Living polymerization of aryl substituted acetylenes by MoCl5 and WCl6 based initiators: The ortho...

Living Polymerization of Aryl Substituted Acetylenes by MoCl, and WCl, Based

Initiators: The Ortho Phenyl Substituent Effect

JAY KUNZLER and VIRGIL PERCEC,* Department of Macromolecular Science, Case Western Reserve University,

Cleveland, Ohio 441 06-2699

Synopsis

The polymerization of phenylacetylene initiated by MoCl, and WC1, based initiators was monitored directly in the NMR sample tube and demonstrated the presence of backbiting and intramolecular cyclization reactions. It was shown that the ratio of cis to trans structural units obtained by isomerization prior to double bond formation dictates the degree of backbiting and intramolecular cyclization reactions. This cis-trans ratio determines the length of cis-transoidal sequences present in the polymer backbone which are available for both backbiting and intrachain cyclization reactions. The cyclic trimers obtained in the metathesis polymerization of phenylacetylene are formed only through the cis-cisoidal-induced backbiting and/or intramolecular reactions. The o-trimethylsilylphenylacetylene follows a living mechanism of polymerization. This is due to the fact that the size of the ortho substituent suppresses the cis-transoidal to cis-cisoidal isomerization reactions and therefore eliminates the backbiting reactions. The steric hindrance provided by the size of the ortho substituent also eliminates interchain and intrachain reactions.

INTRODUCTION

The first homolog of the aryl acetylenes, phenylacetylene, was first polymerized by Natta in 1955 using Ziegler-Natta coordinative catalysts.' Since then phenylacetylene has also been shown to polymerize by ~ a t i o n i c , ~ . ~ thermal,4*5 r a d i ~ a l , ~ - ~ and transition metal halide based catalysts which are considered to promote polymerization by a metathesis mechanism?-" How- ever, only the coordinative and metathesis catalysts have resulted in high yields of high molecular weight polymers.

The coordinative polymerization of phenylacetylene has been extensively studied."-14 Unlike polyacetylene, polyphenylacetylene possesses several conformational isomers which are soluble in organic solvents. As a result, conformational analysis of polyphenylacetylene could be studied by NMR spectroscopy. This has led to a clearer understanding of phenylacetylene polymerization and polyphenylacetylene micr~structure.'~~ 15-18 Studies have shown that the opening of the triple bond occurs in the cis position and that trans sequences occur by thermal isomerization. The microstructure, as well as the physical properties of the purified polymer, are dependent on the thermal history of the polymer preparati~n.'~-'' Two geometric isomers are obtained

*To whom correspondence should be addressed.

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 28, 1221-1236 (1990) 0 1990 John Wiley & Sons, Inc. CCC 0887 -624X/90/05122 1 - 16$04.OO

1222 KUNZLER AND PERCEC

when using Ziegler-Natta coordinative catalysts under conditions when the thermal effects responsible for thermally induced isomerization reactions are removed: cis-cisoidal and cis-transoidal. The cis-cisoidal isomer is thought to occur through the isomerization of the cis-transoidal isomer during propagation. The thermally-induced isomerization reaction leads to trans structural units which appear mostly by the double bonds migration which accompanies an intramolecular cyclization reaction.

Ph

The thermal effects arise from either (a) the heat of polymerization (b), the polymerization temperature, or (c) degradation of the catalyst. Monitoring the thermal isomerization of a purified polymer by NMR spectroscopy results in a decrease in the cis-polyenic proton signal and the appearance of the 2,4,6-pro- tons of 1,3,5-triphenylbenzene and the cyclohexadiene unit signals. This was proof for a concurrent cyclization and chain scission reaction during the thermal cis-trans isomerization. A mechanism was proposed to explain the chain scission and cyclization products.14

In 1974 Higashimura and Masuda first polymerized phenylacetylene using transition metal halide catalysts? The mechanism of polymerization is believed to involve carbene initiation followed by propagation through metal carbenes and cyclometallabutene intermediates as shown below1’- 28:

