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Thermoresponsive properties of poly(7-methacryloyloxycoumarin) ( P1a ) and its derivatives as homopolymers containing a photoreactive coumarin unit were systematically investigated. P1a showed a lower critical solution temperature (LCST)-type phase separation in dichloromethane, chloroform, and 1,1,2-trichloroethane. The temperature producing a 50% transmittance ( T c ) of 0.1 wt% P1a in distilled chloroform was 26 °C, while the T c value decreased to 21 °C in deuterated chloroform i.e., the deuterium isotope effect. The T c values in chloroform were adjustable from 28 to 57 °C by the introduction of the ethyleneoxy spacer and the substitution of α-methyl group by hydrogen. While the polymers containing 4-substituted coumarins were soluble in chloroform and dichloromethane from 0 °C to their boiling points. In the distilled chloroform solution, the coumarin units in P1a underwent a [2 + 2] cycloaddition and the transmittance at 500 nm decreased from 98% to 29% at 25 °C by a 180-s photoirradiation (326 mJ/cm 2 , emission band at 365 nm). [ABSTRACT FROM AUTHOR]