Disentangled Ultrahigh-Molecular-Weight Polyethylene by Boroxide Complexes of Half-Titanocenes via Hydrogen Bonding Interaction

Abstract

The disentangled state of ultrahigh-molecular-weight polyethylene (UHMWPE) is of practical importance to its processability, but the direct synthesis of disentangled UHMWPE (dis-UHMWPE) remains highly challenging. Herein, a series of half-titanocenes Cp′Ti(L)Cl2 (Cp′ = cyclopentadienyl derivatives; L = (Ar2)BO–) bearing electron-withdrawing boroxide ancillary ligands have been developed for ethylene polymerization. Although the steric and electronic modifications of Cp′ and boroxide ancillary ligands impact catalytic activity, the substituted boroxide-containing half-titanocenes produce UHMWPE with viscosity-average molecular weights (Mv) of >1 × 106 g/mol under regular polymerization conditions. The resultant UHMWPEs feature a low degree of chain entanglement as confirmed by differential scanning calorimetry (DSC) annealing studies and melt rheological tests. Density functional theory (DFT) calculations further reveal that the formation of dis-UHMWPE originates from the hydrogen bonding interaction between the electron-withdrawing boroxide ancillary ligand and β-H on the growing polymer chain, which promotes chain growth and suppresses β-H elimination chain transfer.