\ /R

RC /r” ,c-c

\ H R

tram

The polymer microstructure is determined by two mechanisms: isomerization prior to double bond formation, and thermal isomerization after double

THE ORTHO PHENYL SUBSTITUENT EFFECT 1223

bond formation. The former, unlike in Ziegler-Natta polymerizations, can result in cis-transoidal or trans-cisoidal isomers depending on the mode of cyclometallabutene ~pening.”-~’ During this step the C3 - C2 bond rotates around its axis to form a coplanar double bond. The polymer configuration is determined by this rotation. The C3 - C2 bond will rotate in a manner which will minimize the steric interaction between the coordinated metal and the R substituent of the polymer backbone. This step is dependent on the solvent nature (solvation of metal complex) and the size of the transition metal. The latter, similar to the case of Ziegler-Natta polymerizations, results in the cis isomerization to trans and 1,3-cyclohexadiene sequences. The cyclohexadiene structures can undergo further rearrangement leading to chain scission and aromatization p r o d ~ c t s . ~ ~ - ~ ~ However, independent of the polymerization conditions used, no cis-cisoidal polyphenylacetylene could be synthesized by using metathesis initiators.*-”? 14,18,29-31 No explanation for this difference between the structural isomers of polyphenylacetylene generated from Ziegler-Natta and metathesis polymerizations could be advanced so far.

The two primary mechanisms responsible for polymer degradation during the polymerization of phenylacetylene using metathesis initiators are the intrachain backbiting of a propagating carbene to form 1,3,5- and 1,2,4- triphenylbenzene derivatives, and the intramolecular cyclization of cis se- q u e n c e ~ . ~ ~ - ~ ~ Trans sequences can not lead to intrachain backbiting and intramolecular cyclization reactions. Consequently, when the trans polymer sequences are obtained prior to double bond formation, very high molecular weight polymers will result by suppressing cis-induced backbiting and cyclization reactions, i.e., k,, approaches 0. This, however, does not preclude interchain reactions.

Previous work has shown that in the polymerization of alkyl substituted acetylenic monomers initiated with metathesis initiators, living polymers could be ~ b t a i n e d . ~ ~ - ~ ~ The living nature of these polymers is thought to be due to the steric nature of the backbone where an optimum substituent size is required to suppress interchain and intrachain termination and transfer reactions, i.e., k, , , k , = 0. The large substituents suppress the isomerization of cis-transoidal sequences to cis-cisoidal sequences and subsequently propagating carbene interchain and intrachain reactions are also ~uppressed .~~ Re- cently, i t has been shown that the o-trimethylsilylphenylacetylene follows a living mechanism of p~lymerization.~~

This paper will summarize our results on the polymerization of phenylacetylene and ortho-substituted phenylacetylenes and will demonstrate that bulky substituents in the ortho position of the phenyl ring eliminate the intramolecular cyclization reactions and suppress transfer and termination reactions leading to a living mechanism of polymerization.

EXPERIMENTAL

Materials

Phenylacetylene was purchased from Farchan Chem. Go. The o-methylphenylacetylene was supplied by Prof. G. Wegner of the Max-Planck Insti- tute, Mainz, West Germany. All monomers were distilled from calcium hydride prior to use. Purity of the phenylacetylene was 99.2% (GC) and of


theo-methylphenylacetylene was 85.4% pure (GC). MoCl, and WC1, (99.9 + %) were purchased from Aldrich Chem. Co. and used without further purifica- tion. Tetramethyltin, (CH,),Sn, (Aldrich) was stored over activated 4 A molecular sieves prior to use. Toluene used as polymerization solvent was first refluxed over sodium under argon and then distilled. All other solvents and reagents were purified by standard methods.

