*4.3.2. Non-metallic catalysts and ionic liquids*

of crystallinity of ZnGA was promoted and resulted in achieving higher yield and larger molecular weight PPC. In yet another study, Pluronic PE6400 was used as an amphiphilic template to increase the degree of crystallinity and enhance the surface area of ZnGA [79]. It is important to note that in this study a mixture of ethanol/water was used as a solvent for the synthesis of ZnGA, and they achieved 83 g PPC/g activity for ZnGA. The yield of PPC production was increased nearly 20% when this catalyst was used for the copolymerization

78 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

Reducing the particle size of crystalline particles of ZnGA is another strategy that has been carried out to increase the catalyst activity [9]. Ball milling method can be used to reduce the particle size; however, it is important to optimize the processing time as long-hour milling might reduce the degree of crystallinity. It was also attempted to increase the surface area to elevate the ZnGA catalyst activity. As an example, PPC with the yield of 115.2 g PPC/g catalyst was obtained when high surface montmorillonite (MMT) was used for ZnGA support [80].

Optimization of the reaction condition was the subject of several studies with the aim of increasing the yield of polymerization. High yield of 126 g PPC/g catalyst was reported for a zinc glutarate supported on a perfluorinated compound with PO/catalyst ratio of 200 ml/g under mechanical stirring [81]. An optimum PO/catalyst ratio of 312 ml/g resulted in PPC yield of 160 g PPC/g catalyst for ZnGA prepared using ultrasonic stirring method [9]. Duan et al. in 2010 investigated the effect of pressure on the yield of copolymerization, product composition, molecular structure and thermal stability of PPC [82]. The results showed a significant enhancement in catalyst selectivity towards the formation of PPC and reduction of byproducts. Dehghani research group established a green method to synthesis ZnGA in super‐ critical CO2 and eliminated usage of the organic solvent [2]. This study demonstrated that the crystallinity of catalyst produced in this solvent-free process was a function of processing parameters such as temperature and pressure. In addition, the yield of PPC copolymerization was significantly improved when catalyst processed by this technique was compared to ZnGA

To enhance the catalytic activity and selectivity towards the copolymerization of CO2 and epoxides, several measures have been taken such as bimetallic catalyst systems, non-metallic

The idea behind the bimetallic pathway for the copolymerization of carbon dioxide and epoxides is to conduct the ring opening of epoxide and activation of carbon dioxide at the same time. In fact, one of the metal centres is in charge of the coordination and activation of the epoxide and the other metal centre associates with coordination and activation of CO2 [28]. It was found that the bimetallic pathway involved a second-order dependence on the metal concentration, whereas the monometallic pathway associated with a first order kinetic [30]. For instance, a double zinc centre complex showed high activity and selectivity in the conver‐

reaction.

that was synthesized in toluene.

*4.3.1. Bimetallic catalyst systems*

**4.3. New advances in the copolymerization of CO2 and epoxides**

catalysts, ionic liquids (ILs), and supported catalysts.

An ionic liquid is a salt in which the ions are poorly coordinated. Accordingly, these solvents are liquid at the temperatures below 100°C, or even at room temperature. ILs are broadly used as reaction media, co-catalyst and catalyst for the chemical reactions. In the case of the reaction of epoxides and CO2, several researchers have focused on the application of ILs. The mecha‐ nism of the reaction involved ring opening of the epoxide through a nucleophilic attack by the anion followed by insertion of carbon dioxide and ring closure [28]. Indeed, the absence of metallic centre as Lewis acid resulted in the selectivity of reaction towards the formation of cyclic carbonate compounds [30]. 1-*n*-butyl-3-methylimidazolium tetrafluoroborate success‐ fully activated CO2 and propylene oxide for the synthesis of propylene carbonate [84]. Under high-pressure condition, 100% conversion of propylene oxide was achieved by using 1-octyl-3 methylimidazolium tetrafluoroborate as catalyst [85]. However, the reaction was only selective towards the formation of cPC, and no polymer was achieved.

## *4.3.3. Supported catalytic systems*

In these systems, the active catalyst should be anchored to the supporting material through a strong covalent bond in order to avoid possible leaching during the reaction. In fact, the immobilization of a catalyst on the solid support enables its rapid recovery and recycling. However, in some cases, immobilization shows adverse effect on catalyst activity due to limiting the diffusion of the reactants and intermediate compounds into and from the active sites of the catalyst [30]. Mesoporous materials with high surface area such as MCM-41 or SBA-15 have been used as supports to tackle this hurdle [28]. The zn β-diiminate catalyst was supported on controlled pore glass, and mesoporous SBA-15. This catalyst showed significant activity in the alternating copolymerization of CHO and CO2. However, the resulted copoly‐ mer contains higher ether linkages than their corresponding non-supported catalysts due to the starvation of the active sites, particularly in the last stages of the reaction [86].

Recoverable aluminium salen catalysts bonded to poly(ethylene glycol bismethacrylate) and polystyrene exhibited comparable catalyst activity in the copolymerization of CO2 and epoxides [87]. Chromium salen catalysts anchored on the aminopropylsilyl-modified silica in combination with an IL were used to catalyze the reaction of the styrene oxide and CO2 [88]. The resulted catalysts exhibited 100% conversion yield; however, they were only selective towards the formation of cyclic carbonates.

Metal salen complexes were also immobilized on the surface of supporting material to facilitate their recovery after synthesis. For instance, zinc β-diiminate complex is immobilized on the surface of mesoporous silica (SBA-15), and this catalyst was used for the copolymerization of CHO and CO2. However, ether linkage also occurred in the presence of this catalyst. In another study, chromium salen catalyst, immobilized on the surface of silicates, was used for the copolymerization of styrene oxide and CO2 with nearly 50% activity and 80% selectivity [88]. In yet another example, aluminium salen catalyst, bonded on the surface of the polystyrene, was used for the copolymerization of CO2 and styrene oxide [87]. In addition to the potential of being recovered, the supported aluminium salen catalyst exhibited similar activity to the unsupported aluminium salen compounds.
