We report that the anionic polymerization of P-mesityl and m-xylyl-substituted phosphaalkenes follows an unusual addition-isomerization mechanism. Specifically, the polymerization of ArP=CPh2 [Ar = Mes (1a), m-Xyl (1b)] involves the hindered nucleophilic anion intermediate, (P)-P(Ar)-CPh2-, which undergoes a proton migration from the o-CH3 of the Mes/m-Xyl moiety to the -CPh2 moiety to afford a propagating benzylic anion. This mechanism is supported by the preparation of model compounds MeP(CHPh2)-4,6-Me2C6H2-2-CH2-CPh3 (2a) or MeP(CHPh2)-6-MeC6H3-2-CH2-CPh3 (2b), which were both crystallographically characterized. Polymerization of la or 1b in THF solution using n-BuLi (2 mol %) revealed H-1 and C-13 NMR signals assigned to -CH2- and -CHPh2 groups consistent with an addition-isomerization polymerization mechanism to afford poly(methylenephosphine) 3a or 3b. A large kinetic isotope effect (<= 23) was determined for the n-BuLi-initiated polymerization of 1a-d(9) compared to la in THF at 50 degrees C, consistent with C-H (or C-D) activation as the rate-determining step. This C-H activation step was modeled using DFT computations which revealed that the intramolecular proton transfer from the o-CH3 of the Mes moiety to the -CPh2 moiety has an activation energy (E-a = +18.5 kcal mol(-1)). For comparison, this computational value was quite close to the experimentally measured activation energy of propagation ArP=CPh2 in THF [E-a = 14.0 +/- 0.9 kcal mol(-1) (1a), 15.6 +/- 2.8 kcal mol(-1) (1b)].