Correlation of structure and molecular weight distributions during the formation of poly(methylphenylsilylene) by the Wurtz reductive-coupling reaction

Variations of yields and molecular weight parameters of poly(methylphenylsilylene) formed through sodium-mediated Wurtz-type reductive coupling of dichloromethylphenylsilane in refluxing toluene are described. The intermediate molecular weight fractions within the polymodal distributions that result from such syntheses are shown to correlate well with the length of all-trans sequences within the polymer as revealed through the near-ultraviolet spectra of the fractionated polymers. The origins of the polymodal distributions are explained as being the consequence of a competition between;termination through a backbiting reaction and a continued growth reaction whenever additions in gauche conformation arise. The likelihoods of such additions are reasoned to increase with the lengths of the all-traits sequences-that arise from the preceding additions, and the termination reaction is rationalized as having a maximum probability when the polymer consists of only one such sequence. The reasoning is shown to correlate with the polydispersities of the intermediate and high molecular weight fractions within the overall molecular weight distribution of the polymer. Reasoning from this explanation of the mechanism of the polymerization, it is shown that it is possible to obtain high molecular weight, monomodal poly(methylphenylsilylene) in high yield from syntheses conducted in refluxing tetrahydrofuran.