The dehydropolymerization of the primary phosphine–boranes, RPH2•BH3 (1a–f) (R = 3,4-(OCH2O)C6H3 (a), Ph (b), p-(CF3O)C6H4 (c), 3,5-(CF3)2C6H3 (d), 2,4,6-(CH3)3C6H2 (e), 2,4,6-tBu3C6H2 (f)) is explored using the precatalyst [CpFe(CO)2OTf] (I) (OTf = OS(O)2CF3), based on the earth abundant element Fe. Formation of polyphosphinoboranes [RPH-BH2]n (2a–e) is confirmed by multinuclear NMR spectroscopy, but no conversion of 1f to 2f is detected. Analysis by electrospray ionization mass spectrometry confirms the presence of the anticipated polymer repeat units for 2a–e. Gel permeation chromatography (GPC) confirms the polymeric nature of 2a–e and indicates number-average molecular weights (Mn) of 12 000–209 000 Da and polydispersity indices between 1.14 and 2.17. By contrast, thermal dehydropolymerization of 1a–e in the absence of added precatalyst leads to formation of oligomeric material. Interestingly, polyphosphinoboranes 2c and 2d display GPC behavior typical of polyelectrolytes, with a hydrodynamic radius dependent on concentration. The thermal transition behavior, thermal stability, and surface properties of thin films are also studied.