Kinetic and thermal study of ethylene-propylene copolymerization catalyzed by ansa-zirconocene activated with Alkylaluminium/borate: Effects of linear and branched alkylaluminium compounds as cocatalyst

Abstract

Ethylene-propylene (EP) copolymerizations was carried out with two ansa-metallocene: rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2 (Mt-I) and rac-Et(Ind)2ZrCl2 (Mt-II) combined with borates activators like ([Me2NPh][B(C6F5)4] (Borate-I), [Ph3C][B(C6F5)4] (Borate-II). Different structures of alkylaluminum( triethylaluminium (TEA), triisobutylaluminium (TIBA) and TEA/TIBA mixture of 25/75, 50/50, 75/25 mol ratios were used as cocatalysts. The choice of ligand structure and, more importantly, the nature of the cocatalyst significantly impact these systems' activity and the polymeric materials' properties. Borate-II has been shown as giving higher activities than Borate-I with both ansa-metallocene, attending 5.6 × 106 g/mol Zr*h. Mt-I gives higher activities and molecular weight but produced copolymers with low ethylene content, melting points, and crystallinity than Mt-II. The activities were very close to each other with 100% TIBA. Still, Mt-I became more active than Mt-II when TEA was more than 25% in the cocatalyst system. The effects of alkylaluminiums catalysts cocatalyst on EP copolymer molecular weight (Mw) and molecular weight distribution (MWD) were much more complicated. The MWD curve changed from broad to narrow with Mt-I when TIBA was replaced by a different mole ratio of TEA/TIBA ratio. The technique used for the assessment of active centers [C*]/[Zr] fraction in the ansa-metallocene catalyst was quenched label by using 2-thiophenecarbonyl chloride (TPCC) to determine the effects of alkylaluminiums catalysts structure on the propagation rate constants (kP) of EP copolymers. The [C*]/[Zr] value of Mt-I/Borate-I/TIBA is lower than those of the Mt-I/Borate-I/TEA and Mt-I/Borate-I/TEA/TIBA system. The main differences appear between catalysts containing pure TIBA and those containing TEA, possibly due to the faster rate of chain transfer with Al—Et than with bulky Al—iBu. Adding TEA in metallocene/borate/alkylaluminiums catalysts catalysts can be applied as an effective method to regulate the molecular weight of EP copolymer with a high degree of activity