Bulk or concentrated solution polymerizations

Such polymerizations often exhibit a second, discontinuous phase. They frequently exhibit high exothermicity, but this is more characteristic of the reaction mechanism than of bulk polymerization as such. Bulk polymerizations of the free-radical variety are most common, although commercially important condensation processes also have a highly viscous continuous phase. 

Robust equipment has been developed for the various processing steps, including stirred-tank and tubular reactors, flash devolatilizers, extruder reactors, and extruder devolatilizers. Equipment costs are high based on working volume, but the volumetric efficiency of bulk polymerization is also high. If a polymer can be made in bulk, manufacturing economics will most likely favor this approach. 

It is tempting to suggest that polymer processes will gradually evolve toward bulk. Recently, the suspension process for impact polystyrene has been supplanted by the bulk process, and the emulsion process for ABS may similarly be replaced. However, the modern gas-phase process for polyethylene appears to represent an opposite trend. It seems that polymerization technology tends to eliminate solvents and suspending fluids other than the monomers themselves. When the monomer is a solvent for the polymer, bulk processes are preferred. When the monomer is not a solvent, suspension and slurry processes like those for polyethylene and polypropylene are employed. 

it is worthwhile avoiding a highly viscous continuous phase, but not at the price of introducing an extraneous material. 

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