Techniques

The relative molecular weights of polymers were determined by gel perme- ation chromatography (GPC) with a Perkin-Elmer Series 10 LC equipped with a Nelson Analytical 900 Series data station. The measurements were made using THF as solvent (1 mL/min at 40°C) with a series of PL-gel columns (lo2, 5 x lo2, lo3, lo4 and lo5 A) and a calibration plot constructed with polystyrene standards. Monomer conversions and purity were determined on a Hewlett-Packard HP 5890 GC using a 6.1 x 0.32 cm column of 10% SP-1000 8 0 / l O O Supelcoport. Monomer and polymer structures were confirmed by an XL 200 Varian 200MHz spectrophotometer. UV spectra were recorded from CC1 solutions on Perkin-Elmer Lambda 9 W-Visible Spectrophotometer a t 25°C.

Synthesis of o-trimethylsilylphenylacetylene

The o-trimethylsilylphenylacetylene was prepared according to a literature pr~cedure.~' This involved the dimetallation of phenylacetylene (0.1 mol) using a mixture of potassium t-butoxide (0.11 mol) and butyllithium (0.22 mol) (1M in hexane) at - 20°C for 2 h in 100 mL of dry diethylether. Addition of trimethylchlorosilane (0.3 mol) gave trimethylsilyl(2-trimethyl- silylpheny1)acetylene. Deprotection using a catalytic amount of sodium hy- droxide in ethanol, followed by distillation, afforded a 90% yield (98% pure by GC) of o-trimethylsilylphenylacetylene: 'H-NMR (CDCl,, 6, TMS): 0.25 (s, 9H, -Si(CH,),), 3.05 (s, lH, HC'-C-), 6.9 and 7.2 (2m, 5H, ArH).

Synthesis of Polymers

MoCL, Initiator

In a typical polymerization experiment 0.027 g (0.1 mmol) MoCl, was added under an argon atmosphere to 10 mL of dried toluene at room temperature in a flame dried 25 mL Schlenk tube. Following 20 min of rapid stirring, the acetylene monomer was added (amount determined by desired [MI/[ I] ratio). The reaction mixture was stirred for 24 h. A t various reaction intervals, aliquots were removed and added to methanol. The obtained polymer was evaluated for molecular weight by GPC and the resultant solution was monitored for monomer conversion by GC.

MoCL5/(CH3),Sn Initiator

In a typical polymerization experiment 0.027 g (0.1 mmol) MoCl, was added to 10 mL of dried toluene under an argon atmosphere at room temperature in a dried 25 mL Schlenk tube and allowed to dissolve with vigorous stirring. To


this reaction mixture was added 14 pl(0.1 mmol) of (CH,),Sn. Following a 20 min reaction time, the acetylenic monomer ([M]/[I] = 50) was added and the polymerization reaction was monitored as above.

MoCl, /(CH,),Sn / EtOH Initiator

In a typical polymerization, 0.027 g (0.1 mmol) MoCl, was added to 10 mL of dried toluene in a 25 mL Schlenk tube under an argon atmosphere and allowed to dissolve with vigorous stirring. To this reaction mixture was added 3 pL of 100 proof ethanol (0.05 mmol) and after a 5 min reaction time, 14 pL (0.1 mmol) of (CH3),Sn was added. After an additional 20 min of reaction, the acetylenic monomer ([M]/[I] = 50) was added and the polymerization reaction was monitored as previously described.

MoCl, Initiator Polymerization Monitoring in NMR Sample Tube

In a typical polymerization, 0.5 mL of alumina dried d,-benzene was added to a flame dried NMR tube under an argon atmosphere. MoCl, (0.027 g, 0.1 mmol) was added to this solution and allowed to dissolve for 20 min. Phenyl- acetylene (51 pL, 5.0 mmol) was added to the reaction mixture and the NMR sample tube was immediately placed in the NMR instrument. Polymerization monitoring was performed by 200 MHz ‘H-NMR spectroscopy using an XL 200 Varian spectrometer (spectra width 5000 Hz, 16 transients with an acquisition time of 2 s, time between acquisition of 1 s and 20K FID data points).

