Poly(<scp>l</scp>,<scp>l</scp>-lactide) Microspheres by Ring-Opening Polymerization

Abstract

Poly(l,l-lactide) microspheres were synthesized directly during the ring-opening precipitation polymerizations of l,l-lactide. The polymerizations were carried out in heptane−1,4-dioxane (4:1 v/v) mixed solvent in the presence of various poly(dodecyl acrylate)-g-poly(ε-caprolactone) copolymers (poly(DA−CL)) used as surface active agents. Copolymers with different molecular weights, different average number of poly(ε-caprolactone) grafts per macromolecule, and different molecular weights of these grafts were used. Poly(l,l-lactide) microspheres with number-average diameters (D̄n) from 2.46 to 4.07 μm and with polydispersity parameters (D̄v/D̄n, where D̄v denotes the volume-average diameter) from 1.08 to 1.45 were obtained. The dependence of D̄n and D̄v/D̄n on the structure and on the concentration of poly(DA−CL) was investigated. The narrow diameter polydispersity (D̄v/D̄n = 1.08) was obtained when poly(DA−CL), containing an average of 1.3 poly(ε-caprolactone) grafts (with M̄n = 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-lactide) microspheres contained from 1.5 to 6.6 wt % of the unreacted lactide. The optical purity of monomer (95.4%) and of the corresponding polylactide from microspheres (from 91.0% to 94.9%) indicated that polymerization in the investigated heterogeneous system proceeds with retention of configuration on the chiral carbon atom. The DSC studies revealed that the crystallinity of the poly(l,l-lactide) microspheres was strongly dependent on the thermal history of these materials and on the concentration of the surface active agent used for the polymerization. Microspheres partly composed of crystalline and/or amorphous poly(l,l-lactide) were obtained.