Making accessible soluble silicon-containing polynorbornenes: hydrosilylation of vinyl-addition poly(5-vinyl-2-norbornene)†
Silicon-containing vinyl-addition polynorbornenes are of interest as membrane materials for gas separation processes, microporous materials for gas storage and stationary phases for chromatography. However, the syntheses of the corresponding monomers and their polymerization with the formation of high-molecular-weight products are often problematic. Herein, we report a facilitated and simple approach to synthesizing various silicon-containing vinyl-addition polynorbornenes via hydrosilylation of side chains in a parent polymer bearing vinyl groups. For producing soluble Si-containing vinyl-addition polynorbornenes, a series of Rh- and Pt-complexes were systematically studied as catalysts in model hydrosilylation reactions of 2-vinylnorbornane and poly(5-vinyl-2-norbornene) (PVNB) with pentamethyldisiloxane. While traditional Rh- and Pt- (Karstedt's) catalysts exhibited high activity, they provided insoluble products in the case of the modification of high-molecular-weight PVNB. At the same time, (NHC)Pt-complexes (where NHC is N-heterocyclic carbene ligand) were less active, but gave soluble modified vinyl-addition polymers. Tuning the size of the NHC-ligand and the nature of the aryl-group in this ligand made it possible to find out the most efficient (NHC)Pt-catalyst among the studied complexes. This catalyst allowed us to reach high selectivities and conversions (>90%) of vinyl groups in PVNB. The efficiency of the (NHC)Pt-catalyst was also evaluated in the hydrosilylation reaction of PVNB with more hindered silanes (1,1,1,3,5,5,5-heptamethyltrisiloxane and triethylsilane). The prepared polymers exhibited good film-forming properties, while the incorporated silyl-groups led to enhanced gas permeability over the parent PVNB.