**6. Conclusions**

**Patent Number Catalyst Epoxide Activity**

dicarboxylic acid and 1,10-decane dicarboxylic acid) and aromatic dicarboxylic acids (phthalic acid, isophthalic acid, terephthalic acid, 1,2 naphthalene dicarboxylic acid, 1,3 naphthalene dicarboxylic acid, 1,4 naphthalene dicarboxylic acid, 1,5 naphthalene dicarboxylic acid, 1,6 naphthalene dicarboxylic acid, 1,7 naphthalene dicarboxylic acid, 1,8 naphthalene dicarboxylic acid, 2,3 naphthalene dicarboxylic acid, 2,5 naphthalene dicarboxylic acid, 2,6 naphthalene dicarboxylic acid and 2,7 naphthalene dicarboxylic acid) a

82 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

Zinc carboxylate catalyst by reaction of zinc compounds (zinc oxide, Zn(OH)2, Zn(CO3)2 and Zn(OAc)2) with a dicarboxylic acid (glutaric acid or adipic acid) a

double metal cyanide catalyst a

Catalysts from the oxidizing of a dicarboxylic acid precursor (chosen from pentanediol, hexanediol, 1,5 dibromopentane, 1,5-dichloropentane, 1,6-dichlorohexane, 1,6-dibromohexane, glutaronitrile, adiponitrile, glutarimide, adipaimide, glutaraldehyde and adipaldehyde) and a zinc precursor (chosen from zinc acetate dihydrate, zinc hydroxide, zinc nitrate hexahydrate, zinc perchlorate hexahydrate, zinc oxide and zinc sulphate) a

Bimetallic complex of Zn(II), Co(II), Mn(II), Mg(II), Fe(II), Cr(III)-X or Fe(III)- Xb

WO 2003029325 Zinc glutarate and zinc-cobalt based

US 5026676

WO 2004000912

WO 2009130470

**(g/g catalyst)**

Propylene oxide 26 (max) [93]

Ethylene oxide [97]

Propylene oxide [95]

Propylene oxide [99]

Propylene oxide

Cyclohexene oxide

US 20060089252 Cobalt salen catalyst b Propylene oxide [98]

WO 2012136658 Double-metal cyanide compound b Propylene oxide [100]

**Reference**

This review demonstrated the significant advances that have been accomplished for develop‐ ing active catalysts for the copolymerization of carbon dioxide and epoxides in the last four decades. In fact, a broad range of catalysts have been designed and examined with high activity and selectivity for the synthesis of biodegradable poly(alkylene carbonates). In addition, the mechanistic aspects of CO2 cycloaddition to epoxides have been recognized and reported. Regardless of the structure of the catalyst and some slight differences, the overall mechanism proposed for the copolymerization of CO2 and epoxides is coordination–insertion mechanism catalyzed via metal compounds with Lewis acid and Lewis base active sites.

Homogeneous catalysts for CO2–epoxide copolymerization were proposed such as phenoxide, β-diiminate and metal salen complexes. The two former groups only showed activity for copolymerization of CHO and CO2. In addition, salen catalysts exhibited activity and selec‐ tivity for a broader range of epoxides copolymerized by CO2. However, none of these homo‐ genous catalysts was used in commercial scale due to complicated synthesis process and low selectivity. Among several heterogeneous catalysts designed, organometallic complexes, particularly zinc glutarate, are the most effective catalysts because of both high activity and selectivity, low cost, non-toxicity and simple manufacturing process. In the last decade, attempts have been undertaken to design bimetallic catalysts to enhance activity towards copolymerization of CO2 with epoxides. However, these studies are still in their infancy and have not yet used in the commercial scale.

In summary, the biodegradable polycarbonates are promising polymers that are produced from bonding CO2 with epoxides that are partially renewable with huge market value for production of broad range of products such as food packaging materials, healthcare devices and agricultural mulches. Developing an efficient catalyst that reduces the cost and minimizes the catalyst consumption and by-product is still attractive as an alternative to non-degradable polymers in many applications to remedy the global issue of plastic wastes in landfills.
