The rate of homogeneous polymerization of N-dodecylmaleimide (DMIm) with dimethyl 2,2′-azobisisobutyrate depended significantly on the kind of solvents used. The polymerization systems involved ESR-observable propagating poly(DMIm) radical. The rate constants of propagation(kp) and termination(kt) were determined at 50°C in various solvents. The kp value was smaller in aromatic solvents than in aliphatic ones. The Hammett's plot of kp against σ-value of the substituents on the aromatic solvents showed that kp took higher values in the solvents with either electron-accepting or donating substituents. The solvent effects on kp seemed to stem from complexion of poly(DMIm) radical with solvents.