Zirconocene Propylene Polymerisation: Controlling Termination Reactions

Abstract

Summary: Propylene was polymerised at varying trimethylaluminium (TMA) concentration with two metallocenes activated by methylaluminoxane (MAO) in an attempt to better understand the effect of TMA on the activation process, catalyst activity and termination reactions. A chemical treatment of MAO solution with 2,6‐di‐tert‐butyl‐p‐cresol was used to obtain TMA‐free polymerisation conditions. The metallocene precursors under investigation were diphenylmethyl(cyclopentadienyl)(9‐fluorenyl)zirconium dichloride (1) and rac‐dimethylsilylbis(4‐tert‐butyl‐2‐methyl‐cyclopentadienyl)zirconium dichloride (2). Chain transfer to aluminium was the dominating termination route for 1/MAO accompanied with β‐H/β‐CH3 transfer to Zr and β‐H transfer to monomer. It was found that β‐H/β‐CH3 transfer to Zr was favoured over the β‐H transfer to monomer at elevated temperatures, and that polymerisation and β‐H transfer to monomer depended on the same critical reaction. For 2/MAO the detected termination routes were β‐H transfer to Zr and chain transfer to aluminium. A strong activity dependency on TMA concentration was observed. With 1/MAO high TMA concentration decreased and stabilised the activity, whereas TMA free polymerisation conditions at 40 °C increased markedly the activity, indicating that TMA coordinated to the active site of 1/MAO. Surprisingly, with more sterically hindered 2/MAO, high TMA concentration increased the activity and nearly complete activity loss occurred at TMA‐free polymerisation conditions at 40 °C.Metallocene precursors for investigation of propylene polymerisation.magnified imageMetallocene precursors for investigation of propylene polymerisation.

Alkylation and activation of metallocene polymerization catalysts by reactions with trimethylaluminum: A computational study

“…While TMA has been shown to monoalkylate dichloride catalyst precursor, [7e13] its role in the activation process itself is elusive. Usually TMA is found to reduce the activity of the catalysts, [7e11] but addition of TMA has also been reported to increase the activity, [14,15] or even to act as the actual cocatalyst [4,15]. A number of related computational studies have been reported on aspects concerning ligand abstraction, ion pair formation and monomer incorporation with various cocatalysts and catalyst precursors [10,13,16e25].…”

Chain transfer reactions of propylene polymerization catalyzed by AlEt3 activated TiCl4/MgCl2 catalyst under very low monomer addition rate

“…In the monomer-starved polymerization system, chain transfer with cocatalyst (AlEt 3 in this work) may become the dominant termination reaction in the absence of hydrogen [6,14,24,25], as the AlEt 3 concentration will be much higher than [M]. As reported in literatures, the product of transfer with alkylaluminum, the aluminum-terminated PP chains, can be converted to hydroxyterminated PP chains after quenching the polymerization system with oxygen and subsequent hydrolysis [24,25].…”

“…Since the 1990s, chain transfer reactions in olefin polymerization with metallocene catalysts have been studied in detail [4][5][6][7][8][9][10][11][12][13]. However, study of chain transfer reactions in MgCl 2 -supported catalyst systems was limited, partly because of the difficulties in tracing the low level of active sites in these heterogeneous systems.…”

Versatile Pathways for In Situ Polyolefin Functionalization with Heteroatoms: Catalytic Chain Transfer

“…Additionally, there have also been several studies of alane chain transfer in fluorenyl‐ and Cp‐based zirconocene‐mediated propylene polymerization systems 8a,c,d. These studies again indicate that addition of AlMe 3 or excess MAO to the polymerization processes results in depressed product polymer M n values along with polymer microstructures containing saturated isopropyl end groups, suggesting alane chain transfer as the dominant chain‐termination pathway.…”