RESULTS AND DISCUSSION

As described previ~usly,~’ there are three approaches to the development of the living polymerization of acetylenes. The first is to design a preformed metal carbene initiator complex whose reactivity can be controlled by the choice of ligands and carbene substituent. This involves the preparation of preformed metal carbenes of varying reactivity to study the polymerization of a particular acetylenic monomer. The ideal metal carbene will provide a higher rate constant of initiation than the rate constant of propagation. The growing metal carbene should be unreactive to transfer and termination reactions with the polymer backbone.

The second approach to the living polymerization of acetylenes involves the use of a single, highly active generated carbene complex while varying the size and polarizability of the acetylene monomer substituents. An optimum substituent size along the polymer backbone suppresses the termination and transfer reactions of the growing propagating carbene complex.

The third approach, which is of limited scope, is to prepare polyacetylenes having rigid rod-like conformations. This can theoretically be accomplished by the appropriate combination of acetylene monomer substituent, initiator structure, and polymerization solvent. The rigid rod-like structures will suppress the concentration of cis-cisoidal configurations and thus suppress the cis-induced termination and transfer reactions. The intrachain termination and transfer reactions of the growing propagating metal carbene occur by the backbiting and intramolecular cyclization reactions of a cis-cisoidal propagat-


ing chain. This, however, does not preclude interchain termination and transfer reactions.

It has been shown that the MoC1,-initiated polymerization of phenylacetylene leads to polymers containing higher concentrations of cis sequences than the WC1,-initiated polymerization of phenyla~etylene.~'-~~ The higher the trans content prior to double bond formation, the higher the polymer molecular weight. Trans sequences can not lead to intramolecular cyclization and backbiting reactions. This has been shown experimentally. The WC1 ,-initiated polymerization of phenylacetylene in dioxane results in an almost all trans structure (as determined by NMR) and affords the highest reported molecular weight obtained for polyphenylacetylene (ii?, = 90,000).40 The MoCl ,-initiated polymerization of phenylacetylene in dioxane, however, results in a low yield of low molecular weight polymer with undetermined ~tructure.~' The high molecular weight for the dioxane/WCl, polymerization is due to the lack of cis sequences which promote backbiting and intramolecular cyclization reactions. This polymerization, however, does not follow a living polymerization mechanism. The WCl,/dioxane-initiated polymerization of phenylacetylene when monitored for number average molecular weight and conversion results in a nonlinear plot.40 An initial rise in molecular weight followed by a leveling-off occurs. A linear relationship is indicative of a living polymerization. The deviation from linearity is probably the result of interchain termination and transfer reactions.

The polymerization of o-trimethylsilylphenylacetylene using metathesis initiators has recently been shown to follow a living mechanism of polymerization.,, Monitoring the polymerization for number average molecular weight versus conversion resulted in a linear relationship. At 100% conversion of the monomer, a second equivalent of monomer was added to the reaction mixture and the molecular weight-conversion dependence remained linear for this second polymerization process. No details on the microstructure or mechanis- tic explanation for the living nature of this polymerization were given.

Polymerization studies were performed for phenylacetylene, o-methylphenylacetylene and o-trimethylsilylphenylacetylene using metal halide based initiators. These initiators included MoCl ,, MoCl,/(CH,),Sn/EtOH, and WCl,/(CH,),Sn. These initiation systems have been described in a previous report .,'

The dependence between molecular weight and conversion for the MoC1,- initiated polymerization of phenylacetylene is shown in Figure 1. This plot illustrates a classical example of a nonliving polymerization. Monitoring the polymerization for number-average molecular weight and conversion resulted in deviation from linearity a t low conversion (10-25%). An initial rise in molecular weight followed by a leveling-off and decrease occurred. The polymer polydispersity decreases with monomer conversion. At 100% conversion of monomer, a second portion of monomer was added to the reaction mixture and the polymerization monitoring continued. An initial rise followed by a decrease in molecular weight was again observed. A 15% conversion of monomer after an additional 10 h reaction period was observed. Table I summarizes reaction time, conversion, and molecular weight data for the MoCl ,/phenylacetylene polymerization.


