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THE STEREOCHEMISTRY OF GROUP TRANSFER POLYMERIZATION OF METHYL, DIPHENYLMETHYL, AND TRIPHENYLMETHYL METHACRYLATES By KRISHNA G. BANERJEE A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1988 EU- F, UBFIUA aIF This dissertation is dedicated to my spiritual master, Om Visnupada Hridayananda dasa Goswami Acaryadeva, (ardent disciple of His Divine Grace A. C. Bhaktivedanta Swami Prabhupada) who taught me how to dedicate one's so called assets in this world in the loving service of the Almighty. ACKNOWLEDGEMENTS I am gratefully indebted to the members of my supervisory committee, Dr. George B. Butler, Dr. John F. Helling, Dr. John Eyler, and Dr. Charles Beaty. Special thanks and deep appreciation are due to Dr. Thieo E. Hogen- Esch for his invaluable guidance, moral support, encouragement, patience, and friendship. Especially valuable was his encouragement during the most difficult times of the research project, when nothing but failures stared me in the face. Thanks are also due to the glassblowers Dick Mosier and Rudy Strohschein for their fine glassblowing and ever-joking mood; to Mrs. Lorraine Williams, the polymer floor secretary, for her kindness and motherly affection; to Dr. Ken Wagener, for his encouragement; and to the other members of the "polymer floor" for their association and encouragement. Last, but not the least, I would like to acknowledge the encouragement and support of my parents, without whom none of this work would have been possible. TABLE OF CONTENTS ACKNOWLEDGEMENTS ............................................................................. iii ABSTRACT ........................................... ....................................................... vi CHAPTER I INTRODUCTION ........................................................................... 1 Background ................................................................................... 1 Objectives ...................................................................................... 16 II EXPERIMENTAL ...................................................................... ... 21 Catalyst Syntheses ..................................................................... 22 Synthesis and Purification of Initiators ........................................24 Monomer Syntheses and Purification .........................................35 Methyl Methacrylate (MMA) .................................. ............. 35 Silver Methacrylate ................................................................35 Triphenylmethyl (Trityl) Methacrylate........................................37 Diphenylmethyl Methacrylate..............................................40 Polymerization Reactions ............................................................41 Group Transfer Polymerization of Methyl Methacrylate .......41 Methylation of Chain End of PMMA prepared by GTP.........45 Polymer Isolation for PMMA prepared by GTP.................... 45 Group-Transfer Polymerization of Diphenylmethyl and Triphenylmethyl Methacrylate........................................... 47 Anionic Polymerization of MMA.................................... ..47 Anionic Polymerization of TrMA..................................... ...49 Polymer Hydrolysis ...................................................................... 52 Poly TrMa ........................................................................... 52 PDMA........................................................................................52 Diazomethane Methylation .......................................... .......... 53 Titration of Alkyl Lithium Solutions.................................. .....53 Instrumentation .............................................................................. 55 CHAPTER age Gas Chromatography .......................................................... 55 Preparative Liquid Chromatography .....................................56 NMR Spectroscopy ............................................................... 57 Size Exclusion Chromatography (SEC) ...............................58 III GROUP TRANSFER POLYMERIZATION OF METHYL METHACRYLATE ...................................................................... 59 Background ...................................... ........................................ 59 Stereochemical Kinetics: 13C NMR Analysis of PMMA Terminated with Labelled End Groups..................................67 Side Reactions in Group Transfer Polymerization....................82 IV GROUP TRANSFER POLYMERIZATION OF DIPHENYL- METHYL AND TRIPHENYLMETHYL METHACRYLATES..87 Background ...................................... ..............................................87 Group Transfer Polymerization of TrMA......................................88 Methylation Attempts and the Possibility of a Dissociative Mechanism for the GTP of TrMA.......................................94 Stereochemistry of GTP of TrMA.......................................... 101 Group Transfer Polymerization of DMA................................. 114 Experimental Conditions and SEC Results........................ 114 Stereochemistry of GTP of DMA............................................. 115 REFERENCES ................................................................................................ 120 BIOGRAPHICAL SKETCH............................................................................. 125 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 THE STEREOCHEMISTRY OF GROUP TRANSFER POLYMERIZATION OF METHYL, DIPHENYLMETHYL, AND TRIPHENYLMETHYL METHACRYLATES By Krishna G. Banerjee August 1988 Chairman: Thieo E. Hogen-Esch Major Department: Chemistry The stereochemistry of the chain-end of PMMA prepared by group transfer polymerization (GTP) and terminated with 13CH31 in the presence of tris(dimethylamino)sulfonium difluorotrimethyl siliconate (TASSiMe3F2) was determined by 13C NMR and compared with the tacticity of the chain. The propagation statistics reveal consistency with a Bernoullian process for the entire temperature range studied (-960C to 45oC), confirming previous reports based on main chain triad tacticity data alone. The results indicate that the E and Z stereoisomers demonstrated for these systems propagate with identical stereochemistry and also confirm a previous suggestion that a comparison of the stereochemistry of the end group and the main chain is a valuable new method for analyzing the statistics of vinyl polymers. The GTP of diphenylmethyl methacrylate (DMA) and triphenylmethyl methacrylate (TrMA) at various temperatures using various nucleophilic catalysts is also described. The strong fluoride ion donor catalysts appear to be the most effective in polymerizing TrMA but are required in much higher concentrations than for methyl methacrylate (MMA) polymerizations. The GTP of both DMA and TrMA are affected by side reactions competing with initiation, resulting in partial destruction of the silyl ketene acetal initiator. Molecular weight control is poor but the molecular weight distributions remain moderately narrow (1 < Mw/Mn < 2). Attempts to methylate the chain-end of poly (TrMA) prepared by GTP were unsuccessful. In contrast to the radical and anionic polymerizations of TrMA, the stereoselectivity of GTP of TrMA increases with increasing temperature with a higher isotactic content at room temperature. A sigmoidal curve is obtained when the tactic content of poly(TrMA) is plotted as In (kr/km) versus 1/T. The possibility is discussed of the polymerization proceeding through two active species such an enolate and a silyl ketene acetal which are interconverting rapidly during the polymerization. On the other hand, the tacticity results for the GTP of DMA are similar to those of anionic polymerization in that the syndiotactic content decreases and the heterotactic content increases with increasing temperature. CHAPTER I INTRODUCTION Background Group transfer polymerization (GTP) is a new technique for the polymerization of acrylic monomers discovered by the scientists at Dupont.1-4 This technique gives "living" polymers (polymers that are capable of further increase in molecular weight upon the addition of additional monomer) and can be carried out at room temperature or above, in contrast to the corresponding anionic polymerization that only works well at low temperatures (below -50oC). Although the method works best for methacrylates, other monomers such as acrylates, acrylonitrile, maleimides and vinyl ketones can also be polymerized. As block and graft copolymers have found increasing use as elastomers, compatibilizers, adhesives and components of high performance finishes, there has been a a great emphasis on new synthetic methods to prepare well-characterized blocks with functional end groups which could serve as building blocks for copolymers of predetermined architecture. Group transfer polymerization appears to overcome some of the major disadvantages of other types of polymerization for the preparation of well defined functional blocks. For example, although anionic polymerization of styrene and butadiene has been carried out commercially, the anionic polymerization of methacrylates is uneconomical due to the low temperatures (< -50oC) required to maintain "living" conditions. Condensation polymerization leads to polymers with broad molecular weight distribution and hydrolytically unstable backbone linkages although they are 2 very useful polymers of great commercial importance. Due to chain transfer and termination reactions, and the resulting inability to rigorously control the MW distribution, free radical polymerization is also unsuitable. Group transfer polymerization however offers a great deal of practical advantages not offered by any other methods, namely operability over a broad temperature range, a wide choice of solvents, good molecular weight control, the ability to functionalize the polymer ends and the ability to prepare block and random copolymers. In addition, although group transfer polymerization requires highly purified reagents and scrupulously dry conditions, a rigorously oxygen- free atmosphere is not required. Group transfer polymerization is an example of Michael addition of silyl ketene acetal to an alpha,B- unsaturated carbonyl compound. It is a first application of such chemistry to polymer formation by sequential additions. Figure 1-1 illustrates the polymerization of methyl methacrylate (MMA) with dimethylketene methyl trimethylsilyl acetal, 1, as initiator. The trimethylsilyl group is transferred from the initiator and the growing end to the incoming monomer (hence the name, "group transfer polymerization"). A catalyst is required for the polymerization to proceed and these can be classified into two types, the anion catalysts1-3 and the Lewis acid catalysts.1, 4 Of the anion type, tris(dimethylamino)sulfonium bifluoride (TASHF2) has given the best overall results and has been used most extensively. Several other catalysts of both types used for GTP are listed in Table 1-1. Me OMe Me OSiMe3 CO2Me MeO OSiMe3 TASHF2 Me Me Me CO2Me TAS = [N(CH3)2]3S+ Me CO2Me Me OMe COgMe Me Me Me I FCH I O Me PMMA. CH2-CHCO2Me --- Me --(CH2C-- OSiMe Me + CO2Me CO2Me M3SIOR Fig. 1-1. Scheme illustrating group transfer polymerization of MMA. Table 1-1. Various Anion and Lewis Acid Catalysts for GTP Nucleophilic Catalysts (Activate Initiator) TASSiMe3F2 TASHF2 TASCN TASN3 RCO2 H(RCO2)2 Lewis Acid Catalysts (Activate Monomer) ZnBr2 Znl2 ZnCI2 (i-Bu2AI)20 i-Bu2AICI Et2AICI __ The mechanism of anion catalyzed GTP5, 6 is rather complex and is far from being completely worked out and established, in detail. However, several experiments by the Dupont scientists strongly indicate an associative silyl transfer mechanism rather than a dissociative (i.e., through enolate anions) one, at least for the bifluoride catalyzed GTP of MMA. The proposed mechanism is illustrated in Fig. 1-2. In the first step, the nucleophilic catalyst activates the initiator by coordination to silicon, to generate a pentacoordinated silicon species 2 (active species). The activated initiator and monomer a were originally proposed to form a hypervalent silicon intermediate 4_(hexacoordinated) and new C-C and Si-O bonds are formed, cleaving the old Si-O bond. More recently however, 4 has been considered to be unlikely as an intermediate, both by the Dupont group7 and by others working in the GTP field.8 Instead, it has been recognized that monomer addition may well take place in two steps, as illustrated in Figure 1-2.To test the involvement of the hypothesized silicon species 2, a stable pentacoordinate siliconate Z (equation 1-1) was synthesized5, 6 as a model compound by treatment of the silane 5 with the lithium enolate of methyl isobutyrate, i. 3 3 L,6 < CF3 CH3 (1-1) + Nu- Rmo R i R Si R'LACt, .OMe R= CH3 Nu- = nucleophilic catalyst Nu- + 0 Nu Si- R R* k ( 0 OMe Nu R., O Si- R RI 0 OMe MeO' R R =w-. Si.