*4.2.3. Rare earth metal catalysts*

epoxides as it reduced the catalyst loading significantly. Similarly, a cobalt complex containing two tertiary amine cations on pendant arms was designed and showed high activity and above 90% conversion yield for very low catalyst loadings such as 1:50,000 (catalyst/ PO) [60].

Many research activities have focused on the area of homogeneous catalysts for CO2–epoxide copolymerization due to the design flexibility and high activity. However, none of them has been used in large scale due to complicated synthesis process and low selectivity for PACs copolymerization. On the other hand, heterogeneous catalysts are generally non-toxic and economically viable due to the simpler synthesis process and easier handling. In the next section, the heterogeneous catalysts that are designed for the synthesis of PACs are described.

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‐

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

ization process more environmentally friendly and economically viable.

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

copolymerization reaction [70].

**4.2. Heterogeneous catalysts**

76 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

*4.2.1. Organometallic compounds*

In the very first attempts, rare earth metal complexes such as Y(P204)3–Al(*i*-Bu)3–glycerine were used for the copolymerization of PO and CO2 and formation of high molecular weight PPC. However, the structure of the resulted copolymer consisted of up to 90% of ether linkages [71]. In the next discoveries, rare earth metal catalysts showed an increase in selectivity and reduction in synthesis time for copolymerization of CO2 with epoxides [72–74]. Yttrium carboxylate in combination with ZnEt2 significantly improved carbonate linkage percentage on the PPC and PCHC backbone [75]. A ternary catalytic system of rare earth complex, diethyl zinc and glycerine resulted in an extremely high molecular weight PPC [76]. Likewise, PO was successfully copolymerized with CO2 and formed PPC by using Y(CCl3COO)3, ZnEt2 and glycerine ternary system [76]. Rare earth metal catalytic systems, in general, can be used to adjust the microstructure of the polymer chain. For instance, head-to-tail linkage and molec‐ ular weight distribution of PPC was successfully adjusted by using yttrium carboxylate as a catalyst.

Comparing all heterogeneous catalytic systems created so far, conventional zinc glutarate is the only catalyst that has been used commercially for alternative copolymerization of CO2 and PO. Zinc glutarate exhibits high activity and favourable selectivity [62, 77]. It is cheap, nontoxic and easy to synthesize and can be used for manufacturing relatively high molecular weight PPC with superior carbonate linkage percentage [5]. However, its activity is still one or two order of magnitude lower than the common catalysts used for synthesis of conventional polyolefins [78]. Therefore, enhancement of the catalytic activity of ZnGA was the topic of several studies [7–9].
