Alternating Copolymerization of Cyclohexene Oxide and Carbon Dioxide Catalyzed by Organo Rare Earth Metal Complexes

The mono(cyclopentadienyl)-ligated rare earth metal bis(alkyl) complexes (C 5 Me 4 SiMe 3 )Ln(CH 2 SiMe 3 ) 2 (THF) (Ln = Y ( 1a ), Dy ( 1b ), Lu ( 1c ), Sc ( 1d )) and polyhydride complexes [(C 5 Me 4 SiMe 3 )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 CO 2 . 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 n = (50−80) × 10 3 and M w / M n ≅ 2 in most cases. The copolymerization of CHO and CO 2 by 1a − c and 2a − c at 70−110 °C under 12 atm of CO 2 yielded the corresponding polycarbonate with M n = (14−40) × 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 Sc 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 1a, c, and d with CO 2 afforded quantitatively the corresponding bis(carboxylate) complexes [(C 5 Me 4 SiMe 3 )Ln(μ-η 1:η 1 -O 2 CCH 2 SiMe 3 ) 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 CO 2, 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.