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 H2BNMeH (ΔG
‡
calc = 19.6 kcal mol–1). H2BNMeH 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.