-w R o Fig. 1-2. Proposed mechanistic scheme for bifluoride catalyzed group transfer polymerization of MMA. MeO OMe It was hypothesized that a species such as 2 should react with MMA without added nucleophilic catalysts. Compound Z indeed reacted with MMA at room temperature, without added catalyst to give PMMA of reasonably narrow molecular weight distribution. Although the ligands of 2 and Z are admittedly different, this result strongly supports (along with other corroborative evidence) the involvement of a pentacoordinate siliconate species in GTP. Additional evidence for the associative mechanism comes from detailed labelling studies5, 6 (see chapter IV). In bifluoride catalyzed GTP, there is no exchange of the silyl group on the chain end with added trialkylsilyl fluoride. Therefore, if the function of the catalyst is to generate a small amount of enolate anion for anionic polymerization, these anions should be recapped by silyl fluoride, which they are not (Fig. 1-3). In addition, in a double labelling experiment,5, 6 it was shown that exchange does not occur between living poly(butyl methacrylate) chain ends and living poly(methyl methacrylate) chain ends. These results rules out the dissociative route depicted in Fig. 1-4 where at any one time there would be only trace amounts of PMMA enolate ions present. MeC COSiR3 + Nu- Me2C CO SiNu 2 + NR3SiNu Me Me I R'3SiF PMMA-C-=COSiR'3 --- PMMA-C CO I I OMe OMe Fig. 1-3. Scheme illustrating the attempted recapping of enolate ions in GTP of MMA. \ OSiMe3 0O O Me + Nu 7 PMMA O > OMe PMMA O PMMA OSiMe3 PMMA OSiMe3 e OMe < OMe PMMA 0 OMe Fig. 1-4. Dissociative scheme showing recapping of PMMA enolate ions by PMMA silyl ketene acetal. In the case of Lewis acid catalyzed GTP,1, 4 the monomer is believed to be activated by coordination of the Lewis acid to the carbonyl oxygen, for Michael addition of the silyl ketene acetal to MMA. Polymerization of MMA by GTP is rapid and exothermic. Because initiators and living polymer sites are very water-sensitive, equipment and reagents must be scrupulously dry. The monomer to initiator ratio determines the molecular weight which may be varied over a wide range (1000 < M > 105). Only trace amounts of catalyst (0.1-1.0 mole % of initiator for anion catalysts and 10% for Lewis acid catalysts) are required for MMA polymerizations. Monodisperse PMMA with molecular weight as high as 100,000 has been claimed by Dupont using highly purified solvents and reagents. Tetrahydrofuran (THF), toluene, and acetonitrile are typical solvents for nucleophile anionn) catalyzed GTP and toluene and halocarbons for Lewis acid catalyzed polymerization. PMMA prepared by GTP with the anion catalysts at ambient temperature contains approximately a 55:45 ratio of syndiotactic and heterotactic sequences respectively, with no measurable isotactic components in all solvents examined. The syndiotactic content increases from 50% at 600C to 75% at -950C for anion catalyzed GTP in THF as the temperature is lowered, with a final syndiotactic: heterotactic ratio of 3:1. Lewis acid catalysis of GTP of MMA generally provides a much more syndiotactic PMMA than do anion catalysts, but detailed temperature dependence of tacticity studies have not been reported for this system. The tacticity of PMMA prepared by GTP appears to be independent of solvent but is dependent on the reaction temperature and the nature of the catalyst (i.e., anionic vs. Lewis acid). This is in sharp contrast to the anionic polymerization of MMA initiated by alkyllithium reagents, where the tacticity of PMMA is dependent on the polarity of the solvent media. Thus for the anionic polymerization of vinyl monomers of the structure CH2=C(R)C(Y)=X (X,Y = O or N, R = H or alkyl) isotactic PMMA is obtained in the presence of Li or Mg initiators in non-polar solvents (e.g. toluene), while polar solvents (e.g., THF) give predominantly syndiotactic polymers.9-12 In many cases, the silyl ketene acetal is stable and may be used for further reactions or for preparing block polymers by changing to a second vinyl monomer.1, 3 Figure 1-5 illustrates the formation of a triblock polymer from MMA, n-butyl methacrylate and allyl methacrylate. OMe PMMA OMe + MMA - > OSiMe3 OSiMe3 n-BUTYL METHACRYLATE PMMA-PBMA-PAMA OMe PMMA-PBMA OBu 57% 32% 11% < __ / \ ALLYL METHACRYLATE \ OSiMe3 OSiMe3 Fig. 1-5. Scheme illustrating the synthesis of a triblock copolymer by GTP The figure also illustrates the point that methacrylates bearing free radical sensitive groups can be introduced into the polymer chains by GTP. A polymer with 11% allyl methacrylate would form a gel if prepared by free radical initiation. Polymers with thermally sensitive functionality such as glycidyl groups can also be prepared, but polymerization temperatures must be maintained at OoC or lower. When mixtures of various methacrylate monomers are used, random copolymers form. End functionalized polyacrylates can be obtained by employing properly designed initiators.1, 3 The corresponding living polymers can be coupled readily to telechelic polymers. Thus, use of a (Fig. 1-6) as an initiator for polymerization of MMA or ethyl acrylate (EA) gave polymers with protected terminal functional groups. The existence of ia or 92 in solution was demonstrated by reaction with a proton source (e.g. methanol) or with a suitable alkylating agent (e.g. benzyl bromide) in the presence of stoichiometric amounts of a strong fluoride ion source (such as TASSi(CH3)3F2) to give 10 or 11 (Figure 1-7). v OCH2CH20SiMe3 + (n+1) H2C= C--COg2R OSiMe3 OCH R' 8 I I C-C-(CH2-C -)n OR Me3SiOCH2CH20 I C CH3 CO2R R' OSiMe3 9a R= R=CH3 9b R = C2H5, R'= H Fig. 1-6. Scheme showing the synthesis of telechelic polymers by GTP. Deprotection of 1Q and 11 with n-Bu4NF (TBAF) gave the corresponding polymers with 100% hydroxyl groups in one terminal position. The functionalized polymers may be readily distinguished from the non- functionalized species by high performance liquid chromatography (HPLC). Size exclusion chromatography (SEC) analyses showed that the polymers had narrow molecular weight distribution (Mw/Mn = 1.0-1.3). When the alkylation reaction was carried out with 1,4-bis (bromomethyl) benzene, the intermediate 9a gave after deprotection, alpha, omega - dihydroxypoly(methyl methacrylate) 12 in a quantitative yield (Figure 1-8). The coupling reaction was carried out in the presence of one equivalent (with respect to the initiator) fluoride ion (TASSiMe3F2). The extent of coupling was determined by GPC, HPLC and hydroxyl group analysis. CH3 R' C-C-(CH2-C --CH2 X Me3SiOCH2CH20 I I CH3 CO2R 11 X'= H, R = R'= CH3 1. BrCH2-OX' 2. TASSiMe3F2 (1 equivalent) MeOH or Br2 CH3 R' O\ F C -C(CH2-C X Me3SiOCH2CH20 OI I CH3 CO2R 10aX H, R R'=CH3 10bX=Br, R=R'=CH3 10cX=H, R= C2Hs,R=H Figure 1-7. Alkylation, halogenation and protonation of living PMMA by GTP. 9a 1. BrCH2-QCH2Br S2. TASSiMe3F2 (1 equivalent) SCH R' CH3 CH3 OC-[C H2CCH CH2 CH 2F CHCC-CH Me3SiO(CH2)20 OH n CH OCH2CH2OH CO2R CO2R n-Bu4NF O CH3 R' CH3 CH, 0 C-C-(CH2-C -CH, CH2.' CH) -C. HOCH2CH20 0I In f ni OCH2CH2OH CH3 CO2R CH3 CH3 12 Fig. 1-8. Coupling of two PMMA chains with a difunctional coupling agent. Similarly, alpha, omega-dicarboxylpoly(methyl methacrylate) was prepared by initiating the polymerization with 1,1'-bis (trimethylsiloxy)-2- methylpropene-1 (13) and hydrolyzing the end group of the resulting polymer (Figure 1-9.). A control sample was removed before the addition of the coupling agent. Comparison of the control with the coupled product enables one to determine the extent of coupling. Thus the monofunctional polymer (control) + CO2aMe OSiMe3 H2C 13 S CH3 CH3 CH3 OSiMe3 0C C---(CH2--C CH2- C=C Me3SiO' I Me CH3 CH3 OMe 14 H CH3 CH3 CH3 CH3 O C-C-(CH C-rCH, CH-C-CH -CO HO CH3 COPCH Ca CH3 15 1. BrCH2-- CH2Br 2. H3Ct 14 SH3C 0 CH3 CH3 -I I HO II S CH3 C02CH3 16 Fig. 1-9. Synthesis of alpha, omega- dicarboxy-PMMA. had Mn = 2100, Mw = 2700, while the alpha, omega-difunctionalized coupled product 16 had Mn = 4200 and Mw = 5600, indicating quantitative coupling. The reaction of silyl enol ethers with tertiary alkyl halides and the direct coupling of silyl ketene acetals with titanium tetrachloride (TiCI4) are known.13, 14 Telechelic polymers were prepared taking advantage of these reactions. Using a mixture of Br2 and TiCl4 as the coupling agent, living polymers 9a and J14were reacted independently (Figure 1-10), to yield the coupled polymers 192a and 19 b. Star polymers can be synthesized by generating polyfunctional initiators in situ by the Michael addition reaction of polyfunctional monomers with silicon reagents (Fig. 1-11).1 The alternative approach would involve initiation of GTP with polyfunctional initiators, which presents the problem of synthesis of such large, reactive, initiators. Br2-TiCI, 9a 2 4 - Or CH2C2. 0C 14 - CH, CH3 S I I C-C-H,-CH cn---Br + CH3 CO2CH3 17a R = CH2CH2OSiMe3 17b R SiMe3 SCH3 CH3 CH3 OTiCI3 C.C---+CH2 C CH2-C =C R CH3 CH3 OMe 18a R= CH2CH2OSiMe3 8b R = SiMe3 -BrTICI3 H30 CH3 C COACH3 2 19a R=CH2CHO2a- 19b R=H Fig. 1-10. Coupling of PMMA chains with bromine/TiCl4. - CH2OOC 3 20 EA = Ethyl acrylate > OSiMe3 (i-BuAI)20 + 3)== ------- + OMe -1 4CO2Et CO2 Me -__ 3 Fig. 1-11. Synthesis of star polymers by GTP. When one equivalent of a n-functional monomer is allowed to react with n-equivalents of ketene silyl acetal initiator (1) in the presence of a Lewis acid catalyst (such as dialkylaluminum chloride or dialkylaluminum oxide) followed by the addition of excess monofunctional monomer, cross linking does not occur, but, instead, star polymers are formed. Thus the reaction of 1 with a 0.33 molar equivalent of trimethylolpropane triacrylate 20 at -780C followed by ten molar equivalents of EA gave a quantitative yield of soluble star polymer containing no residual unsaturation with Mn = 2190, Mw = 3040, and D = 1.39 (theoretical Mn = 3300) (Fig. 1-11). Similarly, treatment of 1 at -78oC with 0.25 molar equivalent of pentaerythritol tetraacrylate, 21, in the presence of diisobutylaluminum oxide followed by ten equivalents of ethyl acrylate gave a quantitative yield of soluble polymer with Mn = 2400, Mw = 2970, and polydispersity 1.24 (theoretical Mn = 4752)(equation 1- 2) C C 0H OOC> 1 C(poly EA)4 E 4 2. EA 21 (1-2) Objectives One of our original objectives was to investigate the effect of alkyl or aryl substitution on Si in the initiator molecule on the tacticity of PMMA prepared by GTP. We also wanted to investigate what substituent size could be tolerated on silicon to yield PMMA by GTP. In this regard, the effect of chiral silicon in the initiator molecule on the stereochemistry of the prochiral carbon was of special interest. Chiral silicon PMMA OSIR1 R2 R3 CHa3 OMe Prochiral carbon We were invested in exploring the possibility for chirality transfer on the prochiral carbon ensuing in possible tacticity control. Hathaway and Paquette15 tested the concept of chiral transfer with the following reaction: 'OCH3 BF.Et2 OCH3 CH3.,I Si + Ph H CH2=CH-CH2----C I I' OPh CH2CH=CH2 OCH3 H h 22 23 24 (1-3) Using the allylsilane 22 and the dimethyl acetal, 2a he found about 5% enantiomeric excess of 24. Thus there was clear evidence for the transfer of chirality from silicon to carbon. We were interested in investigating the degree of specificity for chirality transfer in the GTP process and how this may be optimized by Si substitution. Pentacoordinate species such as 2. formed by complexation of the catalyst with the initiator may racemize through pseudorotation16-23 (an intramolecular ligand exchange process). If the rate of pseudorotation is slow enough for measurement (and therefore slow on the polymerization time scale) then it may be possible to monitor the racemization by optical rotation. Racemization by pseudorotation would diminish chirality transfer to prochiral carbon. We wanted to carry out some exploratory work in this area by first trying to synthesize racemic and then optically active (1- naphthyl)phenylmethylsilyl enolate of the GTP initiator, 2& (equation 1-4) Initially we wanted to prepare the racemic compound. Preparation of the silyl chloride, 25, involved four steps following Sommer's procedure.24 CI- Si- Ph Me 25 However, we could not synthesise it by the usual method (equation 1-4). Me2CHCO2Me 1. N(iPr)2Li 2. (a-Np)(P)(Me)SiCI 2. (a-Np)(Ph)(Me)SiCl Me O-Si-Ph Me Me OMe (1-4) We did not try further to synthesize it, thinking the project highly risky for various reasons. It is to be noted that the Dupont group, in trying to test the intermediacy of a hexacoordinate structure 4 (which would require retention of configuration at silicon during the transfer step as the silyl group has been shown to remain invariant in the bifluoride catalyzed GTP process) synthesized both diastereomers of silacyclopentane initiators (27a and 2ZL).6 Me Me Me O- Si 27a cis Me OMe Me OMe 27b trans But these cyclic silyl ketene acetals underwent rapid pseudorotation (stereomutation) at silicon under GTP reaction conditions. Thus, it is likely that even if the synthesis of the target compound 26 were possible, the process of pseudorotation would have made the optical rotation studies difficult, if the pseudorotation process were too fast to measure, as it now seems likely. Initially, when work on GTP began in this laboratory, Dupont was still engaged in fundamental research in this area. As it turned out, some of our areas of investigation were explored by them and the results published while we were still actively engaged in that area. For example, in order to explore the effects of alkyl (or aryl) substitution in Si on PMMA tacticity, we synthesized a variety of silyl ketene acetals to be used as initiators for this purpose (see experimental section), and experienced some difficulty in polymerizing MMA using some of them under conditions of very low catalyst level with respect to the initiator. However, before we could proceed further we learned that much of what we were about to do was already investigated by Dupont and going to be published.1 We also found out in this connection that substitution on silicon in the initiator essentially had no effect on the tacticity of PMMA prepared by GTP. This information was never published but was obtained by private communication.25 We therefore shifted our attention to other areas of investigation in GTP, such as the trapping of PMMA living ends with labelled groups, in order to obtain information on the chain end stereochemistry. Comparison of the stereochemistry of the main chain with that of the chain end has been recently shown to be an independent method for analysing the propagation statistics in the polymerization of vinyl monomers.26 Thus, we wanted to extend the application of the end-group method (so far used only for anionic polymerization in these labs) to GTP, to elucidate the propagation statistics of PMMA prepared by GTP. Another area of investigation has been to examine the GTP of monomers other than MMA, and the tacticity of the corresponding polymers. Thus the GTP of diphenylmethyl methacrylate (DMA) and the unusual monomer triphenylmethyl methacrylate (TrMA) have been investigated in detail and the tacticities of the polymers compared to those prepared by anionic and radical methods. In this regard, we have obtained some evidence that the associative mechanism proposed for the GTP of MMA by the Dupont group may not be true for other systems, such as TrMA. CHAPTER II EXPERIMENTAL General Much of the work involving carbanions was carried out under high vacuum (10-6mm Hg). The vacuum was generated by the combination of a mechanical pump (Welch Duo-Seal Vacuum Pump) and a mercury diffusion pump. All stopcocks and ground glass joints were lubricated with Dow Corning high vacuum silicone grease. All glassware was constructed from pyrex using a natural gas/oxygen torch. The glassware used in the vacuum line work was treated in the following manner. First, an apparatus was rinsed successively with 5% HF, water, and acetone. Then it was dried at 110oC in the drying oven. Prior to all reactions on the vacuum line, further drying was carried out on the vacuum line by flame degassing which involved heating an evacuated apparatus with a torch. Solvents Tetrahydrofuran (THF) was dried by first refluxing over Na/K (approximately 1:2 ratio) for 24 hours. About two liters of THF was then distilled at atmospheric pressure. After flushing the distillate flask for several minutes with argon, fresh Na and K metals were added to it (1:2 ratio of sodium to potassium) along with 0.5 g benzophenone. The flask was then attached to the vacuum line and degassed. The solvent turned purple within a half hour indicating the presence of benzophenone dianion which indicated the absence of water and oxygen. Unless stated otherwise, all other solvents used were purified by first stirring with CaH2 for several hours, followed by distillation from CaH2. Catalyst Syntheses Tris(dimethylamino)sulfonium bifluoride (TASHF2) This was prepared1 3 from commercially available TASSiMe3F2 (Me- TAS-SiMe3F2; Aldrich, technical grade, containing about 10% TASHF2). 