Ethylene Polymerization Using Novel Homogeneous Single Site Catalysts

Abstract

A kinetic study of ethylene polymerization with a novel homogeneous single site catalyst (SSC) is conducted in Buchi Glasuster Polyclave Slurry Reactor. Under optimized polymerization conditions, the catalyst efficiently polymerizes ethylene into polyethylene (PE) having very high molecular weight (MW). The investigation into kinetics of this type of polymerization includes the effects of operational parameters such as polymerization time starting from starting point to 3 hrs. and polymerization temperature ranging from 25OC to 50OC, while keeping other process conditions like ethylene pressure, co-catalyst to catalyst ratio and polymerization temperature constant .The study illustrates that the polymerization is first order with respect to catalyst concentration, with an overall rate constant of k1 = 457.8539 (g of PE/g hexane)/ (min. (g of catalyst/g hexane). (g of Ethylene/g hexane)) and rate of catalyst deactivation, kd = 0.1678 min-1. The rate of polymerization dramatically decreases in the course of polymerization, whereas MW of resulting PE increases with time. It seems at the initial point in time of polymerization the catalytic system generates few active sites which produce low molecular weight PE of MW 1-2 million g/mole. These centres can undergo gradual deactivation or transformation into new centres that produce higher molecular weight PE of MW more than 4 million g/mole. As a result, a product with broad molecular weight distribution (MWD) is obtained. It seems the polymerization time and temperature does not have any impact on thermal properties and crytallinity of resulting polymer, as all PE samples obtained after different polymerization time show melting temperature in the range of 141-145OC with enthalpy of melting ranging from 180-245 J/g and % of crystallinity in the range of 90-95%. With an aim to study the effect of substituents on the SSC described in the above paragraph, two Schiff base imine ligands and their corresponding metal ligand complexes leading to the formation of two novel SSCs (Catalyst 1 & Catalyst 2) were synthesized, purified and characterized. The performance of pure catalysts was evaluated for ethylene polymerization. It was found that both catalysts efficiently polymerize ethylene into PE. However in both cases, catalyst productivity goes down by 20% as compared to performance of regular catalyst. Both catalysts significantly enhance MW of resulting polymer while keeping their MWD not much broad. Catalyst 1 produces PE having MW of 5.5 million g/mole and MWD of ~ 9.0. Catalyst 2 produces same polymer having MW of 3.78 million g/mole and MWD of ~ 9.7. For both catalysts, as polymerization time increases from 1 hr to 3 hr, MW of resulting PE increases, indicating pseudo-living characteristics of catalyst 1 & 2 under the employed polymerization conditions. The introduction of an electron withdrawing substituent in the present catalyst-framework changes the electronic and steric environment around the metal center in such a manner that the new catalysts showed different performance characteristics towards ethylene polymerization.