A systematic study of the effect of monomer [M] and initiator [I] concentration on the rate of polymerization, R(p), for bilayers of acryloyl-and methacryloyl-substituted lipids is described. In a companion paper (Macromolecules 1994,27, 226) Sells and O’Brien describe the effect of these variables on the degree of polymerization for the thermally initiated radical polymerization of an acryloyl-substituted phosphatidylcholine (mono-AcrylPC) in bilayer assemblies. Interestingly, the relative degree of polymerization (X(n)), for the polymerization was proportional to [M]2 and [I]-1, which indicates that chain termination at high conversion to polymer was dominated by primary termination. Both the polymerizations of mono-AcrylPC and mono-MethPC bilayers exhibit a R(p) dependence on [M] and [I]0.5 at the start of the polymerization, which indicates the bilayer-constrained polymer chains terminate by chain coupling and/or chain disproportionation. As the polymerization approaches high conversion, the kinetic behavior changes. Both the R(p) of mono-AcrylPC and the mono-MethPC bilayer polymerizations at high conversions show an increased dependence on [M], 1.86 and 1.6, respectively, and a diminished dependence on [I]. The course of the polymerization appears to progressively modify the bilayer in a manner that reduces the probability of bimolecular chain termination reactions and favors increased domination by primary termination.