2 (NMe2)3S+ SiMe3F2- + H20 -------->2 (NMe2)3S+ HF2- +(SiMe3)20 (2-1) To the TASSiMe3F2 (5.0 g; 16 mmol) was added 0.14 mL(7.78 mmol) of water dissolved in 6 ml of acetonitrile (distilled from CaH2). Two layers formed. The volatiles were pumped off and to the residue were added 3ml of acetonitrile first and then 30 ml of THF (CH3CN/THF = 1/10 ratio). Precipitation or crystallization occurred immediately. Filtration under argon (in a dry box) followed by drying with high vacuum gave 3.1 g (95% yield) of TASHF2 in crystalline form. The 13C NMR (CD3CN, 25MHz) showed only one absorbance at 37.3 ppm for the carbon corresponding to the TAS moiety. The absence of any Si-CH3 signal indicated quantitative conversion of the starting material (TASSiMe3F2). In the proton NMR, the protons of the TAS moiety appeared at 3.05 ppm. N-tetrabutylammonium acetate (NBu40Ac; Alfa) and N- tetrabutylammonium fluoride (NBu4F; Aldrich; 1.0M solution in THF, containing less than 5 wt% water) were used without further purification. Tris(diethylamino)sulfonium trimethyldifluorosiliconate (Et-TASF) This was prepared according to a patent procedure27. SF4 (Matheson) was first condensed into a graduated 10 mL cylinder under vacuum at -780C. Some of it (1.4 ml; 0.0252 mole) of this was then distilled through the vacuum line into the reaction flask. Diethyl ether (previously distilled once from CaH2) was distilled again from CaH2 in vacuo into the reaction flask. Then N,N,-Diethyltrimethylsilylamine (TMSDEA, 97%, Aldrich, 10.98 g, 75.6 mmole) was added by means of a syringe to the reaction vessel under argon at -780C. The reaction mixture was stirred for five days (warming up to room temp). Two liquid layers formed, the bottom layer being dark brown. The top layer most probably was SiMe3F (a by-product of the reaction): 3 Et2NSiMe3 + SF4---->(Et2N)3S+ Me3F2Si- + 2 SiMe3F (2-2) The ether and trimethylfluorosilane (TMSF) were removed by evaporation in vacuo. According to the patent procedure, one is erroneously led to believe that at this point, that the (Et2N)3S+ Me3F2Si- will appear as light grey crystals. However, this was not the case, thus proving the general statement that "patents are made to claim, and not to reveal information". Even private communication with Dupont scientists revealed that they themselves were unable to obtain results according to the exact wordings of the patent procedure. A dark brown oil was the final product, which even after continuous evacuation at high vacuum (10-6 torr) for more than 24 hours did not yield a solid. The reaction apparatus was then transferred under argon to a dry box for preparation of NMR samples. Analysis by 1H, 13C, and 19F NMR revealed the presence of a mixture of 2 products, (Et2N)3S+SiMe3F2 (Et-TAS-SiMe3F2) and (Et2N)3S+HF2 in approximately 85:15 ratio in quantitative yield. Characterization: NMR (13C, THF-d8, 50MHZ): 7.3 ppm (Si-CH3), 13.2 ppm (NCH2CH3 of ((Et2N)3S+SiMe3F2), 42.0 ppm (N-CH2 of (Et2N)3S+SiMe3F2), 14.7 ppm (NCH2CH3 of TASHF2), 38.7 ppm (NCH2 of TASHF2). NMR (IH, THF-dS, 200 MHz): 0.0 ppm(s, Si-CH3), 1.28 ppm (t, N- CH2CH3 of TASHF2, 1.43 ppm (t, NCH2C.3 of (Et2N)3S+SiMe3F2) 3.22 ppm (q, NCH2 of TASHF2), 3.54 ppm (q, NCH2 of (Et2N)3S+SiMe3F2). NMR (19F, THF-d8, 188 MHz, CFCI3 = 0.00 ppm): -57.00 ppm (s,TASSiMe3E2), -149.8 ppm (d, JHF = 120 Hz, TASHE2). Synthesis and Purification of Initiators Ketene Trialkylsilvl Acetals These compounds were prepared according to published procedures28-30 with some modifications. Compound 1(1-methoxv-1- trimethylsiloxy-2-methyl-l -propene: Aldrich) was distilled first from CaH2 (350C, 15 mm Hg) and stored under argon. To this was added fresh CaH2 and the round bottom flask A containing it (see Fig. 2-1) was attached to the side arm B. The apparatus was then attached to the vacuum line and the contents of A were stirred for an additional 1-2 hours with CaH2 with occasional degassing. It was finally distilled into C by heating A intermittently with a low temperature heat gun (actually a hair dryer), while C was immersed in a dry ice/isopropanol bath (-780C). While A and C were kept cold, the ampoule was sealed off at the constriction D n vacuo. HIGH VACUUM Fig. 2-1. Apparatus used for the in-vacuo distillation of the GTP initiator into an ampoule. A number of ketene trialkylsilyl acetals (some are new compounds) have been prepared according to general published preocedures (see Table 2-1) in the hopes of using them as GTP initiators. The preparation of 28 and 32 are illustrative general procedures. Synthesis of Methyl dimethylethylsilyl dimethyl ketene acetal(28). This was prepared according to a literature procedure28. A three neck round bottomed flask was flame dried under vacuum. THF (50 ml) was distilled in through the vacuum line at -780C. Diisopropylamine (5 g; 3.6 ml; 49.4 mmol), previously distilled from CaH2 was then added to the flask under argon followed by addition of 30 ml of 1.5 M BuLi (45 mmol) with a syringe. The mixture was stirred for 30 mins. while the dry ice bath was replaced by an ice- water bath. Then methyl isobutyrate, 34. CH(CH3)2CO2Me 34 Table 2-1. Different Silyl Ketene Acetals Synthesized for Use As Initiators in the GTP of MMA. (CH3)2C=C(OMe)(OSiR1 R2R3) Compound No. B1B2B3 Method used (ref) 28 Me2Et 28 29 Me2iPr 28 30 Me2tBu 29, 30 31 Me, C6H5, vinyl 28 32 Me2, C6H5 28 33 Et3 29,30 (Aldrich; 3.0 g; 29.4 mmol), distilled from CaH2 was added dropwise under argon with a syringe at 0oC. The reaction mixture was stirred for an additional 30 minutes at 0 C and then was treated with excess chloroethyldimethylsilane (Aldrich) distilled from CaH2 through the vacuum line. The mixture was stirred for an additional hour at 0oC and then allowed to warm slowly to room temperature. It was filtered several times to remove the inorganic salts (LiCI). Evaporation of solvent gave an oily residue which was filtered again through glass wool. The oil was then distilled in vacuo with a fractionating column, several fractions were collected and their purity checked by GC. The purest fraction was determined to be 98% pure. The yield was approximately 30%. Compounds 28, 29, and 32 were prepared according to the above general procedure using methyl isobutyrate and the corresponding silyl chlorides. The yields of isolated silyl ketene acetals ranged from 20-40%. Synthesis of methyl triethylsilyl dimethylketene acetal (3).29,30 The reaction is represented by the following equation: CH2=C(CH3) (CO2Me) + R3SiH ----> CH2=C(CH3)(OSiR3) (2-3) A mixture of methyl methacrylate (2.6g; 26 mmol, distilled from CaH2), triethylsilane (3.2 g; 27.5 mole) and tris (triphenylphosphine) rhodium chloride (Wilkinson's catalyst; Aldrich; gold label; 72.2 mg; 0.0780 mmol) was heated at 1000C under vacuum for 2 minutes. The ketene silyl acetal was obtained by fractional vacuum distillation in various fractions (best fraction: 90% pure by GC). The yield of combined fractions was 75%. Compound Q0 was once unsuccessfully attempted to be prepared by the method of Ainsworth. However the method of Yoshi29 gave a quantitative yield of 3Q prior to fractional distillation. The purity of the crude product was determined by GC to be 77%. All of the silyl ketene acetals prepared above had reasonable 1H and 13C NMR spectra which are listed below. Compound 28: 1H NMR (CDCl3, 200 MHz): 0.16 ppm (s,Si-CHL3),0.66 ppm (q, CJt2CH3), 0.99(t, CH2CH3), 1.52, 1.57 ppm (s, (CH3)2C=), 3.49 ppm (s, OC13). NMR (13C, CDCl3, 200 MHz): -2.1 ppm (Si-CQH3), 0.64 ppm (CH2CH3), 0.82 (CH2CH3),16.00 and 18.00 ppm ((CH3)2C=), 56.4 (OCH3), 90.90 ppm ((CH3)2C=), 149.60 ppm (=C(OSiMe3)(OMe)). Compound 29: 1H NMR (CDCI3, 200 MHz): 0.38 ppm (s, Si(CHi3)2), 1.1-1.38 (m, CHL(CH3)2), 1.75 and 1.80 ppm (s,(CH3)2C=), 3.75 ppm (OCJi3) NMR (13C, 50 MHz, CDC13): -4.00 ppm (Si-.H3), 14.80 and 16.00 ppm ((CQH3)2C=), 16.80 ppm (HC(QH3)2-), 56.80 ppm (OQH3), 23.40 ppm ((CH(CH3)2), 91.0 ppm ((CH3)2C=), 150.00 ppm (.(OSiMe3)(OMe)). Compound 30: 1H NMR (CDC13, 60 MHz): 0.22 ppm (s, Si-CH3), 1.05 ppm (s, (CH3)3C), 1.61 and 1.65 ppm ((C03)2C=), 3.57 ppm (s, OCH3). Compound 31: 1H NMR (200 MHz, CDCI3): 0.55 ppm (Si-CH3), 1.42 and 1.46 ppm (s, (C0.3)2C=), 3.41 ppm (s, OC.3), 5.8-6.46 ppm (m, Si- CH-=CH2), 7.33-7.80 ppm (aromatic H's). NMR (13C, 50 MHz, CDCl3): -3.4 ppm (Si-C.H3), 16.0 and 16.4 ((CH3)2C=), 57.2 (OH3), 91.40 (Me2Q=), 124.80-135.50 (vinyl and aromatic C's), 149.50 ppm (=Q(OSiMe3)(OMe)). Compound U2: 1H NMR (CDCI3, 200MHz): 0.48 ppm (Si-Mh), 1.51 and 1.57 ppm (s, (Ch.3)2C=), 3.42 (OMe), 7.32-7.45 ppm (3 aromatic H's), 7.52-7.72 ppm (2 aromatic H's). NMR (13C, 50 MHz, CDCI3): -1.40 ppm (Si-CH3), 16.2 and 17.0 ppm ((CH3)2C=), 57.1 ppm (OMt), 91.50 ppm (Me2.=), 127.6, 129.8, 133.30, 137.20 ppm (aromatic C's), 149.3 ppm (=Q(OSiMe3)(OMe). Compound 3: 1H (200MHz, CDCI3): 0.70 ppm (q, Si-Cj2Me, fine complicated splitting), 1.00 ppm ( complicated t, Si-CH2Cd3), 1.53, 1.56 ppm ((CH3)2C=), 3.50 ppm (OCC3). NMR (13C, 50 MHz, CDCI3): -4.8 ppm (Si-CH2CH3), 6.5 ppm (SiCH2MQ), 15.9 and 16.60 ppm ((CH3)2C=); 56.8 ppm (OSH3), 90.80 ppm ((CH3)2C=), 149.90 ppm (=C(OSiMe3)(OMe). In the literature, most of the silyl ketene acetals were described as having been purified by distillation and preparative GC.28-31 They are sensitive to moisture and decompose upon attempted separation or purification by column chromatography.31,32 Thus, for example, the attempted separation of 31 with neutral alumina and very dry solvents resulted in the cleavage of the O-Si bond and the corresponding silyl alcohol (Si(C6H5)(CH3)(CH=CH2)(OH)) was isolated as a by-product, identified by 1 H NMR spectroscopy: (60 MHz, CDCI3): 0.5 ppm (s,Si-ChL3), 2.26 ppm (broad s, Si-OHl), 5.6-6.73 ppm multiplee, CH=CH2), 7.27-7.8 ppm (aromatic H's). Even the C -silyl compounds decompose upon attempted separation by column chromatography.32 The scheme for the synthesis of (1-Naphthyl)phenylmethylchlorosilane was the same as that of Sommer24 and is shown in Fig. 2-2. Synthesis of (1-Naphthyl) phenvlmethylmethoxvsilane (36) This was prepared according to the literature procedure24. First the Grignard reagent (1-NpMgBr), was prepared. A 3-neck round bottom flask was equipped with a condenser, a drying tube and an addition funnel. Excess magnesium turnings and a solvent mixture consisting of 2 parts (by volume) of ether, 3 parts of toluene and one part of THF was placed inside the flask. In the addition funnel was placed 2.99 g (14.4 mmol) of 1-bromonaphthalene, 35. and 12 ml of the above solvent mixture. The Grignard reagent was prepared by gradual addition of the contents of the addition funnel to the 3- neck flask. An oil bath (temp. maintained at 60-70oC) was used to warm the mixture and iodine was added to initiate the Grignard reaction. The dark purple color of the iodine faded as the reaction started, together with gradual depletion of magnesium. At the end of the reaction, the color of the reaction mixture was yellow. The apparatus was cooled using an ice-water bath and 2.73 g (15.0 mmol) of methylphenyldimethoxysilane dissolved in the above solvent mixture was added. A white precipitate formed upon the addition of the reagent. The ice/water bath was removed and replaced with an oil bath, and the mixture was stirred overnight at 500C. It was then treated wlith cold aqueous ammonium chloride (saturated) and washed three times with 30 ml portions of water in a separatory funnel. An inorganic green precipitate was left behind in the reaction vessel (probably HOMgBr). The organic layer was dried over anhydrous sodiumsulfate. Filtration and removal of the solvent provided a viscous syrup which was chromatographed by HPLC using hexane/THF solvent mixture. At first, pure hexane was used as an eluent. This eluted the desired compound. Hexane/THF mixture (programmed to slowly change the composition of solvent mixture to pure THF) removed other + Mg Br 1-NpBr Ph I 1 -Np-Si--OCH3 0 ,CH(CH3)2 MgBr 2. H30 Menthol =it---- CH30-Si-Ph CH CH3 H3C' SLiAIH4 Ph I cy 1-Np--Si-CH3 - I H Ph 1-Np-Si- CH3 Cl Fig. 2-2. Scheme illustrating the synthesis of (1-naphthyl) phenylmethyl chlorosilane. compounds from the column. GC showed the compound (36) to be at least 95% pure. It was characterized by 1H and 13C NMR. NMR (1H, 60 MHz, CDCl3): 0.80 ppm (s, Si-CH3), 3.55 ppm (s, OCH3),7.2-8.4 ppm (m, 12 aromatic H's). NMR (13C, 50 MHz, CDC13) -2.44 ppm (Si-CH3), 50.78 ppm (OCH3), 34.69, 34.79 (C1-Si(1-NP)(Ph)(OMe)),124.96 137.34 (aromatic C's). Synthesis of (1-Naphthyl) phenylmethylmenthoxysilane (37) This compound was also prepared according to Sommer's procedure24. Into a round bottom flask was placed 1.05 g (3.78 mmol) of 8, 0.300 g (1.92 X 10-3 mole) of menthol and a few grains of powdered KOH (powdered from KOH pellets). Toluene as the reaction solvent was then added. The reaction mixture was maintained at 125-1350 for six hours while any MeOH-toluene azeotrope was distilled through a fractionating column. The toluene was evaporated off from the reaction mixture and the latter was passed through a column of silica (eluent: CHCI3 first, then THF) to remove the basic KOH and any other fine inorganic material. Separation of the product by HPLC (with hexane as the eluent) yielded pure 37. as judged from GC. The material was a viscous colorless syrup. The reactions involved in the preparation of 3Z are Menthol + KOH -------> MenO- K+ + H20 (2-4) MenO-K++Si(a-Naph)(Ph)(Me)(OMe) ---> Si(1-Naph)(Ph)(Me)(OMen) + CH3OH + KOH (2-5) The procedure in the literature24 called for fractional distillation (173-177oC; 0.07 mm Hg), but in this case, it