The catalytic dehydrocoupling/dehydrogenation of N-methylamine-borane, MeNH2 center dot BH3 (7), to yield the soluble, high molecular weight poly(N-methylaminoborane) (8a), [MeNH-BH2](n) (M-w > 20 000), has been achieved at 20 degrees C using Brookhart's Ir(III) pincer complex IrH2POCOP (5) (POCOP = [mu(3)-1,3-(OPtBu2)(2)C6H3]) as a catalyst. The analogous reaction with ammonia-borane, NH3 center dot BH3 (4), gave an insoluble product, [NH2-BH2](n) (8d), but copolymerization with MeNH2 center dot BH3 gave soluble random copolymers, [MeNH-BH2](n)-r-[NH2-BH2](m) (8b and 8c). The structures of polyaminoborane 8a and copolymers 8b and 8c were further analyzed by ultrahigh resolution electrospray mass spectrometry (ESI-MS), and 8a, together with insoluble homopolymer 8d, was also characterized by B-11 and H-1 solid-state NMR, IR, and wide-angle X-ray scattering (WAXS). The data indicate that 8a-8c are essentially linear, high molecular weight materials and that the insoluble polyaminoborane 8d possesses a similar structure but is of lower molecular weight (ca. 20 repeat units), presumably due to premature precipitation during its formation. The yield and molecular weight of polymer 8a was found to be relatively robust toward the influence of different temperatures, solvents, and adduct concentrations, while higher catalyst loadings led to higher molecular weight materials. It was therefore unexpected that the polymerization of 7 using 5 was found to be a chain-growth rather than a step-growth process, where high molecular weights were already attained at about 40% conversion of 7. The results obtained are consistent with a two stage polymerization mechanism where, first, the Ir catalyst 5 dehydrogenates 7 to afford the monomer MeNH=BH2 and, second, the same catalyst effects the subsequent polymerization of this species. A wide range of other catalysts based on Ru, Rh, and Pd were also found to be effective for the transformation of 7 to polyaminoborane 8a. For example, polyaminoborane 8a was even isolated from the initial stage of the dehydrocoupling/dehydrogenation of 7 with [Rh(mu-Cl)(1,5-cod)](2) (2) as the catalyst at 20 degrees C, a reaction reported to give the N,N,N-trimethyl borazine, [MeN-BH](3), under different conditions (dimethoxyethane, 45 degrees C). The ability to use a variety of catalysts to prepare polyaminoboranes suggests that the synthetic strategy should be applicable to a broad range of amine-borane precursors and is a promising development for this new class of inorganic polymers.