High molar mass polyphosphinoboranes substituted with an alkyl group at phosphorus [RPH-BH2](n) (R = tBu, 1-Ad, iPr, Cy, nHex, and Me) have been successfully prepared via dehydropolymerization of the phosphine-boranes RPH2 center dot BH3 using an iron precatalyst, [CpFe(CO)(2)(OTf)] (100 degrees C, toluene, 2.0 M, 10-100 h). Substrate purity and the reaction conditions were found to be crucial in obtaining a high molar mass material as the major product (M-n = 18,200-57,200 g mol(-1) and D = 1.24-3.40). For example, the addition of small quantities of primary phosphines, a potential monomer contaminant, was found to lead to a lower molar mass oligomeric material [RPH-BH2](x). Our experiments indicated that the added Lewis basic primary phosphine does not induce main-chain scission post-polymerization. In contrast, a phosphine-mediated termination process during the catalytic cycle appears to compete with the polymerization of the phosphine-borane monomer. The resulting poly(alkylphosphinoboranes) were characterized by multinuclear NMR spectroscopy, gel permeation chromatography, and electrospray ionization mass spectrometry. The thermal properties were also investigated by thermogravimetric analysis, which showed the materials to be stable to weight loss up to 100-120 degrees C, and differential scanning calorimetry, which revealed strongly side group-dependent T-g values that ranged from -76 to 87 degrees C.