Unexpected side reactions and chain transfer for zinc-catalyzed copolymerization of cyclohexene oxide and carbon dioxide

A detailed investigation was conducted into the chain microstructure and reaction mechanisms of polycyclohexene carbonate (PCHC) synthesized with ß-diketiminato zinc catalysts (EtBDI)ZnOEt, (EtBDI)ZnOMe (EtBDI = 2-(2,6-diethylphenyl)amido-4-(2,6-diethylphenyl)imino-2-pentene), and a bis(phenoxy)zinc catalyst [(2,6-F2C6H3O)2Zn·THF]2. For these complexes several different initiation, propagation, and termination reactions were identified by analyzing the MALDI-TOF-MS spectra of the resulting polymers. The bis(phenoxy)zinc system showed some ether linkages while the ß-diketiminato zinc catalyzed system produces perfectly alternating cyclohexene oxide-CO2 copolymers. The presence of a mixture of cyclopentyl, cyclohexyl, and cyclohexenyl end groups for all three catalyst systems points to an unexpected side reaction that is initiated by traces of water. This study also led to the discovery that, under conditions where traces of water or alcohols are present, the ß-diketiminato zinc catalyst is not a truly living system since a very rapid reversible chain transfer mechanism is operating, where alcohols and/or hydroxyl-terminated polymer chains act as the chain transfer agents.