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

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 × 10 5 g·mol Zr –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 × 10 6 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.