was obtained simply by HPLC separation. The 1H and 13C NMR were completely consistent with the structure. NMR (1H, 200 MHz, CDCI3): 7.2-8.2 ppm (m, 12 aromatic H's), 3.55 ppm (m, OCHR2), 0.4-2.5 ppm (m, 17 aliphatic H's). Synthesis of (1-Naphthyl)phenvlmethylsilane (3) This compound was prepared according to Sommer's procedure.24 Compound 3Z (1.92 g; 4.76 mmol) was dissolved in 15 ml of anhydrous ether and the solution placed in a round bottom flask. LiAIH4 (240 mg; 6.33 mmol; 5.31 equivalents) and 15 ml of di-n-butyl ether were then added. Most of the diethyl ether was removed by distillation as the mixture was slowly heated to 800C. Heating was continued at 80-900C for 18 hours. After decomposition of the excess metallic hydride with acetone, treatment with crushed ice and concentrated hydrochloric acid was followed by drying over sodium sulfate, and filtering. Solvents and menthol were removed by heating to 1700C at 1.5 mm Hg. The remaining material was further filtered to remove some solid impurities and chromatographed by HPLC (hexane being the eluent to afford 38 in pure form as judged from GC. The compound was characterized by 1H and 13C NMR. NMR (1H, 200 MHz, CDCI3): 5.40 ppm (q, Si-H), 0.70 ppm (d, Si-CJ3), 7.2-8.15 ppm (aromatic H's). NMR (13C, 50 MHz, CDCI3): -4.0 ppm (Si-CH3), 124-138 ppm (11 aromatic absorptions). Synthesis of (1-naphthyl)phenylmethychlorosilane (25) This compound was prepared using Sommer's procedure.24 In a three-neck round bottom flask was placed 1 g (4.03 mmol) of 2a dissolved in dry CCl4. While cooling the reaction flask in an ice bath and magnetically stirring the contents, chlorine gas was passed into the solution by means of a fritted glass gas dispersion tube. The reaction was very rapid (approximately one min.), and it was possible to observe a greenish yellow end point when the reaction of Si-H was completed. The solvent was removed by a rotary evaporator to afford the silyl chloride 25, in quantitative yield. The compound at this stage was 90% pure (GC) and was characterized by both 1H and 13C NMR which were consistent with the structure. NMR (1H, 60 MHz, CDCI3): 0.96 ppm (s, Si-Cli3), 6.90-8.33 ppm (12 aromatic H's). NMR (13C, 50 MHz, CDCI3): 2.70 ppm (Si-QH3), 125.00-137.00 (13 aromatic C's). An attempt to improve the purity of 25 by HPLC separation elutingg solvent being hexane) resulted in a mess, completely degrading the silyl chloride. Presumably, there is a reaction involving Si-OH group of the silica gel column with the silyl chloride yielding the corresponding silyl alcohol. Anionic Initiators Samples of diphenylmethyllithium (DPML) were kindly donated by other members of the research group. They were prepared according to established procedure.33 The DPML initiator concentrations were determined by GC using the following procedure. About one ml of DPML solution (in THF) was reacted on the vacuum line with dry Mel (Mel dried over CaH2). The terminated DPML initiator was analyzed by GC to determine the presence of diphenylmethane (as unreacted starting material or as unexpected protonated product). From the relative intensities of the diphenylmethane and 1,1-diphenylethane peaks, the fraction of the methylated product and therefore that of DPML was calculated. In most cases, only negligible amounts of diphenylmethane were detected. To a known amount of diphenylmethane was added a known volume of the Mel terminated DPML solution. The mixture was analyzed by GC and the concentration of the unknown DPML initiator was determined by direct comparison of the Mel terminated DPML with diphenylmethane standard solution. Even though the GC used employed a flame ionization detector which responded to the number of carbon atoms a molecule possessed (diphenylmethane, 13 C; diphenylethane 14 C), no corrections were made for detector response in the concentration determination of DPML. It has been previously verified33 that such analysis without corrections (for compounds having a lot of carbon atoms but differing from each other by only one carbon atom) results in practically no error. Monomer Syntheses and Purification Methyl Methacrylate (MMA) In a 100 ml round bottom flask equipped with a reflux condenser and CaSO4 drying tube, 50 ml MMA (Aldrich) was stirred over CaH2 at room temperature for several hours (12-24). The MMA was then distilled under atmospheric pressure (99-101oC) discarding the first and last few milliliters of distillate. Fresh CaH2 was added to the distillate. The distillated was then attached to the vacuum line and distilled in vacuo into flask A (cooled at -780C) of the apparatus shown in Fig. 2-3. The flask was warmed to room temperature and after the introduction of argon into the appartus, triethylaluminum34-36 was added dropwise into A through sidearm B by means of a double tipped needle, until the MMA turned slightly but a persistent greenish yellow color. The flask A was cooled to -780C and evacuated, and the constriction in B sealed off under vacuum. Then the MMA was distilled into the side ampoule C after A was warmed to room temperature and C cooled at -780C. Ampoule C was sealed off from the vacuum line and the MMA later subdivided into smaller breakseal-equipped ampoules. The MMA ampoules were stored in the freezer at -200C. Silver Methacrylate This was prepared according to literature procedures.33,37 Methacrylic acid was first distilled in vacuo to remove the hydroquinone monomethyl ether inhibitor. Some of it (50 ml; 50.75 g; 590 mmol) was placed in a 500 ml three-neck round bottom flask equipped with a mechanical stirrer and two addition funnels. At room temperature, 35.82 ml (590 mmol) of aqueous 28% ammonium hydroxide solution was added dropwise. The 100.2 g (590 mmol) of silver nitrate (dissolved in 200 ml ofdeionized water) was added dropwise to the ammonium methacrylate. The silver methacrylate (AgMA) precipitated as a grey solid. The reaction was stirred for an additional two hours. The AgMA was separated by filtration and recrystallized from boiling water. The final product was either greyish or slightly purple in color. It was first dried in the vacuum oven overnight at room temperature and then further dried on the vacuum line (10-6 torr) for 48 hours. It was stored under high vacuum in flasks equipped I HIGH VACUUM Fig. 2-3. Apparatus used for the purification of MMA. with high vacuum stopcocks (Fig. 2-4). The yield after recrystallization was about 70%. Triphenylmethyl (Tritvl) Methacrylate Thils was prepared according to literature procedures33,37,38 AgMA (10.56 g; 54.7 mmol) suspended in dry ether was placed in a 500 ml three- neck round bottom flask equipped with an addition funnel, a reflux condenser (with calcium sulfate drying tube), a magnetic stirrer and an oil bath. Trityl chloride (Aldrich; 11.70 g; 42.0 mmol) was dissolved in 150 ml of dry ether and added to the AgMA-ether suspension. The reaction was refluxed overnight. The AgCI was collected by vacuum filtrations and the ether filtrate was condensed on a rotary evaporator. The crude trityl trityl methacrylate looks slightly yellowish. It is purified by a hot filtration using celite and dry ether and then a simple recrystallization using ether. Final product yields were 50% or less. The TrMA was ground to a fine powder and stored under high vacuum (as in Fig. 2-3). It was characterized by melting point, elemental analysis and 1H and 13C NMR. M.P. 99-101oC (lit. 101-103oC).37,38 Elemental analysis: Found : C, 84.08; H, 6.15%. Calculated for C23H2002: C, 84.12; H, 6.14%. NMR (1H, 200 MHz, CDCI3): 7.35 ppm (m, 15 aromatic H's); 6.30 ppm J(s, 1 vinyl H); 5.60 ppm (s, 1 vinyl H), 2.0 ppm (s, C-CJJ3). NMR (13C, 50 MHz, CDCI3): 18.5 ppm (C-CH3), 90.0 ppm (O-CPh3), 125.5, 137,5 (vinyl C's); 127.3, 127.6, 128.2 (para, meta and ortho aromatic C's); 143.5 (ipso aromatic C); 165.0 ppm (C=O). Fig. 2-4. Apparatus for the storage of solid materials under high vacuum. According to a previous report,33 the trityl chloride was recrystallized before use. In the present case, it didn't seem to make any difference whether the trityl chloride was recrystallized (from a mixture of benzene and acetyl chloride in a 4:1 v/v ratio) or not. Therefore, in most cases unrecrystallized trityl chloride (as directly obtained from Aldrich) was used. It appears that both trityl chloride and trityl methacrylate cannot be analyzed for purity by GC. For example, although the 1H and the 13C NMR of TrMA looked very good in terms of purity, the GC of the same sample often showed three peaks with the major component being about 75%. Thus there appeared to be too much discrepancy between GC and NMR results, indicating possible decomposition of the material in the GC column. The synthesis of TrMA is illustrated in Figure 2-5. Diphenylmethyl methacrvlate (DMA) This was prepared according to literature procedures.33,37,38 Silver methacrylate (AgMA; 17.26 g; 89.4 mmol) and dry ether were placed in a 500 three-neck round bottom flask equipped with a reflux condenser, magnetic stirrer, addition funnel, and an oil bath. Diphenylmethyl chloride (Aldrich; 15.0 g; 0.074 mole) was added to the flask at room temp. The reaction mixture was refluxed overnight with stirring. AgCI was separated by vacuum filtration, and the ether filtrate was concentrated to yield crude diphenylmethyl methacrylate (DMA). A fraction of the crude product was kept for analysis by melting point, elemental composition and NMR. The rest of the DMA was recrystallized by one hot filtration using celite and ether and a simple recrystalllization from ether. The mother liquors were kept and concentrated to give additional DMA. CH2gC.COOH + NH CH-- CH2aCCOONH + + 0 CH3 C4l' Methacryllc add Ammonium methacrylate SAgNO3 CH2-CCOO- Ag I + NIJ N03 C Hs C(C.Hs)2-Cl Silver methacrylate C(C.H), -C CH,.C.COCH(CH,), CH-gC.COC(C.H,), C H, C H, Dlphenylmethyl methacrylate Triphenylmethyl methacrytate Fig. 2-5. Scheme illustrating the synthesis of DMA and TrMA. The crude yield was 76%, and the recrystallized yield was about 40%, but elemental analysis and NMR spectra showed no difference in purity between the crude and recrystallized product, although the latter looked somewhat whiter. In view of the analysis results, the DMA obtained from concentration of the mother liquor filtrate from the recrystallization was kept and used for subsequent polymerizations. M.P.: 78-79oC (literature 790C).33,37,38 Elemental analysis: Found : C, 80.86; H, 6.39%. Calculated for C17H1602: C, 80.95%; H, 6.35%. 1H NMR (200 MHz; CDCI3) : 7.30 ppm (m, 10 aromatic H's); 6.95 ppm (s, Ph2CH), 6.25 ppm (s, 1 vinyl H); 5.60 (s, 1 vinyl H's); 2.00 ppm (s, C-C.-3). 13C NMR (50 MHz, CDCI3): 18.3 ppm (C-CH3), 126-128.8 ppm (3 aromatic C's and -CHPh2), 136.6 and 140.5 (vinyl C's), 166.5 (.=O). The synthesis of DMA is illustrated in Fig. 2-6. HIGH VACUUM (D) / Fig. 2-6. Apparatus for the GTP of MMA according to Method A Polymerization Reactions Group Transfer Polymerization of Methyl methacrylate (MMA) Method A. In this method, low temperature polymerizations (-100 to - 220C) were carried out, where the MMA was added by in vacuo slow vapor distillation to a mixture of initiator and catalyst in THF. The apparatus used is shown in Fig. 2-6. First, the whole apparatus was evacuated on the vacuum line and flame dried. Then flask A was cooled to -780C with dry ice/isopropanol. Argon was admitted into the apparatus through the line A solution of TASHF2 catalyst in acetonitrile was introduced through the side arm B by a long syringe through the septum into the flask A. A was cooled to - 78 C and vacuum was applied slowly to the apparatus. The constriction in side arm B sealed off under vacuum and the acetonitrile was slowly pumped out of A (after removal of the dry ice/isopropanol bath) leaving a very thin film of the catalyst in A. Dry THF was allowed to distil into A through the line and the mixture of catalyst and solvent stirred for 30 mins. The break seal of the ampoule D containing the initiator solution in THF was broken allowing the initiator to run down into A, the contents of which were being stirred continuously. Stopcock E was closed and the flask C was cooled with an ice bath. The MMA ampoule breakseal (F) was broken allowing the monomer to run down into C. With the MMA stirring in C, the stopcock in C was carefully opened in order to allow MMA to slowly distill into A. Thirty minutes after the addition of all of the monomer, the polymer solution in A was terminated by distillation of methanol through the vacuum line into A. The polymer solution was precipitated in a 10-fold volume excess of either hexane or cyclohexane. The polymer was collected by filtration and dried in vacuum oven at 500 C for at least 2 days. Method B. In this method, used for higher temperature polymerization (00C and 25oC) in vacuo, the monomer was slowly poured into a solution of initiator and catalyst. The apparatus is shown in Fig 2-7. After evacuation and flame drying of the apparatus, the catalyst was introduced into A under argon through side tube B as an acetonitrile solution. The constriction in side tube B was sealed off, and the acetonitrile was slowly pumped off, leaving a thin film of the catalyst on the glass surface in A. After cooling A to -780C, THF was distilled in. The apparatus was then sealed off from the vacuum line at constriction C. The initiator ampoule (F) was then broken and the initiator solution allowed to run down into A. The contents of A were stirred vigorously. Flask D was cooled to -78oC and the monomer from the MMA ampoule (G) was allowed to drain into C. The low temp. dry ice baths cooling A and C were both removed and the apparatus was allowed to reach room temperature. While the contents of A were being continuously stirred, the monomer in flask D was slowly and carefully poured into A intermittently, allowing it to polymerize. Having allowed enough time for the polymerization, the apparatus was again hooked up to the vacuum line through side arm E and the break seal in E broken. MeOH was distilled through the line into flask A to terminate the reaction. Method C. The procedure used was the same as in method B except that a mixed solution of initiator and monomer were slowly added to the catalyst in THF at room temperature. Method D. The procedure was the same as in method D except that the polymerization was carried under argon. Method E. In this method the catalyst is added as an acetonitrile solution by means of a syringe to a premixed solution of initiator and monomer. HIGH VACUUM Fig. 2-7. Apparatus for