Fine Tuning of Catalyst Active Sites to Access Advanced Ultrahigh-Melting Polyolefins

Polyolefins with melting temperatures above 200°C are rare, as their synthesis typically relies on the polymerization of highly sterically congested α-olefins, which is challenging for both industrial Ziegler-Natta and homogeneous catalysts. Pyridylamido-Hf complexes are molecular catalysts known for their high activity and ability to incorporate sterically demanding monomers, for example, allyltrimethylsilane (ATMS) giving isotactic poly(allyltrimethylsilane) with a melting temperature of 310°C and high turnover frequency. However, they are unable to polymerize efficiently monomers such as vinylcyclohexane (VCH) or 4,4-dimethyl-1-pentene (44DMP). In this study, we report a novel synthetic strategy that enables the polymerization of such sterically challenging monomers, based on a pre-activation of the Hf catalyst by using ATMS as ligand modifier in situ. We show that ATMS acts as an effective pre-activator of the Hf complex, enabling controlled formation of highly active species and unlocking the polymerization of sterically demanding α-olefins. This afforded the first synthesis of isotactic poly(4,4-dimethyl-1-pentene) with an unprecedented melting temperature of 420°C, and efficient polymerization of VCH and 3-methyl-1-butene. These findings demonstrate how the tuning of the pyridylamido-Hf active species through rational choice of the activating monomer can be leveraged to expand the access to ultrahigh melting temperature polyolefins.