RADICAL POLYMERIZATION OF AMPHIPHILES IN A 2-DIMENSIONAL SOLUTION (MIXED VESICLES)

We studied the size distribution of macromolecular amphiphiles dissolv ed in a Lipid bilayer matrix of phospholipids, which were polymerized photochemically or by thermal decomposition of a radical-forming initi ator. The degree of polymerization, N, was measured by gel permeation chromatography using chloroform/methanol (93/7, v/v) as solvent. The r eliability of the method was checked by comparison; with a transesteri fied polymer, electron microscopy, and quasi-elastic light scattering. Evidence is provided that polymerization yields larger chains in two- dimensional than in three-dimensional solutions. The photochemically i nduced polymerization yields the largest polymers (N approximate to 10 (4)) by short-time irradiation (less than or equal to 5 min at 208 nm) , which is just long enough to produce one or two radicals per vesicle ; Longer irradiation times lead to photodecomposition of the polymers, and after 10 min, only oligomers of N 4 remain. High sample turbiditi es and multilamellar vesicles require longer times due to light scatte ring. Polymerization mediated by thermal decomposition of a water solu ble initiator at 73 degrees C yields sharp distributions of long chain polymers (N approximate to 10(4)) by adjusting the initiator to amphi phile ratio in such a way that polymerization occurs within a reasonab le time (approximate to 1 h). With increasing initiator concentration, the size distribution is shifted to smaller chains (N approximate to 300) due to simultaneous initiation of several chains per vesicle. Lar ge polymers (N greater than or equal to 10(4)) are formed (both photoc hemically and by initiator decomposition) by both methods for amphiphi le concentrations as small as 25 mol %. From the finding of a threshol d concentration above which large polymers are formed, the lifetime of the radical at the growing end is estimated as t(r) approximate to 1- 3 x 10(-7)s. Formation of large polymers leads to lateral phase separa tion of macrolipids. The resulting domain structure of the vesicle is unstable, leading to budding and subsequent detachment of small vesicl es which may be essentially composed of only one large macrolipid. The budding could be a consequence of a strong spontaneous curvature of t he polymerized domains caused by a sterically induced contraction of t he head groups with respect to the chains.