the GTP of MMA according to Method B Methylation of Chain End of PMMA Prepared by GTP Several attempts (both at -780C and higher temperature) to methylate the chain end of PMMA by first using MeLi and then 13CH31 (99% enriched with carbon 13 label) proved to be unsuccessful (see chapter III). The successful methylation procedure consisted of addition of 13CH31 (between one and two equivalents) to the polymerization mixture followed by one equivalent (with respect to the initiator) of TASSiMe3F2 under argon. Reversing the order of addition of the reagents resulted in practically no methylation at all as evidenced by the absence of the labelled methyl end group in the 13C NMR spectrum of the polymer. Polymer Isolation for PMMA Prepared by GTP PMMA prepared by GTP with catalytic amounts (0.1 to 1 mole % with respect to initiator) of nucleophilic catalysts were precipitated in either hexane or cyclohexane. Reasons for employing cyclohexane in some cases was that hexane was often not completely removed even after two days of drying in the vacuum oven at 50oC, with the result that its signals interfered in the NMR studies of PMMA. There is only one absorption for cyclohexane in NMR and so even if it was not completely removed by drying, it did not interfere with other signals from the PMMA itself. It should be noted that precipitating the PMMA homopolymers in this way does not remove the trace amount of catalyst which is insoluble in hexane and remains associated with the polymer. The catalyst however does not interfere in any way with the NMR analysis of PMMA samples. The PMMA homopolymers prepared by GTP which are methylated contain a much higher amount of catalyst, because of the much higher levels of catalyst required for methylation (see "Methylation of Chain End of PMMA prepared by GTP" below). The catalyst (actually TASI) could be removed from the PMMA by extracting the methylated polymers with water/CHCI3, after all the THF from the polymer solution is removed. The TASI was removed in the aqueous layer. The chloroform layer containing the methylated PMMA was dried with anhydrous sodium sulfate and filtered. It was then precipitated in a 10-fold excess of hexanes or cyclohexane, collected by vacuum filtration, and dried in a vacuum oven at 50oC for at least two days. The TAS salt can also be removed by precipitating the polymer into excess methanol (in which the TAS salt is soluble); however, MeOH precipitation often removes low MW oligomers of PMMA, hence it is not as good a precipitating solvent as hexanes. Group-Transfer Polymerization of Diphenylmethyl and Triphenvlmethyl Methacrvlate The apparatus used is shown in Fig. 2-8. After evacuation and flame drying of the apparatus in vacuo, argon was introduced into the apparatus, and the rubber septum in side arm (A) removed. A weighed amount of dry monomer is placed into the flask (B) through a funnel. The septum was replaced and the apparatus evacuated for 30 minutes. Dry THF was then distilled into the flask through the vacuum line and any monomer clinging to the sides of the flask was carefully washed off with THF by application of a cold dauber to the appropriate regions of the flask. The monomer was made to dissolve in THF by continuous stirring. Ampoule C was broken and the initiator solution was allowed to run down into the flask, the contents of which were being stirred continuously. Argon was introduced into the flask and a solution of the catalyst in acetonitrile was injected through the septum into the flask. Aliquots of samples were removed periodically by a long syringe under argon to monitor the conversion with time by 1H NMR examination of residual Fig. 2-8. Apparatus for the GTP of TrMA or DMA. monomer (vinyl absorption). The polymerization was complete in less than one minute at -780C. MeOH was finally introduced into the flask either by distillation through the vacuum line or under argon to terminate the reaction. Anionic Polymerization of MMA The method used was the same as that for GTP, using Method A above, except that a solution of DPML in THF was used as the initiator which was added to the reaction vessel by means of an ampoule equipped with a breakseal. Anionic Polymerization of TrMA A predetermined amount of solid monomer was placed in a vessel under argon illustrated in Fig. 2-9, and the monomer addition opening was sealed with a torch. The monomer vessel was evacuated on the vacuum line (10-6 mm Hg) for about one hour. The ampoule was then cooled to -780C and THF was distilled through the vacuum line into it. The vessel was sealed from the line and stored int the freezer at -20oC. Anionic homopolymerization of TrMA was carried out in an apparatus depicted in Fig. 2-10. The apparatus was placed on the vacuum line (10-6 torr), flame dried and cooled to -78oC. Approximately 75 ml of dry THF was vacuum distilled into (A). The cold bath was removed and the flask was allowed to warm to room temperature. A THF solution containing the initiator (DPML) ampoulee (B)) was then added to the flask through the breakseal. Any residual initiator clinging to the ampoule was washed into the THF by the application of a cold dauber to the initiator ampoule. The vessel was cooled to -780C, and the monomer solution (C) was added to the initiator. After monomer addition, the initiator solution immediately became colorless. After Fig. 2-9. Apparatus used to prepare THF solutions of TrMA. HIGH VACUUM Fig. 2.10. Apparatus used in the anionic homopolmerization of TrMA. allowing the reaction to proceed for a given time, the polymerization was terminated by vacuum distillation of MeOH through the line. If the polymer (poly TrMA) was soluble in THF or CHCI3, it was precipitated in a 10 fold excess of MeOH to remove the catalyst and unreacted monomer (if any), and the polymer collected by vacuum filtration. If the poly (TrMA) was insoluble on account of its high molecular weight, then it was simply collected either by filtration or centrifugation, and the filtrate from the centrifugation was further concentrated and precipitated in MeOH to attempt to recover any additional polymer. The polymer was dried in a vacuum oven at 50oC for at least two days. Polymer Hydrolysis The hydrolysis of PTrMA and PDMA were performed according to literature procedures.38 Polv(TrMA) About one gram of poly TrMA was refluxed in 50 ml of methanol containing 1% aqueous HCI for about 4 hours. During the hydrolysis the originally insoluble PMMA went into solution (this took less than 1 hour). The solution then was treated by any one of two methods. In one method, the polymer solution was condensed and the residue was dissolved in a minimum amount of MeOH. The resulting poly methacrylicc acid) (PMA) was precipitated in cold ether and collected by vacuum filtration. The polymer was dried at 50oC for 2 days in a vacuum oven. NMR measurements of the corresponding PMMA (see diazomethane methylation) indicated quantitative hydrolysis. In the second method, after hydrolysis of poly TrMA with acidified MeOH, all the solvent was pumped out, and the dry mixture of PMA and trityl alcohol was kept for diazomethane methylation. PDMA Approximately one gram of PDMA was refluxed in 50 ml of methanol containing 5% HCI for at least 7 days. During the first five days, the originally insoluble polymer slowly went into solution. After hydrolysis the solution was condensed and dissolved in a minimum amount of MeOH. The PMA was precipitated in ether and collected by vacuum filtraation. Alternately, after hydrolysis, the solution was evaporated to dryness and diazomethane methylation performed on the mixture of diphenyl methanol and PMA. Diazomethane Methylation Poly methacrylicc acid) (PMA) was methylated to form PMMA using the following procedure:39 A diazomethane (CH2N2) generating apparatus (Fig. 2-11) was constructed from three 250 ml thick wall centrifuge bottles, condensing jacket with screw cap fittings, long stem separatory funnel with a teflon stopcock, rubber stoppers, and fire polished glass tubing. Approximately 10 ml of ether was placed in both collection vessels (B and C) which were cooled in an ice-salt bath. KOH (2.0g; 0.036 mole), 4 ml water, 4 ml ether and 13 ml of 2-(2-ethoxy)-ethoxyethane (Kodak) were placed in centrifuge bottle A. The solution was heated to 70oC using a water bath. N-methyl-N-nitroso-p-toluenesulfonamide (Diazald, Aldrich) dissolved in 40 ml of ether was added dropwise to A, and immediately CH2N2 was generated (it was yellow in color), and distilled with ether into B and C. 54 4CC 0 mm 0 CL C CD LT- The diazomethane-ether solution was added to dry hydrolyzed polymer samples with a pasteur pipette. Rapid bubbling due to gas evolution was detected. Additional diazomethane was added to each sample until there was no further bubbling with the addition of fresh diazomethane. The PMMA samples were usually insoluble in ether, so they could be collected either by filtration or by evaporation of the ether and excess diazomethane. The samples were dissolved in chloroform and precipitated in a 10 fold excess of hexane or cyclohexane. They were then collected by vacuum filtration and dried for several days in a vacuum oven at 50oC. Titration of Alkyl Lithium Solutions The concentration of MeLi in diethyl ether (Aldrich) and BuLi in hexane (Aldrich) did not appear to change much with time as determined by periodic titrations by the method of Winkle et al.40 employing 2,5-dimethoxybenzyl alcohol (DMBA, Aldrich). The first equivalent of alkyl lithium deprotonates the alcohol functionality giving the benzoxide salt which is colorless (equation 2- 6). OMe OMe CH20H Oe CH2O LiU + n-BuLi + n-BuH OMe OMe (2-6) The dianion is dark red in THF and its presence indicates the end point. A few drops of DMBA were added to a preweighed dry flask and the flask again weighed to yield the weight of DMBA. This flask was degassed on the vacuum line after cooling it with liquid nitrogen. Dry THF was distilled in. A syringe with a teflon plunger was flushed with argon and the alkyl lithium solution. The syringe was refilled with the solution. With the DMBA in THF stirring smoothly under argon at room temperature, the alkyl lithium solution was added dropwise after pushing the syring needle through a septum. The persistence of the red coloration longer than 15 seconds indicated the end point. This procedure was repeated twice and an average molarity was calculated from these runs. The results were reproducible within 5%. Instrumentation Gas Chromatography Routine analyses of samples for purity determination as well as quantitative determination of reaction products were done on a Hewlett- Packard Model 5880A gas chromatograph equipped with a capillary column and a flame ionization detector. The column used (HP#19091-60750) was a fused silica capillary (50 m long, 0.2 mm ID) coated with 0.11 uM ffilm of silicone gum (General Electric Co. SE-54, which was methyl 5% phenyl, 1% vinyl cross-linked polysiloxane). The carrier gas was helium. Depending on the nature of the analysis, the column was either heated to a fixed temperature or various step programs were used to increase the oven temp. after specified time intervals. This all depended on the relative importance of the desired speed of analysis and resolution. The microprocessor reported peak retention time (minutes), integrated areas, type, and percent of total area. Although retention times were highly reproducible (+ 0.1% for consecutive injections), standards were nonetheless kept and used to avoid ambiguity as to the identity of peaks. Preparative Liquid Chromatographv Practically all of the HPLC work was done on the separation of the naphthylsilanes. The high performance liquid chromatograph used was an Altex Model 332 system (now Beckman Co.) with programmable gradient elution. The two solvent pumps were fitted with preparative heads. A preparative cell was used in the constant wavelength (254 nm) UV detector Model 153 for dection of the highly absorbing naphthylsilanes. The preparative SiO2 column used was Merck's Lobar B (310 X 25 (ID)mm) packed with 40-63 uM silica gel. It was a glass column with a pressure limit of 90 psi; the system was fitted with a pressure release valve in-line before the injection port. The gradient elution for most of the separations was simple in that only hexane was used to elute the compound of interest. After this, any material still remaining in the column was eluted by using a mixture of hexane and THF with solvent composition being 100% THF at the end (15 minutes to change from 100% hexane to 100% THF). The flow rate was maintained at 2 ml/min. The LC fractions were also analyzed by GC for purity determination. NMR Spectroscopy NMR (1 H) spectra were obtained on either a Varian EM-360 L (60 MHz) or a Varian 200 XL Superconducting spectrometer. Chemical shifts are expressed in parts per million (ppm) downfield from tetramethylsilane (TMS) unless otherwise noted. NMR studies of PMMA stereochemistry were carried out in either CDCI3 (500C) or deuterated tetrachloroethane (TCE-d2) at 90oC at concentrations of about 200 mg per ml of solvent. The triad tacticities of the chain were determined both by integration of the alpha-methyl protons (0.7-1.3 ppm) as well as by integration of the alpha-methyl carbon signals (17- 23 ppm). For compounds with a carbonyl group the pulse delay was set at 1 sec. The triad fractions of the chain end were calculated from the up-and downfield NMR signals at 23-24 and 29-30 ppm respectively corresponding to the diastereotopic labelled methyl end groups. The relative areas of the peaks for the chain end were determined by electronic integration and direct determination of the relative peak areas. Enhanced resolution for many spectra was obtained by use of the resolution enhancement (RE) processing function. This is a line-narrowing technique that resolves Lorentzian lines that are highly overlapped, provided the spacing between the lines is greater than 1/AT (AT= acquisition time) and there is sufficient signal-to-noise to accomplish the desired enhancement. Size Exclusion Chromatography (SEC) SEC analyses were carried out at room temperature using a Waters 6000 Liquid Chromatograph. The columns used Phenomenex TSK G3000 (7.8 mm X 30 cm; 103A), TSK gel type G5000 HXL (105A) columns in series following a filter. THF was the eluent in all cases and the flow rates used were typically 0.7-1.5 ml/min. Both refractive index and UV (222 nm) detectors were used. The column set was calibrated with PMMA standards (Polymer Standards