Influence of Tiny Amounts of 1,4-Pentadiene on Propylene Polymerization and Polymer Properties Catalyzed by a Classical Ziegler-Natta Catalyst

MgCl2/TiCl4 catalyst with 9,9-bis (methoxymethyl) fluorine (BMMF) as internal electron donor has been paired with triethylaluminum (TEA) and used to catalyze propylene polymerization in bulk, to which tiny amounts of 1, 4-pentadiene (PD) are added and their influence on the polymerization behavior and polymer properties are investigated. Gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and dynamic shear rheology are used as the main polymer characterization tools. The results indicate that, within a very low [PD]/[propylene] molar ratio range ([PD]/[propylene] < 0.1%), the catalyst activity decreases sharply following PD concentration increasing. At a [PD]/[propylene] of 0.08%, the activity is barely a quarter of that when PD is absent. However, it is found that PD does not cause a monotonous decrease in viscosity-average molecular weight (M-v) as its concentration increases. Matter-of-factly, at the lowest PD concentration an abrupt increase in M-v is observed for the copolymer as compared to the homopolymer. GPC results further confirm that increase and indicate no branching has occurred to the PP chains due to PD. Results from dynamic shear rheology tests reveal that tiny amounts of PD raise polymer shear viscosity and modulus and prolong the relaxation time, so does the tensile viscosity in extensional flow tests. We argue that a synergism may exist between PD and H-2 that is used in the polymerizations as a polymer molecular weight controller (chain transfer agent), of which a too low concentration of PD may presume "chain extension" for polymer chain growth, while higher PD concentrations will inevitably slow down the rate of chain growth and cause reduction in polymer molecular weight.