**3. PACs synthesis**

The first discovery of PACs synthesis goes back to 1969 when Inoue et al. used a mixture of diethyl zinc (ZnEt2) and water as a catalyst to conduct the copolymerization of CO2 and propylene oxide and form PPC [22]. Shortly after that, the same group successfully used triethylaluminium as another organometallic catalyst for the synthesis of PPC [6]. However, the yield of the copolymerization reaction in the presence of these catalysts was low due to the formation of side products and presence of ether linkages on the polymer backbone. To address the issue of low catalyst activity, other hydrogen donor compounds rather than water have been used in combination with ZnEt2 [23, 24]. It was found that donor compounds with two or three active hydrogen sites formed multi-site catalytic systems with higher activity and selectivity compared with mono-site components. Therefore, the reaction was conducted towards alternative copolymerization of CO2 and PO, and the amount of undesirable cyclic propylene carbonate (cPC) reduced. Furthermore, a series of metal salts of acetic acid were used to catalyze the copolymerization of carbon dioxide and PO [25, 26]. Besides, a combina‐ tion of zinc hydroxide with dicarboxylic acids was used to enhance the yield of desirable polymer product; among all, catalyst system derived from zinc, hydroxide and glutaric acid showed the superior activity [27].

One common mechanism proposed for the copolymerization of CO2 and epoxides is coordi‐ nation–insertion mechanism catalyzed via metal compounds with Lewis acid and Lewis base active sites [10, 28]. In the coordination step, the epoxide molecule is coordinated by the metallic centre of a catalyst (Lewis acid active site) and then attacked by nucleophile site (Lewis base site) and undergone ring opening to form metal-bound alkoxide [10]. The nucleophilic attack can take place by either the nucleophile active site on the metal catalyst (bifunctional homogenous catalysts) or a separate compound (binary catalysts) and resulted to activation of alkoxide [20]. CO2 molecule then inserted into the metal-oxygen bond and initiated the reaction by forming metal carbonate. Up to this stage, all the steps were associated with the activity of the catalyst; however, the pathway of the reaction after this step relies on the selectivity of the catalyst. In fact, selectivity is a function of the type of alkoxide. Commonly, the metal carbonate goes towards its ring closure and forms propylene carbonate or propagates by multiple coordination and insertion of CO2 and produces polycarbonate chain [29]. If the second pathway is followed by the alternative coordination–insertion mechanism, the resulted polycarbonate has 100% carbonate linkage in its structure; however, some catalysts can also homopolymerize epoxides and form ether linkages on the backbone of polymer [10]. The schematic of suggested coordination–insertion mechanism for the copolymerization of epoxides and CO2 is demonstrated in Figure 1.

with soil and dirt. PACs can also be used for designing biodegradable packaging products as

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

The first discovery of PACs synthesis goes back to 1969 when Inoue et al. used a mixture of diethyl zinc (ZnEt2) and water as a catalyst to conduct the copolymerization of CO2 and propylene oxide and form PPC [22]. Shortly after that, the same group successfully used triethylaluminium as another organometallic catalyst for the synthesis of PPC [6]. However, the yield of the copolymerization reaction in the presence of these catalysts was low due to the formation of side products and presence of ether linkages on the polymer backbone. To address the issue of low catalyst activity, other hydrogen donor compounds rather than water have been used in combination with ZnEt2 [23, 24]. It was found that donor compounds with two or three active hydrogen sites formed multi-site catalytic systems with higher activity and selectivity compared with mono-site components. Therefore, the reaction was conducted towards alternative copolymerization of CO2 and PO, and the amount of undesirable cyclic propylene carbonate (cPC) reduced. Furthermore, a series of metal salts of acetic acid were used to catalyze the copolymerization of carbon dioxide and PO [25, 26]. Besides, a combina‐ tion of zinc hydroxide with dicarboxylic acids was used to enhance the yield of desirable polymer product; among all, catalyst system derived from zinc, hydroxide and glutaric acid

One common mechanism proposed for the copolymerization of CO2 and epoxides is coordi‐ nation–insertion mechanism catalyzed via metal compounds with Lewis acid and Lewis base active sites [10, 28]. In the coordination step, the epoxide molecule is coordinated by the metallic centre of a catalyst (Lewis acid active site) and then attacked by nucleophile site (Lewis base site) and undergone ring opening to form metal-bound alkoxide [10]. The nucleophilic attack can take place by either the nucleophile active site on the metal catalyst (bifunctional homogenous catalysts) or a separate compound (binary catalysts) and resulted to activation of alkoxide [20]. CO2 molecule then inserted into the metal-oxygen bond and initiated the reaction by forming metal carbonate. Up to this stage, all the steps were associated with the activity of the catalyst; however, the pathway of the reaction after this step relies on the

they are transparent and have minimal permeability to oxygen and water [19].

72 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

copolymer and reducing the rate of reaction of undesirable products.

**3. PACs synthesis**

showed the superior activity [27].

**Figure 1.** Coordination–insertion mechanism suggested for the copolymerization of epoxides with CO2
