The use of polymers in heterophase polymerizations

Abstract

Polymers can play an important role in heterophase polymerizations either as initiators, or as stabilizers, or as dispersion media. The use of polymeric radicals derived from poly(ethylene glycol) (PEG) to start the polymerization as well as to stabilize the particles leads to the formation of block copolymer particles. The block copolymer architecture depends on the number of radicals per PEG chain as well as on the termination mode of the growing polymer chain. PEG azo initiators resulting in one radical per PEG chain and styrene as monomer lead to PEG-polystyrene-PEG triblock copolymers whereas methyl methacrylate and PEG chains with a radical on both ends lead to poly(methyl methacrylate)-PEG-poly(methyl methacrylate) triblock copolymers. In both cases the particle morphology deviates from the ordinary solid sphere morphology. Block copolymers with one poly(styrene sulfonate) block are effective stabilizers and lead to different particle decorations depending on the length as well as on the degree of sulfonation of the polyelectrolyte block. Fully sulfonated poly(styrene sulfonate) blocks with a contour length of 113 nm lead to electrosterically stabilized particles with a polyelectrolyte brush corona thickness of approx. 60 nm (`porcupine' particles). The same poly(styrene sulfonate) block with a degree of sulfonation of approx. 50% results in `ringlet' particles due to the formation of loops and multiple adsorption points. The `porcupine' particles posses an extraordinary salt stability associated with the entropic elasticity of the polyelectrolyte chain. The use of a homologous series of PEG as a dispersion medium and PEGA initiators leads to polymer in polymer dispersions where the molecular weight ratio of the free PEG chains and the confined PEG chains determines the particle properties. The average particle size is smaller and the polydispersity of the particle size distribution is larger, the higher the molecular weight of the molecules in the dispersion medium compared to the confined PEG chains stabilizing the particles. This is a new way to control particle as well as polymer properties in heterophase polymerizations.