Understanding the Chemical and Physical Transformations of a Ziegler-Natta Catalyst at the Initial Stage of Polymerization Kinetics: The Key Role of Alkylaluminum in the Catalyst Activation Process

Development of Large-Scale Stopped-Flow Technique and its Application in Elucidation of Initial Ziegler-Natta Olefin Polymerization Kinetics

The stopped-flow (SF) technique has been extensively applied to study Ziegler-Natta (ZN) olefin polymerization kinetics within an extremely short period (typically <0.2 s) for understanding the nature of the active sites as well as the polymerization mechanisms through microstructure analyses of obtained polymers. In spite of its great applicability, a small amount of polymer that is yielded in a short-time polymerization has been a major bottleneck for polymer characterizations. In order to overcome this limitation, a large-scale SF (LSF) system has been developed, which offers stable and scalable polymerization over an expanded time range from a few tens milliseconds to several seconds. The scalability of the LSF technique has been further improved by introducing a new quenching protocol. With these advantages, the LSF technique has been effectively applied to address several unknown issues in ZN catalysis, such as the role of physical and chemical transformations of a catalyst on the initial polymerization kinetics, and regiochemistry of ZN propylene polymerization. Here, we review the development of the LSF technique and recent efforts for understanding heterogeneous ZN olefin polymerization catalysis with this new system.

Comparative Study on Kinetics of Ethylene and Propylene Polymerizations with Supported Ziegler⁻Natta Catalyst: Catalyst Fragmentation Promoted by Polymer Crystalline Lamellae

The kinetic behaviors of ethylene and propylene polymerizations with the same MgCl₂-supported Ziegler–Natta (Z–N) catalyst containing an internal electron donor were compared. Changes of polymerization activity and active center concentration ([C*]) with time in the first 10 min were determined. Activity of ethylene polymerization was only 25% of that of propylene, and the polymerization rate (R_p) quickly decayed with time (t_p) in the former system, in contrast to stable R_p in the latter. The ethylene system showed a very low [C*]/[Ti] ratio (<0.6%), in contrast to a much higher [C*]/[Ti] ratio (1.5%–4.9%) in propylene polymerization. The two systems showed noticeably different morphologies of the nascent polymer/catalyst particles, with the PP/catalyst particles being more compact and homogeneous than the PE/catalyst particles. The different kinetic behaviors of the two systems were explained by faster and more sufficient catalyst fragmentation in propylene polymerization than the ethylene system. The smaller lamellar thickness (<20 nm) in nascent polypropylene compared with the size of nanopores (15–25 nm) in the catalyst was considered the key factor for efficient catalyst fragmentation in propylene polymerization, as the PP lamellae may grow inside the nanopores and break up the catalyst particles.

Adsorption of Titanium Tetrachloride on Magnesium Dichloride Clusters

Magnesium dichloride and titanium tetrachloride are key components in heterogeneous Ziegler-Natta olefin polymerization catalysis. We have determined the preferred binding modes of titanium tetrachloride on magnesium dichloride clusters at the M06-2X level of theory, thus accounting for dispersion. A systematic study was carried out to locate the lowest energy isomers of (MgCl₂) _n (TiCl₄) _m complexes. Altogether ca. 1000 complexes with n ranging from 1 to 19 and m from 1 to 13 were studied. In line with the previous literature, the results consistently show that adsorption of TiCl₄ onto MgCl₂ preferably leads to octahedral six-coordination for both Mg and Ti atoms. This can be achieved by mononuclear binding of TiCl₄ and binuclear binding of Ti₂Cl₈ on (110) and (104) surfaces of MgCl₂, respectively. The preferred octahedral six-coordination is also achieved by binding of TiCl₄ on defect sites, of which the most striking example is trinuclear binding mode at corners of the clusters. Overall, the results highlight the relevance of multinuclear binding of TiCl₄ on MgCl₂.

Accurate experimental and theoretical enthalpies of association of TiCl4 with typical Lewis bases used in heterogeneous Ziegler–Natta catalysis

The enthalpy of association of Lewis bases with TiCl₄ is analyzed using experimental and computational techniques.