Abstract: A powdered copolymer of an ethylenically unsaturated compound having a terminal double bond such as diisobutylene, styrene, an acrylate or an methacrylate and maleic anhydride or an ester thereof can directly be obtained by a polymerization process which comprises polymerizing the monomer mixture in a kneader or a Banbury type mixer in the presence or absence of a poor solvent for the copolymer to be produced to form a highly viscous resin phase containing the copolymer produced and the starting monomers dissolving said copolymer, kneading the highly viscous resin phase, proceeding further the polymerization reaction while kneading and shearing the resin phase to produce solid aggregates, and powdering said aggregates to produce a powdered copolymer.