PURPOSE: The titled modified copolymer freed from shrinkage during crosslinking/curing and suitable for use in molding materials, paints, etc., prepared by radical-copolymerizing an ethylenically unsaturated compound with a 2-vinyl- spiroorthoester compound.

CONSTITUTION: The titled modified copolymer having structural units of formula III, wherein A is a monomer unit derived from an ethylenically unsaturated compound and x and y are each a molar fraction of each monomer, is prepared by radical-polymerizing an ethylenically unsaturated compound of formulaI(wherein R1 is H or an alkyl, R2 is -CN, -COOR3 or a like group and R3 is an alkyl, cycloalkyl or a like group), e.g., maleic anhydride or fumaric acid, with a 2- vinyl-spiroorthoester compound of formula II, wherein n is 3W5, obtained by an addition reaction between butadiene monoxide and a lactone. The copolymer thus produced is freed from volume shrinkage during crosslinking/curing because it contains a volume-expandable spiroorthoester groups.