Polymeric modifier for polyvinylhalides

A graft copolymer compsn. prepd. in 3 consecutive steps, comprises (A) 5 - 50 p.b.w. of a non-rubbery hard polymer obtd. by polymsn. of 50 - 100 wt.% of a vinylaromatic cpd., 0 - 50% of a monovinylidene comonomer, and 0 - 10% of a polyfunctional crosslinking monomer; (B) 20 - 70 p.b.w. of a rubbery polymer prepd. by copolymsn. of 50 - 100 wt.% butadiene, isoprene, chloroprene and/or an alkylacrylate (3 - 8C alkyl), 0 - 50% of a monovinylidene comonomer, and 0 - 10% of a polyfunctional crosslinking comonomer, in the presence of the hard polymer of (A); and (C) 15 - 40 p.b.w. of a polymer obtd. by polymsn.of 50 - 100 wt.% of an alkylmethacrylate (1 - 4C alkyl), 0 - 50% of an alkylmercaptan (3 - 12C atoms) and/or 0 - 10% of a polyfunctional crosslinking comonomer, in presence of polymers of steps (A) and (B).Prods. may be admixed to polyvinylhalide (e.g. PVC) resins to improve their processability, impact strength and other desirable characteristics