closely by loss of the

triethylamine fragment. The initial weight loss event in the degradation of PPT-NEt2[28]

occurs at 250 C corresponding to the loss of this same triethylamine type fragment. Both

polymers have a final degradation occurring over 400 C, attributed to the breakdown of

the conjugated backbone and little residual mass remains.



.. .. : ..:











100-


80


60-
0-0
2 .........
S 40. -


20-


0

0 100 200 300 400 500 600 700 800
Temperature (oC)



Figure 3-12. TGA thermograms for neutral and water soluble PPT-NEt under N2.
a) -- PPT-NEt2[28]
b) ----- PPT-NEt3+[31]



Conclusions


A water soluble poly(p-phenylene-co-thiophene) (PPT-NEt3+[31]) has been

synthesized by a variety of modifications of Stille polymerization techniques. Maximum

molecular weight was achieved in DMF using PdCl2(PPh3)2 catalyst with slow addition

of the 2,5-bis(trimethylstannyl)thiophene reagent. Again, as in the case of the PPP-NEt2

system, the polymer does begin to precipitate from the reaction over the coarse of the

reaction, limiting the molecular weights. Polymerizations attempted in THF as solvent

did not produce polymeric materials. Unfortunately, the use of PPT-NEt3+[31] in

electroluminescent devices appears unlikely due to low light emission in such devices.

However, preliminary work has shown that the material may hold promise in

electrosttically deposited thin layer systems for control over refractive index properties.

... : .. .." : :

..





70



Adjustment of the number of PPT-NEt3+[31] layers deposited and thickness of the


layers dramatically changes and allows fine tuning of the refractive index of the


transparent "window" created by the device.


Investigations of test reactions using Suzuki coupling techniques were successful


using a 2,5 thiophene diboronate ester, however, the reagent was too susceptible to


hydrolysis to allow the synthesis of high molecular weight polymers as evidenced by a


lower absorption wavelength maximum than the Stille polymers. The 2,5 thiophene


diboronate ester is a viable alternative to more hazardous and toxic 2,5-


bis(trialkylstannyl)thiophene reagents for Pd coupling reactions to di-substitute thiophene


in the 2,5 positions.


4 .2


,;i
........
,
;;;


.:.... ..


.1*












CHAPTER 4
CATIONIC POLY(p-PHENYLENE-ETHYNYLENE)'s


Introduction


Early Synthetic Attempts

Poly(p-phenyleneethynylene)'s [PPE's] are a class of polymers that are composed

of alternating phenyl rings and triple bonds. They are structurally very similar to the

much studied polymer, poly(p-phenylenevinylene) [PPV], in which electroluminescense

from a conjugated polymer was first observed. PPE's did not receive the early attention

of PPV, but research efforts have increased as the luminescent and conducting properties

of PPE have been shown to be useful for explosive detection," molecular wires that

bridge nanogaps,83 and polarizers for liquid crystalline displays.

The first synthesis of PPE oligomers was reported in 1983 and consisted of

heating cuprous acetylide with diiodobenzene to a degree of polymerization of 10-12

(Figure 4-la).84 This type of approach, along with dehydrobromination of halogenated

PPV's (Figure 4-lb),85 and generation of PPE by electrochemical reduction of hexahalo-

p-xylene (Figure 4-1c)86, was unsuccessful in preparing well-defined systems without

defects and solubility of the resulting species was low. PPE's have also been synthesized

by ring-forming polycondensations, such as the reaction of acetylendicarboxylic amides

with hydrazine sulfate in polyphosphoric acid (PPA) followed by thermal cyclization of

the hydrazide groups (Figure 4-1d),87 and modifications to synthesize a wide variety of

rigid conjugated polyquinolines (Figure 4-Ie).88


71












Cu- -Cu + 1- -1


heat


PPE oligomers


S Br2
S CHCI3




n ---


200 300 C


Cu electr. -1.7 V, 24 h

0.1M BU4N'(CIO4)


0N
H2N NH2


+ H2N-NH2 'H2S04


PPA
-


N-N


0 1.0


di-m-cresyl phosphate
m-cresol
140 C


Figure 4-1. Early synthetic methodologies toward poly(p-phenyleneethynylene)'s [PPE].




