Step-growth titanium-catalysed dehydropolymerisation of amine-boranes

Precatalysts active for the dehydropolymerisation of primary amine-boranes are generally based on mid- or late transition metals. We have found that the activity of the precatalyst system formed from CpR2TiCl2 and 2 nBuLi towards the dehydrogenation of the secondary amine-borane Me2NH·BH3, to yield the cyclic diborazane [Me2N-BH2]2, increases dramatically with increasing electron-donating character of the cyclopentadienyl rings (CpR). Application of the most active precatalyst system (CpR = ƞ-C5Me5) to the primary amine-borane MeNH2·BH3 enabled the first synthesis of high molar mass poly(N-methylaminoborane), [MeNH-BH2]n, the BN analogue of polypropylene, by an early transition metal catalyst. Significantly unlike other dehydropolymerization precatalysts for MeNH2·BH3 such as [Ir(POCOP)H2], skeletal nickel, and [Rh(COD)Cl]2, the Ti precatalyst system was also active towards a range of substrates including the N-benzyl (Bz) substituted amine-borane BzNH2·BH3 yielding high molar mass polymer and copolymers. Moreover, in contrast to the late transition metal catalysed dehydropolymerisation of MeNH2·BH3 and also the Ziegler-Natta polymerisation of olefins, studies indicate that the Ti-catalyzed dehydropolymerizations reactions proceed by a step growth rather than a chain growth mechanism.