The chemical functionality of poly(methylenephosphine) n-Bu[MesP-CPh2]nH (2) is examined in reactions with two isoelectronic species, namely BH3 and CH3+. The potential reactivity of polymer 2 is modelled by examining the reactivity of molecular phosphines bearing similar substituents as the polymer. In particular, the phosphine-borane adducts Mes(Me)P(BH3)-CPh2H (4a) and Mes(Me)P(BH3)-CPh2SiMe2H (4b) are prepared from the reaction of BH3.SMe2 with Mes(Me)P-CPh2H (3a) or Mes(Me)P-CPh2SiMe2H (3b), respectively. Treating 3a with MeOTf affords the methylated model compound, [Mes(Me)2P-CPh2H]OTf (5). X-Ray crystal structures are reported for each model compound. The reaction of n-Bu[MesP-CPh2]nH (Mn = 3.89 x 10(4), PDI = 1.34) with BH3.SMe2 affords the phosphine-borane polymer n-Bu[MesP(BH3)-CPh2]nH (6) (Mn = 4.13 x 10(4), PDI = 1.26). In contrast, methylation of phosphine polymer 2 gives n-Bu[MesP-CPh2]x-/-[MesP(Me)-CPh2]yH.(OTf)y (7) where approximately 50% of the phosphine moieties are methylated (from 31P NMR).