Impact resistant resins and method for making the same

A continuous emulsion polymerization method using a system of reactors for making an aqueous dispersion of an impact resistant resin having a Vicat softening temperature greater than 90.degree. C. and thus adaptable to use as a component of impact resistant polymethacrylate forming masses, which method consists essentially of

• (1) in a first reactor, emulsion polymerizing a first monomer component comprising from 70 to 90 percent by weight of a C.sub.1 -C.sub.4 alkyl acrylate, from 5 to 30 percent of styrene or lower alkyl-substituted styrene, 0 to 10 percent of a C.sub.1 -C.sub.4 alkyl methacrylate, and 0 to 10 percent of at least one further different, copolymerizable, monomer, which first component when polymerized forms a rubbery polymer phase having a T.sub..lambda.max less than 45.degree. C., onto core polymer particles of completely polymerized polymethyl methacrylate crosslinked with 0.5 to 10 percent of a crosslinking agent, in the presence of an initiator and an emulsifier to form an emulsion of particles of an intermediate product comprising said core polymer particles covered with a rubbery polymer phase, said core polymer particles having an average particle size of less than 5 nanometers, said polmerization being effected only until a degree of polymerization of said first monomer component to form said rubbery polymer phase of 80 to 95 percent is attained;
• (2) transferring said emulsion of said intermediate product to a second reactor and emulsion polymerizing methyl methacrylate onto the particles of said intermediate product in the presence of unpolymerized portions of said first monomer component and of further added initiator but without addition of further emulsifier, whereby an emulsion of a further product comprising particles of said intermediate product covered with a shell of harder polymethyl methacrylate polymer is obtained;
• and then
• (3) transferring said emulsion of said further product to a third reactor and continuing polymerization to a degree of more than 99.5 percent.