Palladium (0) Coupling Reactions

Due to the limitations of the above routes, palladium cross coupling of terminal

alkynes to aromatic bromides or iodides in amine solvents is often the preferred

methodology to synthesize well-defined and soluble PPE's. This procedure is called the

.. ...'.:... .
'; I i
~1.1


PPE


PPE








Heck-Cassar-Sonogashira-Hagihara reaction and is one of the most frequently used

carbon-carbon bond forming processes in organic chemistry.8 Figure 4-2 outlines the

simple mono-coupling reaction scheme. Obviously, for polymer synthesis both reagents

are made difunctional and added to the reaction in precise stoichiometric amounts.






R Pd/ Cul R
+ 9 Amine


R = Alkyl, Aryl, X = Br, I
OH, Ether, Ester Y = Ester, Nitrile,
OR, NR3, Alkyl

Figure 4-2. General reaction scheme for the Heck-Cassar-Sonogashira-Hagihara
reaction.



Iodoaromatic compounds react faster and at lower temperatures than their

corresponding bromoaromatic analogs. Electron withdrawing groups on the halo-

aromatic compound increase the rate of the oxidative addition to the Pd(0). Elevated

temperatures necessary for the bromo reagents can lead to cross-linking and defect

formation. Choice of the amine solvent can have a dramatic effect on the reaction and it

has been found that diisopropylamine is an excellent choice for use with iodoarenes. PPE

polymerizations are conducted in concentrated solutions and amine solvents alone are not

good solvents for PPE's, therefore THF, ethyl ether, and toluene are commonly used

choices for co-solvents in the polymerizations.

The air stable, commercially available Pd(II) catalyst, PdCl2(PPh3)2, is often used

as tht source of Pd(0) in the coupling reaction and must be reduced to the active Pd(O)


"-.. ;i'.' .. ". ...... ....








species as outlined in Figure 4-3. Two molecules of a cuprated alkyne transmetallate the

Pd catalyst precursor and a symmetrical butadiyne is reductively eliminated, leaving an

active Pd(0) catalyst. PdCI2(PPh3)2 is used in 0.1-5 mol % amounts and varying amounts

of CuI are used as an alkynyl activator.90 Activation of the Pd(II) catalyst requires

consumption of the alkyne reagent which must be adjusted accordingly in

polymerizations to ensure a 1:1 stoichiometric balance with the haloaromatic compound.

A possible approach to solving the stoichiometric balance problem is the "pre-activation"

of the catalyst by addition of a monofunctional alkyne (such as phenylacetylene) to the

Pd(II) catalyst, thereby converting it to Pd(0). The catalyst solution could then be added

to the polymerization reagents and the diyne by-product of the catalyst activation would

not interfere with the stoichiometric balance.



R R
Amine / Cul C L2PdCl2
Ammonium Iodide -Cu2C12
R


L+ll L\
Pd Pd


Active Catalyst

_- R R


Reductive Elimination
Product

Figure 4-3. Activation of Pd(II) compound to active Pd(0) catalyst.








Dialkoxv-Polv(p-phenyleneethynylene)'s

The first soluble PPE derivatives were synthesized by Giesa with the

incorporation of long alkoxy groups to the rigid PPE backbone.91 Degrees of

polymerization on the order of 10-15 were achieved as a deeply colored solid was

recovered. Solubility was low and the deep coloration indicated extreme interchain

packing or some degree of crosslinking. Crosslinking can occur between internal or

terminal triple bonds in the polymer chain, lowering solubility and darkening the color of

the material and subsequently promoting inter-chain packing. An improved synthesis (see

Scheme 4-4) of similar PPE's was achieved by Moroni et al. with degrees of

polymerization of approximately 20.92 These workers did not take into account the

activation of the palladium catalyst, thus lowering molecular weights and introducing

diyne defects in the backbone.



