PURPOSE: To obtain a mixture imparting PVC with a high heat distortion temperature and an excellent impact resistance by addition copolymerization of each prescribed amount of a monomer, acrylnitrile, methyl methacrylate, styrene or α-methylstyrene and indene in the presence of an unsaturated rubbery polymer.

CONSTITUTION: Normally solid polymer is produced by addition polymerization (A) in the presence of an unsaturated rubbery polymer produced beforehand, of monomers, (B) acrylnitrile, (C) methyl methacrylate, (D) (a) styrene and/or (b) α-methylstyrene and (E) indene and contain 10 to 200 pts.wt. of A, 10 to 40 pts.wt. of B, 5 to 50 pts.wt. of C, 5 to 45 pts.wt. of D and 4 to 45 pts.wt. of E in the polymer structure, per 100 pts.wt. of the total of A, B, C, D and E. The polymer is a high-molecular blending agent which imparts PVC with a high distortion temperature and excellent izod impact resistance by one material.