Services, Mainz, W. Germany). From the SEC chromatogram, number average molecular weights (Mn), weight average molecular weights (Mw), peak molecular weights (Mp) and molecular weight distributions (Mw/Mn) were determined. The Mn and Mw values were determined by computer analysis of a SEC chromatogram using a PMMA calibration curve. In all analyses, corrections were made for column band broadening caused by diffusion. CHAPTER III GROUP TRANSFER POLYMERIZATION OF METHYL METHACRYLATE Background The trapping of stereoisomers of a "living" polymer by a suitable trapping agent has been demonstrated.26, 41 Thus the meso and racemic chain ends of a propagating polymer anion, for instance, can be effectively trapped with an electrophile, E+, (Fig. 3-1) and the stereochemical compositions of the propagating chain ends can be determined. Thus the trapping of lithium salts of stereoisomeric "living" poly (2- vinylpyridine) and poly (4-vinylpyridine) anions by reaction with 13C- labelled methyl iodide followed by 13C NMR analysis of the labelled end group has H E| 1-,1,- Tr -- l-- L C(H)(E) I T Trap C1 R R R R R R R R m m m m R H E R -- A a ^- C(H)(E) %: Trap I I I I R R R R R R m r m r Fig. 3-1. The trapping of stereosomeric anions with an electrophile. been reported.26'41 It has also been shown that a comparison of the stereochemistry of the chain end (as obtained from the 13C NMR of a labelled methyl end group) with that of the main chain may be an independent and sensitive method to test chain statistics in vinyl polymerizations.26 There are several stereochemical models42 of chain statistics. The simplest one is the one-parameter or Bernoulli model (Fig. 3-2). This model assumes that only the last assymetric center in the propagating chain is important in determining polymer stereochemistry. The stereochemistry is not affected by the penultimate asymmetric or preceding centers. Pm and Pr, the transition or conditional probabilities of forming meso and racemic dyads, respectively, are defined by Pm= Rm/(Rm+ Rr); Pr = Rr/(Rm+ Rr); Pm+Pr = 1 (3-1) where Rm and Rr are the rates for meso and racemic dyad placements respectively. The term "n-ad" refers to a unit created by "n" adjacent asymmetric (chiral) centers. Thus dyad tacticity Pm and Pr are synonomous with the dyad tacticity fraction fm and fr defined as the fraction of adjacent repeating units which are meso or isotactic-like and racemic or syndiotactic- like, respectively. The probabilities of formation of mm (isotactic), mr (heterotactic) and rr (syndiotactic) triads are given by fmm = Pm2; mr=2 -Pm); frr(1-Pm)2 (3-2) Thus the probability of forming a particular triad is the product of the probabilities of forming the two dyads comprising the triad. The coefficient of 2 RPr R R PmPr = 1 R indicates prochiral propagating center. Fig. 3-2. The Bernoulli model of chain propagation. for the heterotactic triad accounts for the formation of both mr and rm triads. Analogous expressions may be derived for tetrads and higher sequences. The next more complex stereochemical model is the 1st order Markoff model42 which describes a polymerization where the penultimate assymetric center is important in determining the stereochemistry of monomer addition. Meso and racemic dyads can add in two ways as shown in Figure 3-3. There are now four probabilities, Pmm, Pmr, Prm and Prr, characterizing the addition process (the designation Pmr means the probability that the monomer adds in r-fashion to an m chain end, etc.). We also have the relationships: Pmr + Pmm = 1; Prm + Prr = 1 (3-3) The fractions of triads, tetrads, and higher sequences are given in the literature.42 * R R R I I |rR m m R R Ir R m r S RR R ? ^ R ',^ P',r m- R I R r r Fig. 3-3. The first order Markoff model of chain cropagation. Higher order Markoff models have been described to ascribe effects to assymetric centers further back than the penultimate one. Also, non- Markoffian and non-Bernoullian models such as the Coleman-Fox43 and the E-Z models44-46 have also been proposed. The Coleman-Fox propagation mechanism postulates that a growing polymer chain has two reaction states in dynamic equilibrium, both capable of adding monomer, but each with its own stereospecificity. This mechanism is proposed to explain the "stereoblock" structures which occasionally result from homogeneous anionic polymerizations initiated by metal alkyls in which runs of m's and r's are produced.43 The E-Z model will be discussed in some detail as there is ample experimental evidence for the participation of E and Z geometric isomers as intermediates in anionic polymerization of vinyl monomers of the type CH2=C(R)C(Y)=X where X,Y = O, N, or C and R = H or alkyl. The model is based on the demonstrated presence of E and Z isomers and their slow interconversion relative to monomer addition. E and Z isomers have been found as intermediates in anionic vinyl polymerization of 2-vinylpyridine, MMA and t-butylvinyl ketone and in the group transfer polymerization (GTP) of MMA. They may also play a role in other systems such as in the cationic polymerization of vinyl ethers as a result of charge delocalization onto oxygen. The interconversion between the lithio derivatives of E- and Z-39 (Figure 3-4) generated by the reaction of n- butyllithium (BuLi) with 2-ethylpyridine in THF has a half life of about of 5 hours at 50oC.46,47 Thus this interconversion should not occur at -780C during polymerization since this polymerization is quite fast even at -78oC. The lithio salt 32 is generated predominantly as the E isomer in 16:1 (E:Z) ratio at -780C upon treatment of 2-ethylpyridine with n- BuLi. However, upon the addition of 2-vinylpyridine, the corresponding E:Z ratio in the dimer anion 40 and the trimer and tetramer anions was found to be close to one.46,47 In view of the absence of E-Z interconversion in the polymerization time scale, this distribution therefore reflects the mode of monomer presentation during the transition state, s-cis and s-trans monomer generating Z- and E carbanions respectively (Fig. 3-4). In the case of GTP, propagation has been shown to be about 2000 times as fast as E-Z equilibration. In view of the E:Z ratio (1:1) found in the anionic oligomerization of 2-vinylpyridine in THF in the presence of lithium ion, the monomer appears completely unselective with regard to the mode of monomer presentation (s-cis or s-trans) to the propagating carbanion. Even though the E-Z isomers do not directly interconvert during polymerization, an indirect "interconversion" is taking place through monomer addition. Being diastereomers, the E and Z geometric isomers are expected to behave differently with regard to the kinetics and stereochemistry of polymerization. The stereochemical pathways are illustrated in Fig. 3-5. \--Oi H3C Fig. 3-4. Scheme illustrating the the influence of monomer conformation on stereochemistry of propagating species. (Source: 44: Editor, Polymer Preprints) S krET kmr E kmEl km Z zc Scheme illustrating the various stereochemical pathways of the E- Z statistical model of chain propagation. (Source: 44: Editor, Polymer Preprints) Fig. 3- 5. The propagating species may be E or Z and these isomers may be preceded by a meso or racemic diad. Thus [mE] + [rE] = [E]; [rE + [rZ] = [Z] (3-4) If several dyads are specified, the one next to the active center is indicated last. Thus, rmZ specifies a Z active center preceded by a meso dyad which is in turn preceded by a racemic dyad. The active center may be E or Z, the monomer presentation may be s-cis or s-trans and meso or racemic dyads may be formed. As a result, there are eight different processes specified by bimolecular rate constants kRx,y in which x specifies the reacting carbanion isomer (E or Z), y specifies the mode of monomer presentation, s-cis or s-trans (C or T) and R denotes the formation of new dyad (m or r). There are several assumptions inherent in the E-Z model: (a). Only one type of ionic species is present (ion pairs for instance). (b). The stereochemistry of vinyl addition is only dependent upon the type of carbanion isomer (E or Z) and the mode of monomer addition, s-cis (C) or s-trans (T) and not upon the stereochemistry of the chain adjacent to the carbanion. Thus both sites, E and Z propagate according to Bernoullian statistics. In other words, the mE and rE centers have the same reactivity and react in a stereochemically identical manner. (c). Steady state conditions will hold for a particular species, e.g., d[mE]/dt = d[rmZ]/dt = 0 (3-5) It may be shown46 that the E-Z statistical model does not lead to Bernoullian or first-order Markoff statistics. It, is, however, reducible to Bernoullian or first-order Markoff chains under certain limiting conditions (Table 3-1). Since E-Z diastereomers have been demonstrated as intermediates in GTP of MMA,7 we were surprised by the results of Stickler and Mueller49 who reported the statistics of GTP of MMA to be consistent with a Bernoullian process. The E and Z sites are each expected with a different stereochemistry and individually propagate according to Bernoullian model giving overall non- Bernoullian chain statistics for the GTP process. We therefore decided to verify the results of Stickler and Mueller49 by our independent and sensitive method involving the comparison of main chain and chain end stereochemistry, which has been previously developed in this group. Table 3-1. Limiting Conditions Reducing the E-Z Scheme to Bernoullian or First-Order Markoff Chains. No. Condition (Scheme) Statistics 1 kmET=kmZT, kmEC=kmZC, krET=KrZT, krEC=krZC Bernoulliana 2 kmETkmZC, kmEC=kmZT, krET=krZC,krEC=krZT Bernoullianb 3/4 kmZT=krZT=0 or kmEC=krEC=0 Bernoullianc 5 kmZT=kmZC=krET=krEC=O 1st Markoffd 6 kmET=kmEC=krZT=krZC=- 1st Markoffe aE and Z sites show identical behavior, bS-trans addition to E is identical to S-cis addition to Z and vice versa, cOnly Z or E sites are present respectively, dE sites lead to m-dyads, Z sites lead to r-dyads, eE sites lead to r-dyads and Z sites lead to m dyads. (Source: Ref. 44: Editor, Polymer Preprints) Stereochemical Kinetics: 13p NMR Analysis of PMMA Terminated with Labelled End Groups Initially, conditions had to be found where the living PMMA prepared by GTP could be successfully trapped by reaction with a suitable electrophile. Since methylated oligomers of MMA have been separated and the various stereoisomers completely characterized by NMR spectroscopy in this group, the stereoisomeric composition of the chain-end could be unambiguously ascertained for a methylated PMMA. Thus 13CH31 has been used in this group as the electrophile for trapping anionic living polymers. The obvious reason for employing labelled material is to be able to detect the methyl end group of a polymer as a signal of sufficiently high intensity. Since the living chain end of a PMMA prepared by GTP is not an anion, but a silyl ketene acetal, mere addition of 13CH31 is not expected to methylate the chain end. One of the first attempts to methylate the chain end was to use methyl lithium (MeLi) first, followed by 13CH31. It was expected that MeLi would generate the lithio enolate of PMMA which would then subsequently methylate with 13CH31 (Fig. 3- 6). However, this method was unsuccessful when MeLi was added either at -780C or room temperature, followed by addition of Mel at the same temperatures as demonstrated by the lack of a detectable 13CH3 end group signal in 13C NMR. Experiments with the initiator 1 as a model compound for the chain-end of PMMA showed that there was practically no methylation when MeU was added at -78oC, followed by CH31 for several hours at -780C. However when MeLi was added at room temperature to react with the GTP initiator, followed by CH31 at 0oC or -780C, methyl pivalate, 41 was isolated as the methylated product together with some other unidentified side products (equations 3-6 and 3-7). PMMA OSi(CH3)3 O-13Li PMMA O. > -+ Si(CH3)4 CF6 C)CH3 CH3 COOH 13cH 31 + Lil aMe Fig. 3- 6. Scheme illustrating the attempted methylation of GTP PMMA chain end with methyl lithium/methyl iodide. 1. MeLi, 250C (CH3)2C=C(OMe)(OSiMe3)-----------------> 2. Mel, OOC or -780C 1. MeLi, -780C (CH3)2C=C(OMe)(OSiMe3)-------X---------> 2. Mel, -780C Me3CCO2Me 41 As the Dupont scientists reported successful alkylation of PMMA prepared by GTP with benzyl bromide and other electrophiles1,3 using an equivalent of TASSiMe3F2 (with respect to the initiator), the same method was used for successful methylation of the PMMA chain end. In other words, 13CH31 was added first followed by one equivalent of TASSiMe3F2. Based on Noyori's reports50-52, the methylation should proceed by means of an enolate with tris (dimethylamino) sulfonium counterion, i.e. a TAS enolate (Fig. 3-7). (3-6) (3-7) 13CH3 SiPMMA O TAS PMMA OSi(CH3)3 PMMA C OC3 TASSi(CH3)3F2 3OC H TAS =[N(CH,),]3S+ PMWA 1. 