There are essentially five different industrial polymerization processes: bulk polymerization; precipitation polymerization, solution polymerization, . suspension polymerization and emulsion polymerization. Bulk polymerization is carried out in the absence of a diluent – the reaction mixture consists only of the monomer and a catalyst. If the polymer is insoluble in the monomer, or the monomer but not the polymer is soluble in the solvent added, the process is called precipitation polymerization, because the polymer will be precipitated as soon as it is formed in the reaction medium, usually in the form of small particles.
In solution polymerization both the monomer and the polymer are soluble in the reaction solvent, so that polymer solutions are obtained. If polymerization takes place in an aqueous (water) phase, the process is termed suspension polymerization. In this case the monomers are present as small spherical droplets at the beginning of the polymerization, and the activated species are formed within them. If the activated species are formed in the aqueous phase rather than the monomer droplets, and emulsifiers are added to the reaction medium, the process is called emulsion polymerization. The emulsifier causes the monomers to become finely dispersed in the reaction medium, so the monomer and polymer particles formed are considerably smaller than those in suspension polymerization.
The most important plastics obtained by direct polymerization are also those that are the most important in terms of production volume, namely the polyolefins, the vinyl chloride polymers and the styrene polymers. These three polymers account for about 67 per cent (polyolefins 33 per cent, vinyl chloride polymers 20 per cent and styrene polymers 14 per cent) of the total annual plastics production in the West. The most important polyolefins are the polyethylenes and polypropylene.