Synthesis, Characterization, and Properties of the Polyphosphinoboranes [RPH−BH2]n(R = Ph,iBu,p-nBuC6H4,p-dodecylC6H4): Inorganic Polymers with a Phosphorus−Boron Backbone
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Abstract
The polyphosphinoboranes [PhPH−BH2]n (1), [iBuPH−BH2]n (2), [(p-nBuC6H4)PH−BH2]n (3), and [(p-dodecylC6H4)PH−BH2]n (4) were prepared from the corresponding phosphine−borane adducts RPH2·BH3 (R = Ph, iBu, p-nBuC6H4, p-dodecylC6H4) 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 CH2Cl2 solutions. The absolute weight-average molecular weight of 2 was determined by static light scattering and found to be Mw = 13 100, and values of Mw 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 Mw = ca. 80 000, Mn = ca. 10 000 were determined for 3 and Mw = ca. 168 000, Mn = ca. 12 000 for 4. The chemical stability of 1 in THF toward HNEt2 or nBu3P 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 (Tg) 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 T5% 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.
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