7.5- 0

x 5 . -

&

25 50 75 100 CONVERSION (O/o)

Fig. 1. The dependence of number-average molecular weight versus conversion for the MoCI,- initiated polymerization of phenylacetylene. Polymerization solvent, toluene; polymerization temperature, 25°C; monomer concentration, [MI, = 1 mol/L; initiator concentration, [MoCl,] =

20 mmol/L.

The analysis of the polymerization reaction by 'H-NMR spectroscopy has provided further insight into this polymerization. The 'H-NMR spectra of the MoCl ,-initiated polymerization of phenylacetylene directly in the NMR sample tube are shown in Figure 2(a)-(c). The formation of cis-transoidal polyphenylacetylene (6 = 6.2)14 and 1,3,5-triphenylbenzene ( 6 = 7.6) was observed [Fig. 2(a)] at low conversion of phenylacetylene monomer. The two singlets a t S = 5.4 and 5.4 are of unknown origin. A t 70% conversion of phenylacetylene monomer [Fig. 2(b)], a significant decrease in the concentration of cis-transoidal polyphenylacetylene structural units and an increase in the concentration of 1,3,5-triphenylbenzene was observed. In addition, a broad singlet a t S = 3.5-4.5 appeared. This corresponds to the methynic proton signal arising from the 1,3- and/or 1,4-~yclohexadiene structures. The concentration of cis-polyphenylacetylene continued to decrease and the concentration of 1,3,5-triphenylbenzene continued to increase with monomer conversion. It reached a maximum concentration a t complete monomer conversion. No

TABLE I Polymerization of Phenylacetylene Initiated by MoCI,"

Reaction time Conversion (h) ( W ) Mw Mn Mw/Mn

0.02 0.08 0.17 1 .oo 3.00

17.50 18.50 27.50

20.4 31.5 37.6 73.7 84.3

100.0 110.0 115.0

10,537 14,142 13,341 17,055 15,808 8,725

13,000 11,446

2,630 4,146 4,075 6,400 6,137 3,853 5,400 5,038

4.0 3.4 3.3 2.7 2.6 2.3 2.3 2.3

"Polymerization solvent, toluene; polymerization temperature, 25°C; monomer concentration,

'Second monomer feed. [MI, = 1.0 mol/L; MoCI, concentration, [Cat] = 20 mmol/L.


C -

b -

A

L

h

r tl 10 9 8 T I 5 4 s 2 I 0

6 , P P ~

Fig. 2. 'H-NMR monitoring (polymerization solvent, C6D6) of the MoCl,-initiated polymerization of phenylacetylene showing (a) the formation of cis-polyphenylacetylene and the intramolecular cyclization products, 1,3,5-triphenylbenzene and cyclohexadiene sequences (1 h reaction time, 10% conversion); (b) the decrease in cis-polyphenylacetylene (5 h reaction time, 70% conversion); and (c) the complete loss of cis-polyphenylacetylene (19 h reaction time, 95% conversion).


increase in 1,3,5-triphenylbenzene was observed following complete monomer conversion. The concentration of cyclohexadiene seems to reach a maximum concentration at approximately 70% conversion and remains constant there- after. Figure 2(c) shows the 'H-NMR spectrum a t complete phenylacetylene conversion.

