**4.2. Heterogeneous catalysts**

## *4.2.1. Organometallic compounds*

The organometallic compounds for the copolymerization of CO2 and epoxides are designed from the combination of a hydrogen donor compound and a metal-based complex to activate CO2 and open the ring of the epoxide. The commonly used hydrogen donors are water, primary amines, dihydric phenols, trihydric phenols, aromatic dicarboxylic acids and aromatic hydroxycarboxylic acids [61]. Several efficient organometallic catalysts with well-defined structures have been developed for the copolymerization of carbon dioxide and epoxide. However, the activity of the catalyst derived from zinc hydroxide and glutaric acid was superior compared to other compounds [27]. Ree et al. copolymerized PO and CO2 using zinc glutarate (ZnGA) obtained from various zinc sources. As a result, zinc glutaric derived from zinc oxide and glutaric acid yielded the highest catalyst activity of 64 g /g of catalyst [62]. Discovery of the catalyst activity of zinc glutarate for copolymerization of CO2 and epoxides was a breakthrough in the field especially after 1995 when Darensbourg et al. substituted organic solvents, as the reaction media, by supercritical fluid [63]. This makes the copolymer‐ ization process more environmentally friendly and economically viable.

## *4.2.2. Double-Metal Cyanide (DMC) complexes*

DMCs are another group of heterogeneous catalyst that are efficiently used for the homopol‐ ymers of epoxides. The first DMCs that showed average activity for copolymerization of PO and CO2 to produce PPC were Zn3[Fe(CN)6]2 and Zn3[Co(CN)6]2, a double-metal cyanide compound based on Zn and Fe or Co [64–66]. The activity of zinc–cobalt–DMC catalyst was comparable with zinc glutarate [3, 21]. However, it was found that the system suffered from low selectivity at low temperatures and poor activity at high temperatures [67]. In addition to these catalysts, Darensbourg et al. and Robertson et al. attempted to modify the catalyst structure by increasing the crystallinity of DMC-based catalyst [68, 69]. It was also found that the low molecular weight polyols could act as an initiator to activate DMCs and promote the copolymerization reaction [70].
