By polymerizing MMA during a solvent, many of the disadvantages of the majority polymerizations are overcome. Thermal control is far better in solution polymerizations compared to bulk polymerizations. The solvent must be chosen with care, however. Many organic solvents act as chain transfer agents for free of charge radical reactions, thus lowering the polymer relative molecular mass. Aromatic solvents can accept and stabilize a radical, in effect inhibiting the polymerization.
Suspension Polymerization
Small beads of PMMA are often readily produced using suspension polymerization. The monomer is stirred with approximately twice its volume of water and dispersants. This forms a droplet like suspension of the monomer introducing the aqueous phase. Dispersants stabilize the polymerization and stop the droplets from adhering to at least one another. Dispersants are water-soluble compounds like gelatin, cellulose derivatives, or water-soluble polymers like poly(vinyl alcohol). Dispersants also can be finely suspended inorganic compounds, like kaolin, magnesium carbonate, or aluminum hydroxide. The bead or particle size is often controlled by choosing appropriate stirring conditions and dispersants, to supply beads starting from 50 mm to 1000 mm. Suspension polymerization is initiated with radical initiators that favor the monomer phase, like peroxide. Suspension polymerization provides for the removal of warmth from the polymerization by the aqueous phase while allowing high polymerization rates within the monomer phase.
Emulsion Polymerization
Emulsion polymerization is employed to form colloidal dispersions of PMMA. Emulsion polymerization involves the radical polymerization of monomers that are during a colloidal suspension. Surfactants are added to the (typically) aqueous phase to make micelles. The radical initiators used are water-soluble, like potassium persulfate. Monomers, including a fraction of radical initiated monomers, migrate to the micelles. Within the micelles, the monomer concentration is extremely high, and this is often where the polymerization takes place. Under optimal conditions, theoretically, there's one propagating chain end per micelle. Emulsion polymerization provides for top conversion and high relative molecular mass polymer within the micelles. The dispersed PMMA particles produced are very small, starting from 0.001 mm to 0.100 mm.166,167 Emulsion polymerizations of MMA are used to produce paint resins, paper coating agents and paper processing agents, textile binders, and additives.
Chain Transfer
Chain transfer is that the premature termination of a growing polymer chain and therefore the
initiation of another polymer chain by the transfer of hydrogen, or other atom or species, to the growing polymer chain from some compound present in the system, which could be the monomer, initiator, solvent, or polymer. Chain transfer to the initiator is referred to as induced initiator decomposition. When reinitiation is rapid, with no change within the polymerization rate, in other words, the chain reinitiation rate is constant approximately adequate to the propagation rate constant KP, then two different scenarios can occur.
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