**4.1. Homogenous catalysts**

Triphenyl phosphate (TPP) compounds with a metal atom in the centre were the first group of homogeneous catalyst reported for copolymerization of CO2 and epoxides [31, 32]. TPP compounds showed high activity; however, this catalysts were not very efficient, and the yield of PACs was very low even after more than one weak reaction. Nitrogen donors compounds such as N-methylimidazole and (4-dimethylamino)pyridine (DMAP) were added to TPP compounds to enhance the yield of reaction. However, this catalyst system could only be used for the copolymerization of COH and CO2. For the other epoxides, only cyclic alkylene carbonates were produced [33, 34].

Chen et al. investigated the effect of metal type located in the centre of TPP compounds on the activation of propylene oxide. It was found that the type of metal influenced the acidity strength of catalysts and, therefore, the strength of epoxide metal bond and the yield of copolymer synthesis [35]. Among three types of metal used in this study, chromium-based catalyst exhibited higher selectivity compared to Al and Co towards the formation of PPC from the reaction between PO and CO2.

In a metal complex catalyst, the metal acts as Lewis acid to attack the epoxide and after that the nucleophile part of the catalyst opens the epoxide ring. The nucleophile part of the catalyst can be either a ligand attached to the metal complex or a co-catalyst. In the former group, the nucleophile ligand can often be a salen, a porphyrin, a phthalocyanine or an amido macrocycle [30]. The nature of metal in the catalyst is critical in its activity and selectivity and also the quality of the resulted polymer. Indeed, the strength of epoxide metal bond can determine the pathway of reaction. When this bond is too weak, the reaction proceeds towards formation of ether linkages and backbiting while the high strength of the bond may lead to closure of the ring and formation of cyclic carbonates [36].

After TPP compounds, great number of metal complex catalytic systems such as phenoxide [37–39] and β-diiminate (BDI) [29, 40] were investigated in terms of their activity for copoly‐ merization of CO2 and epoxides that are discussed in this section.

## *4.1.1. Phenoxide complexes*

Phenoxide compounds include an aryl ligand that plays the role of nucleophile site of a catalyst [41]. It was found that the activity of a catalyst is dramatically higher when the phenoxide compounds contain zinc atom as a metal centre [42]. It was found that this group of metal complexes are only able to activate and catalyze CHO to react with CO2 and are not efficient in activation of other epoxides such as PO [37, 43, 44].

## *4.1.2. β-diiminate complexes*

BDI is a family of complexes that include an aryl or acyl ligand with the molecular formula of LnMOR (R: alkyl, acyl) [40]. BDIs have broad applications in inorganic as well as organic and polymer synthesis, especially in the polymerization of lactides [45]. In the case of copolymer‐ ization of epoxides and CO2, BDIs have been used in a large number of studies. The results demonstrated that both the nature of metal centre and the aryl ligand have significant effect on the activity and selectivity of the catalyst [29, 40, 46]. For example, only zinc metal showed remarkable efficiency and selectivity towards the copolymerization of propylene oxide and carbon dioxide [47, 48].

## *4.1.3. Metal salen complexes*

**4.1. Homogenous catalysts**

74 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

carbonates were produced [33, 34].

the reaction between PO and CO2.

ring and formation of cyclic carbonates [36].

*4.1.1. Phenoxide complexes*

*4.1.2. β-diiminate complexes*

merization of CO2 and epoxides that are discussed in this section.

in activation of other epoxides such as PO [37, 43, 44].

Triphenyl phosphate (TPP) compounds with a metal atom in the centre were the first group of homogeneous catalyst reported for copolymerization of CO2 and epoxides [31, 32]. TPP compounds showed high activity; however, this catalysts were not very efficient, and the yield of PACs was very low even after more than one weak reaction. Nitrogen donors compounds such as N-methylimidazole and (4-dimethylamino)pyridine (DMAP) were added to TPP compounds to enhance the yield of reaction. However, this catalyst system could only be used for the copolymerization of COH and CO2. For the other epoxides, only cyclic alkylene

Chen et al. investigated the effect of metal type located in the centre of TPP compounds on the activation of propylene oxide. It was found that the type of metal influenced the acidity strength of catalysts and, therefore, the strength of epoxide metal bond and the yield of copolymer synthesis [35]. Among three types of metal used in this study, chromium-based catalyst exhibited higher selectivity compared to Al and Co towards the formation of PPC from

