Intramolecular cyclization in the polymerization of AB_x monomers: approaches to the control of molecular weight and polydispersity in hyperbranched poly(siloxysilane)

Different approaches to control the molecular weight of hyperbranched poly(siloxysilane) have been explored. Because the intramolecular hydrosilylation competitively consumes the vinyl groups of the monomer and other oligomeric intermediates, the conventional single-batch bulk process generally affords polymer with a relatively low molecular weight (M_w = 5000) in ca. 60% yield. We have developed a progressive slow addition process that effectively increases the molecular weight of the final polymers and improves preparation yield by reducing the occurrence of cyclization. Using this gradual growth method, polymers with molecular weights in the range of 6-86,000 (M_w) and polydispersities in the range 2-15 were easily obtained in yields of ca. 70-80%. More importantly, both the molecular weight and the polydispersity could be controlled by changing the rate of addition or the amount of monomer fed. The slower the addition, or the larger the amount of monomer added, the higher the molecular weight and polydispersity of the resulting polymer. In seeded polymerizations, a similar trend was observed with a maximum M_w near 84,000 and a yield of 80%, values that are very significantly higher than those obtained by the single batch process described earlier.