The mono(cyclopentadienyl)-ligated rare earth metal bis(alkyl) complexes (C5Me4SiMe3)Ln(CH2SiMe3)(2)(THF) (Ln = Y (1a), Dy (1b), Lu (1c), Sc (1d)) and polyhydride complexes [(C5Me4SiMe3)Ln(μ-H)(2)](4)(THF)(x) (2a: Ln = Y, x = 1; 2b: Ln = Dy, x = 2; 2c: Ln = Lu, x = 1) are active as single-component catalysts, not only for the ring-opening homopolymerization of cyclohexene oxide (CHO), but also for the alternating copolymerization of CHO and CO2. The homopolymerization of CHO in bulk took place much more rapidly than that in solution and afforded in high yields the corresponding polyether with M. = (50-80) x 10(3) and M-w/M-n ≅ 2 in most cases. The copolymerization of CHO and CO2 by 1a-c and 2a-c at 70-110 ° C under 12 atm of CO2 yielded the corresponding polycarbonate with M-n = (14-40) x 10(3), M-w/M-n = 4-6, and carbonate linkages = 90-99% with TOF ranging from 1000 to 2000 g polymer/(mol-Ln h). In contrast, the Se alkyl complex 1d gave a polymer containing high ether linkages (carbonate linkages = 23%) under the similar conditions because of its higher activity for CHO homopolymerization. The stoichiometric reaction of the bis(alkyl) complexes la, c, and d with CO2 afforded quantitatively the corresponding bis(carboxylate) complexes [(C5Me4SiMe3)Ln(μ-η(1):η(1)-O2CCH2SiMe3)(2)](2) (Ln = Y (3a), Lu (3b), Sc (3c)), which adopt a dimeric structure through the carboxylate bridges. The isolated carboxylate complexes 3a, b also showed moderate activity for the alternating copolymerization of CHO and CO2, which thus constituted a rare example of a well-defined, catalytically active carboxylate intermediate that was isolated directly from the reaction of a true catalyst system.