In a metal complex catalyst, the metal acts as Lewis acid to attack the epoxide and after that the nucleophile part of the catalyst opens the epoxide ring. The nucleophile part of the catalyst can be either a ligand attached to the metal complex or a co-catalyst. In the former group, the nucleophile ligand can often be a salen, a porphyrin, a phthalocyanine or an amido macrocycle [30]. The nature of metal in the catalyst is critical in its activity and selectivity and also the quality of the resulted polymer. Indeed, the strength of epoxide metal bond can determine the pathway of reaction. When this bond is too weak, the reaction proceeds towards formation of ether linkages and backbiting while the high strength of the bond may lead to closure of the

After TPP compounds, great number of metal complex catalytic systems such as phenoxide [37–39] and β-diiminate (BDI) [29, 40] were investigated in terms of their activity for copoly‐

Phenoxide compounds include an aryl ligand that plays the role of nucleophile site of a catalyst [41]. It was found that the activity of a catalyst is dramatically higher when the phenoxide compounds contain zinc atom as a metal centre [42]. It was found that this group of metal complexes are only able to activate and catalyze CHO to react with CO2 and are not efficient

BDI is a family of complexes that include an aryl or acyl ligand with the molecular formula of LnMOR (R: alkyl, acyl) [40]. BDIs have broad applications in inorganic as well as organic and polymer synthesis, especially in the polymerization of lactides [45]. In the case of copolymer‐ ization of epoxides and CO2, BDIs have been used in a large number of studies. The results

Salens are a class of organic compounds including 1,6-bis(2-hydroxyphenyl)-2,5-diaza‐ hexa-1,5-diene ligand that are broadly used in the synthetic chemistry [49]. Metal salen complexes were first used for the ring opening copolymerization of epoxides in 1995 [50]. This family of catalysts has several advantages over other complexes for the copolymerization of epoxides and CO2. They are highly selective towards the synthesis of polycarbonates in the mild reaction condition (ambient temperature) [51]. Metal salen catalysts can be categorized according to the type of metal in the centre.

Chromium-based salen complexes ((salen)CrCl) were efficient in the copolymerization of epoxides and CO2 [52]. Two years after the first discovery, Jacobsen et al. successfully synthe‐ sized PCHC with 100% carbonate linkage using chiral salen chromium chloride catalyst [53]. The chromium-based salen complexes, however, showed low activity in the copolymerization of CO2 and PO due to the negligible differences between the activation energy of PO and PPC compared to cyclohexene carbonate (CHC) versus poly(cyclohexene carbonate) (PCHC) [54]. Therefore, the selectivity of the catalyst is a critical factor for the copolymerization of PO and CO2. The addition of co-catalyst was found to be an efficient approach to tackle this issue. In 2003, Rieger et al. attempted to add co-catalyst 4-(N,N-dimethylamino)pyridine (DMAP) to chromium salen and were able to successfully synthesize PPC with 98% carbonate linkage and minimal PC side product [55]. They proposed that strong coordination of DMAP to chromium promoted the propagation of polymer chain and formation of carbonate linkages. The type of initiator was another factor that had an impact on the selectivity of chromium salen catalysts for the synthesis of PPC. It was found that changing the initiator from phosphines to azide in combination with Cr(salen)N3 catalyst significantly stimulated the reaction pathway towards the formation of PPC rather than cyclic propylene carbonate [56].

The metal centre has a pivotal role in catalyst activity and selectivity of the metal salen complexes. The first cobalt salen complexes (Co(salen)AOc) was introduced in 2003 for the copolymerization of PO and CO2, which resulted in producing PPC with 99% carbonate linkage [57]. In addition, it was found that similar to the chromium salen catalysts, the addition of co-catalyst to the cobalt salen complexes has an impact on their activity and selectivity. For instance, the addition of sub-stoichiometric amount of bis(triphenylphosphine)iminium (PPN) as co-catalyst to a cobalt salen increased the yield of PPC synthesis to 36% with 99% carbonate linkage [58]. However, any further effect on yield enhancement led to the formation of cyclic propylene compounds due to backbiting degradation of the polymer chain. To accomplish high activity of a catalyst without losing the selectivity, Nakano et al. designed a cobalt salen complex with piperidinyl and piperidinium arms [59]. This modification resulted in producing 99% PPC with 97% conversion yield. Basically in this reaction, the side arms played the role of in situ co-catalysts. This approach was a breakthrough in the copolymerization of CO2 and 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.
