*4.2.4. Catalyst activity of zinc glutarate*

Particle size, crystallinity, microstructure and morphology are the key factors that have impacts on activity and yield of the final product. These characteristics can be tuned by changing the source of zinc and glutarate, their particle size, purity, synthesis method and processing condition.

Ree et al. studied the effect of various zinc sources on the activity of ZnGA catalyst [62]. Their results showed that the highest activity of ZnGA was achieved when zinc was derived from zinc oxide. Zinc glutarate from various sources of glutarate was synthesized in another study carried out by Ree et al. [7]. Results of catalyst activity demonstrated that the yield of PPC was significant when zinc oxide and glutaric acid were used for the synthesis of ZnGA. The effect of zinc and glutarate source and synthesis media on the microstructure of ZnGA was also investigated in a study carried out by Kim et al. [8]. This study demonstrated that the highest activity was acquired when ZnGA was synthesized in toluene media due to achieving low surface area and perfect crystallinity.

Particle size and purity of reactants are other parameters that have effect on the activity of ZnGA [9]. It was shown that highly pure ZnO with large particle size resulted in ZnGA with the superior crystallinity and large particle size. In addition, in a bench-scale study, it was shown that when magnetic stirring was used rather than other methods of mixing, the degree 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 reaction.

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 that was synthesized in toluene.
