PURPOSE: To prepare a new polymer having crosslinked structure, useful for coatings, adhesives, etc. and having improved heat resistance, solvent resistance and adhesive properties, by reacting a high molecular polymer having specific acid anhydride structure in the molecule with a specific oxazoline derivative.

CONSTITUTION: (A) A high molecular polymer having the partial structure I in the molecule (e.g. maleic anhydride-styrene copolymer) and (B) an oxazoline derivative II (n is integer of 1W4; R is n-valent aromatic hydrocarbon residue; R1WR4 are H or 1W3C hydrocarbon) (e.g. 2-phenyl-Δ2-oxazoline) are dissolved in a solvent, e.g. butyl acetate, applied to a tinplate, etc. and dried to give the desired high molecular polymer having crosslinked structure.