A new monomer, methyl 4-(2-methyl-1-oxoprop-2-en-1-yl)benzoate (p-(methoxycarbonyl)phenyl isopropenyl ketone, MeOCO-PIPK), was synthesized and copolymerized with styrene and methyl methacrylate (MMA). The copolymers of MeOCO-PIPK and 2-methyl-1-phenylprop-2-en-1-one (phenyl isopropenyl ketone, PIPK) with styrene and MMA were photolyzed by deep-, mid- and near-UV light in dilute solution; and the quantum yields of scission, phi(s), and the UV absorption spectra were measured. The p-methoxycarbonyl substitution increased the molar extinction coefficients of the ketone monomer units extensively, but slightly lowered the phi(s) values in styrene and MMA copolymers. This is expected to increase the net sensitivity of solid alms of the polymers. The phi(s) was found independent of the wavelength, despite the concurrent absorption by styrene units in the styrene copolymers. Larger phi(s) values were obtained for the MMA copolymers than the corresponding styrene copolymers. Solvents with larger dielectric constants gave larger phi(s) for the copolymer of MMA with PIPK; but when the dielectric constants were similar, lower phi(s) values were observed in the solvents with more easily abstractable hydrogens. A large bleaching effect was seen in MMA copolymers, which should make possible the formation of resist patterns with steep profiles when used in photolithography.