Influence of Branching on the Thermal and Crystallization Behavior of Bimodal Polyethylenes Synthesized with Binary Late-Transition-Metal Catalyst Combinations

Two reactor blends of linear and branched polyethylene resins with bimodal molecular weight distributions were synthesized in a one-reactor polymerization process through the combination of 2,6-bis[1-(2,6-dimethyphenylimino)pyridyl]cobalt dichloride (1) and 2,3-bis(2,6-diisopropylphenyl)butanediimine nickel dibromide (2) or 1,2-bis(2,6-diisopropylphenyl)cyclohexene diimine nickel dibromide (3) in the presence of modified methylaluminoxane. The linear correlation between the catalyst activity and concentration of the nickel compounds suggested that the catalysts performed independently of one another. The molecular weights, molecular weight distributions, and crystalline and phase structures of the blends were investigated with a combination of high-temperature get permeation chromatography, differential scanning calorimetry, wide-angle X-ray diffraction, and small-angle X-ray scattering techniques. The branching degree of the polyethylene produced with 3 was much higher than the branching degree of the sample produced with 2, although their molecular weights were relatively close. In addition, the crystallization rate, melting temperature, degree of crystallinity, and crystallization temperature of more highly branched blends produced with 1/3 were lower. The long periods and thickness of the crystalline region were greatly influenced by the addition of highly branched polyethylene. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 3045-3055, 2010