Step-growth titanium-catalysed dehydropolymerisation of amine–boranes† †Electronic supplementary information (ESI) available. See DOI: 10.1039/c7sc05395a

Titanium-catalysed dehydropolymerisation of primary amine–boranes was found to proceed via a step-growth rather than a chain-growth mechanism.

Integration of 11 B NMR spectra was performed using MestReNova Version 7.1.1-9649 with an estimated accuracy of ± 5%. Gel permeation chromatography (GPC) was performed on a Viscotek RImax chromatograph, equipped with an automatic sampler, a pump, an injector and inline degasser. The columns were contained within an oven (35 °C) and consisted of styrene/divinyl benzene gels with pore sizes ranging from 500 Å to 100,000 Å. THF containing 1 % w/w [nBu 4 N]Br and 1% v/v toluene was used as the eluent at a flow rate of 1.0 mL min -1 . All samples analysed by GPC were dissolved in the eluent (2 mg mL -1 in THF), stirred for 1 h at room temperature and passed through a membrane filter (200 nm pores) before analysis. The calibration was conducted using a series of monodisperse polystyrene standards obtained from Aldrich. The elemental analysis was carried out on a CE Instruments (now Thermo) elemental analyser model EA1110. Mass spectra was obtained on a Water Synapt G2S instrument equipped with a nanospray ionisation module using a spray voltage of 1.5 kV (Advion TriVersa Nanomate). Samples were prepared as CH 2 Cl 2 /THF solutions (conc. 1 mg/ml). the procedure was identical, except no nBuLi was required. A 0.5 mL aliquot of the solution was then charged into a quartz J Young NMR tube with approximately 1.7 mL of headspace and a sealed capillary of B(OiPr) 3 as standard, then sealed and allowed to react at 22 °C . Solutions gradually changed from a dark green to a dark indigo colour, and formation of H 2 was immediately noticeable for all precatalysts; readily observed by formation of bubbles and substantial pressure release upon opening the NMR tube post reaction. Reactions were monitored by 11 B{ 1 H} NMR (96 MHz) spectroscopy for the formation of products 2 (δ 11 B -13.8 ppm for terminal BH 3 moiety and 1.6 ppm for internal BH 2 moiety), and 3 (δ 11 B 4.9 ppm). Also present in minor amounts were diaminoborane (Me 2 N) 2 BH (δ 11 B 28.4 ppm) and S 3 aminoborane Me 2 N=BH 2 (δ 11 B 37.4 ppm). Spectra were acquired at 5 min intervals for the first 60 min, followed by acquisitions at 30 min intervals for a total of 690 min. (11.5 h). See figures S1-S6. Figure S1: 11 2 BH. Note, the first 12 spectra (red) were acquired at 5 min intervals, the remainder (black) were acquired at 30 min intervals. * = very small amount of unidentified species at δ -11.6 ppm, as previously reported for reactions utilizing 6a (+ 2nBuLi) as precatalyst. 8 Figure S2: 11 2 BH. Note, the first 12 spectra (red) were acquired at 5 min intervals, the remainder (black) were acquired at 30 min intervals. * = very small amount of unidentified species at δ -11.6 ppm, as previously reported for reactions utilizing 6a (+ 2nBuLi) as precatalyst. 8 Figure S4: 11 2 BH. Note, the first 12 spectra (red) were acquired at 5 min intervals, the remainder (black) were acquired at 30 min intervals. * = very small amount of unidentified species at δ -11.6 ppm, as previously reported for reactions utilizing 6a (+ 2nBuLi) as precatalyst. 8 Figure S5: 11 2 BH. Note, the first 12 spectra (red) were acquired at 5 min intervals, the remainder (black) were acquired at 30 min intervals. * = very small amount of unidentified species at δ -11.6 ppm, as previously reported for reactions utilizing 6a (+ 2nBuLi) as precatalyst. 8

S 8
Spectra were acquired at 5 min intervals for the first 60 min, followed by acquisitions at 30 min intervals for a total of 720 min (12 h), and for precatalyst 6e, additionally at 23 h. See Figures S9 and S10. Figure S8: 11  present (see Figure S18). The very small shoulder at δ 11 B +2 ppm (Figures S10-13) may be tentatively assigned to B(NMeH) 4 moieties arising from chain branching based on previous work. 9 Resulting molecular weights were determined by GPC analysis ( Figure S19) and are summarised in Table S2.        3 , 13 (δ 11 B 32 ppm), (BzNH) 2 BH, and 14 (δ 11 B 28 ppm). Spectra were acquired at 5 min intervals for the first 60 min, followed by acquisitions at 30 min intervals for a total of 720 min (12 h). See Figure S20. Figure S20: 11  (minor signal) connected thiophene substituent (as no effect on the 11        S 20

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Catalytic dehydropolymerisation of MeNH 2 ·BH 3 (4) by precatalyst 6e (variable reaction time). To a solution of substrate 4 (135 mg, 3.0 mmol), in 1 mL of anhydrous toluene was added a solution of precatalyst [6e + 2nBuLi] (0.21 mmol) in 1 mL toluene. The solution was allowed to stir at 22 °C for 0.5, 1, 2 or 4 h. The colour gradually changed from a dark green to a dark indigo/brown, and formation of gas was immediately noticeable. Upon completion, a 0.5 mL aliquot was taken and measured by 11 B{ 1 H} NMR (96 MHz) spectroscopy ( Figures S19-S22). The formation of 5 (δ 11 B -6.1 ppm) was evident from the broad peak at ca. δ 11 B -18 ppm, which had previously been assigned to the polymer end-group      The solution was allowed to react at 22 °C for 1 h, and was monitored by 11 B{ 1 H} NMR (96 MHz) spectroscopy ( Figure S41). In this time, 7 was quantitatively dehydrogenated to borazine 8 (δ 11 B 32.5 ppm, J BH = 135 Hz), and no other byproducts were detected.

Catalytic polycondensation of [MeNH-BH 2 ] x (5) by [IrH 2 (POCOP)].
To a solution of 5 (M n = 3,100 g mol -1 , PDI = 2.7) (30 mg, 0.668 mmol) in 0.35 mL of anhydrous THF at 0 °C was added a solution of [IrH 2 (POCOP)] (0.0068 mmol) in 0.35 mL THF (also at 0 °C ). The reaction solution was allowed to warm to 22 °C then stirred for a further 20 minutes. The yellow solution was then filtered into cold hexanes (-40 °C ), upon which a pale yellow solid precipitated. This was isolated by centrifugation and dried in vacuo (yield: 0.033 g, >100%*). *Note: the high yield presumably results from the presence of residual catalyst in the sample, which would also account for the colour. The product was characterised by 11 B{ 1 H} NMR spectroscopy and GPC (M n = 6,700 g mol -1 , PDI = 2.5) ( Fig   S44 and S45).

Catalytic dehydropolymerisation of MeNH 2 ·BH 3 (4) by [IrH 2 (POCOP)].
To a solution of [IrH 2 (POCOP)] (7 mg, 0.011 mmol) in 0.57 mL anhydrous THF at 0 °C was added a solution of 4 (51 mg, 1.14 mmol), in 0.57 mL of anhydrous THF also at 0 °C . The solution was allowed to warm to 22 o C then stirred for a further 20 min. The yellow solution was then filtered into cold hexanes (-40 °C ), upon which a pale yellow solid precipitated. The solid was isolated by centrifugation and dried