OR' OR3
S= + x Pd cat.
amine
R20 RO Cul
X= Br or I


OR3 OR1 OR3

endgroup endgroup

R40 R20 R40

Figure 4-4. Synthesis of dialkoxy poly(p-phenyleneethynylene)'s via the Sonogashira
reaction.



Reduction of reaction temperatures to lower than 70 oC by Wrighton et al. in the

coupling of 2,5-diiodo-1,4-dialkoxybenzenes to 2,5-diethynyl-l,4-dialkoxybenzenes in a
C'i ."''. .







diisopropylamine/toluene mixture under PdCl2(PPh3)2/Cul catalysis led to polymers

without crosslinking and degrees of polymerization of up to 100.93 The same group

prepared interesting dialkoxy-substituted copolymers with 3-(dimethylamino)propyl and

7-carboxy-heptyl groups.94 Weder et al.95 utilized the branched solubilizing

ethylhexyloxy and linear octyloxy groups to prepare a polymer with a reported degree of

polymerization of 230, which were summarily reflected in the similar work of Swager

and coworkers who limited the molecular weight by the use of an imbalanced reaction

stoichiometry to ensure defined iodine endgroups.96

Other classes of PPE's have been created via the Sonogashira reaction that mix

di-alkoxy-substituted diiodides with different aromatic diynes. Examples include West's

use of 1,4-diethynylbenzene97 (Figure 4-5a) and Swager's use of al,4-

diethynylpentiptycene monomer to provide bulky chain spacing side-groups9 (Figure 4-

5b) or a bisamide compound better film forming properties (Figure 4-5c).99 Aryl- and

alkyl-substituted PPE's, which resemble a "true" unsubstituted PPE the most, were first

reported in 1995 by Bunz and Millen (Figure 4-5d).t10 A complete coverage of all PPE

type polymers synthesized by Pd(0) coupling methodologies would be impossible in this

dissertation, however, two excellent reviews by Giesa and Bunz on the subject matter are

available for reference.'10 Extensive work has also been accomplished in the field of

metal to ligand charge transfer between PPE's and coordinated metallic species.I02

The utility of the Sonogashira reaction for synthesizing well-defined PPE's, along

with its tolerance for functional groups, makes it applicable for incorporation of 2,5-

dialkoxyamine-phenylene units into a PPE backbone structure. These units can be

protonated with acidic treatment or quaternized with ethylbromide to provide an








interesting new class of polyelectrolyte. In general, such polymers should be yellow in

color and emit in the green region of the visible color spectrum. Due to the extensive

rigid-rod character of PPE's, special care will have to be taken with the resulting

materials to determine the effect different side chains on the second phenylene ring in the

repeat unit will have on the solubility of the initial neutral polymer and subsequently the

effect of bulky organic groups on the properties of the post-polymerization quaternized

polymer. If successful, this set of PPE polymers may provide polymers that emit in a

similar wavelength range as the poly(p-phenylene-co-thiophenes) [PPT's] discussed in

Chapter 3 and are more efficient emitters, which are capable of being cast as free

standing thin films.






C12H25 14 2 )9



OC12H25 OC4H29



(a) (b)



N(COaH7)2
C2oH21 O= C06H13 CH13



OC1oH21 jH=0i3 CeH13
N(CeH17)2

(c) (d)

Figure 4-5. Representative structures of synthetic modifications to poly(p-
phenyleneethynylene)'s.



Pr

.. ........ .
..".. .... ..:. T... .
... IEi : fi "" .i : :. ":" .





