Copolymers having improved optical clarity

Claim 1

IN A PROCESS FOR COPOLYMERIZING 5-95 PARTS BY WEIGHT OF A MONOVINYLIDENE AROMATIC HYDROCARBON AND, CORRESPONDINGLY, 95-5 PARTS BY WEIGHT OF AN UNSATURATED NITRILE OF THE GROUP CONSISTING OF ACRYLONITRILE, METHACRYLONITRILE, AND MIXTURES THEREOF IN AN AQUEOUS SYSTEM IN THE PRESENCE OF A FREE RADICAL-GENERATING POLYMERIZATION INITIATOR AND A SUSPENDING AGENT, THE IMPROVEMENT WHICH COMPRISES (1) INITIATING COPOLYMERIZATION OF THE MONOVINYLIDENE AROMATIC HYDROCARBON AND UNSATURATED NITRILE MONOMERS IN THE ABSENCE OF A SUSPENDING AGENT, (2) CONTINUING COPOLYMERIZATION IN THE BSECNE OF A SUSPENDING AGENT UNTIL 20-40% OF THE MONOMERS HAVE BEEN CONVERTED TO POLYMER, (3) ADDING A WATER-SOLUBLE SUSPENDING AGENT TO THE REACTION MIXTURE AT A STAGE OF THE REACTION BETWEEN 20 AND 40% CONVERSION, (4) ADDING 0.05-1% OF A WATERSOLUBLE INORGANIC SALT, BASED ON THE WEIGHT OF THE MONOMERS, TO THE REACTION MIXTURE PRIOR TO THE STAGE OF ABOUT 65% CONVERSION, (5) MAINTAINING A SUBSTANTIALLY CONSTANT MONOMER RATIO IN THE REACTION MIXTURE BY ADDING SUPPLEMENTAL AMOUNTS OF THE MORE RAPIDLY-CONSUMED MONOMER BETWEEN 30 AND 95% CONVERSION, (6) TERMINATING THE COPOLYMERIZATION REACTION AT 95-98% CONVERSION, AND (7) REMOVING UNREACTED MONOMERS BY DISTILLATION.