No abstract

No abstract

No abstract

The influence of the ligand structures of hafnocene polymerization catalysts on ethene insertion and chain propagation was systematically studied by quantum chemical methods. Altogether 54 hafnocenes were studied as a function of the ligand structures. Two consecutive ethene insertions and chain propagations were performed for the catalysts, giving rise to 15 intermediate structures along the reaction pathway. The behavior of the catalysts was analyzed as a function of ancillary ligands, ligand substituents, and bridging units. The differences along the reaction pathway are dominated by the changes in relative stabilities of the catalytic intermediate products. Large aromatic ancillary ligands and electron-donating ligand substituents strongly stabilize the catalyst cations. Steric effects introduced by the ligand framework mostly affect the feasibility of ethene π-coordination and the activation energy for chain propagation. The dominant effect of the relative stabilities of the catalyst intermediates sheds light on the catalytic performance of metallocenes, which may turn out to be useful in further catalyst development.

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.magnified image