13CH 2. TASSi(CH) HF2 Me 13cH Fig. 3- 7. Scheme illustrating the successful methylation of the GTP living PMMA chain. Reversing the order of addition of the reagents (i.e. TASSiMe3F2 first and then 13CH31), fails to give methylation of the PMMA chain end. Presumably, this is due to to a fast competing side reaction of the fleeting TAS enolate species resulting ultimately in protonation of the active center. Methylation experiments with the GTP initiator using the crude THF soluble ethyl-TAS-SiMe3F2 (i.e. TAS+ = [N(CH2CH3)213S+, actually a 85:15 mixture of ethyl-TAS-SiMe3F2 and ethyl-TAS HF2) and methyl iodide using vinyl pivalate as a GC internal standard showed that the yield of the methylated product, methyl pivalate (4.L) corresponded approximately to the mole amounts of the limiting reagent. Thus when ethyl-TAS-SiMe3F2 catalyst was used in slight molar excess of one equivalent compared to the inititor, with a large molar excess (approx 2-3 equivlants compared to the initiator) of methyl iodide, the yield of methylated product was essentially quantitative and when the catalyst was used in amounts less than one equivalent compared to the initiator, the yield of methyl pivalate reflected approximately the proportion of the catalyst Methylation results of the GTP initiator with Me-TAS-SiMe3F2 were often not reproducible, presumably as a result of the insolubility of the catalyst. These results are shown in Table 3- 2. The results are also consistent with the view that the alkylation proceeds through a TAS enolate species formed by a 1:1 reaction of the initiator and the catalyst. In addition, in methylation experiments of the GTP initiator with the THF insoluble catalyst TASSiMe3F2, both methyl pivalate and TASI were isolated and characterized, the former by 1H and 13C NMR and the latter by C,H, N analysis. Table 3-3 summarizes the experimental conditions of the various polymerization reactions of MMA and the results obtained from SEC. The reasons for employing methods C and D in some of these polymerizations were the frequent low yields due to incomplete monomer Table 3-2. Methylation of the GTP Initiator Under Various Conditions. Catalyst moles of initiatora moles of catalyst moles MPb (X 103) (X 103) (X 103) Ac 0.60 0.73 0.57 AC 0.73 0.37 0.41 Bd 2.18 1.74 1.23 Bd 0.60 1.05 0.28 ainitiator was methyl trimethylsilyl dimethyl ketene acetal (1); bMP = methyl pivalate (41); yield determined by GC using vinyl pivalate as internal standard, added after quenching reaction with methanol.; cA = Et-TAS-SiMe3F2; dB = Me-TAS-SiMe3F2 71 Table 3-3. Experimental Conditions and Results of SEC Analyses of PMMA Prepared by GTP Expt. T/oC Mne Mn Mw Mw/Mn Yieldf No. (calc.) % la -96 2255 1847 2390 1.29 100 2a -85 2460 2024 2782 1.37 100 3a -78 1800 1602 1929 1.20 100 4a -40 2200 1944 2220 1.14 100 5a -23 2915 3172 4198 1.32 100 6b 0 2050 1662 1796 1.08 23 7c 25 1500 2799 3917 1.40 91 8d 45 1200 2520 3324 1.32 70 amonomer distilled into mixture of initiator + catalyst in vacuo (method A; see experimental); bmonomer poured slowly into initiator + catalyst mixture in vacuo (method B); initiator and monomer mixture added slowly to catalyst in vacuo (method C); dinitiator + monomer mixture added to catalyst under argon (method D); number average molecular weight calculated from mole ratio of monomer to initiator; based on weight of polymer isolated. conversion obtained using method B at 0oC and higher temperatures, when monomer was slowly added to the initiator and catalyst. This is due to competing side reactions between the initiator and catalyst (possibly involving initiator-catalyst complex and initiator or initiator-catalyst complex and catalyst; see the section on "Side reactions in GTP" in this chapter), resulting in protonation of the living chain. Although there is no mention of side reactions in GTP in the very early papers by the Dupont group, they are mentioned in their more recent publications1 and in publications of other scientists working in this area.53 The rates of side reactions must be lower than the polymerization rates however on account of the excellent yields and good MW control at lower temperatures (runs 1-5, Table 3-3). In addition, the side control at lower temperatures (runs 1-5, Table 3-3). In addition, the side reactions at higher temperatures can be avoided by changing the order of adding the reagents for the polymerization, i.e., by adding the catalyst last to a mixture of initiator and monomer (batch polymerization) or adding a mixture of monomer and catalyst to initiator It can be seen from Table 1, runs 7, and 8 that the yields improve drastically by employing one of the latter techniques (mixture of monomer and initiator added to catalyst) to minimize the side reactions. In these cases, optimum MW control appears to be lost somewhat but the polydispersity is still well below 2. The methylation of the chain end of PMMA prepared by GTP is shown schematically in Fig. 3-8 .It is clear that the methylation neither creates any new chiral center nor affects the stereochemistry of the assymetric carbons adjoining the chain ends. Furthermore, if the stereochemical composition of the last three dyads is known, the proportions of mm*, rm*, mr* and rr* chain ends may be determined: fmm* = fmmm* + frmm*; fmr* = fmmr* + frmr* frr* = frrr* + fmrr*; frm* = frrm* + fmrm* (3- 8) Fig. 3-9 is an entire spectrum of PMMA prepared by GTP at -230C. The assignments of various peaks are indicated in the spectrum. Figures 3-10 and 3-11 are expanded regions of Fig. 3-9 showing the main chain alpha-methyl region, and methyl end group region respectively. The fractions of the syndiotactic (frr), heterotactic (fmr) and isotactic triads (fmm) from the main chain can be obtained directly from the NMR by integration or direct determination of relative peak areas at 16.5, 18.5 and 21- 22 ppm respectively from Fig. 3-10. The absence of nuclear overhauser effect R = -CO2CH3 TAS = -(NMe2)3S+ kmm R R OCH3" m ' krm R-13 R 0-43 OCH3 OSiMe3 kr r a QCH CHa3 3 13 I I 4I CHI R R R R mm* '1) 3 CH31 2) TASSiMe3F2 CH3 C3 H31 aH3 mm* OHa-I OSIMe3 a2!3 3 R ICH3 R R CH OCH3 OSiMe3 mr* 1) 3CH31 2) TASSIMe3F2 CH3- &CH3 R CH3 1 CH3 0-1 3 I H1 3 I 3 R R CH3 R mr* 1) '3 c 31 rm* - 2) TASSiMe3F2 1) 13CH31 rr* ------- 2) TASSiMe3F2 rm* rr* Fig. 3- 8. Scheme illustrating monomer addition to and methylation of GTP PMMA. (NOE) has been shown by the use of model compounds.54 The fractions of racemic (fr) and meso dyads (fm) from the main chain can be calculated from: fr = frr + 0.5 fmr; fm = fmm + 0.5 fmr (3-9) -i I Sco O( ' -* 5 Q4 flo CL C;) 0 3: --| -L U..) oi C6 il LL I a. I- 0 m caJ CL) a. 0 C- (DJ E Cb -o CL ca 0 0 t5r C- Q. C 0 0f) E Ti 0 LO cii T-- Ch cm, u, I . rrr* mrr* mr* rm* rrr* mr* Smrr rm* 30 224 23 PPM Fig. 3-11. 50MHz 13C NMR spectrum of upfield and downfield regions of 13CH3 end group of PMMA prepared by GTP at -230 in THF. The persistence ratio, p, is defined as : p = 2 fmfr/fmr (3-10) This should be unity for a Bernoullian process. Pmr, and Prm, the first order Markoff probabilities can be calculated from:42 Pmr = fmr/(2fmm + fmr); Prm = mr(2frr + mr) (3-11) For a Bernoullian process, the sum of these two probabilities, ,P(= Pmr + Prm) should be equal to one. From Fig. 3-11 the two diastereomeric methyl end groups are seen as expected, to absorb at markedly different fields (23- 24 vs. 29-30 ppm). The downfield absorption is more intense, but the tacticity signals are somewhat better resolved for the upfield absorption. The triad tacticity assignments (given in the spectrum) are based on well defined oligomers of MMA.54 The fraction of the chain end heterotactic triads (fmr* and frm*) are obtained directly from the NMR as are the fractions of chain-end tetrads (frrr* and fmrr*). The fraction frr* was calculated from equation (3-7). The relative peak areas were determined directly by the method of cutting and weighing of the peaks after extrapolation of overlapping peaks to Lorenzian peak shapes. Both the methyl end groups were used for direct determination of relative peak areas. The first order Markoff conditional probabilites P*mr and P*rm from the chain end, can be calculated from equation (3-12).26 P*mr = frm*/fm*; P*rm = fmr*/fr* (3- 12) Equation 3- 12 may be derived as follows:26 Elias and co-workers55-57 have shown that differences in the stereochemistry of the chain end and main chain of vinyl polymers are consistent with the occurence of Markoff processes. Thus, using Fig. 3-13 and following steady state conditions, we have for chains of sufficiently high degree of polymerization: d[m*]/dt = krm = [r*][M] kmrlm*][M] = 0 (3-13) so that fm* = [m*]/([m*] + [r*]) = krm/(krm + kmr) (3-14) and fr* = kmr/ (krm + kmr) (3-15) The meso content, fm of the chain itself is given by fm = krm/(krm + kmr k/ km) (3-16) where kr = krr + krm and km = kmm + kmr denote the rate constants of propagation for r* and m* silyl ketene acetals respectively. We also have the following relationships, using steady state conditions: d[mr*/dt = kmr[m*][M] kr(mr*][M] = 0 d[rm*]/dt = krm[r*][M] km[rm*][M] = 0 (3-17) Substituting equations 3-14 and 3-15 into equation 3-17 leads to [rm*] = krm [r*]/km or frm* = krmkmr/km(kmr + krm) (3-18) where frm* = [rm*]/([r*] + [m*]). (3-19) Similarly fmr* = krmkmr/(kr[kmr + krm]) (3-20) and using equations 3-14 and 3-15 leads to frm* = (kmr/km)fm* = P*mrfm* and fmr = (krmkr)fr* = P*rmfr* (3-21) Rearrangement of equation 3-21 leads directly to equation 3-12. For a genuinely Bernoullian process, it can be shown that the qualities fm = fm*; fr = fr (3-22) and Pmr = P*mr; Prm = P*rm (3-23) will hold true.26 Thus for a Bernoullian process, the stereochemistry of main chain and chain-end should be the same. Noncompliance with equation 3-22 but compliance with equation 3-23 is consistent with a first order Markov process. All of the tacticity results and the stereochemical parameters from the main chain and chain end are listed in Table 3-4. It can be seen from this table that the persistence ratio (p) and the sum of the two first order Markov probabilities from the main chain, 7P (= Pmr + Prm) calculated from the main chain triads are close to one, consistent with Bernoullian statistics as reported in the literature. Table 3-4. 13C NMR Triad Tacticity and Stereochemical Parameters for Main Chain and Chain End of PMMA Prepared by GTP Main Chain Expt. T/oC fmma frra fmra frb pC Pmrd Prmd fr/fr* No. 1 -96 <0.02 0.75 0.25 0.88 0.88 1.00 0.14 0.99 2 -87 <0.02 0.74 0.26 0.87 0.87 1.00 0.15 1.00 3 -78 0.031 0.71 0.26 0.84 1.03 0.81 0.16 0.97 4 -40 0.034 0.66 0.30 0.82 0.99 0.82 0.19 0.95 5 -23 0.027 0.64 0.33 0.81 0.94 0.86 0.20 0.91 6 0 0.054 0.61 0.34 0.78 1.03 0.76 0.22 0.99 7 25 0.046 0.58 0.38 0.77 0.95 0.80 0.25 0.93 8 45 0.054 0.56 0.39 0.75 0.97 0.78 0.26 1.04 Chain End No.. fr*e 0.89 0.87 0.87 0.86 0.89 0.79 0.83 0.72 frm*a 0.11 0.13 0.14 0.14 0.11 0.21 0.17 0.28 fmr*a 0.12 0.14 0.15 0.15 0.19 0.20 0.21 0.18 frr*a 0.77 0.74 0.72 0.72 0.70 0.59 0.62 0.54 P*rmf 0.13 0.16 0.17 0.17 0.22 0.25 0.25 0.25 aObtained directly from 13C NMR; calculated from equation (3-9); Calculated from equation (3-10); calculated from equation (3-11); calculated from chain end triads as frr* + fmr* and equation (3-8); calculated from equation (3-12); P'mr = 1 since frm* = fm*, no detectable mm* signal. 81 The fraction of racemic dyads in the main chain (fr) agrees very well with that of the chain end (fm*) at all temperatures, indicating consistency with Bernoullian propagation statistics. In addition, the conditional first order Markoff probability (P*rm) calculated from the chain end stereochemistry is in excellent agreement with that from the main chain (Prm), indicating consistency with first order Markoff chain propagation statistics. P*mr could not be determined accurately since there was no detectable end mm* triad signal (equation 3-8). The temperature of methylation was either -780C or OoC. This temperature had no effect on the chain end tacticity but it had a slight effect on the stereochemistry of methylation. The latter is given by the ratio of the peak areas at 29-30 ppm to that at 23-24 ppm, corresponding to the two diastereotopic methyl groups. This ratio at -780C is 5.70 while at 0OC, it is 3.33. The increase in stereoselectivity with decreasing temperature is normal and expected. Comparison with the analogous model oligomers of 2- isopropenylpyridine58,59 indicate that the most shielded position at 23-24 ppm should correspond to the (a) methyl group with the (b) methyl group absorbing at 29-30 ppm (Fig. 3-12). (a) 03 C-13 R -i3 13, i--- I II I I a3 R R 0-1 R (b) mr* R = -CO2CH3 Fig. 3-12. The two diastereotopic methyl groups at the chain-end of PMMA. (a) meso-like (b) racemic-like. Thus the methylation appears to have the same preferred stereochemistry as the addition of monomer. Both occur predominantly in syndiotactic-like manner. It appears from our method of comparison of main chain and chain end stereochemistry, that GTP of MMA is indeed is consistent with a Bernoullian process. These results are indeed surprising in view of the reasons cited earlier. The results appear to conform to the limiting cases in the E-Z model (Table 3-5). One possibility is that the stereochemical behavior of the E and Z sites are identical (limiting condition 1). Another possibility is that the rate of s- trans monomer addition to the E isomer is identical to the rate of s-cis addition to the Z isomer or vice versa (limiting condition 2). The other limiting condition reducing the E-Z scheme to Bernoullian sites, namely the presence of only Z or E sites appears unlikely in this case since a 70/30 E/Z mixture of silyl ketene acetals is always found in the GTP of MMA. In conclusion, it has been shown that the stereochemisty of the GTP of MMA is consistent with simple Bernoullian statistics. In addition, it has been shown that a comparison of the stereochemistry of the main chain with that of the chain-end of a polymer is a sensitive and independent method for the verification of statistics of vinyl polymerization and is applicable to systems other than anionic polymerization. Side Reactions in Group Transfer Polymerization Considerable evidence has been given for the occurence of side reactions in GTP. For example, in the GTP of MMA, low monomer conversions were encountered in cases where MMA was added to a mixture of initiator and catalyst at temperatures 0oC and higher. Also, in the GTP of TrMA (Chapter IV) it was not possible to methylate the chain end. All these are illustrative of side reactions involving the silyl ketene acetal functionality and catalyst, leading to the destruction of the living character of the chain-end of the polymer or even the ketene acetal end group. In this section, the results are described for an experiment where the GTP initiator (1) and TASHF2 catalyst are mixed together in the absence of monomer, and changes are followed by NMR. The purpose of this experiment was to see what side products could be formed by reaction of the initiator and TASHF2 catalyst. A 11:1 (mole ratio) mixture of the initiator and TASHF2 catalyst respectively were mixed together in vacuo at -780C in CD3CN (The catalyst is insoluble in THF). The mixture was poured into an NMR tube after filtration in vacuo through a coarse glass frit and sealed. The course of the reaction was followed by 1H NMR. Figure 3-13 shows the 1H NMR spectrum of the GTP initiator alone and the spectra of the reaction mixture after approximately 45, 75 and 120 minutes at room temperature. The initiator is seen to disappear with time as can be seen by the diminishing intensity of the two singlets at 1.6 ppm and the methoxy signal at 3.45 ppm. Additional peaks are seen to be appearing at 0- 0.3, 1-1.5, 2.5, and 3.6-3.7 ppm regions. The TASHF2 catalyst has only one absorbance at about 3 ppm (in the entire region of the spectrum shown), corresponding to the N-CH3 protons of the TAS moiety. Comparison of the final 1H and 13C NMR spectra of the product mixtures at the completion of the reaction with both 1H and 13C NMR spectra of authentic methyl isobutyrate indicates one of the reaction products to be methyl isobutyrate in approximately an 85:15 mixture of of deuterated and nondeuterated compounds. The presence of methyl isobutyrate was also confirmed by a GC examination of the product mixtures. CH3 OCH3 CH, OSi(CH3)3 (d) (a) (c) (d) , I. ..1 I .I .I ..... 