Barrier resins and impact modifiers therefor

An impact-modified barrier resin composition having a superior balance of properties comprising a blend of (A) from about 60 to 80 weight percent of a polymer of from about 50 to 90 weight percent acrylonitrile, from about 5 to 28 weight percent of a vinylidene aromatic monomer of the formula ##EQU2## wherein R.sub.1 is hydrogen, chlorine or methyl and R.sub.2 is a substituted or unsubstituted aromatic radical having from 6 to 10 carbon atoms and from about 2 to about 25 weight percent of an olefinic ester monomer of the formula ##EQU3## wherein R.sub.3 is hydrogen, an alkyl group having from 1 to 4 carbon atoms or a halogen, and R.sub.4 is an alkyl group of from 1 to 8 carbon atoms, and (B) from about 20 to 40 weight percent of a graft copolymer of (1) from 20 to 60 weight percent of a superstrate polymer polymerized from a monomer mixture comprising from 50 to 90 weight percent acrylonitrile, from 5 to 28 weight percent of vinylidene aromatic monomer and from 2 to 25 weight percent of olefinic ester of the formula ##EQU4## wherein R.sub.3 is hydrogen, halogen, or an alkyl group having from 1 to 4 carbon atoms and R.sub.4 is an alkyl group of from 1 to 6 carbon atoms and (2) from 80 to 40 weight percent of a substrate polymer having a glass transition termperature of temperature than -40.degree.C. comprising 50 to 100 weight percent of a conjugated diene selected from the group consisting of 1,3-butadiene and isoprene, from 50 to 0 weight percent of an ethylenically unsaturated monomer selected from the group consisting of vinylidene aromatic compounds, acrylonitrile, and mixtures thereof, and from 0 to 10 weight percent olefinic ester, provided that said superstrate polymer is at least about 50 percent grafted onto said substrate polymer.