**1. Introduction**

Carbon dioxide is one of the main greenhouse gases that contributes to global warming [1]. One of the approaches to tackle this issue is to fix CO2 as a raw material into valuable polymers [2]. The polymer industry is an emerging market that mostly relies on petrochemical products derived from fossil-fuel feedstocks. However, CO2 is a cheap, renewable, non-toxic and the most abundant carbon source. The consumption of CO2 as a reactant for the synthesis of carbon-based products is a key step to cut down the reliance on the fossil fuels.

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Alternative copolymerization of CO2 with epoxides, such as propylene oxide (PO) and cyclohexene oxide (CHO), results in the formation of biodegradable PACs. As the physical properties of PACs are comparable to conventional polymers, they can be used for a broad range of applications such as packaging, agricultural and biomedical industries [3]. One of the obstacles in the synthesis of PACs is the design of an efficient catalyst that can reduce the activation energy of thermodynamically stable CO2 for such polymerization reaction [4]. In addition to activity, it is pivotal to design a selective catalyst that reduces the yield of byproducts during the PAC copolymerization reactions [5]. Other factors that are contemplated in developing a catalyst are cost and toxicity.

Since 1696 that poly(propylene carbonate) (PPC) was synthesized, many efforts have been attempted to design catalysts [6]. These catalysts are mainly classified into two categories: (1) homogeneous and (2) heterogeneous. This classification is based on their solubility in the reaction media; therefore, the homogenous catalysts are those that are in the same phase as reactants and heterogeneous are those that are in another phase. Regardless of large number of research in this area, only a few of conventional metal-based heterogeneous catalysts have been used for the commercial PACs synthesis that possess acceptable activity and selectivity for large-scale production [3]. However, physical properties of this type of catalysts affect their selectivity and activity remarkably. For example, particle size, crystallinity, microstructure and morphology are key factors that show impact on the activity of a catalyst and the yield of the final product [7–9]. Many bench-scale studies have been conducted to promote the catalyst activity for the synthesis of PACs, and particularly poly(propylene carbonate) [3, 10]. In this chapter, we provide an insight about these types of catalysts. Prior to a discussion about catalyst, the main advantages of PACs are described. Then various catalysts that are available for their synthesis are introduced, followed by discussion about strategies that have been undertaken to promote the activity of these catalysts and finally, an overview of patents filed for the synthesis of PACs are briefly reviewed.
