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<p>Polymerizing monomers into periodic two-dimensional (2D) networks provides structurally
precise, atomically thin macromolecular sheets linked by robust, covalent bonds. These materials
exhibit desirable mechanical, optoelectrotronic, and molecular transport properties derived from
their designed structure and permanent porosity. 2D covalent organic frameworks (COFs) offer
broad monomer scope, but are generally isolated as polycrystalline, insoluble powders with limited
processability. Here we overcome this limitation by controlling 2D COF formation using a two-
step procedure. In the first step, 2D COF nanoparticle seeds are prepared with approximate
diameters of 30 nm. Next, monomers are slowly added to suppress new nucleation while
promoting epitaxial growth on the existing seeds to sizes of several microns. The resulting COF
nanoparticles are of exceptional and unprecedented quality, isolated as single crystalline materials
with micron-scale domain sizes. These findings advance the controlled synthesis of 2D layered
COFs and will enable a broad exploration of synthetic 2D polymer structures and properties. </p>
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