Aqueous suspension polymerization process

A process for forming a copolymer of styrene and butadiene comprising providing an aqueous phase comprising an aqueous mixture comprising water, styrene monomer, butadiene monomer, a suspension stabilizing agent, and a chain propogating amount of a free radical polymerization initiator insoluble in water, soluble in said styrene monomer, soluble in said butadiene monomer and having a 1 hour half-life between about 50.degree. C. and about 130.degree. C., the ratio of said styrene monomer and said butadiene monomer being between about 80:20 and about 95:5 by weight, the weight proportion of water to the combination of said styrene monomer and said butadiene monomer is between about 0.8:1 and about 2:1, said suspension stabilizing agent consisting essentially of a finely-divided, difficultly water-soluble powder and a vapour phase comprising an inert gas and butadiene monomer, heating said aqueous phase and said vapour phase in accordance with a predetermined heating profile comprising heating said aqueous mixture to a first temperature between about 50.degree. C. and less than about 100.degree. C. at a pressure between about 20 psi and about 140 psi, maintaining said first temperature in accordance with said predetermined heating profile, removing butadiene monomer from said vapour phase after at least about 75 percent by weight of said butadiene monomer and said styrene monomer in said aqueous phase are converted to a copolymer and prior to conversion of more than about 98 percent by weight of said butadiene monomer and said styrene monomer to a copolymer in said aqueous phase, heating said aqueous mixture to a second temperature at least about 15.degree. C. greater than said first temperature and greater than about 90.degree. C. and less than about 130.degree. C. in accordance with said predetermined heating profile, and maintaining said second temperature in accordance with said predetermined heating profile at a pressure between about 20 psi and about 140 psi until at least about 90 percent by weight of said styrene monomer and said butadiene monomer are copolymerized to form an aqueous suspension of discrete copolymer particles having a Tg value of between about 45.degree. C. and about 65.degree. C., a weight average molecular weight of between about 10,000 and about 400,000, a molecular weight distribution of said copolymer between about 2 and about 9 and a butadiene monomer concentration of less than about 10 parts per million by weight.

A process for forming a copolymer of styrene and butadiene comprising providing an aqueous phase comprising an aqueous mixture comprising water, styrene monomer, butadiene monomer, a suspension stabilizing agent, and a chain propogating amount of a free radical polymerization initiator insoluble in water, soluble in said styrene monomer, soluble in said butadiene monomer and having a 1 hour half-life between about 50.degree. C. and about 130.degree. C., the ratio of said styrene monomer and said butadiene monomer being between about 80:20 and about 95:5 by weight, the weight proportion of water to the combination of said styrene monomer and said butadiene monomer is between about 08:1 and about 2:1, said suspension stabilizing agent consisting essentially of a finely-divided, difficultly water-soluble powder and a vapour phase comprising an inert gas and butadiene monomer, heating said aqueous phase and said vapour phase to a temperature between about 50.degree. C. and about 130.degree. C. at a pressure between about 20 psi and about 140 psi in the absence of redox initiators and mercaptan compounds, introducing additional styrene monomer, butadiene monomer, free radical polymerization initiator and suspension stabilizing agent to said aqueous mixture at least once after heating to said temperature, the proportions of said additional styrene monomer and butadiene monomer being between about 80:20 and about 95:5 by weight, and said additional free radical polymerization initiator being between about 0.05 and about 6.5 percent by weight based on the total weight of said monomers to extend the molecular weight distribution of said copolymer, heating said aqueous phase and said vapour phase to a temperature between about 50.degree. C. and about 130.degree. C. at a pressure between about 20 psi and about 140 psi in the absence of redox initiators and mercaptan compounds until said styrene monomer and said butadiene monomer form an aqueous suspension of discrete copolymer particles, removing butadiene monomer from said vapour phase after at least about 75 percent by weight of said butadiene monomer and said styrene monomer in said aqueous phase are converted to a copolymer and prior to conversion of more than about 98 percent by weight of said butadiene monomer and said styrene monomer to a copolymer in said aqueous phase, and heating said aqueous phase at a temperature between about 50.degree. C. and about 130.degree. C. at a pressure between about 20 psi and about 140 psi until at least about 90 percent by weight of said styrene monomer and said butadiene monomer are copolymerized to form an aqueous suspension of discrete copolymer particles having a Tg value of between about 45.degree. C. and about 65.degree. C., a weight average molecular weight of between about 10,000 and about 400,000, a molecular weight distribution of said copolymer between about 2 and about 9 and a butadiene monomer concentration of less than about 10 parts per million by weight.