Theoretical Studies On The Olefin Polymerization Catalyzed By Nonbridged Half-Titanocene Complexes

In the past 20 years,half-metallocene complexes exhibit unique properties in the olefin polymerization.However,limited by the existing experimental method and screening technique,the polymerization mechanism could not be explored clearly,especially,the effect of the ancillary ligands on the olefin polymerization.In order to solve the above problems,quantum chemistry methods have been used to study the mechanisms of ethylene?co?polymerization and non-conjugated diene homopolymerization reaction catalyzed by half-titanocene complexes in this paper.The main research results are as summarized as follows:1.Density functional theory?DFT?method was utilized to explore the effect of the ancillary ligand toward the catalytic activity and molecular weight of polyethylene in ethylene homopolymerization catalyzed by half-titanocene complexes,Cp^*TiCl₂?O-2,6-ⁱPr₂C₆H₃??1?,Cp^*TiCl₂?N=C^tBu₂??2?,(Me₂Si?Me₄Cp??NtBu?TiC12?3?,?Cp^* = ?5-C5Me5,Cp = ?5-C5H5?Nonbridged Half-Titanocene complexes 1 and 2 showed slightly lower catalytical activity than Bridged Half-Titanocene complexes 3,which is attributed to the existance of the bridged groups reducing the deformation of active species in the 3.However,the former catalytic systems 1 and 2 could yield polyethylene with greater molecular weight than that of the latter 3,which is ascribed to the chain termination??-HE and ?-HT process?is more difficult to occur in the nonbridged half-titanocene catalyst than CGC on the thermodynamics and kinetics.For nonbridged half-titanocene catalytic systems,the volume of the auxiliary ligands should be reduced to improve catalytical activity.2.The copolymerization of ethylene and styrene can be efficiently carried out by using Cp^*TiCl₂?N=C^tBu₂?Cl₂/MAO,yielding the poly?ethylene-co-styrene?s with isolated styrene units.In order to investigate the reasons for formation of the structure,the mechanism of copolymerization,especially the selective insertion of ethylene and styrene,is studied in detail by DFT method.At the initiation stage,insertion of ethylene shows kinetically more favorable than insertion of styrene,and insertion of styrene kinetically and thermodynamically prefers 2,1-insertion.That is different from the conventional half-titanocene system,in which the 1,2-insertion is favorable.At chain propagation stage,the continuous insertion of styrene is hard to carry out at room temperature due to the high free energy barriers?28.90 and 35.04 kcal/mol for 1,2-insertion,and 29.15 and 34.00 kcal/mol for 2,1-insertion?and thermodynamically unfavorable factors in two different conditions.That is mainly attributed to the steric hindrance between the coming styrene and chain-end styrene or ketimide ligand.The computational results are in good agreement with the experimental data.3.The mechanism of 1,7-octadiene polymerization catalyzed by Cp^*TiCl₂?O-2,6-ⁱPr₂C₆H₃?/MAO,system was investigated by using the DFT method.The monomer's insertion mode,especially the selectivity of repeated insertion and intramolecular cyclization,was explored by concerning energies and structures in detail.At initiation stage,the 1,2-si-insertion is optimal pathway among four insertion modes kinetically and thermodynamically.At chain propagation stage,the competitions between repeated insertion and intramolecular cyclization were discussed in detail,concerning the insertion into different growing chains.The computational results indicated that the repeated insertion of 1,7-octadiene showed kinetically favorable than the intramolecular cyclization in three different situations,with the free energy barrier differences between two insertion modes of 3.75 kcal/mol,3.88 kcal/mol and 7.43 kcal/mol respectively.Moreover,the sequential insertion of seven-membered rings exhibited disadvantageous kinetically and thermodynamically,probably due to the steric repulsion effect between two cyclic rings.Those are in good agreement with the experimental data.It is the first time to study on the selectivity of the insertion mode of nonconjugated dienes theoretically as far as we know.