The decrease in cis concentration during the polymerization of phenylacetylene is a consequence of thermally induced double bond isomerizations and formation of cyclohexadiene sequences and 1,3,5-triphenylbenzene. The decrease in cis concentration corresponds with an increase in the concentration of 1,3,5-triphenylbenzene, the appearance of the cyclohexadiene signal, and a decrease in polymer molecular weight. The 1,3,5-triphenylbenzene can theoretically result from either a direct cyclotrimerization r e a c t i ~ n ~ l - ~ or from the backbiting of a cis-cisoidal growing chain. A metathesis mechanism can explain the formation of cyclic trimers through a backbiting mechanism only. The cyclohexadiene structures are believed to be the result of thermal-induced intramolecular cyclization reactions which can be accompanied by polymer chain scission. Therefore, these results provide strong evidence that the nonliving character of the MoCl,/phenylacetylene polymerization is the result of cis-cisoidal induced backbiting and intramolecular cyclization reactions, i.e., k , > k . The complete loss of the cis signal (19 h reaction time) was not surpr is ina

The mechanism for the formation of the 1,3-cyclohexadiene sequences is shown in Scheme l.14 The cyclohexadiene structure can on thermal treatment lead to polymer aromatization, chain scission and release of cyclic trimers. Another plausible mechanism to form an intramolecular cyclic having a methynic proton is presented in Scheme 2. This reaction involves a 477 + 277 Diels-Alder cyclization to form a tetraline-like backbone sequence and is similar to the Diels-Alder thermally induced dimerization of styrene according to the Mayo m e ~ h a n i s m . ~ ~ - ~ ~ This mechanism requires a cis-trans-polyphenylacetylene diad. Recent spectroscopic evidence which substantiates the Mayo mechanism has been provided.47 It was determined that the thermally induced Diels-Alder dimer of styrene resulted in the compound l-phenyl- 1,2,3,4-tetrahydronaphthalenyl. 'H-NMR spectroscopy of this compound show the presence of a benzylic proton a t S 3.8! These results provide evidence that the 6 3.5-4.5 signal observed during the polymerization of phenylacetylene may be a result of a Diels-Alder cycloaddition reaction.

The polymerization of o-methylphenylacetylene was monitored for molecular weight and conversion in a similar fashion. Figure 3 shows the number- average molecular weight-conversion plot obtained to complete monomer conversion. A linear molecular weight-conversion to approximately 60% conversion followed by a rapid decrease in molecular weight occurred. Compared to the phenylacetylene polymerization, this was a four-fold increase in molecular weight and three-fold increase in monomer conversion. The 'H-NMR spectrum of the purified poly( o-methylphenylacetylene) did not show the presence of cyclohexadiene sequences and only a moderate concentration of cis isomer (6 5.5) (Fig. 4).

Similar experiments were also performed for the polymerization of o-trimethylsilylphenylacetylene (Table 11). Figure 5 shows the linear dependence


H P h H h H P h H P h H P h

1 H Ph ti Ph H Ph

+ 2 C6Hs

'Ph P h w

1 Ph H Ph H Ph

I2

Scheme 1. Possible mechanism for the formation of cyclohexadiene sequences, 1,3,5-triphenylbenzene and chain scission products during the thermal isomerization of cis-transoidal-polyphenylacetylene.

of the number-average molecular weight versus conversion for a MoCl, initiated polymerization. The linear relationship is indicative of a living polymerization. This polymerization resulted in a quantitative yield of narrow polydispersity and high molecular weight polymer which did not undergo any changes in molecular weight if the polymer was left for several days in the polymerization system. Monitoring of this polymerization by 'H-NMR spectroscopy did not show the presence of intramolecular cyclization and backbiting products. Figure 6 shows a 'H-NMR spectrum of purified poly(o- trimethylsilylphenylacetylene) (MoC1,-initiated) showing a high concentration of cis isomer ( 8 = 6.0 signal). It is important to note that the conversion and product yields of the trimerization reaction of substituted phenylacetylene derivatives is not affected by the presence of large ortho substituents on the phenyl r i r ~ g . ~ ' - ~ ~ The trimerization of o-trimethylsilylphenylacetylene should theoretically result in high conversion and yields (using trimerization catalysts). Therefore, this provides strong evidence that the 1,3,5-triphenylbenzene is the result of an intramolecular cis-cisoidal backbiting product only.

