Molecular weight polysilylenes

Table II. Order-Disorder Transition Temperatures and Widths of High-Molecular-Weight Polysilylenes ...

In order to avoid the SET process, we chose diphenylmethylsilyl anions (PI MeSiM 8a, M = K 8b, M = Na 8c, M = Li) as initiators for 7 instead of alkyllithium and benzene as a solvent. The polymerization did not take place in benzene with silyl anions alone. However, in the presence of an equimolar amount of suitable cryptands, the silyl anions initiated the polymerization. The results are summarized in Table 2. The molecular weights of polysilylenes thus obtained were in good agreement with the calculated values within experimental error. 

Although many kinds of polysilylene have been prepared by this method, the structure of the polymers is difficult to control. Molecular weight and polydispersity are also unmanageable. ... 

Symmetrical Dialkyl-Substituted Polysilylenes Because of their extremely sharp order-disorder transitions, the nonpolar, symmetrical dialkyl-substituted polysilylenes are almost ideal systems with which to test the predictions discussed earlier. The predicted solvent dependence of Tc was tested by performing a series of experiments with high-molecular-weight poly(di-n-hexylsilylene) in dilute solution. Experimental results for six solvents are listed in Table II, and the theoretically defined solvation coupling constants and solvent parameters are collected in Table III. 

The predicted intrinsic width of the order-disorder transition of a mono-disperse, flnite-molecular-weight polymer solution was also tested. The average molecular weights of dialkyl-substituted polysilylenes are in the order of 6 X 10, which implies that N is 3000-5000 silicon atoms. With equation 9, the theory predicts that ATq/Tc is 0.004-0,006, which for Tc = -30 corresponds to an intrinsic width of roughly 1 or 2 C. This result is in good agreement with the experimental observations summarized in Table II. 

Polysilylenes have been known for many years but were not considered interesting until the mid-1970s, when Yajima (16) and West (IT) used them as precursors for Si-C ceramics. In 1980, the first soluble high-molecular-weight polymers were synthesized (iS). Photoconductivity in poly(meth-ylphenylsilylene)-TNF (2,4,7-trinitrofiuorenone) was reported by Kepler et al. (14). Preliminary charge-transport studies showed that at 295 K hole mobility in poly(methylphenylsilylene) (PMPS) is 10 cm /V-s at about 10 V/cm, quite high for a polymer 14, 19, 20). 

High-molecular-weight PM PS is soluble in common solvents and is a good film former. Qualitatively, the overall handling characteristics resemble those of polystyrene. Films of PM PS were cast from toluene solution by a solvent evaporation technique. Other polysilylenes, poly(methylcyclohexylsilylene), poly(methyl-n-pro-pylsilylene), poly(methyl-n-octylsilylene), and a copolymer of dimethyl- and meth-ylphenylsilylene (1 1 ratio) were also prepared by Wurtz coupling by the method of Zhang and West (47). Films of these polymers were also cast from toluene. All polymers form clear transparent colorless films. 

While it is only recently that high molecular weight silicon backbone polymers have become available for detailed study, compounds containing the silicon-silicon bond. Including oligomeric polysilylenes, have attracted considerable Interest for many years. Thus there exists a number of papers of fundamental Importance with particular relevance to the polysilylenes. Paramount among these are the contributions of Pitt and coworkers on the optical spectroscopy (18,23,2k)... 

Polysilanes (or polysilylenes) are usually prepared by the Wurtz-type couphng reaction of dichlorodialkylsilanes or, alternatively, via a transition metal-catalyzed dehydrogenation of dialkylsilanes both approaches often exhibit difficulties in terms of controlling the molecular weight and chain-structure, however. Nonetheless, by using masked silylene monomers (l-phenyl-7,8-disilabicyclo[2.2.2]octa-2,5-dienes), a series of novel, well-defined linear poly(silylene)s (M /Mn 1.3) was successfully obtained via an anionic ROP (Scheme 5.11) [147-150]. 

This chlorinated polysilylene after reaction with sodium in toluene at 110°C remained unchanged in molecular weight. The SEC trace of the sodium treated chlorinated polymer coincided with that of the original chlorinated polymer. The Si NMR spectrum of the solution before it was deactivated was also unchanged. 

The molecular weights of polysilylenes (see Table 1) depend on the method of synthesis and the substituents on silicon. Although size-exclusion chromatography (SEC) may be used to estimate molecular weight and polydispersity, more accurate values of M , and radius of gyration (RG) are obtained from light-scattering... 

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