78



Results and Discussion


Monomer Syntheses

As illustrated in Figure 4-4, Sonogashira couplings require the usage of a di-

haloaromatic and a di-ethynylaromatic for an AA-BB type polymerization. Initial

monomer synthesis focused on the di-ethynyl reagent; the substitution of which will

greatly affect the solubility characteristics of the resulting polymer. Figure 4-6 outlines

the Williamson etherification procedure used to alkylate hydroquinone with

primary alkyl bromides.103 A suspension of powdered KOH was stirred in dry DMSO for

one hour followed by addition of hydroquinone and either hexyl- or nonyl-bromide. The

reactions were heated to 80 oC for 12 hours, cooled, poured into ice water, and extracted

with hexanes. The organic layer was subsequently washed with IM NaOH, water, brine,

OC6H13

4 eq. 5
OH B'Br KOH H13CO 32
+ or DMSO OCgH1i
H -- Br 80 0o (
HO

HigCgO 33

Compound % Yeld
32 83
33 79
Figure 4-6. Williamson etherification to synthesize various 1,4-dialkoxyphenylene's.



and dried over MgSO4. Removal of the solvent via reduced pressure evaporation led to

the isolation of reddish solids. The solids, 1,4-bis(hexyloxy)benzene (32) and 1,4-

bis(nonyloxy)benzene (33), were purified by recrystallization fromiethanol giving white

solids in 83 and 79 percent yields, respectively.



.: :....2
.: :. ::: ..:. : : E: E:I: : ...[[










OCeHi3
OCCH1H13
OCI I


SKO4,12 H13C60 34
H13C60 32
or AcOH / H20 / H2SO4 or
C9H70 C /12 h



H19C90 33 H19C90 35

Compound % Yield
34 79
35 85

Figure 4-7. Iodination of various 1,4-dialkoxybenzene's.



The 1,4-bis(alkoxy)benzenes (32 and 33) were iodinated under acidic conditions

using potassium periodate, iodine, and a mixed solvent system consisting of 90:7:3

HOAc/ H20/ HSO4 by volume with heating to yield 1,4-dialkoxy-2,5-diiodobenzenes

(34 and 35) as shown in Figure 4-7 in 79 and 85 percent yields as white crystals. The di-

iodides, 34 and 35, along with 1,4-diidobenzene were then subjected to a Sonogashira

coupling with (trimethylsilyl)acetylene in the presence of PdCl2(PPh3)2 and Cul catalysts

in an amine solvent. The 1,4-bis((trimethylsilyl)ethynyl) compounds[36-38] were isolated

via filtration of the reaction to remove amine salts and passed through a filter plug of

silica gel using toluene as eluent. After removal of the solvent, crude red solids were

obtained and recrystallized twice from ethanol to yield white crystals. The 1,4-

bis((trimethylsilyl)ethynyl) compounds were treated with either tetrabutylammonium

fluoride or aqueous KOH in THF to remove the TMS groups. The diethynyl compounds

(39-41) shown in Figure 4-8 were recovered in overall 70 to 82 percent yields based on



......... ./......








the appropriate starting di-iodide compound as light yellow or white crystals. Elemental

analysis results for compounds 39-41 are listed in Table 4-1.




OC8H13 OC6H13 OCgH13
I- TMS--TM-S H H

H13C60 34 H13C60 36 H13CSO 39
or C9H19 2.2 eq. or or
H SiH- TBAF or aq. KOH
I I TMS -- TMS H H
I PdCl2(PPh3)2 THF
H13CgO 35 Cul H1Cg9 37 H19C90 40
Et3N
or or or

-Q TMS -- TMS H-- = H
38 41

Compound % Yield
39 75
40 70
41 82


Figure 4-8. Synthesis of various 1,4-diethynylphenylene monomers.




The previously described di-iodide compound 2,5-bis(3-[NN-diethylamino]-l-

oxapropyl)-1,4-diiodobenzene (DINEt) was used in conjunction with the above di-

ethynyl compounds for Sonogashira polymerizations to provide a functional amine site to

be quaterized after polymerization (see Chapter 1). Synthesis of 2,5-bis(3-[N,N-

diethylamino]-1-oxapropyl)-1,4-dietihynylbenzene, produced by Pd(0) coupling of DINEt
i, b. a:..

with trimethylsilylacetylene and treatment with base, was attempted in order to have a

S companion reagent to DINEt, which upon Sonogashira polymerization with DINEt

would produce a PPE with every phenylene ring possessing alkoxyamine side chains.


