The radical copolymerization of dialkyl citraconate (DRC, R˭CH3, C2H5, n-C3H7, i-C3H7, n-C4H9, i-C4H9, s-C4H9, C6H11, C6H5CH2) (M1) with styrene (ST, M2) was performed at 60°C, using azobisisobutyronitrile as the initiator in tetrahydrofuran in order to clarify the polymerization behavior of DRC and the substituent effects on copolymerization. The monomer reactivity ratios r1and r2and the Q1and e1values were determined from the results obtained. It was found that the relative reactivities 1/r2of DRC toward an attack by a polystyryl radical could be correlated not by the steric-substituent constant ESof the alkyl group in DRC, but by the polar-substituent constants σ* in Taft' equation: log (1/r2) = ρσ+δES. It was also observed that the e1values are associated with Taft' σ constant. It was found that the weight-average molecular weights of the copolymers are between 8.5 × 103and 1.4 × 104.