Mechanistic\nStudies of Two-Dimensional Covalent Organic\nFrameworks Rapidly Polymerized from Initially Homogenous Conditions

Abstract

Covalent\norganic frameworks (COFs) are periodic two- and three-dimensional\n(2D and 3D) polymer networks with high surface areas, low densities,\nand designed structures. Despite intense interest in framework materials,\nthe nucleation and growth processes of COFs, and even of more established\nmetal–organic frameworks (MOFs), are poorly understood. The\nkinetics of COF growth under varied reaction conditions provides mechanistic\ninsight needed to improve their crystallinity and rationally synthesize\nnew materials. Such kinetic measurements are unprecedented and difficult\nto perform on typical heterogeneous COF reaction mixtures. Here we\nsynthesize 2D boronate ester-linked COF-5 under conditions in which\nthe monomers are fully soluble. These homogeneous growth conditions\nprovide equal or better material quality compared to any previous\nreport and enable the first rigorous studies of the early stages of\nCOF growth. COF-5 forms within minutes, and the precipitation rate\nis readily quantified from optical turbidity measurements. COF-5 formation\nfollows an Arrhenius temperature dependence between 60–90 °C\nwith an activation energy of 22–27 kcal/mol. The measured rate\nlaw includes a second order in both boronic acid and catechol moieties,\nand inverse second order in MeOH concentration. A competitive monofunctional\ncatechol slows COF-5 formation but does not redissolve already precipitated\nCOF, indicating both dynamic covalent bond formation and irreversible\nprecipitation. Finally, stoichiometric H₂O provides a 4-fold\nincrease in crystallite domain areas, representing the first rational\nlink between reaction conditions and material quality.