.. .. .. L

S. ... .... ; ..
"..'-.* ..... .A .** : .. .: ." : : .. "i "'*:. ""i6 W ." :". !;:,,,"







Purification of the 2,5-bis(3-[N,N-diethylamino]-1-oxapropyl)-1,4-diethynylbenzene to a

level satisfactory for use in step growth polymerizations was hindered greatly by the


Table 4-1. Elemental analysis results for PPE monomers and polymers.
Species %C %H %N %I or Br Anal. Calcd.
for
Theo. 80.93 9.27 C22H3002
Compound
39 Exp. 81.20 9.15

Compound Theo. 81.89 10.32 C28H4202
40
40 Exp. 82.02 10.68

Compound Theo. 95.20 4.80 CoH
41 Exp. 95.60 4.60

PPE-NEtH Theo 71.78 7.26 5.98 1.05 C28H34N20210.04
PPE-NEtj/H
[51] Exp. 70.25 7.08 5.35 1.05

PPE-NEt2/ Theo. 75.64 9.20 4.41 0.67 C4o H58N204I. 10
OC6
[52] Exp. 72.10 7.89 4.01 0.67
PPE-NEtz/ Theo. 76.84 9.81 3.90 0.55 (1) C46H7oN2041.0o3
OC9(High)
[53] Exp. 76.11 9.79 3.47 0.55 ()
PPE-NEt2/ Theo. 76.98 9.83 3.90 0.37 (I) C46H7oN204lo.02
OC9(20)
[54] Exp. 75.66 9.64 3.41 0.37 (1)
C4QH7oN204
PPE-NEt3+/ Theo. 64.37 8.64 3.00 17.13 (Br) C46HN204
OC9(20)2 C2H5Br
OC9(20)
[56] Exp. 66.06 8.92 3.03 12.58 (Br) 35 C2HBr
-1.35 C2H5Br

extremely polar amine sites, which prevented column chromatography. Other means of

purification were unsuccessful in that distillation under reduced pressure resulted in the







h i.. .







cleavage of the triethylamine side groups and attempted recrystallization led to both

impurities and the desired compound to crystallized from the chosen solvents.

Taking into account the limitations imposed to purification by the polar amine

side groups of DINEt, it was desirous to have an alternate di-iodo monomer that

possesses the ability to form cationic amine sites, but does not contain amine sites from

the initial compound synthesis A route found in the literature used bromo-terminated

alkyl groups as side chains on benzene that were treated with triethylamine, imparting

water solubility with the resulting quatemized amine functionalities.04 Figure 4-9

outlines the synthesis of 2,5-bis(6-bromohexyl)-1,4-diiodobenzene which provides an

optional monomer to DINEt. It should be noted that this monomer does not have alkoxy,

but rather alkyl side chains and upon incorporation into a polymer backbone would raise

the energy of the 7 to 7I* transition compared to alkoxy containing PPE's.

Isolation of intermediate 43 was a challenging step in that that the presence of

unreacted starting material 42, which will prevent isolation of compound 44 in later steps,

must be removed by careful spinning band distillation under reduced pressure. The

distillation technique and equipment are dependent on user ability and several trials had

to be performed in order to maximize the separation ability of the apparatus. A rather

large spinning band column was used as 100g batches of compound 43 were typically

synthesized. Figure 4-10 shows the GC chromatogram of the reduced pressure

distillation of compound 43 using a simple vigreux column followed by a purification

using the spinning band technique. The initial simple distillation is not necessary for

purification, but was conducted to show the separation advantages of the spinning band

column. The small amount of 1,6-dimethoxyhexane present in the post- spinning band




:" .":` ." I "








distillation GC will not affect the next reaction and its presence is due to the fact that only

one major fraction was collected with priority on avoiding the higher boiling starting

material 42. Additional glassware pieces for the distillation apparatus have been

designed and allow for the collection of multiple fractions without disturbing the reduced

pressure of the column. Small changes in the pressure of the distillation during operation

will negate the separation benefits of the technique.




