The chemical reactivity of the amphiphiles in supramolecular assemblies is dependent on the constraints imposed by the two-dimensional matrix of the assembly. As part of a systematic study of the variables that control the polymerization of reactive lipids in lipid bilayers, the effect of both monomer [M] and initiator [I] concentrations on the degree of polymerization of acryloyl-substituted phospholipids was analyzed. The AIBN-initiated polymerization of hydrated bilayers of 1-palmitoyl-2-[12-(acryloyloxy)dodecanoyl]-sn-glycero-3-phosphocholine (mono-AcrylPC) at 70 degrees C produced linear polymers, which were characterized after transesterification to remove the lipid head group. The resulting copolymers, which were composed of random repeat units of methyl acrylate and methyl carboxyundecylacrylate, were soluble in chloroform and analyzed by size-exclusion chromatography relative to PMMA standards. The number-average relative degree of polymerization (X(n)) ranged from 50 to nearly 2000 and was proportional to [M](2) and [I](-1). These results suggest that chain termination at high conversion of mono-AcrylPC to polymer is dominated by primary termination. The observed effects are probably due to reduced mobility of the long polymer chains in the constrained environment of the bilayer interior, which diminishes the opportunity for bimolecular chain termination.