Styrenic polymers can be produced by many techniques, but relatively few have proven successful on a commercial scale. In recent years, the so-called bulk, continuous process has dominated industrial production and has largely replaced competitive routes such as suspension and emulsion. It is now used for most molding and extrusion grades of crystal polystyrene, for rubber- modified polystyrene, for styrene-acrylonitrile copolymer, and for certain varieties of ABS. The historic definition of “bulk” supposed use of the neat, undiluted monomers, but more recent terminology follows industrial practice in allowing substantial amounts of solvents (e.g., ethylbenzene) as chain- transfer agents and processing aids [l].