Unraveling two-dimensional polymerization in the single crystal

Two-dimensional single-crystal-to-single-crystal polymerization and depolymerization are described in detail. The results are based on in-house and synchrotron X-ray diffraction experiments conducted on several samples at 100 K and room temperature. The reactions are associated with considerable molecular motions of all components (monomer, template and incorporated solvent molecules), which can be as large as 1 Å. Continuous polymerization leads to a gradual gap opening between the emerging two-dimensional polymer layers, which allows for increased mobility of the solvent molecules. The positional flexibility of both the solvents and the weakly bound templates buffers the local strain induced by polymerization through a complex chain of movements. As a consequence, the accumulated global strain remains small enough to essentially preserve the single-crystalline state in the course of a complete polymerization/depolymerization cycle. The unit-cell parameters evolve in an unusual way. The a and c axes of the trigonal lattice slightly increase during polymerization, even though van der Waals interactions are replaced by shorter covalent bonds and the involved molecules shrink. However, the c axis experiences a significant drop of more than 1 Å during the first depolymerization step. Progressive depolymerization expands the c axis again, but it does not quite reach the value of the fresh crystal. These effects can be explained by local strain formation and compensation mechanisms and by annealing effects during heat-induced depolymerization. An interesting side effect of the polymerization is the reorientation of incorporated solvent molecules, which give the crystal a tunable dipole moment. Of particular importance for the understanding of two-dimensional polymers is the evolution of the connectivity between molecules during polymerization and depolymerization. Combining reaction kinetics with structural information, such as the polymerization-induced displacement of reactive sites, allowed for the development of a propagation model, in which both polymerization and depolymerization proceed in a self-impeding fashion. This model is supported by Monte Carlo simulations.