LINEAR LOW-DENSITY POLYETHYLENE - CATALYST SYSTEM EFFECT ON POLYMER MICROSTRUCTURE

The microstructure in terms of molecular mass distribution (MMD) and c hemical comonomer distribution (CCD) of linear low density polyethylen e (LLDPE) is greatly influenced by the catalytic system and process co nditions. Furthermore, the 1-olefin insertion in the polymer chain can be distinguished between intramolecular and intermolecular distributi on. To investigate the intermolecular CCD and the MMD, four LLDPEs (et hylene-1-butene copolymers) from different catalytic systems based on Ti/MgCl2 catalysts, a zirconocene complex and a vanadium compound, are studied. The microstructure of the whole polymers and of their fracti ons are examined by size exclusion chromatography (SEC), differential scanning calorimetry (DSC), and C-13 NMR analysis. Further information about CCD is also obtained by DSC analysis on the whole samples by ap plying a particular thermal treatment, named thermal fractionated crys tallization (TFC), based on isothermal crystallization of the polymers from the melt at fixed temperatures. The results obtained with the di fferent analytical techniques show that the examined polymers have a v ery distinct microstructure depending on the catalyst type.