Althoughl ivingp olymerization methods are widely applicable to organic monomers, their applicationt o inorganic monomers is rare. For the first time, we show that the living poly(methylenephosphine) (PMP n À )a nion can function as am acroinitiator for olefins.S pecifically,t he phosphaalkene, MesP=CPh 2 (PA), and methyl methacrylate (MMA) can be sequentially copolymerized using the BnLi-TMEDA initiator system in toluene. As eries of PMP n -b-PMMA m copolymers with narrow dispersities are accessible ( = 1.05-1.10).Analysis of the block copolymers provided evidence for -P-CPh 2 -CH 2 -CMe(CO 2 Me)-switching groups.I mportantly,t his indicates that the -P-CPh 2 À anion directly initiates the anionic polymerization of MMA and stands in stark contrast to the isomerization mechanism followed for the homopolymerization of PA.F or the first time, the glass transition of a PMP n homopolymer has been measured (T g = 45.1 8C, n = 20). The PMP n -b-PMMA m copolymers do not phase separatea nd show as ingle T g which increases with higherP MMA content.Scheme1.Anionic polymerization of RP=CAr 2 (R = Mes or o-Xyl).

We report the attempted synthesis of a “masked” phosphaalkene by treating MgA·3THF (A = anthracenide) with MesPCl(CH2Cl). Although the desired “masked” phosphaalkene could not be isolated, indirect evidence for its formation was obtained. The product of trapping MesP=CH2 with 1,3‐cyclohexadiene was detected. In addition, the oxide MesPO(Me)A was isolated and crystallographically characterized from the MgA·3THF and MesPCl(CH2Cl) experiment described above. Finally, the attempted isolation of “masked” phoshaalkene afforded P‐containing oligomers with a trimodal molecular weight distribution [Mn = 640, 1.7 × 103 and 7.4 × 103 Da].