A hybrid steric ligand modification strategy promotes propylene (co)polymerization

Abstract

Recently, a Hybrid Steric Ligand Modification (HSLM) strategy has been shown to significantly increase the polymerization activity and the branching degree of the resulting (co)polymers in the ethylene (co)polymerization via α-diimine catalysts. In this study, the HSLM strategy was employed for propylene (co)polymerization using a group of α-diimine Pd(II) catalysts with bulky hybrid ligands. The HSLM catalysts (Pd1-4) produced polypropylene with high molecular weights (45.4–156.9 kg/mol) and resulted in significantly high levels of activity (105 g mol−1 h−1). Additionally, nearly half of the propylene underwent chain straightening (1,3-enchainment = 43–46 %). Our findings suggest that Pd1-4 catalysts are more effective than the symmetrical catalysts Pd5-6 in producing high molecular weight polypropylene with high activity. Among all the catalysts tested, Pd4 with 4-tert-butylphenyl displayed the highest polymerization activity and molecular weight, although with lowest branching densities. It is suggested that the HSLM may provide a more open spatial configuration around the metal center, which facilitates the coordination and insertion of propylene molecules as well as chain walking. The hybrid Pd(II) catalysts showed moderate activity during the copolymerization of propylene with MA. The copolymers produced had low molecular weights (3.1–11.2 kg/mol), high branching densities (120–211/1000C), and high incorporation ratios (8.82–18.08 mol%). The copolymerization activity was highest with 4-tert-butylphenyl Pd4. The resulting P-MA copolymers had the highest molecular weight, the largest branching degree, and the lowest incorporation ratios.