<p>Homogeneous
two-dimensional (2D) polymerization is a poorly understood process in which
topologically planar monomers react to form planar macromolecules, often termed
2D covalent organic frameworks (COFs). While these COFs have traditionally been
limited to weakly crystalline aggregated powders, they were recently grown as
micron-sized single crystals by temporally resolving the growth and nucleation
processes. Here, we present a quantitative analysis of the nucleation and
growth rates of 2D COFs via kinetic Monte Carlo (KMC) simulations, which show
that nucleation and growth have second-order and first-order dependences on
monomer concentration, respectively. The computational results were confirmed experimentally
by systematic measurements of COF nucleation and growth rates performed via <i>in situ </i>X-ray
scattering, which validated the respective monomer concentration dependences of
the nucleation and elongation processes. A
major consequence is that there exists a threshold monomer concentration below
which growth dominates over nucleation. Our computational and experimental
findings rationalize recent empirical observations that, in the formation of 2D
COF single crystals, growth dominates over nucleation when monomers are added
slowly, so as to limit their steady-state concentration. This mechanistic
understanding of the nucleation and growth processes will inform the rational
control of polymerization in two dimensions and ultimately enable access to
high-quality samples of designed two-dimensional polymers. </p>
Growth Rates of Sodium Thiosulfate Single Crystals from Solution
