Organosilicon hydrides in macromolecular design: Reactions of chain-transfer and hydrosilylation

The chain-transfer constants through silicon hydrides in bulk polymerization of styrene and methyl methacrylate (MMA) were measured with using 15 organosilicon compounds belonging to four classes: oligoorganohydrosilanes, oligoorganohydrosiloxanes, disilalkanes and alkylhydrosilanes. The linear dependences of the logarithm of chain-transfer constant on the sum of the Taft inductive constants of substituents at silicon atoms of a Si–H group were found. The negative values of reaction constants ρ indicate that an electrophilic attack of macroradical onto a hydrogen atom occurs at the limiting stage of the process. Oligoorganohydrosilanes proved to be the most effective chain-transfer agents that can be explained by high electron-donating properties of R3Si substituents. The increase of the chain-transfer constant values takes place on the accumulation of both trimethylsilyl and silicon hydride groups in a molecule of hydrosilane. The high electrophilicity of PMMA macroradical as compared with a macroradical of polystyrene (PS) is responsible for a greater sensitivity of the polymerization reaction of MMA to the change of the electron density on a hydrogen atom of the organosilicon chain-transfer agent. In the radical polymerization of MMA and styrene in the presence of the chain-transfer agents till high conversion the polymers with a narrower molecular weight distribution (MWD) in comparison with those synthesized by usual polymerization in bulk are formed. The polymers prepared in the presence of multifunctional silanes can be functionalized by the reaction of hydrosilylation and further used in the synthesis of block copolymers.