Core-shell polymer for use in tire treads

A process for the preparation of a rubbery polymer comprising from about 5 to 55 weight percent of at least one C.sub.3 to C.sub.6 alkenyl nitrile monomer, from about 40 to 80 weight percent of at least one C.sub.4 to C.sub.8 conjugated diolefin monomer and from about 5 to 55 weight percent of at least one C.sub.8 to C.sub.12 monovinylidene aromatic monomer which may be substituted in the alpha position on the vinylidene group with at least one C.sub.1 to C.sub.4 alkyl group or on the aromatic ring with at least one C.sub.1 to C.sub.4 alkyl group or a chlorine or bromine atom for a total of 100 weight percent of said rubbery polymer characterized in that the process comprises from 2 to 6 sequential free radical polymerization reactions in aqueous emulsion wherein the polymer formed in each polymerization reaction comprises from 100 to about 40 weight percent of at least one conjugated diolefin monomer, from 0 to about 60 weight percent of at least one C.sub.3 to C.sub.6 alkenyl nitrile monomer and from 0 to about 60 weight percent of at least one C.sub.8 to C.sub.12 monovinylidene aromatic monomer which may be substituted in the alpha position on the vinylidene group with at least one C.sub.1 to C.sub.4 alkyl group or on the aromatic ring with at least one C.sub.1 to C.sub.4 alkyl group or a chlorine or bromine atom for a total of 100 weight percent of said polymer, the monomer composition of the polymer formed in one polymerization reaction differing from the monomer composition of the polymer formed in each of the other polymerization reactions and/or the molecular weight of the polymer formed in one polymerization reaction differing from the molecular weight of the polymer formed in each of the other polymerization reactions, and the polymer formed in one polymerization reaction comprising from 5 to 95 weight percent of the total polymer formed.

A rubber composition the vulcanizate of which has improved dynamic properties comprising:

• (i) 100 to about 10 parts by weight of a rubbery polymer comprising from about 5 to 55 weight percent of at least one C.sub.3 to C.sub.6 alkenyl nitrile monomer, from about 40 to 80 weight percent of at least one C.sub.4 to C.sub.8 conjugated diolefin and from about 5 to 55 weight percent of at least one C.sub.8 to C.sub.12 monovinylidene aromatic monomer which may be substituted in the alpha position on the vinylidene group with at least one C.sub.1 to C.sub.4 group or on the aromatic ring with at least one C.sub.1 to C.sub.4 alkyl group or a chlorine or bromine atom for a total of 100 weight per cent of said rubbery polymer characterized in that the polymer comprises polymer particles which have from 2 to 6 domains wherein each domain comprises from 100 to about 40 weight percent of at least one C.sub.4 to C.sub.8 conjugated diolefin monomer, from 0 to about 60 weight percent of at least one C.sub.3 to C.sub.6 alkenyl nitrile monomer and from 0 to about 60 weight percent of at least one C.sub.8 to C.sub.12 monovinylidene aromatic monomer which may be substituted in the alpha position on the vinylidene group with at least one C.sub.1 to C.sub.4 alkyl group or on the aromatic ring with at least one C.sub.1 to C.sub.4 alkyl group or a chlorine or bromine atom for a total of 100 weight percent of said domain, the monomer composition of one domain of the polymer particles differing from the monomer composition of the other domains and/or the molecular weight of one domain of the polymer particles differing from the molecular weight of the other domains, and any one domain of the polymer particles comprises from about 5 to 95 weight percent of the polymer particles,
• (ii) 0 to about 90 parts by weight of at least one rubber selected from the group consisting of butadiene rubbers, styrenebutadiene random and block rubbery copolymers, isoprene rubber and natural . rubber, for a total of 100 parts by weight of (i) and (ii),
• (iii) about 30 to about 150 parts by weight of at least one carbon black based on 100 parts by weight of the total of (i) and (ii),
• (iv) 0 to about 50 parts by weight of aromatic hydrocarbon extender oil based on 100 parts by weight of the total of (i) and (ii), and
• (v) vulcanization reagents.