Mass transfer and radical flux effects in dispersed-phase polymerization of highly hydrophobic monomers.

摘要

The polymerization of two highly hydrophobic monomers, isooctyl and isobornyl acrylates, was studied under a variety of conditions that gave rise to different aqueous dispersions. Emulsion, miniemulsion, and microsuspension recipes were employed using surfactant levels above and below the critical micelle concentration, as well as with water-soluble (ionic, nonionic) and oil-soluble initiators. Data on conversion, particle size and number, molecular weight, and insoluble polymer content (pseudogel) were collected.; A simple calculation scheme was developed to model the microsuspensions as bulk polymerizations, effectively ignoring the very small amount polymer formed as submicron particles. The model was then adapted for the emulsions and miniemulsions. It was coupled with a mechanism of small-droplet formation that did not rely on diffusion of monomer through the aqueous phase, but instead on stabilization of radicals or droplets by short-chain oligomers. A piecewise model for radical capture was proposed, based on the pseudo-bulk treatment for emulsions and with a transition that roughly corresponded to the onset of the cage effect in suspension polymerization.; Constants for chain transfer and intra-particle termination were estimated for both monomers. In the case of isooctyl acrylate, the extent of transfer was found to be more pronounced within smaller particles. The transition to a regime in which the bulk rate constant applied was seen to occur at a fairly small particle radius. Termination was found to proceed at a much slower rate than in bulk reactions, primarily due to segregation of active radicals.; Emulsion polymerizations with poly(acrylic acid), an aqueous stabilizer often used in suspension recipes, yielded bimodal distributions with sub- and supermicron particles. Electrostatic repulsion, steric hindrance, and increased viscosity were judged to be responsible for this behavior. The partitioning of radicals between large and small particles was found to correspond reasonably well with the fraction of total surface area represented by each class.