Molecular weight control in organochromium olefin polymerization catalysis by hemilabile ligand–metal interactions

A series of Cr(III) complexes based on quinoline-cyclopentadienyl ligands with additional hemilabile side arms were prepared and used as single-site catalyst precursors for ethylene polymerization. The additional donor functions interact with the metal centers only after activation with the co-catalyst. Evidence for this comes from DFT-calculations and from the differing behavior of the complexes in ethylene polymerization. All complexes investigated show very high catalytic activity and the additional side arm minimizes chain-transfer reactions, leading to increase of molecular weights of the resulting polymers.


Computational details
Density functional theory calculations were carried out using the Gaussian 03 program package [15]. Geometries have been fully optimized using the B3LYP functional [16] on a 6-311g* basis set level [17]. The obtained energy values were characterized as minima by analyzing the frequency values with regard to the absence of zero or imaginary frequencies. All energy values shown are zero point vibrational corrected energies.

General numeration
The numeration of the atoms of the quinolyl ligand part for the assignment of both 1 H and 13 C NMR signals is shown below:

General procedure for the syntheses of the ligands
The syntheses of all new ligands was performed according to a modified literature procedure for the synthesis of 3,4,5-trimethyl-1-(8-quinolyl)-2-(trimethylsilyl)cyclopentadiene [1]. To a suspension of potassium hydride in THF 0.90-1.00 equivalents of 2,3,4-trimethyl-1-(8-quinolyl)cyclopentadiene were added slowly, accompanied by a change of color from yellow to dark violet. After stirring overnight 1.05-1.20 equivalents of an appropriate chlorodimethylsilane were added followed by continuous stirring overnight. The product mixture was quenched with iced water/NH 4 Cl, washed with water, extracted with diethyl ether and the ethereal solution was dried over MgSO 4 . The raw product was purified by column chromatography using dichloromethane as solvent and silica as stationary phase. The products are highly viscous orange oils.

Ethylene polymerizations
The co-catalyst PMAO was supplied by AKZO-NOBEL as 7 wt % solution in toluene and was used as received. All polymerizations were carried out at atmospheric pressure and room temperature.
A 250 mL Schlenk flask, equipped with an output flow meter and cooled with a water bath, was filled with toluene (140 mL). In a 25 mL flask 2-10 mol of the appropriate catalyst precursor 1-8 were dissolved in 10 mL of toluene and activated with PMAO. The Cr:Al ratio was 1:1000. After 5 min the catalyst solution was transferred to the prepared toluene flask, immediately followed by ethylene feeding via the Schlenk valve. The polymerization was operated 10-20 min under intense stirring, while ethylene gas was fed through a flow meter into the flask. The surplus of unreacted ethylene gas was measured with a second flow meter. The reaction was terminated by the addition of methanol/HCl (50 mL). The precipitated polyethylene was filtered, stirred in acetone for two hours and dried at 80 °C overnight to constant weight. The polymer was analyzed by differential scanning calorimetry (DSC) and high temperature size exclusion chromatography (SEC). S14  Figure S1: Molecular structures of the compounds analyzed by X-ray diffraction.
Hydrogen atoms as well as co-crystallized solvent (4) or structure of a second independent molecule in the unit cell (8) are omitted for clarity.