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A study of 1-hexene polymerization with ethylene-bis(9-fluorenyl) hafnium dichloride has been carried out using two different cocatalyst systems, methyl-aluminoxane/trimethylaluminum (MAO/TMA) and tris-isobutyl-aluminum/N,N-dimethylanilinium tetrakis(pentafluorophenyl) borate (TIBA/borate). When MAO/TMA was used, 1-hexene polymerized into a low molar mass poly(1-hexene) with low catalytic activity. Activation with TIBA/borate increased polymerization activity drastically as well as the molar mass of the polymers. In order to analyze differences in the activity profiles, UV–Vis spectroscopy was employed to investigate ligand to metal charge transitions (LMCT) of the hafnocene dichloride during the activation process. The low catalytic activity and the fast chain transfer to the cocatalyst with MAO/TMA may originate from strong bonding between the metallocene cation and the MAO/TMA species thus obstructing monomer coordination and insertion.

Ligand effects on chain termination reactions in hafnocene-catalyzed ethene polymerization process have been systematically studied by quantum chemical methods. β-hydrogen transfer to metal, β-hydrogen transfer to monomer and hydrogenolysis were studied for 27 hafnocenes, initiating the chain termination reactions after insertion of the second ethene monomer. The results of the calculations were studied as a function of the ligand structure, focusing on the effects of various ancillary ligands, ligand substituents and bridging units. The ligand effects on chain termination reactions are strongly affected by combined effects of various structural units, in particular, in the cases of β-hydrogen transfer to monomer and hydrogenolysis. The results are expected to aid in design and development of new hafnocene polymerization catalysts.

The influence of ligand structure of hafnocenes on activation of the polymerization catalysts has been studied by quantum chemical methods. Altogether 54 hafnocenes were included in the analysis, supplemented by four zirconocenes for comparison. The trends in structural and electronic parameters relevant in the catalyst activation step were studied for the dichloride, dimethyl and cationic monomethyl forms of the catalysts. The effects of ligand modifications were analyzed as functions of the metal, ancillary cyclopentadienyl-based ligand, ligand substituent and the ligand bridge, making comparisons to experimental data. Generally, large aromatic ligands together with electron donating ligand substituents stabilize the catalytically active species, thus facilitating the catalyst activation process. The obtained trends are expected to aid in the development of new high-performance polymerization catalysts.

Six (Cp)(4-phenyl-indenyl)MCl2 (A) and four (Cp)(1-phenyl-indenyl)MCl2 (C) (M=Zr/Hf) based complexes were tested in ethene homo- and copolymerisation under different conditions to explain how the differences in the complex structure affect the polymerisation process and the formed polymer. Polymerisation experiments reveal that hafnocene catalysts need a higher amount of MAO to reach the maximum activity than zirconocenes. Hafnocenes also incorporate better 1-hexene. Catalysts with the Cp* or 1,2,4-Me 3 Cp ligand show higher activity, and work well with [HNMe2Ph][B(C6F5)4]/TIBA as cocatalyst, but produce polymers with lower M n than the corresponding catalyst with a plain Cp ligand. The Cp* ligand seems to prevent 1-hexene incorporation. A methyl group in the 2-position of (Cp)(4-PhInd)ZrCl 2 decreases the activity and M n but favours 1-hexene insertion. Polymerisation activity is higher with catalysts with a 1-PhInd-ligand than with a 4-PhInd-ligand.

Poly(propylene)s were prepared with metallocene catalyst rac-SiMe2(2-Me-4-PhInd)2ZrMe2/MAO (rac-dimethylsilylbis(2-methyl-4-phenylindenyl)dimethylzirconium/methylaluminoxane) in heptane solution at temperatures from 50 to 80 °C with varying concentrations of monomer, hydrogen, triisobutylaluminium (TIBA) and MAO. Polymer molar mass depended on the monomer, MAO, TIBA, and hydrogen concentrations and on polymerization temperature. The isotacticity was very high (mmmm > 95%), and only a slight decrease was detected at high temperatures. Regio selectivity was also high; the total amount of 2,1- and 3,1-insertions was less than 0.4 mol-%. Lowering the monomer concentration and raising the temperature increased the amount of 3,1 defects over the amount of 2,1 defects. End-group analysis by 13C NMR spectroscopy revealed isobutyl and allyl end-groups. Chain transfer to aluminium and β-CH3 elimination were concluded to be the dominating chain-termination mechanisms. The importance of β-CH3 elimination increased with temperature. Hydrogen addition changed both the initiation and termination mechanisms as indicated by the presence of propyl, butyl and 2,3-dimethylbutyl end-groups. According to modeling studies, the molar mass follows a first-order relationship with propylene and hydrogen concentrations, and a half-order relationship with MAO concentration. Arrhenius-type activation energy coefficients were 125 kJ · mol−1 for β-CH3 elimination, 66 kJ · mol−1 for chain transfer to aluminium, and 53 kJ · mol−1 for chain transfer to hydrogen. A value of 45 kJ · mol−1 was used for the propagation.

The aim of this paper is to compare the results of analytical and rheological characterization techniques with respect to the analysis of long-chain branches (LCB) in polyethylenes. The materials investigated are metallocene-catalyzed ethylene homopolymers into which LCB were introduced by the selection of appropriate polymerization conditions. The samples were characterized analytically by 13C NMR, IR-spectroscopy, and size exclusion chromatography coupled with a multi-angle laser light scattering detector. The rheological characterization was performed using dynamic–mechanical measurements and creep and creep recovery experiments in shear. It was found that the analytical methods were in good qualitative agreement for the most highly branched sample whose LCB content was 0.12 LCB/1000 C as determined by 13C NMR spectroscopy. However, even a 10-times lower LCB content, which is almost beyond the detection limit of 13C NMR measurements, had a significant impact on the rheological behavior. Rheological experiments clearly indicated the presence of LCB by changes in the frequency dependence of dynamic–mechanical material functions and the molecular mass dependence of the zero shear-rate viscosity in comparison to linear polyethylenes.

