BH3·SMe2 addition enables molar mass control via chain stabilization in phosphine–borane dehydropolymerization

Abstract

We report the synthesis of high molar mass polyphosphinoboranes using commercially available reagents through thermal dehydropolymerization in the presence of Lewis acids and bases. These dehydropolymerizations produce materials of higher molecular weight compared to the state-of-the-art catalyst, Cp(CO)₂FeOTf ([PhPH-BH₂]_n (2), 5 mol% LiOTf, 2 M in 2-MeTHF, 100 °C, 24 h; M_n = 80 000 g mol⁻¹, Đ = 1.64 cf. 5 mol% Cp(CO)₂FeOTf, 2 M in toluene, 100 °C, 24 h, M_n = 40 000 g mol⁻¹, Đ = 1.64). We propose a mechanism for the thermal dehydropolymerization of PhPH₂·BH₃ (1) with additives. Initially, the phosphine–borane adduct dissociates, yielding borane in situ, which acts as a (pre)catalyst for the dehydrogenation of 1. Subsequent addition polymerization occurs as described previously, but the addition of Lewis acids and Lewis bases allows for reversible complexation of both termini. Competition between temporary chain capping and termination events results in fewer termination events over time, leading to high molar mass materials. With this mechanism in mind, we were able to show that added BH₃·SMe₂ allows for control over the molar mass of the resulting materials. These results show that transition-metal catalysts are not needed in the thermal dehydropolymerization of PhPH₂·BH₃, and offer a new mechanistic insight that may unlock greater control over the dehydropolymerization of main-group substrates.