Latexes and microspheres by ring-opening polymerization. Polymerization of cyclic esters

Latexes and microspheres were synthesized by pseudoanionic polymerization of lactides initiated with tin(II) 2-ethylhexanoate and pseudoanionic and anionic polymerization of epsilon-caprolactone initiated with (CH3CH2)(2)AlOCH2CH3 and (CH3)(3)SiONa, respectively. Polymerizations were carried out in 1,4-dioxane/heptane mixtures with poly(dodecyl acrylate)-g-poly(epsilon-caprolactone) (poly(DA-CL)) added as a surface active agent. Propagation was initiated in the homogeneous systems. When propagating macromolecules reach their critical lengths ((M) over bar(n) approximate to 1000), they precipitate, and stabilized by macromolecules of poly(DA-CL) form nuclei of microspheres. The number of particles formed in the initial period of the pseudoanionic polymerization of L,L-lactide and epsilon-caprolactone remains constant whereas in the anionic polymerization of epsilon-caprolactone a weak aggregation, manifested by decreasing number of particles, was observed. Determination of partition of monomer and active centers between continuous and condensed (particles) phases revealed that polymeric particles were highly swollen with monomer (e.g. after incubation of poly(epsilon-caprolactone) latex ([poly(CL)] = 2.17 x 10(1) g/l) in a solution containing initially [epsilon-caprolactone] = 7.50 x 10(-2) mol/l, 28% of monomer became incorporated into polymer particles) and that shortly after the initiation period all active centers were located inside growing latex particles. High local concentrations of monomer and active centers resulted in rates of polymerization which were up to ca 25 times higher than the rates of similar polymerization with the same monomer and initiator concentrations averaged over the whole volume of the reaction mixtures. Diameters of obtained poly(epsilon-caprolactone) latex particles were in the region from 0.6 to 0.7 mu m and diameters of poly(lactide) microspheres varied from 2.2 to 4.2 mu m depending on the polymerization conditions. Polydispersity of particle diameters was found to be strongly dependent on the ratio of molecular weight of poly(epsilon-caprolactone) grafts and molecular weight of poly(DA-CL) copolymer. The most uniform poly(L,L-lactide) microspheres ((D) over bar(v)/(D) over bar(n) < 1.05) were obtained when the value of this parameter was close to 0.25. Polymers in poly(lactide) and poly(epsilon-caprolactone) particles were characterized by a narrow molecular weight distribution (1.05 < (M) over bar(w)/(M) over bar(n) < 1.3). In the case of the anionic dispersion polymerization of epsilon-caprolactone, the intra- and intermolecular transesterification side reactions were reduced and latexes were obtained made of polymer with (M) over bar(n) > 100 000, free from the admixture of cyclic oligomers and with (M) over bar(w)/(M) over bar(n) = 1.06. (C) 1999 Elsevier Science B.V. All rights reserved.