Mechanistic Studies of Polysilane Polymerization - American

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Downloaded by UNIV OF PITTSBURGH on March 10, 2015 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0224.ch018

Mechanistic Studies of Polysilane Polymerization Sylvie Gauthier and Denis J. Worsfold Division of Chemistry, National Research Council of Canada, Ottawa K1A 0R6, Canada

The copolymerization of dialkyldichlorosilanes was investigated under conditions of concurrent and consecutive monomer addition. The reactivities of the monomers in initiation and propagation reactions were different. Block copolymers may be formed both by sequential addition to existing active centers and by reactivation of existing polymer. These results were combined with previous findings to suggest general schemes for the pathways taken by the reaction.

POLYSILANE POLYMERS of high molecular weight were first prepared several years ago (1-3). The method commonly used is the reductive coupling of dialkyldichlorosilanes with sodium in refluxing toluene. Relatively few studies of the mechanism of the reaction or discussions of the probable reaction intermediates have been reported. The molecular equation describing the reaction,

SOLUBLE

nRR'SiCl 2 + 2nNa ± * (RR'Si)n + 2nNaCl suggests that the reaction is the silicon equivalent of a Wurtz condensation and that the reaction would follow a condensation polymerization mechanism. The characteristics of the reaction depart considerably from that expected of a condensation polymerization, such as the formation of polyesters. In a condensation polymerization, the monomer is consumed in the formation of the first dimers with some trimers, and only late in the reaction are high-molecular-weight polymers formed. The formation of high-molec-

0065-2393/90/0224-0299$06.00/0 Published 1990 American Chemical Society

In Silicon-Based Polymer Science; Zeigler, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.


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SILICON-BASED POLYMER SCIENCE: A COMPREHENSIVE RESOURCE

ular-weight polymers depends on the exact stoichiometry of the reagents. A prime condition for these characteristics is the independence of the reactivity of the active chain end groups on the molecular weight of the polymer chain. In polysilane formation by the previously mentioned route, the products were complex and contained high-molecular-weight materials (molecular weight > 105) after only 10% of the dichloride had reacted. In the presence of a twofold excess or a 10% deficit of sodium, an appreciable amount of high-molecular-weight polymer was formed. Attempts to isolate dimers after 50% of the dichloride had reacted failed. The product with the lowest molecular weight was a cyclic pentamer. The polymerization, in fact, demonstrates some of the characteristics of a chain reaction as found in addition polymerization. West et al. (4, 5) have suggested a number of possible reaction intermediates, which include anions, radical anions, radicals, and diradicals. Zeigler (6, 7) has proposed, on the basis of some radical-trapping experiments, that the intermediate is, at least at some point, a radical. He showed that the diradical silylene was not an intermediate, and he also stressed the importance of bulk solvent composition on the course of the reaction. The bulk solvent composition determines the expansion of the polymer coil as it interacts with the sodium surface. Miller et al. (8) initially suggested that the reaction, which is promoted by the addition of diethylene glycol dimethyl ether, proceeds by an anionic process, although they later (9) accepted Zeigler's bulk-solvent model. The products of the reaction of the dichloride with melted sodium are complex. Except for sodium chloride, the products are polymeric and display a wide range of molecular weights, which fall into three groups when analyzed by size exclusion chromatography: high molecular weight (usually >105), intermediate molecular weight (103-104), and low molecular weight. The low-molecular-weight products probably consist mostly of cyclic polysilanes, primarily of pentamers starting from phenylmethyl- or hexylmethyldichlorosilane (10). The multiplicity of these products suggests that two or more mechanisms are operating. Interestingly, the distribution of the molecular weights strongly resembles that found in the formation of polysiloxanes by the anionic polymerization of cyclic trisiloxanes and tetrasiloxanes (11). In these anionic polymerizations, an equilibrium exists between linear high-molecular-weight chains, macrocyclic products, and small ring trimers and tetramers. In the polysilane case, however, the thermodynamically stable material in solution at low temperature is probably the low-molecular-weight cyclic product. In fact, if the high-molecular-weight material is refluxed with potassium in T H F (tetrahydrofuran) solution, the polymer rapidly forms the low-molecularweight product. Also, in the reaction of dialkyldichlorosilanes with potassium in THF, considerable high-molecular-weight product is formed during the initial 1-2 min, but this product equally rapidly reverts to the cyclic material.



