Functional polysiloxanes. II. Neighboring effect in the hydrosilylation ofpoly(hydrogenmethylsiloxane-co-dimethylsiloxane)s by allylglycidylether

Polysiloxanes bearing epoxy groups as lateral substituents were prepared by the hydrosilylation of 1-allyloxy-2,3-epoxypropane (allylglycidylether) wi th poly(hydrogenmethylsiloxane-co -dimethylsiloxane)s (D-H-D copolymers) of various compositions. To determine the optimal conditions of the hydrosily lation catalyzed by hexachloroplatinic acid, the kinetics of the reaction w ere investigated for the poly(hydrogenmethylsiloxane) homopolymer. The reac tion was first order in platinum, first order in double bonds, and 0.5 in S iH. This kinetic law was consistent with the hypothesis that hydrogenmethyl siloxane dyads are much more reactive than isolated units, which may be exp lained by the simultaneous insertion of two vicinal SiH's in a binuclear co mplex of platinum. This peculiar type of neighboring effect was investigate d further by comparison of the kinetics of the hydrosilylation of D-D-H cop olymers of various compositions and D-H block lengths and was confirmed by the microstructure of a D-D-H copolymer (50/50) before and after partial hy drosilylation. Triad analysis by Si-29 NMR showed that the resulting copoly mer was not statistical but contained a high proportion of isolated SiH. Th is explains why hydrosilylation proceeded in two steps: a fast reaction of D-H-D-H dyads and a slow reaction of the remaining isolated D-H units. (C) 2000 John Wiley & Sons, Inc.