Molecular Bottlebrush Supported Mono(phenoxy-imine) Metal Complexes: Synthesis and Ethylene Polymerization

We report a poly(norbornene-graft-styrene) (PNB-g-PS) supported mono(phenoxy-imine) metal complex for ethylene polymerization. As a molecular bottlebrush (MB), PNB-g-PS was prepared via grafting-through ring-opening metathesis polymerization (ROMP) of the norbornenyl macromonomer. Postmodification reactions of PNB-g-PS were performed to incorporate phenoxy-imine type ligands in PS side chains, which were then coordinated with early transition metals to generate a series of MB supported mono(phenoxy-imine) catalysts. The loading efficiency of the metals was assessed by inductively coupled plasma mass spectrometry. Using these catalysts with four types of phenoxy-imine ligands and two metals (Ti and Zr), we evaluate their catalytic performances (activity, Mw, and the crystallinity of polyethylenes) for ethylene polymerization. Improved catalytic activities were observed for the catalysts bearing bulky substituents ortho to the phenolic oxygen in the ligands. The highest ones are 369 and 461 kg PE mol-1 h-1 for the Ti and Zr catalysts with a cumyl group. The obtained polyethylenes (PEs) have high crystallinities and tunable molecular weights ranging from 80 to 2000 kDa. We also successfully prepared bimodal PEs in one pot by rationally combining two different phenoxy-imine ligands in the MB supported catalysts.