The polymerization of organic monomers on solid surfaces is a promising and versatile synthetic route for the bottom-up fabrication of 2D polymers. Various coupling reactions are successfully employed for the on-surface synthesis of extended covalent nanostructures that are inaccessible by other synthetic means. Yet, many important reactions, as for instance Ullmann coupling, normally proceed irreversibly. Consequently, the lack of error correction mechanisms during growth inevitably results in networks with high defect densities.
In contrast, supramolecular self-assembly based on reversible intermolecular interactions is well-known to yield networks with high structural quality. The presentation introduces and discusses different concepts, how bond reversibility and non-covalent self-assembly can also be utilized for the on-surface synthesis of covalent nanostructures: A more obvious option is the use of coupling reactions that can proceed reversibly as for instance boronic acid condensation. Yet, on the appropriate surface even a notoriously irreversible reaction as Ullmann coupling can exhibit an organometallic intermediate featuring bond reversibility. We will demonstrate that equilibration of these organometallic intermediates is advantageous to improve the structural quality of covalent networks. Lastly, the most elegant, but most specialized option is to firstly use non-covalent interactions for pre-assembling an ordered structure, and to secondly induce covalent cross-linking while maintaining this initial arrangement. A topochemical photopolymerization on a solid surface will be discussed as an implementation of this principle.