The syntheses of several dialkyl complexes based on rare-earth metal were described. Three beta-diimine compounds with varying N-aryl substituents (HL1 = (2-CH3O(C6H4))N=C(CH3)CH=C(CH3)NH(2-CH3O(C6H4)), HL2 = (2,4,6-(CH3)(3) (C6H2))N=C(CH3)CH=C(CH3)NH(2,4,6-(CH3)(3)(C6H2)), HL3 = PhN=C(CH3)CH(CH3) NHPh) were treated with Ln(CH2SiMe3)(3)(THF)(2) to give dialkyl complexes L(1)Ln (CH2SiMe3)(2) (Ln = Y (1a), Lu (1b), Sc (1c)), L(2)Ln(CH2SiMe3)(2)(THF) (Ln = Y (2a), Lu (2b)), and (LLu)-Lu-3(CH2SiMe3)(2)(THF) (3). All these complexes were applied to the copolymerization of cyclohexene oxide (CHO) and carbon dioxide as single-component catalysts. Systematic investigation revealed that the central metal with larger radii and less steric bulkiness were beneficial for the copolymerization of CHO and CO2. Thus, methoxy-modified beta-diiminato yttrium bis(alkyl) complex 1a, (LY)-Y-1(CH2SiMe3)(2), was identified as the optimal catalyst, which converted CHO and CO2 to polycarbonate with a TOF of 47.4 h(-1) in 1,4-dioxane under a 15 bar of CO2 atmosphere (T-p = 130 degrees C), representing the highest catalytic activity achieved by rare-earth metal catalyst. The resultant copolymer contained high carbonate linkages (> 99%) with molar mass up to 1.9 x 10(4) as well as narrow molar mass distribution (M-w/M-n = 1.7). (c) 2008 Wiley Periodicals, Inc.