Polymerization of liposomes evaluated from molecular weight distribution of diene-type phospholipid polymers

1, 2-Bis(2, 4-octadecadienoyl)-sn-glycero-3-phosphorylcholine (DODPC) or 1-palmitoyl-2-(2, 4-octadecadienoyl)-sn-glycero-3-phosphorylcholine (POPC) was dispersed in water by sonication to form liposomes. DODPC liposomes were polymerized by UV-irradiation or with radical initiator such as azobis(2-amidinopropane)dihydrochloride (AAPD) at 60°C and methanolyzed to obtain THF-soluble polymers. The molecular weight distribution obtained from a gel permeation chromatography reveals that the UV-irradiation provides low-molecular-weight products and a small amount of polymers, while free radical polymerization gives polymers having relatively high-molecular-weight. The molecular weight of the phospholipid polymers is also influenced by the assembling state. The constrained lipid packing of small unilamellar POPC liposomes provides relatively low-molecular-weight polymers in comparison with those of larger liposomes. In addition, POPC liposomes polymerized at the temperature above the gel-to-liquid crystalline phase transition temperature (T_c) show the higher molecular weight of polymers than those polymerized below the T_c.