Mechanism of particle formation and kinetics of the dispersion polymerization of cyclic esters

摘要

Pseudoanionic and anionic polymerizations of epsilon-caprolactone and lactides in 1,4-dioxane:heptane mixtures containing poly(dodecyl acrylate)-g-poly(epsilon-caprolactone) yield polyesters in form of microspheres. Monitoring partition of active centers between solution and microspheres revealed that particles are formed during initial period, when macromolecules reach their critical masses (ca. 1 000) and became insoluble. Then, propagation proceeds inside of microspheres into which monomer diffuses from solution. Monitoring of variation of the number of particles in a unit volume of reaction mixture with time indicated that after a primary nucleation the delayed nucleation and aggregation are absent. In effect, microspheres with narrow diameter distribution are obtained. Kinetic measurements revealed that in the dispersion pseudoanionic (initiator (CH3CH2)(2)AlOCH2CH3) and anionic (initiator (CH3)(3)SiONa) polymerizations of epsilon-caprolactone the overall rates of monomer conversion are from 10 to 30 times higher than for the corresponding polymerizations in solution (THF solvent). Analysis of kinetic equations indicated that the observed faster monomer conversions in polymerizations in dispersed systems are due to the high local concentrations of active centers and monomer in growing microspheres. [References: 10]