Syndioselective propylene polymerization: Comparison of Me2C(Cp)(Flu)ZrMe2 with Et(Cp)(Flu)ZrMe2

Abstract

The kinetics and stereochemical control of propylene polymerization initiated by syndiospecific isopropylidene(1-eta(5)-cyclopentadienyl)(1-eta(5)-fluorenyl)-dimenthylzirconium-methyl aluminoxane (1/MAO) and (1-fluorenyl-2-cyclopentadienylethane)-dimethylzirconium-MAO (2/MAO) were investigated. The influence of MAO concentration and polymerization temperature (T-p) on polymerization kinetics and polypropylene properties, such as molecular weight, molecular weight distribution (MWD), and stereoselectivity, have been studied in detail. The activity of both catalytic systems is very sensitive to the concentration of MAO. The 1/MAO and 2/MAO catalysts record maximum activity when [Al]/[Zr] ratio is around 1300 and 2500, respectively. The activity and the degree of stereochemical control are also sensitive to T-g. The 2/MAO catalyst is much more thermally stable than 1/MAO catalyst; the former shows maximum activity at 80 degrees C, whereas the latter shows maximum activity at 20 degrees C. The cationic active species generated by 2/MAO is not so stereorigid as those by 1/MAO so that 2/MAO catalyst produces sPP of broad MWD (4.43-6.38) and low syndiospecificity at high T-p. When T-p is above 50 degrees C, 2/MAO catalyst produces completely atactic polypropylene. The results of fractionation of sPP samples produced by 1/MAO and 2/MAO demonstrate that 1/MAO catalyst is characterized by uniform active sites, but 2/MAO is characterized by multiple active sites. (C) 1998 John Wiley & Sons, Inc.