POLYESTERS FROM LACTIDES AND EPSILON-CAPROLACTONE - DISPERSION POLYMERIZATION VERSUS POLYMERIZATION IN SOLUTION

Polymerizations of lactides and epsilon-caprolactone, carried out in 1 ,4-dioxane:heptane mixtures in the presence of poly(dodecyl acrylate)- g-poly(epsilon-caprolactone) surface active agent, yield polymers in t he form of microspheres. Polymerizations are initiated in solution. Sh ortly after initiation particles are nucleated and the main part of pr opagation proceeds in heterogeneous systems which consist of growing m icrospheres suspended in the 1,4-dioxane-heptane-monomer media. These dispersion polymerizations differ in many aspects from the correspondi ng polymerizations in solution. Suspensions of microspheres are signif icantly less viscous than polymer solutions with the same polymer cont ent. For example, viscosity of suspension of polylactide microspheres (microsphere diameters (D) over bar(n) = 2.7 mu m) in heptane is 14 ti mes lower than that of the THF solution of the same polylactide (polym er content in suspension and in solution 5 wt%). High local concentrat ions of active centers and monomers inside microspheres result in fast polymerization. For polymerizations of epsilon-caprolactone in disper sed systems, a given degree of monomer conversion is achieved from 10 to 20 times earlier than for the corresponding polymerizations in solu tion. Dispersion polymerizations yield polylactides and poly(epsilon-c aprolactone) with 1.05 < (M) over bar(w)/(M) over bar(n) < 1.29. Polyl actide and poly(epsilon-caprolactone) microspheres with a high values of monodispersity parameter (xi = (D) over bar(n)((D) over bar(v) - (D ) over bar(n)) > 10) are obtained in polymerizations carried out in th e presence of a surface active agent with a ratio of molecular weight of poly(epsilon-caprolactone) grafts and molecular weight of poly(dode cyl acrylate)-g-poly(epsilon-caprolactone) copolymer close to 0.23. Th e microspheres can be isolated from suspension by gravitational sedime ntation. Depending on the post-synthesis treatment it is possible to o btain poly(L,L-lactide) microspheres with controlled degree of crystal linity. (C) 1998 Elsevier Science Limited. All rights reserved.