Abstract
Half-titanocenes are well known to show high activity for ethylene polymerization and good capability for copolymerization of ethylene with other olefins, and the ancillary ligands can crucially affect the catalytic performance. In this paper, the mechanisms of ethylene polymerization catalyzed by three half-metallocenes, (η5-C5Me5)TiCl2(O-2,6-iPr2C6H3) (1), (η5-C5Me5)TiCl2(N=CtBu2) (2) and [Me2Si(η5-C5Me4)(NtBu)]TiCl2 (3), have been investigated by density functional theory (DFT) method. At the initiation stage, a higher free energy barrier was determined for complex 1, probably due to the presence of electronegative O atom in phenoxy ligand. At the propagation stage, front-side insertion of the second ethylene is kinetically more favorable than back-side insertion for complexes 1 and 2, while both side insertion orientations are comparable for complex 3. The energy decomposition showed that the bridged cyclopentadienyl amide ligand could enhance the rigidity of the active species as suggested by the lowest deformation energy derived from 3. At the chain termination stage, β-H transfer was calculated to be a dominant chain termination route over β-H elimination, presumably owing to the thermodynamic perspective.
Highlights
Since the discovery of Ziegler–Natta and Phillips catalysts in 1950s, polyolefins, mainly consisting of polyethylene (PE) and polypropylene (PP), have become the largest class of commercial thermoplastics, making the polyolefin industry a multibillion-dollar business [1]
We focused on the reactivity of the half-titanocene-catalyzed ethylene polymerization to get insights into the roles of different ancillary groups
The complexes (η5-C5Me5)TiCl2(O-2,6-iPr2C6H3) (1), (η5-C5Me5)TiCl2(N=CtBu2) (2) and [Me2Si(η5-C5Me4)(NtBu)]TiCl2 (3) could be considered to generate the alkyl titanium cations [Cp*Ti(O-2,6-iPr2C6H3)CH3]+ (1A), [Cp*Ti(N=CtBu2)CH3]+ (2A) and [(Me2Si(Me4Cp) (NtBu)TiCH3]+ (3A), respectively, which served as the initial active species in the present computational study (Figure 1)
Summary
Since the discovery of Ziegler–Natta and Phillips catalysts in 1950s, polyolefins, mainly consisting of polyethylene (PE) and polypropylene (PP), have become the largest class of commercial thermoplastics, making the polyolefin industry a multibillion-dollar business [1]. Marks et al communicated the preliminary results of the ab initio computational investigation on Me2Si-bridged Cp/amido systems and found that counteranion and solvation effects were substantial [31,32] They found that the ion pair Ti···H3CB interaction was predominantly electrostatic in character [32]. We have been focusing on the experimental and theoretical studies on the coordination polymerization of olefins or conjugated dienes by early transition metal complexes [33,34,35,36,37], especially the theoretical study on mechanism of the non-bridged half-titanocene systems for polymerization of 1,7-octadiene and copolymerization of ethylene and styrene [36,37]. Β-H transfer and the β-H elimination pathway in the termination stage were investigated
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