Latexes and microspheres by ring-opening polymerization. Polymerization ofcyclic esters

Latexes and microspheres were synthesized by pseudoanionic polymerization o f lactides initiated with tin(II) 2-ethylhexanoate and pseudoanionic and an ionic polymerization of epsilon-caprolactone initiated with (CH3CH2)(2)AlOC H2CH3 and (CH3)(3)SiONa, respectively. Polymerizations were carried out in 1,4-dioxane/heptane mixtures with poly(dodecyl acrylate)-g-poly(epsilon-cap rolactone) (poly(DA-CL)) added as a surface active agent. Propagation was i nitiated in the homogeneous systems. When propagating macromolecules reach their critical lengths ((M) over bar(n) approximate to 1000), they precipit ate, and stabilized by macromolecules of poly(DA-CL) form nuclei of microsp heres. The number of particles formed in the initial period of the pseudoan ionic polymerization of L,L-lactide and epsilon-caprolactone remains consta nt whereas in the anionic polymerization of epsilon-caprolactone a weak agg regation, manifested by decreasing number of particles, was observed. Deter mination 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) l atex ([poly(CL)] = 2.17 x 10(1) g/l) in a solution containing initially [ep silon-caprolactone] = 7.50 x 10(-2) mol/l, 28% of monomer became incorporat ed into polymer particles) and that shortly after the initiation period all active centers were located inside growing latex particles. High local con centrations of monomer and active centers resulted in rates of polymerizati on which were up to ca 25 times higher than the rates of similar polymeriza tion with the same monomer and initiator concentrations averaged over the w hole volume of the reaction mixtures. Diameters of obtained poly(epsilon-ca prolactone) latex particles were in the region from 0.6 to 0.7 mu m and dia meters 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(eps ilon-caprolactone) grafts and molecular weight of poly(DA-CL) copolymer. Th e 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 0.2 5. Polymers in poly(lactide) and poly(epsilon-caprolactone) particles were characterized by a narrow molecular weight distribution (1.05 < (M) over ba r(w)/(M) over bar(n) < 1.3). In the case of the anionic dispersion polymeri zation of epsilon-caprolactone, the intra- and intermolecular transesterifi cation side reactions were reduced and latexes were obtained made of polyme r with (M) over bar(n) > 100 000, free from the admixture of cyclic oligome rs and with (M) over bar(w)/(M) over bar(n) = 1.06. (C) 1999 Elsevier Scien ce B.V. All rights reserved.