The polyphosphinoboranes [PhPH-BH2](n) (1), [iBuPH-BH2](n), (2), 1(p-nBuC(6)H(4))PH-BH2](n) (3), and [(p-dodecylC(6)H(4))PH-BH2](n) (4) were prepared from the corresponding phosphine-borane adducts RPH2-BH3 (R = Ph, iBu, p-nBuC(6)H(4), p-dodecylC(6)H(4)) via a rhodium-catalyzed dehydrocoupling procedure at elevated temperatures (ca. 90-130 degreesC). 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 M-w = 13 100, and values of M, 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 HNEt2 or nBu(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 degreesC, respectively. TGA analysis on 1-3 revealed T-5% values (temperature for which 5% or the weight is lost) of 240 degreesC for 1 and ca. 150-160 degreesC for 2 and 3. After heating to 1000 degreesC, 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.