Translucent impact polymers

A process for the free-radical polymerization of a monovinyl aromatic monomer consisting essentially of:

• (a) preparing a solution of about 4 to about 12 parts by weight of a polybutadiene rubber per 100 parts by weight of combined rubber and monovinyl aromatic monomer, said polybutadiene rubber having a cis-1,4 content of about 80% to about 98%, a trans-1,4 content of about 1% to about 19%, and a vinyl-1,2 content of about 1% to about 19%,
• (b) heating the resulting solution with agitation and in the presence of about 0.01 to about 1 mole percent (based on the moles of monovinyl aromatic) of a free-radical initiator of t-butyl peroxybenzoate until about 25 to about 50 weight percent of the monovinyl aromatic is converted to polymer;
• and
• (c) bulk polymerizing the resulting mixture in the absence of additional agitation at progressively increasing temperatures up to about 180.degree. C. until about 75 to about 98 weight percent conversion of monomer to polymer, to provide a monovinyl aromatic polymer having an improved balance of translucency and impact, the structure of the monovinyl aromatic polymer comprising a continuous matrix of poly(monovinyl aromatic) having a dispersed phase of a reinforcing polybutadiene rubber, the rubber being present in a proportion of about 6 to about 9 parts by weight per one hundred parts by weight of the combined poly(monovinyl aromatic) and rubber, the rubber being in the form of a plurality of particles of generally cellular structure and containing occlusions of poly(monovinyl aromatic), the particles being a mixture of coil-shaped, capsule-shaped and bar-shaped particles having a weight average particle diameter not exceeding two microns and cell walls not greater than about 0.05 microns in thickness, the resulting polymer having a toughness-haze index of greater than about 30.