Addition polymerization, the most general method of preparation for organic polymers, has successfully been extended to P=C bonds. The polymerization of a phosphaalkene has been initiated by thermolysis or with alkyllithium reagents. The unprecedented poly(methylenephosphine)s are easily oxidized using oxygen or sulfur to give air stable macromolecules. A molecular weight (M(w)) of 35000 g/mol for the poly(methylenephosphine sulfude) was estimated by light-scattering GPC.