Although living polymerization methods are widely applicable to organic monomers, their application to inorganic monomers is rare. For the first time, we show that the living poly(methylenephosphine) (PMP_n⁻) anion can function as a macroinitiator for olefins. Specifically, the phosphaalkene, MesP=CPh₂ (PA), and methyl methacrylate (MMA) can be sequentially copolymerized using the BnLi‐TMEDA initiator system in toluene. A series of PMP_n‐b‐PMMA_m copolymers with narrow dispersities are accessible (Đ=1.05–1.10). Analysis of the block copolymers provided evidence for ‐P‐CPh₂‐CH₂‐CMe(CO₂Me)‐ switching groups. Importantly, this indicates that the ‐P‐CPh₂⁻ anion directly initiates the anionic polymerization of MMA and stands in stark contrast to the isomerization mechanism followed for the homopolymerization of PA. For the first time, the glass transition of a PMP_n homopolymer has been measured (T_g=45.1 °C, n=20). The PMP_n‐b‐PMMA_m copolymers do not phase separate and show a single T_g which increases with higher PMMA content.