Metal-Free Room-Temperature Vulcanization of Silicones via Borane Hydrosilylation

Vulcanization of silicone networks from commercially available linear poly(dimethyl-co-methylhydro)siloxane (PMHS) and alpha-diketones was achieved using metal-free borane hydrosilylation at room temperature. The Lewis acid catalyst, tris(pentafluorophenyl)borane (B(C6F5)(3)), efficiently cross-linked PMHS at minimal catalyst loadings (200-1000 ppm) to produce polymer networks with mechanical properties, thermal stability, and optical clarity rivaling that achieved from traditional platinum catalysis. Variation of the starting PMHS structure is shown to influence the final characteristics of the network. Increasing molar mass of the PMHS chain results in a higher thermal decomposition temperature, while increasing mole fractions of Si-H moieties along the backbone increase the cross-linking density and the attendant Shore hardness. The degradation behavior of the networks was investigated, with the borane-vulcanized samples showing rapid dissolution upon exposure to acid and high stability to neutral and basic conditions. Functional networks bearing halide and vinyl groups could also be prepared via a preliminary reaction of PMHS with an appropriate monoketone, providing a general and versatile strategy for network derivatization with the potential for postvulcanization functionalization being subsequently demonstrated via thiol-ene click chemistry.