Novel polymer architectures via the selective polymerization of lyotropic liquid crystals of heterobifunctional amphiphiles

The reactivity of polymerizable amphiphiles in supramolecular assemblies, such as lipid bilayers, is dependent on the mode of initiation, the polymerizable group, and the position of the reactive group in the amphiphile. This report describes the synthesis, characterization, and polymerization of a novel heterobifunctional lipid, 1-palmitoyl-2-(2,4,12,14-tetraenehexadecanoyl)phosphatidylcholine (1), that contains a diene and a dienoyl group in the sn-2 acyl chain. The difference in polarity of local sites of each group within the bilayer makes it possible to perform simultaneous or selective polymerizations of these groups. Simultaneous polymerization of both reactive groups was achieved by either redox polymerization or direct photoirradiation. Selective polymerization of the dienoyl was accomplished by photoirradiation with filtered light from a high pressure Hg/Xe lamp. The diene was selectively initiated by thermal decomposition of AIBN. The degree of polymerization depended strongly on the initiation chemistry. Photoirradiation gave oligomers, whereas radical polymerization with redox or AIBN afforded polymers with relative number average degrees of polymerization of 200 and 350, respectively. Both diene and dienoyl groups formed 1,4-poly(diene) regardless of the mode and order of the polymerization applied. The polymers obtained from hydrated bilayers of 1 by either simultaneous polymerization of both reactive groups or sequential selective polymerization of each group were not cross-linked. The absence of cross-linking at high conversions to polymer and the size of the solubilized surfactant-polymer micelles are consistent with the formation of ladder-like polymers with parallel chains tethered together by a hexa-methylene spacer.