The sigma amine-borane complexes Rh(L1)(η :η -H B⋅NRH )OTf (L1=2,6-bis-1-(2,6-diisopropylphenylimino)ethylpyridine, R=Me, Et, Pr) are described, alongside Rh(L1)(NMeH )OTf. Using R=Me as a pre-catalyst (1 mol %) the dehydropolymerization of H B ⋅ NMeH gives H BNMeH selectively. Added NMeH , or the direct use of Rh(L1)(NMeH )OTf, is required for initiation of catalysis, which is suggested to operate through the formation of a neutral hydride complex, Rh(L1)H. The formation of small (1-5 nm) nanoparticles is observed at the end of catalysis, but studies are ambiguous as to whether the catalysis is solely nanoparticle promoted or if there is a molecular homogeneous component. Rh(L1)(NMeH )OTf is shown to operate at 0.025 mol % loadings on a 2 g scale of H B ⋅ NMeH to give polyaminoborane H BNMeH M =30,900 g/mol, Ð=1.8 that can be purified to a low residual Rh (6 μg/g). Addition of NaN(SiMe ) to H BNMeH results in selective depolymerization to form the eee-isomer of N,N,N-trimethylcyclotriborazane H BNMeH : the chemical repurposing of a main-group polymer.