Br -Br NaOMe Br*OMe 1. Mg
42 MeOH / ether 43 2
reflux /2 days 70% Br -Br Ni (0) cat.


HBr
MeO OMe AcOH B r Br

44 45
62% 91%

45 Br Br
K104
H20,H2S04,
AcOH I 46
65%

Figure 4-9. Synthesis of 2,5-bis(6-bromohexyl)-1,4-diiodobenzene.



Pure compound 43 was reacted with Mg metal to form the Grignard reagent and is

added to a solution of 1,4-dibromobenzene and nickel catalyst to form compound 44,

which was isolated by simple vacuum distillation. The methoxy endroups were

converted to bromine by refluxing in a hydrogen bromide/acetic acid solution and

isolated as a colorless solid after recrystallization. This step was necessary as the

iodination conditions used to synthesize compound 46 were shown to cleave the methoxy

groups, giving a complex mixture of inseparable products. Compound 46 was collected



j 4A




84


in 65% yield as a white crystalline solid after recrystallization from methanol, based on

the amount of compound 45 used (overall 26 % yield based on starting compound 42).

Rehahn et al. demonstrated the technique of "protecting" the bromine groups by

etherification with phenol and reacting their 2,5-bis(6-phenyoxyhexyl)-1,4-

dibromobenzene with various phenylene boronic reagents to create poly(p-phenylene)'s.

The phenoxy end groups could be converted post-polymerization into iodo functionalities

by treatment with trimethyliodosilane followed by exposure to triethylamine to produce

charged, cationic amine sites along the backbone of the polymer (see Figure 4-11). These

PPP type polymers possessed significant solubility in water, but it should be noted that

more rigid PPE polyelectrolytes should inherently have more complex and lower

solubility behaviors in water.

The same methodology could be used to synthesize PPE type polymers with 6-

phenoxyhexyl sides that could undergo the above transformation to cationic amine sites.

The Williamson etherification of compound 46 with phenol is outlined in Figure 4-12.

Compound 50 could be used in Sonogashira type polymerizations to create interesting

PPE's, see Figure 4-13, which can be treated in the same manner as the PPP's in Figure

4-11 to achieve charged amine sites along the backbone. As the purpose of this body of

work is to focus on dialkoxy substituted PPE's, polymerizations utilizing 2,5-bis(6-

phenoxyhexyl)-l,4-diiodobenzene [50] were not conducted. However, the somewhat

difficult monomer synthesis has been fine tuned to allow the pursuit of these type of PPE

polymers by future workers.
.;. h. !
*, .:L.... ".



.:
: ** "* "* : ....


." :. ":
*.: ***"* .. ..,,: .. .:^,. ,:..tl .. ::.* i ':' ,i,^^i .,*^ ^, ..


































M"O..1NOMS


I I
4 6


MmOOr..roM.


8 10 12 min


I I I I
4 6 8 10 min
Figure 4-10. Gas chromatography analysis of purification of 6-bromohexylmethylether
(43) by vacuum distillation using (a) simple vigreux column and (b) spinning band
column.











" :..


B'~Br









CH12OPh

Br -Br

CsH12OPh


R
HO POH Suzuki
+ B -
HO ) OH Polymerizaton
R


R P6Hn2OPh

n
R CeH12OPh
47


R ,CH121
47 Trimethyliodosilane t-n

R C6H121
48


R C6H12NEt3s I-
Et3N f
ACN
R C6H12NEt3+ I-
49


Figure 4-11. Rehahn's route to cationic PPP's.


Phenol
Na*t-BuO

toluene / DMF I'
reflux, 16h


Figure 4-12. Williamson etherification to "protect" bromo endgroups.


