Surface active properties of polyoxyethylene macromonomers and their role in radical polymerization in disperse systems

Abstract

Conventional dispersion polymerization and copolymerization of low-molecular weight (conventional) unsaturated monomers allows preparation of monodisperse and micronsize polymer particles. A similar behavior can be found in the surfactant-free dispersion polymerization of non-traditional vinyl monomers, unsaturated macromonomers. The latter systems allow preparation of random, comb-like, star-like and graft copolymers as well. An interesting alternative arises with the use of amphiphilic reactive macromonomers that contain a polymerizable group and aggregate into an organized structure — a micelle. Under such conditions the high rate of polymerization and ultrafine (microparticles) polymer dispersions are generated. Thus, the surface-active macromonomers promote the formation of micelles and polymer growth within the main reaction loci — polymer particles. Furthermore, the surface-active compounds can be formed during the copolymerization of hydrophilic macromonomer and hydrophobic low-molecular weight comonomer. The reactive surface-active oligomeric radicals are incorporated into the polymer matrix or the particle surface layer, which prevents them from subsequent migration. Besides, the covalently bound surface-active groups at the particle surface strongly increase the colloidal stability of final polymer dispersion. This article presents a review of the current literature in the field of the surfactant-free dispersion polymerization of the polyoxyethylene unsaturated macromonomers. Besides a short introduction into some kinetic aspects of radical polymerization of traditional monomers in homogeneous and disperse systems, we mainly focus on the organized aggregation of amphiphilic polyoxyethylene macromonomers, the characterization of amphiphilic graft copolymers and their aggregation properties, and radical copolymerization of polyoxyethylene macromonomers. We discuss the birth and growth of chains, the transfer of reaction loci from the continuous phase to polymer particles, the diffusion-controlled termination, association of amphiphilic reaction by-products, the particle growth by agglomeration, the particle nucleation, the deactivation of polymer chain growth and the colloidal stability. Effects of initiator type and concentration, the surface activity of macromonomer, the macromonomer type and concentration, temperature, additives and the type of continuous phase on the kinetics of polymerization, and colloidal parameters of the reaction system are also evaluated. Variation of the polymer coil density, the polymer–polymer interaction, and polymer–solvent interaction with the molecular weight, diluent and method (light scattering, the size exclusion, etc.) are discussed. Polymerization of macromonomers provides regularly branched polymers with varied branching density. Since both the degree of polymerization and the length of branches may be varied, polymeric materials with specific properties can be prepared.