Five new Zn(II) complexes
bearing pyridylamido ligands with different
substituents at the pyridine and at the amido moieties were synthesized
and evaluated as catalysts for the alternating copolymerization of
carbon dioxide and cyclohexene oxide. Polymers with carbonate linkages
ranging between 86 and 99% and a slightly isotactic-enriched structure
were selectively obtained under a variety of reaction conditions,
including 1 atm of CO2 pressure, without any cocatalyst.
Some experimental and theoretical evidence suggested that different
initiating species are involved in the catalytic cycle, including
dimeric [(L
x
)Zn(μ-OR)]2 and [(L
x
)Zn(μ-O2COR)]
complexes similar to those involved in the classical β-diiminate
Zn(II) catalysts.