4 J N(CH3)2 + (CH3)2N-S + I N(CH3)2 (e) (a) 0 II (CH3)2C-C-OCH3 (1) D(H) (9) (h) (iii) ) K 4 (ii) (g) (f) (c) (d) i a (iv) (g) U~l 1 I 60 MHz 1H NMR spectrum of the GTP initiator 1 and TASHF2 catalyst and initiator mixture (1:11 molar ratio) after various time intervals. (i) the initiator, (ii) reaction mixture after 45 mins., (iii) reaction mixture after 75 mins., (iv) reaction mixture after 120 mins. Fig. 3-13. -- "'"''" Al The mechanism of deuteration is undoubtedly complex owing to the presence of a number of compounds and although a detailed mechanism cannot be given at this time, the experiment does demonstrate that there is indeed a reaction between the initiator and catalyst, leading to the complete destruction of the silyl ketene acetal group (by deuteration in this case). In addition, since the initiator was in approximately 11 fold excess with respect to the catalyst, and none of it remained at the end of the reaction, there is a strong possibility of the polymerization catalyst acting as a deuteration (protonation) catalyst as well. Although several mechanisms may be possible, some of the possible steps are given in Fig. 3-14. A likely first step is the formation of catalyst- initiator complex, which is the same as the step involving initiator activation in the polymerization of MMA (step 1).1 This pentacoordinate species may then abstract a D+ ion from CD3CN (pKa = 25) to give deuterated methyl isobutyrate in a number of ways (steps 2,3 and 5). The formation of 15% nondeuterated methyl isobutyrate may result from the protonation of the initiator (step 6) with HF formed in the second step or from the protonation of the TAS enolate ( step 7). It was not possible to arrive at a detailed mechanism for this complicated system in the context of this research. The identity of any species other than methyl isobutyrate could not be confirmed spectroscopically, due to the complexity of the 1H and 13C NMR spectra resulting from the presence of a number of compounds obtained at the end of the reaction. The catalyst destruction also could not be observed as there was only one absorption for the TAS moiety in both the 1H and the 13C NMR, corresponding to the N-methyl group, and the intensity of this absorption did not change with time. CH3 OCH3 1) + TASHF2 CH, OSi(CH 3)3 CH3 OCH3 2)--c0 TAS* 2) CH3, / -SIMe3 F CD2CN q CH3 OCH3 S0 TASc CH3 OSi(CH3)3 HF - CD(CH3)2CO2CH3+ (CH3)3SiF + CD2HCN + TASF CH, OCH3 )CH3 M 3) C'g J ^SiMe, r FHF D CD2CN 4) TASF + 1 - AS+ C 0 S- NCxOCD- SiMe3 L FHF TAS+ --TAS-CD2CN- I + HF + Me3SiF CD(CH3)CO02CH3 CH3 OCHA CH3, 0O TAS* + Si(CH 3)3F CH3 OCH3 5) CH + CD3CN -TASCD2CN + CD(CH 3)20C0CH3 CH3 O- TAS CH3 OCH3 6) >-H< + HF -- CH3 OSi(CH3)3 CH3 OCH3 CH7) OHa 0- (CH3)2CHCO2CH3 + SIMe3F + HF (CH3)2CHCO2CH3 + TASF- TAS* HF + TAS*CDCN- TAS-F- + CHD2CN Fig. 3-14. Possible steps in mechanism of deuteration of GTP initiator in the presence of TASHF2 in CD3CN. CHAPTER IV GROUP TRANSFER POLYMERIZATION OF DIPHENYLMETHYL AND TRIPHENYLMETHYL METHACRYLATES Background The stereospecific polymerization of various methacrylate monomers has been studied extensively.60 Among these the polymerization of triphenylmethyl methacrylate (TrMA) is of special interest in that its polymerization brings about some most unusual and unexpected results. This monomer forms at -780C highly isotactic polymers (> 90%) with n- Butyllithium (BuLi) not only in toluene but also in THF.11,38 Even the radical polymerization of this monomer at 600C gives a highly isotactic (= 60%) polymer.11,38,61,62 This unique nature of this polymerization has been ascribed to the bulky trityl group and its interaction with the polymer backbone. The large ester group also affects greatly its reactivity in the alkyl lithium initiated copolymerization with other methacrylates, such as methyl methacrylate (MMA) or alpha-methylbenzyl methacrylate (MBMA) in toluene.63-65 Some of the copolymers of (S)-MBMA and TrMA show large positive optical rotations, which are opposite in sign to the homopolymers of (S)-MBMA. This positive rotation is attributed to the isotactic sequence of TrMA units which has helical conformation spiraled in a single screw sense.66-68 It has also been found by Okamoto and coworkers that polymerization of TrMA with chiral anionic initiators also gives an optically active polymer, the chirality of which is caused by helicity.69 This was the first 88 example of an optically active vinyl polymer the optical activity of which arose only from the helicity of the chains. In contrast to trityl methacrylate, the anionic polymerization of diphenylmethyl methacrylate (DMA) and benzyl methacrylate (BMA) in THF using BuLi yields predominantly syndiotactic polymer.38 The tacticity results are generally very similar to MMA. As group transfer polymerization is a new technique for polymerization of acrylates and methacrylates, it was of great interest to look at the tacticity of poly TrMA (PTrMA) and poly DMA (PDMA) prepared by GTP and compare the results with those obtained from anionic and radical polymerizations. Thus the group transfer polymerization of MMA, DMA and TrMA would constitute a systematic study concerning the effect of the ester group on the tacticity of the polymer. Group Transfer Polymerization of TrMA Initially, the GTP of TrMA was attempted under conditions similar to those used for MMA at temperatures higher than OOC, i.e. slow addition of monomer and intiator mixture into the catalyst suspension in THF at -780C. At initiator to catalyst concentration ratio of about 78, virtually no polymer formed, and all of the unreacted monomer was recovered, as judged by 1H NMR. This indicated that conditions different from those of GTP of MMA had to be employed for the polymerization of TrMA. The first successful polymerimation resulted by using a molar ratio of initiator to catalyst of about one. The GTP of TrMA is extremely fast. Even at -70oC, complete monomer conversion was observed in less than one minute. Since the initial failure of polymerization could have been due to insufficient catalyst concentration in the reaction mixture, a variety of catalyst concentrations were used to see what conditions 89 would bring about quantitative conversion of TrMA into polymer. The results of these experiments are summarized in Table 4-1. It is apparent from this table that much higher levels of catalyst are required (i.e. higher values of intiator to catalyst concentration ratios) for quantitative polymerization of TrMA than for the MMA polymerizations, where catalyst levels as low as 0.1 mole % with respect to initiator (methyl trimethyl silyl dimethyl ketene acetal) have been found to yield narrow MW distribution PMMA in quantitative yield. All of the polymerizations in Table 1(except run number 17 where a mixture of monomer and initiator were added to the catalyst) were batch polymerizations in which the catalyst was added at one time as an acetonitrile solution (0.1-1.0 mL depending on the mole amount of initiator) to a mixture of monomer and intitator in THF. Several conclusions can be drawn from the data in Table 1. The molecular weight control appears very poor in that it far exceeds the expected molecular weight, indicating to poor initiator efficiency, which varies from 11-53%. There does appear to be some internal consistency however, in that the agreement between expected and actual MW becomes poorer with increasing degree of polymerization. The poor initiator efficiency indicates that not all of the initiator is used to initiate polymerization. It is somewhat surprising however, that the MW distribution are fairly narrow despite the poor initiator efficiencies. This means that initiation is indeed fast compared to propagation but that much of it is destroyed, before initiation. This is most likely due to side reactions involving the initiator and catalyst. The rates of side reaction and initiation must be more competitive in this case compared to MMA polymerization, where there usually was good agreement between expected and actual Mn. It is not surprising that the termination reactions are more severe in the case of the TrMA polymerization than in the Table 4-1. SEC Results of Group Transfer Polymerization of TrMA. Run No. Temp Catalyst mole. init. [Init.] Yield Mwe Mne Mw Calcul. Mf (OC) X 10-3 %_ [Cat.] Mn 1 -97 Aa 0.25 1.0 > 95 4105 3196 1.28 1337 2 -95 0.70 > 95 3027 2695 1.12 1437 3 -82 0.46 > 95 4442 3642 1.21 1870 4 -70 0.29 > 95 39564 26870 1.47 2930 5 -70 0.31 > 95 3391 3094 1.11 1313 6 -70 0.21 2.6 >95 14103 11233 1.26 3030 7 -70 0.27 93.0 =30 31787 21586 1.47 2881 8 -42 0.49 1.0 > 95% 4218 3458 1.22 1440 9 -21 0.37 > 95% 3223 2768 1.16 678 10 0 1.39 >95% 8655 6953 1.24 850 11 0 0.57 >95% 8088 4951 1.63 1130 12 25 0.32 >95% 28423 15134 1.88 2720 13 25 0.69 70 9839 5846 1.68 966 14 35 0.49 <5 32317 11309 2.86 1000 15 50 0.98 <5 25705 8137 3.16 643 16 -70 Bb 0.55 1.0 100 10910 8554 1.28 1231 17 -70 Bb 0.73 78.0 <5 - 18 -70 Cc 0.61 1.0 =10 ND9 ND ND ND 19 -70 Dd 0.18 1.0 -15 NDh ND ND ND aTASSiMe3F2, Cn-Bu4NF, bTASHF2, dn-Bu40Ac, molecular weight of the transesterified PMMA samples, calculated from the mole ratio of monomer to initiator, sample not transesterified but the elution volume of the poly (TrMA) corresponded to an approximate molecular weight of 199000 based on PMMA calibration curve, Mw/Mn > 2.0. sample not transesterified but the elution volume of the poly (TrMA) corresponded to a molecular weight of approximately 126000 based on PMMA calibration curve, Mw/Mn = 1.5 91 case of MMA since in the latter case, it is reported that it is best to use the minimum amount of catalyst in order to avoid termination reactions and to obtain polymers of lowest polydispersity, particularly when preparing polymers of Mn above about 20,000. Also in MMA polymerizations using a high level of catalyst, it was found that the addition of additional monomer after 30 minutes to a "living GTP PMMA", gave no polymerization, indicating that the polymer formed in the presence of a high level of catalyst was no longer living. Since a high level of catalyst is necessary to obtain complete conversion of monomer to polymer in the case of TrMA, it is not surprising that the polymerization is actually self-terminating. The self-termination was demonstrated by the addition of additional monomer (TrMA) 30 mins. after an initial polymerization of TrMA at -70oC. The second batch of polymer failed to polymerize. The rates of side reactions competing with initiation increases with temperature as was demonstrated during attempted polymerizations at temperatures higher than 25oC. Thus attempted polymerizations at 350C and 50oC, resulted in complete failure, with virtually all the monomer recovered. The failure of the polymerization is apparently due to a lack of initiation caused by the destruction of the silyl ketene acetal initiator. This was demonstrated by cooling the reaction mixture of an attempted 50oC polymerization down to OoC, and adding an additional amount of initiator. In this case, all the monomer polymerized immediately and the color also changed to dark orange. Thus it appears that although at higher temperatures, initiation is inhibited due to destruction of the initiator, there is still sufficient active catalyst left capable of catalyzing the polymerization upon the addition of additional initiator at lower temperature. This indicates that the destruction of the initiator may not be due to a 1:1 equimolar reaction between 92 the initiator and the catalyst. This is completely consistent with our earlier investigations into the side reactions involving the initiator and TASHF2 catalyst in CD3CN at room temperature without the presence of monomer. In this case, it was found that even though a large excess of initiator over catalyst (11:1 mole ratio) was present in the reaction, all the initiator dissapeared in approximately two hours after the start of reaction. Various color changes were seen in the case when the initiator was added at OoC to to the reaction mixture of an unsuccessful 50oC attempted polymerization. As more and more initiator was added drop by drop, the color changed from slight yellow to orange to dark brown to red and finally to green in gradual progression. It was a colorful experiment to say the least! The reasons for these color changes are not entirely clear at present, but could represent reactions involving the initiator catalyst complex (2) or enolate species with a TAS counterion (such as 42 generated in equation (4-1)): CH3 OSI(CH3)3 CO TAS (1 > (4-1) Such an enolate species, could eliminate a methoxide ion to yield a ketene, 43 in a side reaction (equation 4-2). It is to be noted however, that no direct evidence was found for the presence of the ketene; such reactions however, are plausible on the basis of literature findings.28 C000- TAS+ TSC -c O + TAS OMe > = OMe 43 (4-2) Good catalytic activity was expected from NBu4F (Aldrich; containing less than 5 weight % water) on account of its being a highly efficient source of nucleophilic fluoride ion, and its use in the GTP of MMA. However even one equivalent of NBu4F with respect ot initiator was incapable of catalyzing the polymerization to complete monomer conversion (run no. 18, Table 4-1). Undoubtedly, the reagent may still contain moisture which could terminate the polymerization. When halides or tosylates were treated with "anhydrous TBAF" (i.e. NBu4F, 3 H20 warmed at 400C under vacuum for several hours) in the absence of any solvent, hydrolysis to the corresponding alcohol of the tosylate appeared to be a significant side reaction.70 This was explained by the presence of traces of moisture remaining in the "anhydrous" TBAF which are rendered highly nucleophilic by the fluoride ion. However, it is probably unlikely that the presence of water was the reason for the lack of activity of NBu4F. If that were the case, it would not be effective in GTP of MMA either. It is therefore not entirely clear why this catalyst failed to yield a good yield of poly(TrMA), based on the conventional associaive mechanism of GTP. Another catayst, NBu40AC, known to catalyse the GTP of MMA failed to effectively catalyse the polymerization of TrMA in good yield (run no. 19). These findings hint at the possibility of a mechanism for the GTP of TrMA different from that postulated by the Dupont group for MMA polymerization. |