Long-chain branching and rheological properties of ethylene-1-hexene copolymers synthesized from ethylene stock by concurrent tandem catalysis

Concurrent tandem catalysis systems have shown a significant advantage in the convenient synthesis of linear low-density polyethylene (LLDPE) from a sole ethylene monomer stock by uniquely coupling the tandem action between an ethylene oligomerization catalyst and an ethylene copolymerization catalyst in a single reactor. Recently, we have reported the successful synthesis of ethylene-hexene derived LLDPE using an effective concurrent tandem Catalysis system comprising (eta(5)-C5H4CMe2C6H5)TiCl3 (1)/MMAO and a CGC copolymerization catalyst, [(eta(5)-C5Me4)SiMe2((BuN)-Bu-t)]TiCl2 (2)/MMAO. In this work, we report the results from an extensive study on the important rheological properties of LLDPE grades prepared with this tandem catalysis system. Two sets of LLDPE samples having different short-chain branching density (SCBD) were prepared with the tandem catalysis system under various catalyst concentrations and at temperatures of 25 and 45 degrees C. The melt rheological properties of these polymers were evaluated using small-amplitude dynamic oscillation measurements. These polymers have been found to possess typical rheological properties found in long-chain branched (LCB) polymers, such as enhanced zero-shear viscosity (eta(0)), improved shear-thinning, elevated dynamic moduli, and thermorheological complexity, which indicate the presence of long-chain branching in the polymers. The long-chain branching density (LCBD) of the two respective sets of polymers were qualitatively compared and correlated to the polymerization conditions including catalyst ratio and temperature. This work represents the first study on the rheological properties of LLDPE synthesized with concurrent tandem catalysis,and it discloses another appealing feature of this unique approach-its ability to produce LCB LLDPE from a single ethylene monomer stock.