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

The reactivity of polymerizable amphiphiles in the two-dimensional mat rix of supramolecular assemblies, such as Lipid bilayers, is dependent on the monomer structure, the mode of initiation, the monomer to init iator ratio, and the lateral diffusion of the monomer within the assem bly. As part of a study of these factors, me examined the thermally in itiated polymerization and the photopolymerization of SorbPCs. The AIB N-initiated polymerization of hydrated bilayers of 1-palmitoyl-2-[10-( 2',4'-hexadienoyloxy)decanoyl] -sn-glycero-3-phosphocholine (mono-Sorb PC) or xadienoyloxy)decanoyl]-sn-glycero-3-phosphocholine (bis-SorbPC) at 60 degrees C produced linear and cross-linked polymers, respective ly. 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 solub le in tetrahydrofuran and analyzed by size-exclusion chromatography re lative to PMMA standards. The H-1-NMR spectra of the transesterified p olymers 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 ch ain termination at high conversion of SorbPC to polymer is dominated b y primary termination, are similar to the behavior of acryloyl-substit uted lipids (AcrylPC) in bilayers. The poly(AcrylPC)s were 4 times lar ger 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 bilayer s appears to be a singlet-mediated process, which produced oligomers a t temperatures > T-m. The rate of the photoreaction was directly propo rtional to the incident light intensity in a manner consistent with ph otoactivated addition.