Figure 7 shows the ultraviolet spectrum for polyphenylacetylene, poly( o- methylphenylacetylene), and poly( o-trimethylsilylphenylacetylene). A dra-


+ 2-c=c.

Ph

71

FZh + -c=c-c-c- Ph

Scheme 2. Possible mechanism for the formation of a cyclic structural unit sequence possess- ing a methynic proton which involves the intramolecular Diels-Alder cyclization reaction of a cis-trans-polyphenylacetylene diad. An ortho phenyl substituted similar diad will have a lower probability to undergo this reaction.

matic shift to higher wavenumber is observed with increasing size of the ortho substituent and is apparently due to a more conjugated backbone. This is in contrast to the polyalkylacetylenes where an increase in substituent size results in a decrease in conjugation length. The poly (o-trimethylsilylphenylacetylene) has a maximum absorption (535 nm) which approaches the maximum absorption for cis-polyacetylene (580 nm).48 Unlike polyacetylene, the poly( 0- trimethylsilylphenylacetylene) is readily soluble in aro- matic and halogenated solvents, has excellent film forming properties, and

rn I 0 X

li

30 i

20 -

* 0 50 100

CONVERSION ('lo)

Fig. 3. The dependence of number-average molecular weight versus conversion for the MoC1,- initiated polymerization of o-methylphenylacetylene. Polymerization solvent, toluene; polymerization temperature, 25°C; monomer concentration, [MI, = 1 mol/L; initiator concentration, [MoCI,~] = 20 mmol/L.


- I I I I I I 1 1

5 4 3 2 I 0 9 a r 6

6. PPO Fig. 4. H-NMR (CDCl,,, TMS) of poly(o-methylphenylacetylene) initiated with MoC1,.

Polymerization solvent, toluene; polymerization temperature, 25°C; monomer concentration, [MI, = 1 mol/L; initiator concentration, [MoCI,] = 20 mmol/L.

preliminary experiments indicate it possesses excellent thermal and oxidative stability. The maximum absorption for the MoC1,/(CH3),Sn/EtOH- and WC1,/(CH3),Sn-initiated poly( o-trimethylsilylphenylacetylene) are 535 and 518 nm, respectively. The maximum absorption corresponds with the relative concentration of cis isomer. The 'H-NMR spectrum of the purified poly ( o-trimethylsilylphenylacetylene) obtained using MoCl , had the highest concentration of cis isomer ( 6 6.0), the poly( o-trimethylsilylphenylacetylene) obtained using MoC1,/(CH3),Sn/EtOH the next highest and the poly( o-trimethylsilylphenylacetylene) obtained using WCl,/(CH,),Sn the lowest (all with Rn > 50000).

The microstructure of the polyphenylacetylene obtained by metathesis initiators is controlled by both isomerization prior to double bond formation

TABLE I1 Polymerization of o-Trimethylsilylphenylacetylene Initiated by MoClSa

Reaction time Conversion (h) @) MW MIl MW/M,

0.02 0.08 0.50 1.50 6.50

17.50

7.0 14,324 8,680 1.65 10.0 59,142 23,014 2.60 72.0 88,467 52,832 1.70 90.0 146,836 108,419 1.40

100.0 144,215 95,736 1.50 100.0 157,268 100,585 1.60

a Polymerization solvent, toluene; polymerization temperature, 25°C; monomer concentration, [MI, = 1.0 mol/L; MoCI, concentration, [Cat] = 20 mmol/L.