A study of ethene solution polymerization with the rac-dimethylsilylbis(indenyl)-zirconium dichloride/methylaluminoxane catalyst system in a high-temperature (140 °C), continuously stirred tank reactor system was carried out. 13C NMR, gel permeation chromatography, Fourier transform infrared, and rheological measurements were used for polymer analyses. Polyethylenes with low molecular weights (weight-average molecular weight ≈ 35–55 kg/mol) and small amounts of methyl, ethyl, and long-chain branching were produced. 13C NMR measurements showed that the long-chain and methyl branches increased and that the ethyl branch contents decreased with decreasing monomer concentrations. At high monomer concentrations, the chain transfer to the coordinated monomer was concluded to be the predominant chain termination mechanism, whereas the chain transfer to aluminum was dominant at low monomer concentrations, which was evidenced by the fact that the selectivity of end groups was reduced to about 50%. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3292–3301, 2002

Ethene was homo- and copolymerized over the methylaluminoxane (MAO) activated meso- and rac-diastereomers of 1- and 2-tert-butyldimethylsiloxy-substituted ethylenebis(indenyl)zirconium dichlorides (meso-1, rac-1, meso-2, rac-2) at 40 °C in toluene. Homopolyethenes produced at low ethene concentration under slurry conditions revealed significantly modified melt rheological properties resulting from long-chain branching (LCB). In addition, a small amount of ethyl branching was found in polyethenes produced with meso-1, rac-1, and meso-2. The meso-catalysts were found to have an excellent comonomer response; in the copolymerization of ethene with 1-hexene rE(meso-1) = 10 ± 1 and rE(meso-2) = 11 ± 1. Comonomer reactivity ratios indicate a strong preference toward formation of isolated comonomer units in the polymer chain. Chain termination occurs preferentially after regular 1,2-insertion of the comonomer, although irregular 2,1-insertions also inflict some chain terminations with the racemic catalysts but no...

A study was made on the effects of polymerization conditions on the long-chain branching, molecular weight, and end-group types of polyethene produced with the metallocene-catalyst systems Et[Ind]2ZrCl2/MAO, Et[IndH4]2ZrCl2/MAO, and (n-BuCp)2ZrCl2/MAO. Long-chain branching in the polyethenes, as measured by dynamic rheometry, depended heavily on the catalyst and polymerization conditions. In a semibatch flow reactor, the level of branching in the polyethenes produced with Et[Ind]2ZrCl2/MAO increased as the ethene concentration decreased or the polymerization time increased. The introduction of hydrogen or comonomer suppressed branching. Under similar polymerization conditions, the two other catalyst systems, (n-BuCp)2ZrCl2/MAO and Et[IndH4]2ZrCl2/MAO, produced linear or only slightly branched polyethene. On the basis of an end-group analysis by FTIR and molecular weight analysis by GPC, we concluded that a chain transfer to ethene was the prevailing termination mechanism with Et[Ind]2ZrCl2/MAO at 80 °C in toluene. For the other catalyst systems, β-H elimination dominated at low ethene concentrations. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 376–388, 2000

The influence of siloxy substitution of ethylene-bridged bis(indenyl)- and bis(tetrahydroindenyl)zirconium dichlorides on ethylene/α-olefin copolymerization and ethylene homopolymerization performance was investigated. Ethylene was copolymerized with 1-hexene and 1-hexadecene over rac-[ethylenebis(1-(tert-butyldimethylsiloxy)-3-indenyl)]zirconium dichloride (1), rac-[ethylenebis(1-(tert-butyldimethylsiloxy)-4,5,6,7-tetrahydro-3-indenyl)]zirconium dichloride (H1), rac-[ethylenebis(2-(tert-butyldimethylsiloxy)-1-indenyl)]zirconium dichloride (2), and rac-ethylenebis(1-indenyl)zirconium dichloride (EBI) using methylaluminoxane (MAO) as cocatalyst. 1-Siloxy substitution was found to remarkably improve the copolymerization ability and 2-siloxy substitution to enhance the polymerization activity of ethylenebis(indenyl)zirconium dichlorides. Optimum homo- and copolymerization performance was observed at a very low Al(MAO) concentration, which for 2 could be reduced to a level of 0.2 mmol/dm3 by using a small amo...

Ethene homopolymers and ethene copolymers with 1-hexene were prepared by Et[Ind]2ZrCl2/MAO and Et[IndH4]2ZrCl2/MAO catalyst systems in slurry polymerizations. The melt behavior of the polymers was studied with small amplitude dynamic rheological measurements. The low-frequency complex viscosity of the polymers was higher than expected on the basis of their GPC molecular weights. Furthermore, the polymers exhibited elevated activation energy for flow. The polymers catalyzed by Et[Ind]2ZrCl2/MAO had an Arrhenius-type flow activation energy of 50−60 kJ/mol, and those catalyzed by Et[IndH4]2ZrCl2/MAO a somewhat lower value of 40 kJ/mol. Branching could be detected by 13C NMR in homopolyethene samples polymerized by Et[Ind]2ZrCl2/MAO. We suggest that these properties are due to long-chain branching that occurs via in situ incorporation of vinyl-terminated polyethene macromonomers. With Et[Ind]2ZrCl2/MAO the polymerization parameters affecting the rheological behavior most were ethene partial pressure and comon...