Effect of the Degree of Cluster TiCl4 on the Polymerization of Isoprene over Ziegler-Natta Catalysts: A DFT Study

The active center of the TiCl4/MgCl2 Ziegler-Natta catalyst (TMC) has an important impact on alkadiene polymerization. In this paper, density functional theory was used to study the adsorption behavior of monomeric TiCl4 and dimeric Ti2Cl8 on different surfaces of MgCl2 (110) and (104). Additionally, the influence of the distribution of active center TiCl4 on the kinetics of isoprene polymerization catalyzed by TMC was also investigated, aiming to gain a better understanding of the potential active site structures in the TMC for the polymerization of dienes. Three different active models were employed to analyze the catalysts: TMC-1 with isolated mononuclear TiCl4, TMC-2 with clustered mononuclear TiCl4, and TMC-3 with larger-size cluster mononuclear TiCl4. Moreover, TMC-2 was further divided into TMC-2(a) and TMC-2(b) for discussion. In the TMC-2(a) model, the central Ti site was selected as the active center for the coordinating and insertion process with cis-isoprene, while in the TMC-2(b) model, the left Ti site was selected as the coordination and insertion process with cis-isoprene. The calculation results showed that monomeric TiCl4 adsorption on the (110) surface is the most stable adsorption configuration. For cis-isoprene, TMC-1 showed the highest stereoselectivity, while TMC-3 showed the lowest stereoselectivity. In the coordination stage, the TMC-1 model, with isolated mononuclear TiCl4, had a preference for the si configuration, while the TMC-2(a), TMC-2(b), and TMC-3 models, with clustered mononuclear TiCl4, had a preference for the re configuration. In the insertion process of cis-isoprene, the re configuration in TMC-1 had a relatively lower reaction barrier, while the si configuration in TMC-2(b) and TMC-3 had relatively lower reaction barriers.