Synthesis, Characterization, and Properties of the Polyphosphinoboranes [RPH−BH₂]n(R = Ph,iBu,p-nBuC₆H₄,p-dodecylC₆H₄):  Inorganic Polymers with a Phosphorus−Boron Backbone

The polyphosphinoboranes [PhPH−BH 2 ] n ( 1 ), [ i BuPH−BH 2 ] n ( 2 ), [( p - n BuC 6 H 4 )PH−BH 2 ] n ( 3 ), and [( p -dodecylC 6 H 4 )PH−BH 2 ] n ( 4 ) were prepared from the corresponding phosphine−borane adducts RPH 2 ·BH 3 (R = Ph, i Bu, p - n BuC 6 H 4, p -dodecylC 6 H 4 ) via a rhodium-catalyzed dehydrocoupling procedure at elevated temperatures (ca. 90−130 °C). Samples of polymers 1 and 2 and the new materials 3 and 4 were characterized by multinuclear NMR spectroscopy, and the molecular weights were determined by light scattering methods in THF or CH 2 Cl 2 solutions. The absolute weight-average molecular weight of 2 was determined by static light scattering and found to be M w = 13 100, and values of M w of ca. 20 000 were estimated for the sample of polymers 1 and the new material 3 using dynamic light scattering (DLS). The molecular weights of polymers 3 and 4 were also analyzed by gel permeation chromatography using polystyrene standards, and values up to M w = ca. 80 000, M n = ca. 10 000 were determined for 3 and M w = ca. 168 000, M n = ca. 12 000 for 4 . The chemical stability of 1 in THF toward HNEt 2 or n Bu 3 P was demonstrated using NMR spectroscopy and DLS analysis, which indicated that no significant polymer degradation occurred. WAXS analysis of 1 and 3 showed that the polymers are amorphous. The glass transition temperatures ( T g ) of polymers 2, 3, and 4 were analyzed by DSC and were detected at ca. 5, 8, and −1 °C, respectively. TGA analysis on 1 − 3 revealed T 5% values (temperature for which 5% or the weight is lost) of 240 °C for 1 and ca. 150−160 °C for 2 and 3 . After heating to 1000 °C, ceramic yields in the range of 35−80% were obtained. The high ceramic yield for 1 (75−80%) indicates that this material is of interest as a pyrolytic precursor to boron phosphide-based solid-state materials.