Controlled Synthesis of Well-Defined Polyaminoboranes on Scale Using a Robust and Efficient Catalyst.

The air tolerant precatalyst, [Rh(L)(NBD)]Cl () [L = κ-(PrPCHCH)NH, NBD = norbornadiene], mediates the selective synthesis of -methylpolyaminoborane, (HBNMeH), by dehydropolymerization of HB·NMeH. Kinetic, speciation, and DFT studies show an induction period in which the active catalyst, Rh(L)H (), forms, which sits as an outer-sphere adduct as the resting state. At the end of catalysis, dormant Rh(L)HCl () is formed. Reaction of with HB·NMeH returns , alongside the proposed formation of boronium [HB(NMeH)]Cl. Aided by isotopic labeling, Eyring analysis, and DFT calculations, a mechanism is proposed in which the cooperative "PNHP" ligand templates dehydrogenation, releasing HB═NMeH (Δ = 19.6 kcal mol). HB═NMeH is proposed to undergo rapid, low barrier, head-to-tail chain propagation for which is the catalyst/initiator. A high molecular weight polymer is formed that is relatively insensitive to catalyst loading ( ∼71 000 g mol; , of ∼ 1.6). The molecular weight can be controlled using [HB(NMeH)]Cl as a chain transfer agent, = 37 900-78 100 g mol. This polymerization is suggested to arise from an ensemble of processes (catalyst speciation, dehydrogenation, propagation, chain transfer) that are geared around the concentration of HB·NMeH. TGA and DSC thermal analysis of polymer produced on scale (10 g, 0.01 mol % ) show a processing window that allows for melt extrusion of polyaminoborane strands, as well as hot pressing, drop casting, and electrospray deposition. By variation of conditions in the latter, smooth or porous microstructured films or spherical polyaminoboranes beads (∼100 nm) result.