Liposome formation of selectively‐polymerized diene‐containing phospholipids and their postpolymerization

Small and large unilamellar liposomes composed of 1,2‐bis(2,4‐octadecadienoyl)‐sn‐glycero‐3‐phosphorylcholine (DODPC) are prepared by sonication and extrusion, respectively. They are polymerized with water‐insoluble radical initiator, azobis(isobutyronitrile) (AIBN) which can selectively polymerize diene groups in 1‐acyl chains of the lipids. Polymerized liposomes are freeze‐dried to obtain the polymerized liposome powder. There are two methods to redisperse lyophilized liposomes into water. The extrusion is an effective method to disperse them because the energy at extrusion is necessary only for redispersion, whereas the excess energy at sonication gives damage on liposome structure. There is no difference in stability between polymerized liposomes before and after redispersion with extrusion. DODPC polymers, obtained from free radical‐initiated polymerization with AIBN, are linear and have polymerizable diene groups in 2‐acyl chains. The liposome powder is therefore soluble in organic solvents. Reconstruction of polymerized liposomes is performed with lipid polymers having low or high molecular weight. The lipid polymers having high molecular weight provide stable large unilamellar liposomes by ethanol injection, but unstable small unilamellar liposomes are formed by sonication. The liposomes reconstructed from lipid polymers having low molecular weight are unstable regardless of their size. After reconstruction of liposomes selectively polymerized by AIBN, diene groups in 2‐acyl chains are polymerized by water‐soluble radical initiator or UV‐irradiation to yield highly crosslinked structure. Their stability is improved remarkably by this postpolymerization.