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dispersion polymerization
In polymer science, dispersion polymerization is a heterogeneous polymerization process carried out in the presence of a polymeric stabilizer in the reaction medium. Dispersion polymerization is a type of precipitation polymerization, meaning the solvent selected as the reaction medium is a good solvent for the monomer and the initiator, but is a non-solvent for the polymer. As the polymerization reaction proceeds, particles of polymer form, creating a non-homogeneous solution. In dispersion polymerization these particles are the locus of polymerization, with monomer being added to the particle throughout the reaction. In this sense, the mechanism for polymer formation and growth has features similar to that of emulsion polymerization. With typical precipitation polymerization, the continuous phase (the solvent solution) is the main locus of polymerization, which is the main difference between precipitation and dispersion.
==Polymerization mechanism==
At the onset of polymerization, polymers remain in solution until they reach a critical molecular weight (MW), at which point they precipitate. These initial polymer particles are unstable and coagulate with other particles until stabilized particles form. After this point in the polymerization, growth only occurs by addition of monomer to the stabilized particles.〔 As the polymer particles grow, stabilizer (or dispersant) molecules attach covalently to the surface. These stabilizer molecules are generally graft or block copolymers, and can be preformed or can form ''in situ'' during the reaction.〔 Typically, one side of the stabilizer copolymer has an affinity for the solvent while the other side has an affinity for the polymer particle being formed. These molecules play a crucial role in dispersion polymerization by forming a “hairy layer” around the particles that prevents particle coagulation.〔 This controls size and colloidal stability of the particles in the reaction system. The driving force for the particle separation is steric hindrance between the outward-facing tails of the stabilizer layers.〔
Dispersion polymerization can produce nearly monodisperse polymer particles of 0.1–15 micrometers (µm). This is important because it fills the gap between particle size generated by conventional emulsion polymerization (0.006–0.7 µm) in batch process and that of suspension polymerization (50–1000 µm).〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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