Robert J. Perry

Chain transfer processes in dichlorosilane reductive polymerization and their control: A simplified route to high molecular weight polysilylenes

Polysilylenes are usually synthesized by a reductive coupling of diorganodichlorosilanes with molten sodium dispersions in an inert solvent, typically neat toluene or toluene mixed with a cosolvent such as heptane. The surface nature of this reaction leads to highly nonstatistical molecular weight distributions which are broad and poorly reproducible in the absence of rigid control over reaction parameters. Because the electronic and physical properties of polysilylenes are sensitive to molecular weight distribution, it is important to understand the factors which influence the molecular weight distribution obtained during preparation. In this work, we have examined the chain transfer processes in the Na-mediated reductive coupling of representative alkyl and aryl diorganodichlorosilanes by comparing rigorously controlled reactions carried out in toluene, a solvent normally thought to be non-chain-transferring in these reactions, to reactions carried out in benzene, which is thermodynamically incapable of undergoing chain transfer via hydrogen abstraction by a terminal polysilyl radical. We have identified for the first time a low-yield back-biting hydrogen abstraction process on the polysilylene side chain C-H bonds which generates an Si-H terminated polysilylene chain and a reactive site alpha to the silicon. The data also suggest the presence of a second back-biting reaction on the polysilylene Si-Si backbone bonds which produces cyclosilane by-products. An outgrowth of the work is a new modification of the Wurtz coupling process which routinely provides very high molecular weight polysilylenes by the safer and more convenient “normal” addition procedure.