EFFECT OF CATALYST ON REGIOSELECTIVITY AND VINYL H EXCHANGE ON SILICON IN THE HYDROSILYLATION OF VINYL-SILOXANES BY T-8 HYDROGEN SILSESQUIOXANE/

We have recently reported the synthesis of octopus molecules of define d shape and size with molecular weights well into the thousands. These octopus molecules were made by placing eight pendant groups symmetric ally about a central silsesquioxane core via the H2PtCl6 catalyzed hyd rosilylation of 1-alkenes as well as vinyl- and allyl-siloxanes by T-8 hydrogen silsesquioxane, (HSiO3/2)(8). The chemistry of addition was studied and it was found that while the addition of the 1-alkenes to T -8 was regioprecise with only alpha-addition being observed, both alph a- and beta-addition occurred with vinyl-siloxane. In addition, H/viny l exchange on silicon was observed to occur with addition of vinylsilo xane to T-8. In the current studies, the effect of the hydrosilylation catalyst, homogeneous and heterogeneous, on the regioselectivity of a ddition and on the extent of exchange on silicon was evaluated. It was found that the heterogeneous catalysts Pt/C, sulfided Pt/C, and Rh/C required higher temperature and longer times to get complete reaction than the homogeneous catalysts, H2PtCl6 and the tetramethyldivinyldisi loxane complex of Pt. Pd supported catalysts were not effective cataly sts for this hydrosilylation. The extent of exchange on silicon and th e degree of the second mode of addition occurring were higher with the heterogenous catalysts and may be a result of the higher reaction tem peratures.