Direct synthesis of various polar functionalized polypropylene materials with tunable molecular weights and high incorporation ratios

Abstract

In this study, a series of polar functionalized polypropylene materials with tunable molecular weights and high incorporation ratios were synthesized by copolymerizing propylene and various polar monomers (MA, AA, BA, 6FA and HA) using some elaborate iminopyridyl Pd(II) catalysts. In propylene homopolymerization, the rotation-restricted substituent variations of the Pd(II) catalysts are observed to have a systematic influence on the polymerization reaction with respect to the molecular weight of the polymer (Pd1-free rotation: 0.4–0.5 kg mol⁻¹, Pd2-restricted rotation: 0.6–0.8 kg mol⁻¹ and Pd3-fixation: 2.6–31.4 kg mol⁻¹). More importantly, compared with the classical α-diimine palladium catalyst, iminopyridyl Pd(II) catalysts with 8-aryl naphthyl or dibenzosuberyl substituents can lead to a significantly higher molecular weight (M_w up to 99.6 kg mol⁻¹). Meanwhile, the ligand structure of the catalysts can significantly affect the microstructure (BD: 94–251/1000C) of the obtained polypropylene. A similar tendency is also observed during the copolymerization of propylene with polar monomers. Most importantly, iminopyridyl Pd(II) catalysts can produce various polar functionalized polypropylene materials with significantly higher incorporation ratios (up to 30.6 mol%) in comparison to the classical α-diimine palladium catalyst. The semi-open structure of the iminopyridyl ligand allowed the large propylene and polar monomer molecules to easily coordinate to the metal center, thus, leading to an adequate incorporation in the polymer chains.