[Rh(κ 2 -PP-DPEphos){η 2 η 2 -H 2 B(NMe 3 )(CH 2 ) 2 t Bu}][BAr F 4 ] acts as an effective precatalyst for the dehydropolymerization of H 3 B·NMeH 2 to form N -methylpolyaminoborane (H 2 BNMeH) n . Control of polymer molecular weight is achieved by variation of precatalyst loading (0.1–1 mol %, an inverse relationship) and use of the chain-modifying agent H 2 : with M n ranging between 5 500 and 34 900 g/mol and Đ between 1.5 and 1.8. H 2 evolution studies (1,2-F 2 C 6 H 4 solvent) reveal an induction period that gets longer with higher precatalyst loading and complex kinetics with a noninteger order in [Rh] TOTAL . Speciation studies at 10 mol % indicate the initial formation of the amino–borane bridged dimer, [Rh 2 (κ 2 -PP-DPEphos) 2 (μ-H)(μ-H 2 BN=HMe)][BAr F 4 ], followed by the crystallographically characterized amidodiboryl complex [Rh 2 ( cis -κ 2 -PP-DPEphos) 2 (σ,μ-(H 2 B) 2 NHMe)][BAr F 4 ]. Adding ∼2 equiv of NMeH 2 in tetrahydrofuran (THF) solution to the precatalyst removes this induction period, pseudo-first-order kinetics are observed, a half-order relationship to [Rh] TOTAL is revealed with regard to dehydrogenation, and polymer molecular weights are increased (e.g., M n = 40 000 g/mol). Speciation studies suggest that NMeH 2 acts to form the  precatalysts [Rh(κ 2 -DPEphos)(NMeH 2 ) 2 ][BAr F 4 ] and [Rh(κ 2 -DPEphos)(H) 2 (NMeH 2 ) 2 ][BAr F 4 ], which were independently synthesized and shown to follow very similar dehydrogenation kinetics, and produce polymers of molecular weight comparable with [Rh(κ 2 -PP-DPEphos){η 2 -H 2 B(NMe 3 )(CH 2 ) 2 t Bu}][BAr F 4 ], which has been doped with amine. This promoting effect of added amine in situ is shown to be genera...

A detailed study of H3B·NMeH2 dehydropolymerization using the cationic precatalyst [Rh­(DPEphos)­(H2BNMe3(CH2)2 tBu)]­[BArF 4] identifies the resting state as dimeric [Rh­(DPEphos)­H2]2 and boronium [H2B­(NMeH2)2]+ as the chain-control agent. [Rh­(DPEphos)­H2]2 can be generated in situ from Rh­(DPEphos)­(benzyl) and catalyzes polyaminoborane formation (H2BNMeH) n [M n = 15 000 g mol–1]. Closely related Rh­(Xantphos)­(benzyl) operates at 0.1 mol % to give a higher molecular weight polymer [M n = 85 000 g mol–1] on the gram scale with low residual [Rh], 81 ppm. This insight offers a mechanistic template for dehydropolymerization.

The air tolerant precatalyst, [Rh­(L)­(NBD)]Cl ([1]­Cl) [L = κ3-( i Pr2PCH2CH2)2NH, NBD = norbornadiene], mediates the selective synthesis of N-methylpolyaminoborane, (H2BNMeH) n , by dehydropolymerization of H3B·NMeH2. Kinetic, speciation, and DFT studies show an induction period in which the active catalyst, Rh­(L)­H3 (3), forms, which sits as an outer-sphere adduct 3·H 3 BNMeH 2 as the resting state. At the end of catalysis, dormant Rh­(L)­H2Cl (2) is formed. Reaction of 2 with H3B·NMeH2 returns 3, alongside the proposed formation of boronium [H2B­(NMeH2)2]­Cl. Aided by isotopic labeling, Eyring analysis, and DFT calculations, a mechanism is proposed in which the cooperative “PNHP” ligand templates dehydrogenation, releasing H2BNMeH (ΔG ‡ calc = 19.6 kcal mol–1). H2BNMeH is proposed to undergo rapid, low barrier, head-to-tail chain propagation for which 3 is the catalyst/initiator. A high molecular weight polymer is formed that is relatively insensitive to catalyst loading (M n ∼71 000 g mol–1; Đ, of ∼ 1.6). The molecular weight can be controlled using [H2B­(NMe2H)2]Cl as a chain transfer agent, M n = 37 900–78 100 g mol–1. This polymerization is suggested to arise from an ensemble of processes (catalyst speciation, dehydrogenation, propagation, chain transfer) that are geared around the concentration of H3B·NMeH2. TGA and DSC thermal analysis of polymer produced on scale (10 g, 0.01 mol % [1]­Cl) 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.