Synthesis and characterization of hyperbranched polyethylenes containing cross-linking structures by chain walking copolymerization of ethylene with diacrylate comonomer

Abstract

We report the synthesis of higher-molecular-weight hyperbranched polyethylenes containing cross-linking structures via chain walking copolymerization of ethylene with a diacrylate cross-linker, 1,4-butanediol diacrylate, using a Pd-diimine catalyst, [(ArNC(Me)–(Me)CNAr)Pd(CH3)(NCMe)]SbF6 (Ar=2,6-(iPr)2C6H3). The hyperbranched chain topology of these polymers was achieved through the chain walking mechanism of the Pd-diimine catalyst. By controlling the diacrylate feed concentration in the polymerization system, three sets of hyperbranched polyethylenes having various cross-linking levels (including both intermolecular and intramolecular cross-linking) but without macrogelation were synthesized at three different temperatures, 15, 25, and 35°C, respectively. The diacrylate content and cross-linking density in the copolymers were found to increase with the enhancements of diacrylate feed concentration and polymerization temperature. Triple-detection gel permeation chromatography (GPC) measurements confirmed the significant enhancement of polymer average molecular weight and broadening in molecular weight distribution with an increase of diacrylate feed concentration as a result of intermolecular cross-linking. The chain topology of the copolymers becomes more compact, compared to homopolymers, due to the presence of intermolecular cross-linking. Rheological studies show that the copolymers possess characteristic rheological properties typically found in polymers containing intermolecular cross-linking. Our results also indicate qualitatively the presence of intramolecular cross-linking in these hyperbranched copolymers, particularly in those synthesized at 35°C, probably due to their highly compact chain topologies. This work demonstrates the capability of chain walking polymerization for synthesis of hyperbranched polyethylenes of enhanced molecular weights with the simple use of diacrylate as a cross-linker.