Transition Metal Hexacyanoferrate(II) Complexes as Catalysts in the Ring-Opening Copolymerization of CO2 and Propylene Oxide

The catalytic activity of four transition metal hexacyanoferrate(II) complexes (Ni2[Fe(CN)6], Co2[Fe(CN)6], KFe[Fe(CN)6] and Zn2[Fe(CN)6]) in the ring-opening copolymerization (ROCOP) of CO2 and propylene oxide (PO) is reported here for the first time and compared with that of other hexacyanometallate compounds. Complexes were prepared by coprecipitation employing tert-butanol as complexing agent. X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis, elemental analysis, X-ray fluorescence, scanning electron microscopy, transmission electron microscopy and N2 physisorption were used to confirm the identity of the obtained materials. Except for Zn2[Fe(CN)6], which showed an amorphous nature, the complexes were constituted by aggregates of cubic nanocrystals with intra-crystalline micropores and inter-crystalline mesopores. Gas–solid phase titration with NH3 revealed the high potential of hexacyanoferrates as Lewis acid catalysts. In the case of Zn2[Fe(CN)6], the lack of structural organization led to an extremely high density of acid sites (43 μmol m−2). The resulting copolymers were analyzed via nuclear magnetic resonance spectroscopy and gel permeation chromatography. The studied transition metal hexacyanoferrate(II) catalysts showed mild activity in the target reaction, giving rise to polyethercarbonates with moderate CO2 content (9.3–18.1 wt%), random configuration (67.0–92.4% of polyethercarbonate linkages), modest molecular weights (MW, g mol−1 = 3400–20,200) and high dispersity (ĐM = 4.0–5.4). Cyclic propylene carbonate (PC) was also produced (1.4–19.8 wt%). Among all, the Co2[Fe(CN)6] complex stands as a potential catalyst for CO2/PO ROCOP due to its high CO2 uptake, selectivity and molecular weight of the obtained copolymer.