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In the polymerization of dialkyldichlorosilanes with sodium in refluxing toluene, propagation and a concurrent back-biting reaction to cyclic material could give the range of products found if the products are kinetically, instead of thermodynamically, determined (12). No evidence for depolymerization has been found for the reaction in toluene solution. Reaction rate studies (10) have shown that the reaction curves are sigmoidal, with an initial period of increasing rate and a final falling off in rate as the monomer is exhausted. At the maximum central part, the rate is relatively independent of the surface area of sodium, a fact suggesting that the rate-determining propagation step does not take place on the sodium surface. The mechanism suggested to explain these findings involves a twostage reaction whereby initiation occurs at the sodium surface by a slow reaction of the dichloride to form sodium-ended chains. Propagation is by a comparatively rapid reaction of the dichloride with this chain end to reform a chlorine-ended chain, which would interact very rapidly with the sodium surface in a non-rate-determining step to regenerate the sodium end. Copolymerization can often give insights into polymerization mechanisms. Consequently, a number of copolymerizations have been studied. The comonomers were added either concurrently or consecutively.

Experimental Procedures The experimental techniques followed are described elsewhere (10). Sodium block was added to refluxing toluene under nitrogen and stirred to form small particles of the required size by controlling the stirrer speed. The monomer was added by syringe as rapidly as possible, with cooling if necessary to control the reaction. This step takes less than a minute, except for phenylmethyldichlorosilane, which was too reactive. In the sequential copolymerizations, enough time was left between the addition of the two monomers for >90% of the first monomer to react. The disap pearance of the monomers was monitored by gas chromatography.

Results Concurrent Copolymerization. Difficulties are encountered in studying the kinetics of these reactions because of reaction-to-reaction variation probably caused by the heterogeneous nature of the reaction and its sensitivity to the sodium surface area. Also, some batch-to-batch variation in the dichloride is encountered, despite redistillation. For the polymerizations, the relative rates of monomer consumption are compared under comparable conditions and are probably valid. The variations in the reaction rates of dichlorides during homopolymerization are large enough for semiquantitative conclusions to be drawn. Nevertheless, because of the sigmoidal reaction curves found for all reactions slow enough to follow, extracting a rate constant would be rather dubious. Qualitative comparisons of rate, or comparisons of half-lives at best, are all that can be done reliably at present.



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On a qualitative scale, in which >> represents about a 10-fold factor, the relative rates of monomer consumption during homopolymerization are PMDS » HMDS > DMDS » VMDS, in which PMDS, HMDS, DMDS, and VMDS are phenylmethyl- hexylmethyl-, dimethyl-, and vinylmethyldichlorosilane, respectively. HMDS and DMDS have half-lives of 20 and 90 min, respectively. The reactions of PMDS and VMDS are, respectively, too fast and too slow to measure with any precision by our methods. The relative rates of monomer consumption during copolymerization are in a different order. For the following pairs, the relative rates are PMDS > HMDS, PMDS > DMDS, PMDS > VMDS but VMDS > HMDS, VMDS > DMDS, DMDS > HMDS. For a terpolymerization, the relative rates are PMDS > VMDS > HMDS. In these copolymerizations, the polymers with the vinylmethylsilane group are of interest, because the presence of the vinyl group would assist cross-linking if the polymers are used as precursors for SiC. Both IR and NMR data indicated that the vinyl content in the polymer is lower than expected and that some carbosilane may form. The yields of SiC were good (Table I). Table I. Copolymerization with VMDS Starting Monomer Ratio 0

1:9:0 l:9:0d 0.5:9.5:0 2:8:0 1:7:2 1:5:4 1:5:5

Polymer Yield

(%r 69 77 61 48 48 30 24

SiC Yield

(%y 75 73 79 81 62 57 67

The data are the starting VMDS/PMDS/HMDS ratios. *The data are the yields of toluene-soluble polymer precipitated in isopropyl alcohol. The data are percentages of the theoretical yield. rf Diphenylmercury (1%) was added at the start of the reaction to restrict the polymer molecular weight.

