PURPOSE: To obtain a copolymer capable of being formed into various molded products excellent in thermal properties and water resistance without detriment to the excellent mechanical and optical properties of a methacrylic resin.

CONSTITUTION: A copolymer has units of methyl methacrylate and units of a monomer of the formula, and a number-average mol. wt. of 5×103 to 3×106. In the formula, n is 0 or 1; X is a hydrogen atom or a methyl group; Y and Z are each a hydrogen atom, a 10C or lower hydrocarbon group, a group of the general formula: -(CH2)pOR1, a group of the general formula: -(CH2)qOH, a group of the general formula: -(CH2)rOCOR2, a group of the general formula: -(CH2)sCOOR3 or a cyano group, wherein R1, R2 and R3 are each a hydrogen atom or a 10C or lower hydrocarbon group; and p, q, r and s are each an integer of 0 to 10.