Abstract
[Rh(κ2-PP-DPEphos){η2η2-H2B(NMe3)(CH2)2tBu}][BArF4] acts as an effective precatalyst for the dehydropolymerization of H3B·NMeH2 to form N-methylpolyaminoborane (H2BNMeH)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 H2: with Mn ranging between 5 500 and 34 900 g/mol and Đ between 1.5 and 1.8. H2 evolution studies (1,2-F2C6H4 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, [Rh2(κ2-PP-DPEphos)2(μ-H)(μ-H2BN=HMe)][BArF4], followed by the crystallographically characterized amidodiboryl complex [Rh2(cis-κ2-PP-DPEphos)2(σ,μ-(H2B)2NHMe)][BArF4]. Adding ∼2 equiv of NMeH2 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., Mn = 40 000 g/mol). Speciation studies suggest that NMeH2 acts to form the precatalysts [Rh(κ2-DPEphos)(NMeH2)2][BArF4] and [Rh(κ2-DPEphos)(H)2(NMeH2)2][BArF4], which were independently synthesized and shown to follow very similar dehydrogenation kinetics, and produce polymers of molecular weight comparable with [Rh(κ2-PP-DPEphos){η2-H2B(NMe3)(CH2)2tBu}][BArF4], which has been doped with amine. This promoting effect of added amine in situ is shown to be general in other cationic Rh-based systems, and possible mechanistic scenarios are discussed.
Highlights
Polyaminoboranes,[1−4] exemplified by N-methylpolyaminoborane (H2BNMeH)n, have alternating main-chain B−N units and are of interest as precursors to BN-based ceramics or as new unexplored materials that are isosteres of polyolefins
We propose an overall mechanism shown in Scheme 5, in which the induction period relationship between Mn and [Rh]TOTAL, coupled with the sensitivity to H2, suggests a coordination/insertion/chaingrowth mechanism for which NMeH2 modifies chain length possibly by attenuating chain termination
On the basis of the half-order kinetics observed from the dehydrogenation studies, we suggest three possible general motifs for the active catalyst (Scheme 6): one which invokes a monomer−
Summary
Polyaminoboranes,[1−4] exemplified by N-methylpolyaminoborane (H2BNMeH)n, have alternating main-chain B−N units and are of interest as precursors to BN-based ceramics or as new unexplored materials that are isosteres of polyolefins. A number of different propagation scenarios have been proposed for this latter step that show elements of chain-growth,[4,10] step-growth,[15] or hybrid mechanisms.[16] interesting would be systems that demonstrate the potential for control[17] over the polymerization process, holistically defined by degree of polymerization (as measured by Mn), dispersity (Đ), initiation/termination. Dehydropolymerization of Amine−Boranes events, and catalyst lifetime (i.e., TON). While aspects of these performance criteria have been noted,[7−10,15] there is no general approach to their optimization
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