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 metal. We have found that the activity of the precatalyst system formed from CpR 2 TiCl2 and 2 n BuLi towards the dehydrogenation of the secondary amine-borane Me2 NH·BH3 , to yield the cyclic diborazane [Me2 N-BH2 ]2 , increases dramatically with increasing electron-donating character of the cyclopentadienyl rings (CpR ). Application of the most active precatalyst system (CpR = η-C5 Me5 ) 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 such as 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 BzNH2 ·BH3 (Bz = benzyl) yielding high molar mass polymer. 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 dehydropolymerization reactions proceed by a step-growth rather than a chain-growth mechanism.