The polymerization of polar monomers such as methyl methacrylate (MMA), methyl acrylate (MA), methacrylonitrile (MAN), and acrylonitrile (AN) was carried out with gadolinium-based Ziegler-Natta catalysts [Gd(OCOCCl3)(3)-(i-Bu)(3)Al-Et2AlCl] in hexane at 50 degrees C under N-2 to elucidate the effect of the monomer’s HOMO (highest occupied moleculor orbital) and LUMO (lowest unoccupied molecular orbital) levels on the polymerizability. In the case of homopolymerization, all monomers were found to polymerize and the order of relative polymerizability was as follows : MMA > MA > MAN > AN. On the other hand, the result of copolymerization of St with MMA shows that the values of the monomer reactivity ratios are r(1) = 0.06 and r(2) = 1.98 for St(M-1)/MMA (M-2). The monomer reactivity ratios of styrene (St), p-methoxystyrene (PMOS), p-methylstyrene (PMS), and p-chlorostyrene (PCS) evaluated as r(1) = 0.55 and r(2) = 1.07 for St(M-1)/PMOS(M-2), r(1) = 0.38 and r(2) = 0.51 for St(M-1)/PMS(M-2), and r(1) = 0.72 and r(2) = 1.25 for St(M-1)/PCS(M-2) were compared with those for St(M-1)/MMA(M-2). The copolymerization behavior is apparently different from the titanium-based Ziegler-Natta catalyst, regarding a larger monomer reactivity ratio of PCS. The lower LUMO level of PCS and MMA may enhance a back-donation process from the metal catalyst, therefore resulting in high polymerizability. These results are discussed on the basis of the energy level of the gadolinium catalyst and the HOMO and LUMO levels of the monomers.