Polyesters from lactides and ε-caprolactone. Dispersion polymerization versus polymerization in solution

Abstract

Polymerizations of lactides and ε-caprolactone, carried out in 1,4-dioxane:heptane mixtures in the presence of poly(dodecyl acrylate)- g-poly(ε-caprolactone) surface active agent, yield polymers in the form of microspheres. Polymerizations are initiated in solution. Shortly after initiation particles are nucleated and the main part of propagation proceeds in heterogeneous systems which consist of growing microspheres suspended in the 1,4-dioxane-heptane-monomer media. These dispersion polymerizations differ in many aspects from the corresponding polymerizations in solution. Suspensions of microspheres are significantly less viscous than polymer solutions with the same polymer content. For example, viscosity of suspension of polylactide microspheres (microsphere diameters D ¯ n = 2.7 μ m ) in heptane is 14 times lower than that of the THF solution of the same polylactide (polymer content in suspension and in solution 5 wt%). High local concentrations of active centers and monomers inside microspheres result in fast polymerization. For polymerizations of ε-caprolactone in dispersed systems, a given degree of monomer conversion is achieved from 10 to 20 times earlier than for the corresponding polymerizations in solution. Dispersion polymerizations yield polylactides and poly(ε-caprolactone) with 1.05 < M ¯ w / M ¯ n < 1.29 . Polylactide and poly(ε-caprolactone) microspheres with a high values of monodispersity parameter ( ξ = D ¯ n / ( D ¯ v − D ¯ n ) > 10 ) are obtained in polymerizations carried out in the presence of a surface active agent with a ratio of molecular weight of poly(ε-caprolactone) grafts and molecular weight of poly(dodecyl acrylate)- g-poly(ε-caprolactone) copolymer close to 0.23. The microspheres can be isolated from suspension by gravitational sedimentation. Depending on the post-synthesis treatment it is possible to obtain poly(L,L-lactide) microspheres with controlled degree of crystallinity.