Sequential Polymerization. The sigmoidal reaction curves, which indicate a tendency for the molecular weight to increase during the course of the reaction, and other considerations led to the suggestion that the polymerizing chains had long lifetimes (9), similar to the chains in a "living" polymerization. If this is the case, sequential addition of different monomers would lead to block copolymer formation. To check this hypothesis, PMDS and HMDS were polymerized sequentially with sufficient time between additions for the first monomer to be consumed. In one experiment, PMDS was the first monomer, and in another experiment, HMDS was the first. In



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two other experiments, diphenylmercury was added to PMDS in the first addition to limit the molecular weight of this part of the polymer (9). Differential solubility was used to check for the presence of block copolymer. First, the higher molecular weight polymer was isolated from the reaction mixture by precipitation in isopropyl alcohol. Neither poly(hexylmethylsilane) nor poly(phenylmethylsilane) are soluble in this alcohol. Poly(hexylmethylsilane) is soluble in hexane, but poly(phenylmethylsilane) is not. The precipitated polymer was then dissolved in meth ylene dichloride and added to hexane. The polymer that precipitated was recovered byfiltration;the rest was recovered by evaporation of the solution. NMR analysis showed the presence of phenyl and hexyl groups in both the hexane-soluble and the hexane-insoluble fractions, a result indicating the presence of copolymers, presumably block copolymers (Table II). Be cause the phenyl content of the hexane-insoluble polymer was considerably higher than that of the hexane-soluble polymer (Table II), the composition distribution must have been fairly wide. Fractional precipitation of both fractions from toluene solution by methanol was attempted, and some homopoly(phenylmethylsilane) was isolated from the hexane-insoluble fraction. Most fractions contained both phenyl and hexyl groups.

Table Π. Composition of Polymer from Sequential Polymerization Monomer HMDS, PMDS, PMDS, PMDS,

Sequence

0

PMDS HMDS HMDS* HMDSFEC

Hexane-Soluble Polymer (%) 62 58 53 100

Phenyl Groups in Polymer (Ψο) Hexane

Soluble

Hexane

66 60 60 25

Insoluble 81 85 93 0

"Equal volumes of monomers were used. ^Diphenylmercury 1 (%) was added at the start of the reaction. The reaction was analyzed at the end of the PMDS reaction. The reaction mixture was washed, dried, and restarted with fresh Na and the second monomer, HMDS.

Further sequential polymerizations were performed to determine whether the incorporated polymer had in fact to be living, as in an anionic polymerization. Previous studies (10) have shown that even after all the dichloride has been consumed in a PMDS polymerization, increases in mo lecular weight of the middle-range fraction of the polymer (103-104) occur on continued reflux with excess sodium. Also, even if the reaction solution is washed with water and dried, these changes occur on reflux with sodium. This middle-range fraction was maximized by the addition of a little diphenylmercury at the beginning of the reaction. The reaction solution was again washed with water to deactivate any chlorine or sodium chain ends, dried, and refluxed with sodium, and HMDS was added. By analyzing for



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block copolymers, the final reaction product was compared with one from a reaction mixture in which the washing process had been omitted. The washed reaction mixture gave only polymer that was hexane soluble, but the highest molecular weight fraction contained 25% phenyl groups (Table II). The molecular weight of this product was also several times that of the product from the first stage. Undoubtedly, block copolymers were formed in this reaction but not as much as in the comparable reaction that was not washed. In the unwashed reaction, hexane-insoluble high-molecular-weight polymer with a higher phenyl content was formed (Table II). Some type of reactivation is possible.

