Coordination-Engineered Zr Centers in Covalent Organic Frameworks: From O-Zr-O to N-Zr-N for Enhanced Ethylene...

The synthesis of ultrahigh-molecular-weight polyethylene (UHMWPE) is often hampered by the inherent compromise in traditional catalysts, such as Ziegler-Natta systems, which struggle to balance high activity with precise control over molecular parameters. Covalent organic frameworks (COFs) offer a promising alternative, allowing for atomic-level design through their tunable pores and microenvironments. In this work, we address this challenge by designing a series of Zr@COF catalysts─denoted as Zr@PQ-TFPB (O-Zr-O), Zr@PQSO-TFPB (N-Zr-O), and Zr@PQDO-TFPB (N-Zr-N)─to systematically investigate the role of the coordination environment. Spectroscopic and catalytic studies reveal that the N-Zr-N configuration in Zr@PQDO-TFPB optimally reduces the electron density at the Zr center. This electronic modulation, characterized by reduced electron density at the Zr center, facilitates ethylene adsorption/insertion while suppressing β-H elimination, leading to a superior polymerization activity of 3.87 × 105 g·molZr-1·h-1 at 3.0 MPa─66% and 38% higher than its O-Zr-O and N-Zr-O analogues, respectively. This catalyst produces UHMWPE with a high molecular weight of 1.71 × 106 g·mol-1 and a relatively narrow dispersity (Đ = 2.2). Furthermore, the rigid nanochannels of the COF provide a confining effect that stabilizes the growing polyethylene chains. Our findings underscore a critical synergy between coordination-induced electronic effects and spatial nanoconfinement, providing a novel design principle for next-generation polyolefin catalysts.