POLY(L,L-LACTIDE) MICROSPHERES BY RING-OPENING POLYMERIZATION

Poly(L,L-lactide) microspheres were synthesized directly during the ri ng-opening precipitation polymerizations of L,L-lactide. The polymeriz ations were carried out in heptane-1,4-dioxane (4:1 v/v) mixed solvent in the presence of various poly(dodecyl acrylate)-g-poly(epsilon-capr olactone) copolymers (poly(DA-CL)) used as surface active agents. Copo lymers with different molecular weights, different average number of p oly(epsilon-caprolactone) grafts per macromolecule, and different mole cular weights of these grafts were used. Poly(L,L-lactide) microsphere s with number-average diameters ((D) over bar i(n)) from 2.46 to 4.07 mu m and with polydispersity parameters ((D) over bar(v)/(D) over bar( n), where (D) over bar(v) denotes the volume-average diameter) from 1. 08 to 1.45 were obtained. The dependence of (D) over bar(n), and (D) o ver bar(v)/(D) over bar(n) on the structure and on the concentration o f poly (DA-CL) was investigated. The narrow diameter polydispersity (( D) over bar(v)/(D) over bar(n) = 1.08) was obtained when poly(DA-CL), containing an average of 1.3 poly(epsilon-caprolactone) grafts (with ( M) over bar(n) = 4700 g/mol) per copolymer macromolecule, was used at a concentration of 1.6 g/L, i.e., below the critical concentration of micellization of this surface active agent (ccm = 5.1 g/L). Poly(L,L-l actide) microspheres contained from 1.5 to 6.6 wt% of the unreacted la ctide. The optical purity of monomer (95.4%) and of the corresponding polylactide from microspheres (from 91.0% to 94.9%) indicated that pol ymerization in the investigated heterogeneous system proceeds with ret ention of configuration on the chiral carbon atom. The DSC studies rev ealed that the crystallinity of the poly(L,L-lactide) microspheres was strongly dependent on the thermal history of these materials and on t he concentration of the surface active agent used For the polymerizati on. Microspheres partly composed of crystalline and/or amorphous poly( L,L-lactide) were obtained.