Engineering Coordinatively Unsaturated Metal Sites in 2D Metal‐Porphyrin Frameworks for Initiator‐Free, Broadband Photocatalytic Synthesis of Ultrahigh Molecular Weight Polymers

Abstract Metal‐organic frameworks bearing coordinatively unsaturated metal sites (CUSs) exhibit distinctive interactions toward guest molecules, thereby presenting intriguing properties in photocatalytic applications. Herein, an adaptable strategy is established to create 2D metal‐porphyrin frameworks while engineering the CUSs based on the use of bivalent metal nodes (Zn²⁺, Co²⁺, Cu²⁺, Ni²⁺, Cd²⁺) and Zr‐O clusters. The strong surface‐coordination effect reduces the energy barriers to activate acrylate monomers adsorbed to the nanosheet frameworks, enabling the generation of anion radicals through a broadband light‐induced electron transfer pathway. Particularly, the photocatalytic cycle offers a hole‐mediated deactivation mechanism to regulate the radical concentrations in the polymerization system, while the propagating chains exhibit a surface‐confined growth behavior that significantly inhibits recombination terminations. Extensive mechanistic studies and theoretical calculations reveal the molecular‐level guest‐framework interactions and establish the guidelines to screen metal centers for fabricating valid photocatalysts and monomers compatible with photo‐regulated polymerization. A series of ultrahigh molecular weight polymers with M_n > 1 000 000 and relatively low dispersity (Đ ≈ 1.5) are obtained upon exposure to the full spectrum of visible light. This study provides novel inspirations for harnessing natural sunlight to drive macromolecular synthesis with minimal energy consumption.