**2. Copolymerization of CO2 and epoxides**

Direct copolymerization of CO2 and epoxides such as ethylene oxide, propylene oxide, isobutylene oxide, cycloheptene oxide, cyclopentene oxide and cyclohexene oxide results in the formation of PACs, which is a typical example of fixation of CO2 in polymers [11]. PACs have excellent physical properties, including low density, transparency, durability, coloura‐ bility and processability, and also they exhibited decent electrical insulation properties [12]. Accordingly, these types of polymers have a broad range of applications in electronics, optical media and have been used for the preparation of sheets, automotive productions, medical devices and healthcare products [10]. Furthermore, they are commercially used as a binder, plasticizer and raw material for polyurethane synthesis [3]. However, the most important feature of PACs that has attracted attention in recent years, as a potent alternative for conven‐ tional polymers, is their biodegradability. PACs degrade into water and CO2 when exposed to moisture and enzymes or non-sterilized soil such as landfills [3]. PACs can be considered as an alternative to non-degradable polymers to address the growing concerns about the disposal of plastics in municipal waste, shortage of allocated spaces for landfills and consumer pressure for sustainable products. The list of some companies that use PACs in commercial scale for different purposes is presented in Table 1. PPC is used in combination with acrylo‐ nitrile butadiene styrene (ABS) to make a scratch resistance plastic by Bayer MaterialScience [13]. Furthermore, poly(propylene carbonate) and poly(hydroxybutyrate) are blended with other polymeric materials or mixed with inorganic solids to form the alternative plastic to acrylonitrile butadiene styrene by Baden Aniline and Soda Factory (BASF) –a chemical company [14]. Cardia Bioplastics (CO2 Starch Pty Ltd) has developed a method to manufacture commercially PPC/starch blends [15], and they currently manufacture huge amount of degradable plastic bags globally from this blend.


**Table 1.** List of Some Companies That Use PACs in Commercial Scale

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

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

Direct copolymerization of CO2 and epoxides such as ethylene oxide, propylene oxide, isobutylene oxide, cycloheptene oxide, cyclopentene oxide and cyclohexene oxide results in the formation of PACs, which is a typical example of fixation of CO2 in polymers [11]. PACs have excellent physical properties, including low density, transparency, durability, coloura‐ bility and processability, and also they exhibited decent electrical insulation properties [12]. Accordingly, these types of polymers have a broad range of applications in electronics, optical media and have been used for the preparation of sheets, automotive productions, medical devices and healthcare products [10]. Furthermore, they are commercially used as a binder, plasticizer and raw material for polyurethane synthesis [3]. However, the most important feature of PACs that has attracted attention in recent years, as a potent alternative for conven‐ tional polymers, is their biodegradability. PACs degrade into water and CO2 when exposed to moisture and enzymes or non-sterilized soil such as landfills [3]. PACs can be considered as an alternative to non-degradable polymers to address the growing concerns about the

in developing a catalyst are cost and toxicity.

70 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

for the synthesis of PACs are briefly reviewed.

**2. Copolymerization of CO2 and epoxides**

PACs can be used for biomedical applications as an alternative to current biodegradable polymers such as poly(lactic acid) (PLA). For example, enzymatic degradation of PPC leads to producing benign products that include only water and CO2, which does not cause any inflammation in biological environment. However, PLA degrades by hydrolysis and generates lactic acid, reduces the pH in surrounding tissue leads to inflammation. In addition, studies by Zhong et al. in 2012 and Yang et al. in 2013 demonstrated the potential of using PPC for producing scaffolds for tissue engineering application [16–18].

PACs are superior polymers for the agricultural purposes. For example, they can be used for the production of plastic films for agricultural mulches as an alternative to non-degradable polyethylene. It is therefore eradicating the risk of accumulation of plastic residues in agricul‐ tural spots, the high cost of collecting mulches after the harvesting season and contamination with soil and dirt. PACs can also be used for designing biodegradable packaging products as they are transparent and have minimal permeability to oxygen and water [19].

It is pivotal to determine the presence of any impurities that may have an adverse effect on physical properties and also the toxicity of a product fabricated from a polymer for food and biomedical applications. For instance, in processing PACs, the type of epoxide, by-product, catalyst and any other impurities may have an impact on their properties [20]. In an ideal copolymerization, CO2 and epoxide molecules form carbonate linkage. However, in reality, two molecules of epoxide may also bond and produce undesirable products. The presence of ether linkage has adverse effects on properties such as mechanical strength, thermal transition, thermal decomposition temperatures and molecular weight [21]. Therefore, the design of an active catalyst plays a critical role in promoting selectivity for the synthesis of favourable copolymer and reducing the rate of reaction of undesirable products.
