Degree of polymerization in two-dimensional assemblies of acryloyl or methacryloyl lipids.

Abstract

The first systematic study of the effect of variations in initiator and monomer concentrations on two-dimensional polymerizations is described here. The number average degree of polymerization, X(n), was estimated by SEC for the thermal polymerization of bilayers composed of a mono-substituted acryloyl phospholipid. When the monomer to initiator (azobisiso-butyronitrile) molar ratio, [M]/[I], was varied from 5 to 40, holding [M] constant, the X(n) increased from 233 to 1,936. The measured X(n) was observed to be inversely proportional to the initiator concentration. When the monomer concentration was diluted by the incorporation of a nonpolymerizable lipid (DMPC), the X(n) was decreased from 1,476 for pure mono-acryloyl PC to 299 for a 1/1 molar ratio of mono-acryloyl PC and DMPC. The X(n) was found to be dependent on the square of the monomer mole fraction in the bilayers. The polymerization of bis-acryloyl PC produced much shorter polymers than mono-acryloyl PC but appeared to have the same dependency on initiator concentration. These data suggest that this two-dimensional polymerization is preferentially terminated by reaction with initiator fragments rather than by the usual radical coupling and/or disproportionation reactions of the growing polymer chains. It is suggested that the preference for termination by small molecular fragments is a consequence of limited diffusion of the growing polymer chains in the constrained environment of the lipid bilayer. Furthermore these data demonstrate that careful attention to the ratio of [M]/[I] is necessary to reproducibly control the size and properties of polymers formed in supramolecular assemblies.