Bimodal ultra-high molecular weight polyethylene thermoplastic elastomers enabled with "half-sandwich" α-diimine...

Bimodal ultra-high molecular weight polyolefin thermoplastic elastomers (UHMW-P-TPEs) represent an important class of materials that uniquely combine the exceptional mechanical strength of UHMW polymers with the high elasticity and improved processability imparted by a tailored low-molecular-weight fraction. Herein, a new family of half-sandwich α-diimine nickel complexes featuring a distinctive axial η⁶-phenyl coordination was synthesized and thoroughly characterized. During ethylene polymerization, such an unique structural motif not only provides highly effective suppression of chain transfer-enabling access to UHMW elastomeric architectures-but also introduces a dynamic metal-arene interaction that gives rise to two catalytically competent active species. As revealed by detailed experimental studies and DFT calculations, these dual active species promote two distinct monomer-enchainment pathways, i.e. upward and downward insertions, ultimately leading to the formation of bimodal polyethylene elastomers. Notably, the degree of bimodality can be modulated by using polymerization conditions such as temperature and ethylene pressure, which govern the balance between the tightly bound and weakly bound η⁶-phenyl coordination states. Overall, this work establishes an effective catalyst-design strategy for generating structurally tunable bimodal UHMW-P-TPEs and provides mechanistic insights that may guide future development of advanced polyolefin elastomers.