Core-shell impact modifiers for styrenic resins

An impact-property-modified polymer blend, comprising:

• a) from about 60 to about 95 weight percent, based on the polymer blend, of a matrix polymer selected from the group consisting of styrenic polymers and copolymers and their blends, high impact polystyrene and high impact polystyrene blends, polyphenylene oxides, and polypropylene;
• and
• b) from about 5 to about 40 weight percent, based on the polymer blend, of an additive polymer comprising particles having
• i. a rubber core of crosslinked diolefin polymer or copolymer containing mercaptan chain regulating agents derived from a conjugated diolefin monomer, the rubber core further derived from 0 to 50%, based on the total core weight, of a vinyl monomer polymerized with the diolefin and selected from the group consisting of styrene, alkylstyrene having 1 to 8 carbon atoms in the alkyl group, alkyl acrylate having 1 to 8 carbon atoms in the alkyl group, and alkyl methacrylate having 1 to 8 carbon atoms in the alkyl group, and
• ii. the core being surrounded by one or more polymer shells at least one of which is a vinylaromatic polymer or copolymer polymerized from one or more monomers selected from the group consisting of vinylaromatic monomers, alkyl acrylate monomers having 1 to 8 carbon atoms in the alkyl group, alkyl methacrylate monomers having I to 8 carbon atoms in the alkyl group, and polyvinyl unsaturated crosslinking monomers, and the remainder of which shells are vinylaromatic polymers or copolymers or methyl methacrylate polymers or copolymers polymerized from one or more monomers selected from the group consisting of vinylaromatic monomers, methyl methacrylate, alkyl acrylate monomers having 1 to 8 carbon atoms in the alkyl group and polyvinyl unsaturated crosslinking monomers,
• the particles further having an average particle diameter smaller than about 250 nanometers and a toluene swell index from about 13 to about 45.