A series of cationic rare-earth aryloxide complexes,i.e.,[LREOAr']+[B(C6F5)4]-(L = CH3C(NAr)CHC(CH3)(NCH(R)CH2PPh2);RE = Y,Lu;Ar'=2,6-tBu2-C6H3,2,6-(PhCMe2)2-4-Me-C6H2;Ar = 2,6-iPr2-C6H3,2,6-(Ph2CH)2-4-iPr-C6H2;R = H,CH3,iPr,Ph),were prepared and ap-plied to the Lewis pair polymerization of methyl methacrylate(MMA).The stereoregularity of the resulting PMMA was significantly affected by the R substituent on the pendant arm of the tridentate NNP ligand,and was found to increase with increase in the steric hindrance of R.When using a Ph group as R,the Y complex produced a highly isotactic polymer with an mm value of 95％and a Tg of 54.6 ℃.In contrast,the steric hindrance of the Ar and Ar'groups had no effect on the tacticity of the resulting polymer,presumably because these two substituents were situated such that they pointed outward from the cyclic intermediates.Kinetics studies demon-strated thatthe polymerization was a first-order process with regard to the monomer concentration prior to catalyst deactivation.End group analysis indicated that the polymerization was accompanied by two possibly competing chain-termination side reactions that proceeded via intramolecular backbiting cyclization.