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

The reactivity of polymerizable amphiphiles in the two-dimensional matrix of supramolecular assemblies, such as Lipid bilayers, is dependent on the monomer structure, the mode of initiation, the monomer to initiator ratio, and the lateral diffusion of the monomer within the assembly. As part of a study of these factors, me examined the thermally initiated polymerization and the photopolymerization of SorbPCs. The AIBN-initiated polymerization of hydrated bilayers of 1-palmitoyl-2-[10-(2',4'-hexadienoyloxy)decanoyl] -sn-glycero-3-phosphocholine (mono-SorbPC) or 1,2-bis[10-(2',4'-hexadienoyloxy)decanoyl]-sn-glycero-3-phosphocholine (bis-SorbPC) at 60 degrees C produced linear and cross-linked polymers, respectively. in each case the polymers were transesterfied to remove the lipid head group and yield linear copolymers composed of random repeat units of methyl sorbate and methyl carboxynonanyl sorbate, which were soluble in tetrahydrofuran and analyzed by size-exclusion chromatography relative to PMMA standards. The H-1-NMR spectra of the transesterified polymers indicated the structures were 1,4-polymers. The number-average relative degree of polymerization (X(n)) ranged from 50 to nearly 600 and was proportional to [I](-1). These results, which suggest that chain termination at high conversion of SorbPC to polymer is dominated by primary termination, are similar to the behavior of acryloyl-substituted lipids (AcrylPC) in bilayers. The poly(AcrylPC)s were 4 times larger than poly(SorbPC)s at a given ratio of monomer to initiator. This is likely due to greater resonance stabilization of the sorbyl radical compared to the acryloyl radical. The photoreaction of SorbPC bilayers appears to be a singlet-mediated process, which produced oligomers at temperatures > T-m. The rate of the photoreaction was directly proportional to the incident light intensity in a manner consistent with photoactivated addition.