Discussion General Reaction Scheme. The polysilane reaction products from the reaction of dialkyldichlorosilanes with sodium in refluxing toluene are complex, with molecular weights spanning the three groups indicated previously. Any mechanism proposed for this reaction should take into account all the three products described at the start of this chapter, even if more than one mechanism coexist. Before the reaction can be considered at the atomic level, the general pathways for the formation of the three products must be known. The pathway for polysiloxane formation has been mentioned. When applied to polysilanes, this pathway could imply an initiation reaction with a much faster propagation reaction whereby the silane units would add in a stepwise fashion, possibly by a two-stage process as discussed earlier. The concurrent and end-biting and back-biting reactions would chop off the low-molecularweight cyclic materials and, less frequently, the larger cyclic materials to give the intermediate fraction. The distribution would then be determined kinetically by the relative rates of the three processes until final deactivation. Earlier, West et al. (12) described such a kinetic determination of products for DMDS. An alternative scheme that does not involve end biting is possible. As before, reaction of the initial dichloride with sodium is necessary for the initiation step in pure systems. After the first one or two additions of the monomer, the growing short chain would, by the two-step addition process, have alternately sodium or chlorine ends at each end of the short chain. When the chain reaches 5 or 6 units, cyclization can take place ideally, if the two chain ends are different and can react. Only that fraction of chains that does not cyclize at this point could grow to high molecular weight. The intermediate fraction could be composed of chains of moderate length, which would still have a reasonable chance to cyclize once the chain length is greater than 8-12 units; ring formation by chains of 8-12 units is difficult. Also, condensation reactions could still occur. Chain propagation reactions



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may accelerate as the chain lengthens, and increased electron derealization (13) may alter the activity of the chain end groups. The middle-range polymer fraction may contain an appreciable amount of cyclic material. A fraction of PMDS polymer with a molecular weight of 2000 was isolated, and the phenyl and methyl regions were analyzed by 1 H NMR spectroscopy. The phenyl/methyl ratio was very close to 1:1 even if (CH 3 ) 3 SiCl was used to terminate the reaction. This result suggests that this material is cyclic. The 2 9 Si NMR spectrum showed a small extra peak at 8 ppm (TMS [tetramethylsilane] peak at 0 ppm), as well as the usual signal for Si-Si-Si at -40 ppm, corresponding to 4% Si-Si-(CH 3 ) 3 . This finding casts doubt on the totally cyclic nature of this material. Copolymerization Rates. Both of the overall schemes described rely upon a comparatively slow reaction of the initial dichloride with sodium, and this requirement is confirmed by the kinetics of the reactions. These reactions all have an initial period of accelerating rate, suggesting that the initial dichloride itself does not react rapidly with the sodium surface but that some intermediate in the reaction will react more rapidly with the dichloride. Presumably, this intermediate is the growing chain end. The homopolymerization rate is a function of the initiation rate, as well as the rates of subsequent propagation and other steps. Thus, the apparently anomalously rapid incorporation of VMDS or DMDS during copolymerization with HMDS suggests that these two monomers have fairly rapid propagation steps. Because of their very low rates of reaction with sodium in the initiation reaction, their overall homopolymerization rates are slower than their rates of copolymerization, in which initiation may be via the other monomer and faster. Block Copolymer Formation. The formation of block copolymers during sequential polymerization suggests that chain ends do remain reactive even after all the initial dichloride is consumed. The incorporation of the first polymer into block copolymers with the second monomer is, however, compatible with the suggestion that chain extension is by monomer addition alone or by a combination of monomer addition and condensation reactions to form the higher molecular weight fractions. With chain extension by monomer addition alone, triblocks with the first monomer in the middle can be formed. Condensation reactions could give multiblock copolymers. NMR analysis has indicated block copolymer formation, but the block length has not been determined yet. The distribution of block lengths is probably large because of the broad molecular weight distribution of thefirstpolymer before the second addition. The inclusion of apparently "dead" polymer into block copolymer is of interest (Table II). Certainly less of the first PMDS polymer was incorporated in this system than in the corresponding living system, because all the


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copolymer formed in the system with dead polymer was hexane soluble. The results suggest that the active new chain end reacts with the existing polymer and reactivates it. This reaction is necessary in the mechanism that suggests back biting to eliminate cyclic materials. This type of reaction is widely postulated to account for the molecular weight distributions found in cyclic oxide polymerization, for example, T H F (14) and oxetane (15).


