Hemiisotactic poly(propylene) through propene polymerization with the ipr[3‐mecpflu]zrcl2/mao catalyst system: A kinetic and microstructural analysis

Abstract

AbstractThrough polymerizing propene by means of the catalyst system iPr[3‐MeCpFlu]ZrCl2/MAO it was possible for the first time to generate hemiisotactic poly(propylene); the microstructure of this polymer is characterized by an alternating sequence of monomer units with equal and statistical configuration. iPr[3‐MeCpFlu]ZrCl2 is one of the new developed stereorigid ansa‐metallocene catalysts which by means of the structure of the bridged η‐ligand system are considered to give access to the description and calculation of the behaviour of the stereospecifity. In the present paper the α‐olefin polymerization was investigated with the iPr[3‐MeCpFlu]ZrCl2/MAO catalyst system by means of kinetics and product analytics; parameter were the [Al]:[Zr] ratio, the monomer concentration, the polymerization temperature, the polarity of the solvent as well as the chain length of the starting α‐olefin. The order of the polymerization reaction concerning the propene concentration was found to be 1,3; this broken order indicates a complex reaction. Concerning the dependency of the polymerization rate on the [Al]:[Zr] ratio a bell‐shaped curve was found; obviously, at very high [Al]:[Zr] ratios the complex MAO compound inhibits the active zirconocene centre. The microstructure of the poly(propylene)s generated in toluene at 25°C corresponds with the expected hemiisotactic structure. On the contrary, the microstructure of the poly(propylene)s generated in toluene at higher polymerization temperatures and in the polar solvents o‐dichlorobenzene and methylene chloride at 25°C shows considerable deviations from a hemiisotactic structure. Under these polymerization conditions the stereospecifity of the iPr[3‐MeCpFlu]ZrCl2/MAO catalyst can only be satisfyingly explained with isomerization reactions of the active zirconocene species between two insertion steps. These isomerization reactions without monomer coordination occur preferably starting from the unstereospecific state of the catalyst. This interpretation is confirmed by 1‐butene and 1‐hexene polymerization experiments.