A systematic study of the catalyst structure and overall charge for the dehydropolymerization of H₃B·NMeH₂ to form N-methyl polyaminoborane is reported using catalysts based upon neutral and cationic {Rh(Xantphos-R)} fragments in which PR₂ groups are selected from Et, ⁱPr, and ^tBu. The most efficient systems are based upon {Rh(Xantphos-ⁱPr)}, i.e., [Rh(κ³-P,O,P-Xantphos-ⁱPr)(H)₂(η¹-H₃B·NMe₃)][BAr^F₄], 6, and Rh(κ³-P,O,P-Xantphos-ⁱPr)H, 11. While H₂ evolution kinetics show both are fast catalysts (ToF ≈ 1500 h^-1) and polymer growth kinetics for dehydropolymerization suggest a classical chain growth process for both, neutral 11 (M_n = 28 000 g mol^-1, Đ = 1.9) promotes significantly higher degrees of polymerization than cationic 6 (M_n = 9000 g mol^-1, Đ = 2.9). For 6 isotopic labeling studies suggest a rate-determining NH activation, while speciation studies, coupled with DFT calculations, show the formation of a dimetalloborylene [{Rh(κ³-P,O,P-Xantphos-ⁱPr)}₂B]⁺ as the, likely dormant, end product of catalysis. A dual mechanism is proposed for dehydropolymerization in which neutral hydrides (formed by hydride transfer in cationic 6 to form a boronium coproduct) are the active catalysts for dehydrogenation to form aminoborane. Contemporaneous chain-growth polymer propagation is suggested to occur on a separate metal center via head-to-tail end chain B-N bond formation of the aminoborane monomer, templated by an aminoborohydride motif on the metal.

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