Conclusions If the reactions in T H F and the inclusion of dead material in block copolymers are taken into account, the balance of the evidence at present seems to support the back-biting mechanism. However, the mechanisms are not really mutually exclusive, because they both suggest an alternation between a chlorine-ended chain and a sodium-ended chain. The sodium ended-chain reacts first with the initial dichloride in the rate-determining step, and then the chlorine-ended chain reacts with the sodium surface. The difference lies in the mode of formation of the cyclic materials and the importance of any condensation reactions. Both mechanisms might be operative, although one may dominate, depending on the conditions or substituents. The details of the reaction at the atomic level are even more difficult to unravel. The reaction of a chlorine-ended chain with a sodium surface may involve two single-electron-transfer steps that may atfirstform a chain radical and CI or a delocalized radical anion on the chain. The transfer of a second electron from sodium to form a delocalized anion-Na+ ion pair may follow. The ion pair could leave the Na surface, probably as an aggregate of ion pairs, when the chain is long enough to solubilize it. The increasing ability of longer chains to delocalize the charge would account for the great difference between the activities of the initial dichloride and the growing chain in reacting with the sodium surface. In reacting with the initial dichloride or with another chlorine-ended chain, the polysilane sodium chain end could again involve a radical intermediate, as suggested for the reaction of alkyl halides and sodium alkyls. The radical traps in the experiments of Zeigler (6, 7) could operate as chain breakers at both these stages. The promoting effect of the ethers found by Miller (8) could assist by breaking down the ion-pair aggregates in toluene, as happens during the anionic polymerization of carbon-based polymers (16), or accelerate the formation of radical anions.

References 1. Wesson, J. P.; Williams, T. C. J. Polym. Sci., Polym. Chem. Ed. 1980, 18, 959. 2. Trujillo, R. E. J. Organometal. Chem. 1980, 198, C 27. 3. West, R.; David, L. D.; Djurovich, P. I.; Stearley, D. L . ; Srinivasan, K. S. V.; Yu, H . J. Am. Chem. Soc. 1981, 103, 1352.



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4. West, R. J. Organomet. Chem. 1986, 300, 327. 5. Zhang, Y-H.; West, R. J. Polym. Sci., Polym. Chem. Ed. 1984, 22, 225. 6. Zeigler, J. M . Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1986, 27, 109. 7. Zeigler, J. M . Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1987, 28, 424. 8. Miller, R. D.; Hofer, D.; McKean, D. R.; Willson, C. G.; West, R.; Trefonas, P. T. In Materials for Microlithography; Thompson, L. F.; Willson, C. G.; Fréchet, J. M . J., Eds.; ACS Symposium Series 266; American Chemical Society: Washington, D C , 1984;p293. 9. Miller, R. D.; Rabolt, J. F.; Sooriyakumaran, R.; Fleming, W.; Fickes,G.M.; Farmer, B. L.; Kuzmany, H . In Inorganic and Organometallic Polymers: Macromolecules Containing Silicon, Phosphorus, and Other Inorganic Elements; Zeldin, M . ; Wynne, K. J.; Allcock, H . R., Eds.; ACS Symposium Series 360; American Chemical Society: Washington, D C , 1988. 10. Worsfold, D . J. In Inorganic and Organometallic Polymers: Macromolecules Containing Silicon, Phosphorus, and Other Inorganic Elements; Zeldin, M.; Wynne, K. J.; Allcock, H . R., Eds.; ACS Symposium Series 360; American Chemical Society: Washington, D C , 1988; pp 101-111. 11. Brown, J. F.; Slusarczuk, G. M . J. J. Am. Chem. Soc. 1965, 87, 931. 12. Carberry, E . ; West, R. J. Am. Chem. Soc. 1969, 91, 5440. 13. Trefonas, P.; West, R.; Miller, R. D.; Hofer, D. C. J. Polym. Sci., Polym. Lett. Ed. 1983, 21, 823. 14. Dreyfuss, M . P. J. Macromol. Sci., Chem. 1975, 9, 729. 15. Black, P. E.; Worsfold, D. J. Can. J. Chem. 1976, 54, 3325. 16. Bywater, S.; Worsfold, D. J. Can. J. Chem. 1962, 40, 1564. RECEIVED for review May 27, 1988. ACCEPTED revised manuscript March 27, 1989.


Mechanistic Studies of Polysilane Polymerization - American

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