- - I

25 50 7 5 100

CONVERSION (Yo) (R w/Rn)

Fig. 5. The dependence of number-average molecular weight versus conversion for the MoCI,- initiated polymerization of o-trimethylsilylphenylacetylene. Polymerization solvent, toluene; polymerization temperature, 25OC; monomer concentration, [MI, = 1 mol/L; initiator concentration, [MoCI,] = 20 mmol/L. Data between parentheses represent MJM,.

as well as thermally induced reactions after double bond formation. The cis-trans ratio can be controlled prior to double bond formation by the choice of initiator and solvent. The cis-trans ratio obtained by isomerization prior to double bond formation dictates the possible degree of backbiting reactions which occur during cis-transoidal to cis-cisoidal isomerization taking place during the propagation process. It also dictates the length of cis-transoidal sequences present in the polymer backbone which are available for intrachain cyclization reactions. These reactions were shown spectroscopically by monitoring the polymerization directly in the NMR tube. An increase in size of the

ri

6. ppm Fig. 6. 'H-NMR spectrum (CDCI,, TMS) of poly( o-trimethylsilylphenylacetylene) obtained

by the polymerization of o-trimethylsilylphenylacetylene initiated with MoC1,. Polymerization solvent, toulene; polymerization temperature, 25°C; monomer concentration, [MI, = 1 mol/L; initiator concentration, [MoCl,] = 20 mmol/L.


7- H

\

0 ' 1 I I I I I I I I

400 800 700 aw WAVELENGTH [rial

Fig. 7. Ultraviolet spectra (solution in CCl,) for the purified polyphenylacetylene, poly( o- methylphenylacetylene, and poly(o-trimethylsilylphenylacetylene) prepared in toluene using MoCI, as initiator. Monomer concentration, [MI, = 1 mol/L; initiator concentration, [MoCI,] =

20 mmol/L.

ortho substituent suppresses transfer and termination reactions. The polymerization of o-methylphenylacetylene results in a near living-like polymerization. The o-trimethylsilylphenylacetylene, however, follows a living mechanism of polymerization. The large trimethylsilyl substituent dictates the microstructure prior to double bond and after double bond formation. The rotation of the C2-C3 axis is controlled by the size of the trimethylsilyl group. The trimethylsilyl substituent also suppresses the cis-transoidal to cis-cisoidal isomerization and therefore eliminates the backbiting reaction. Alternatively, i t could be that for steric reasons the cis-cisoidal sequence can not backbite.

The poly( 0- trimethylsilylphenylacetylene) is a highly conjugated polymer and as a result the n orbitals adopt a planar geometry. The planar structure maximizes the overlap of the p orbitals permitting strong electronic interac- tions over an array of several atoms. This electron delocalization results in a stabilized backbone of higher resonance energy. This energy may also suppress the termination and transfer reactions by lowering the backbone reactivity.

In conclusion, monitoring the polymerization of phenylacetylene directly in the NMR sample tube has provided evidence for backbiting and intramolecular cyclization reactions. It was shown that the cis-trans ratio obtained by isomerization prior to double-bond formation dictates the possible degree of backbiting reactions. These reactions occur during the cis-transoidal to cis-cisoidal isomerizations. The cis-trans ratio dictates the length of cis- transoidal sequences present in the polymer backbone which are available for intrachain cyclization reactions. This is why no cis-cisoidal sequences have been obtained by the metathesis polymerization of phenylacetylene. The


cyclic trimers formed in the metathesis polymerization of phenylacetylene are formed only through the cis-cisoidal induced backbiting and or intramolecular reactions. An increase in the size of the ortho substituent results in a living mechanism of polymerization. The size of the ortho substituent suppresses the cis-transoidal to cis-cisoidal isomerization reactions and the possible interchain reactions. No evidence of backbiting or cyclohexadiene structures were observed in the polymerization of o-trimethylsilylphenylacetylene.

This work was supported by a grant from Bausch and Lomb Inc. of Rochester, New York.

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Received March 1, 1989 Accepted August 16, 1989

Living polymerization of aryl substituted acetylenes by MoCl5 and WCl6 based initiators: The ortho...

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