C6H12OPh



CsH120Ph


R

+ -'

R


Sonogashira

Coupling


R CH1i2OPh


R CH1IOPh
R COH12OPh


R C6H12OPh
Trimethyliodosilane


R C6H12OPh


Et3N
ACN


R CgH12NEt3+ I



R C6Hi2NEt.+ I'


Figure 4-13. Envisioned application of Rehahn's strategy to PPE's.


ii"




87


Neutral Polymer Syntheses

The general Sonagashira polymerization is outlined in Figure 4-14. DINEt, di-

ethynyl compound, Cul co-catalyst, and Pd catalyst of choice, were stirred in a solution

of toluene and amine (triethylamine or diisopropylamine) with heating to 70 oC.

Temperatures above 70 oC are known to promote crosslinking in the PPE chains, along

with undesired diyne defects, and were therefore avoided. When Pd(II) catalysts are

employed, the amount of di-ethynyl compound should be adjusted to account for

reduction of the catalyst to Pd(0), as shown in Figure 4-3. Pd(0) catalysts, such as

Pd(PPh3)4, are effective for the coupling and do not need to be reduced before beginning

the catalytic cycle, but careful exclusion of 02 from the reaction must be conducted. For

the polymerizations in this study, diisopropylamine and Pd(PPh3)4 were used in all

couplings.




N N/


O R Pd(PPh3)4/ Cul 0 OR
Toluene
I + 1.02 eq. Toluene -
Diisopropylamine -
70 OC
R 700 RO
/ Rand



DINEt Polymer R

Monof l PPE-NEt2/H[51] H
Monofunctional PPE-NEt2/OC6[52] OCsH13
fEndapping Agent PPE-NEt/OC9(high) [53] OC9H19
for PPE-NEt2/QC(20) PPE-NEt2/OC9(20) [54] OCgHig

Figure 4-14. General synthesis for alkoxy-amine containing PPE's.



:,i...

.:,":i: ... ...... ..







Initial synthetic attempts at producing useful PPE's were performed with the

coupling of DINEt to 1,4-diethynylbenzene using toluene / diisopropylamine solvent

system (0.05M in DINEt) and 5 mol% Pd(PPh3)4 / Cul catalysts (PPE-NEt2/H[51]). A

slight excess of di-ethynyl compound is used (-1 mol %), even with an initial Pd(0)

catalyst, to account for unavoidable side reactions of the compound. After a reaction time

of 24 hours, a noticeable amount of material was precipitating from the reaction flask.

After cooling the reaction after 48 hours, the mixture was poured into cold ethanol and a

yellow solid recovered in nearly quantitative yield. This yellow material was insoluble in

hot chloroform, THF, or toluene. This result was not unexpected as PPE's are known for

their high susceptibility to packing when isolated as solids and subsequent poor

solubility. PPE-NEtz/H[51] was extracted with hot ethanol overnight, in an attempt to

remove catalyst residues. The polymer did appear to swell with solvent and was dried

overnight under vacuum. Elemental analysis was performed on the polymer sample to

help determine if crosslinking had occurred during polymerization. Carbon, hydrogen,

and nitrogen values are close to the predicted values for the polymer repeat unit structure.

Determination of molecular weight (via 'H NMR or GPC) was excluded by the

insolubility of the material. Elemental analysis of the material was consistent with the

proposed repeat unit structure (see Table 4-1).

Longer alkoxy groups were then used on the di-ethynyl reagent in hopes of

adding solubility in organic solvents to the neutral PPE's. Polymerization of compound

39, 1,4-diethynyl-2,5-bis(hexyloxy)benzene, with DINEt under the same conditions as

listed above for PPE-NEt2/H[51] was undertaken (PPE-NEt2/OC6[52]). Over the

coarse of 24 hours, the growing polymer remained in solution with no evidence of a




.., ': ".: ":::-. ""a .,




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INGEST IEID ER15J4GUD_MCBSAE INGEST_TIME 2013-02-14T13:43:03Z PACKAGE AA00013535_00001
AGREEMENT_INFO ACCOUNT UF PROJECT UFDC
FILES