An Addition–Isomerization Mechanism for the Anionic Polymerization of MesPCPh2 and m‑XylPCPh2

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(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 1a or 1b in THF solution using n-BuLi (2 mol %) revealed 1H and 13C 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-d9 compared to 1a in THF at 50 °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 (Ea = +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 [Ea = 14.0 ± 0.9 kcal mol–1 (1a), 15.6 ± 2.8 kcal mol–1 (1b)].