2-DIMENSIONAL POLYMERIZATION OF LIPID BILAYERS - DEGREE OF POLYMERIZATION OF ACRYLOYL LIPIDS

The chemical reactivity of the amphiphiles in supramolecular assemblie s is dependent on the constraints imposed by the two-dimensional matri x 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 deg ree of polymerization of acryloyl-substituted phospholipids was analyz ed. The AIBN-initiated polymerization of hydrated bilayers of cryloylo xy)dodecanoyl]-sn-glycero-3-phosphocholine (mono-AcrylPC) at 70 degree s C produced linear polymers, which were characterized after transeste rification to remove the lipid head group. The resulting copolymers, w hich were composed of random repeat units of methyl acrylate and methy l carboxyundecylacrylate, were soluble in chloroform and analyzed by s ize-exclusion chromatography relative to PMMA standards. The number-av erage relative degree of polymerization (X(n)) ranged from 50 to nearl y 2000 and was proportional to [M](2) and [I](-1). These results sugge st that chain termination at high conversion of mono-AcrylPC to polyme r is dominated by primary termination. The observed effects are probab ly due to reduced mobility of the long polymer chains in the constrain ed environment of the bilayer interior, which diminishes the opportuni ty for bimolecular chain termination.