N-Phenyl maleimide (N-PMI) was successfully polymerized by divalent rare-earth complexes (ArO)(2)Sm(THF)(4) (ArO = 2,6-di-tert-butyl-4-methyl phenoxo-; THF = tetrahydrofuran) and (Ar'O)(2)Ln(THF)(3) (Ar'O = 2,6-di-tert-butyl phenoxo-; Ln = Sm, Yb, or Eu). The central metals greatly affected the reactivity, and the reactivity order was Sm(II) > Yb(II) > Eu(II). The activity of (Ar'O)(2)Sm(THF)(3) was higher than that of (ArO)(2)Sm(THF)(4). The polymerization yields were higher in THF than in other solvents, and the maximum yields were obtained around 25 degrees C. A proposed mechanism is discussed. (c) 2005 Wiley Periodicals, Inc.