Linear low density polyethylene: catalyst system effect on polymer microstructure

The microstructure in terms of molecular mass distribution (MMD) and chemical comonomer distribution (CCD) of linear low density polyethylene (LLDPE) is greatly influenced by the catalytic system and process conditions. Furthermore, the 1-olefin insertion in the polymer chain can be distinguished between intramolecular and intermolecular distribution. To investigate the intermolecular CCD and the MMD, four LLDPEs (ethylene—1-butene copolymers) from different catalytic systems based on TiMgCl2 catalysts, a zirconocene complex and a vanadium compound, are studied. The microstructure of the whole polymers and of their fractions are examined by size exclusion chromatography (SEC), differential scanning calorimetry (DSC), and 13C NMR analysis. Further information about CCD is also obtained by DSC analysis on the whole samples by applying a particular thermal treatment, named thermal fractionated crystallization (TFC), based on isothermal crystallization of the polymers from the melt at fixed temperatures. The results obtained with the different analytical techniques show that the examined polymers have a very distinct microstructure depending on the catalyst type.