Since Beckers synthesis of the first stable phosphaalkene in 1976, [1] these compounds, which contain a P=C bond and were once considered exotic, now constitute a major branch of phosphorus chemistry. Applications for these compounds are even being developed. [2][3][4] The ability of P = C bonds, in many instances, to "copy" the well-established chemistry of isolobal C=C bonds has attracted considerable attention. [2, 3a, d] In molecular chemistry, the remarkable analogy between phosphaalkenes and alkenes is evidenced by phospha variants of a number of reactions, including 1,2-addition, [4+2]cycloaddition, Peterson and Wittig olefination, h 2 coordination to metal centers, and Cope and allylic rearrangements. One of the most important reactions of C = C bonds is the addition polymerization of olefins, which is used to produce many commodity polymers. The absence of an analogous polymer chemistry for P=C bonds prompted us to investigate addition polymerization as a potential route to new phosphine polymers. We recently reported the first addition polymerization of the phosphaalkene 1 [5] (Mes = 2,4,6-trimethylphenyl) to give the poly(methylenephosphine) 2 (M ¯w % 10 4 g mol À1 by GPC versus polystyrene), an alternating PÀC polymer. [6] The development of synthetic methodologies to incorporate phosphorus into the backbone of macromolecules or into dendrimers is a challenging frontier in polymer chemistry