Dynamic covalent chemistry with azines

New Advances in Covalent Network Polymers via Dynamic Covalent Chemistry

Covalent network polymers, as materials composed of atoms interconnected by covalent bonds in a continuous network, are known for their thermal and chemical stability. Over the past two decades, these materials have undergone significant transformations, gaining properties such as malleability, environmental responsiveness, recyclability, crystallinity, and customizable porosity, enabled by the development and integration of dynamic covalent chemistry (DCvC). In this review, we explore the innovative realm of covalent network polymers by focusing on the recent advances achieved through the application of DCvC. We start by examining the history and fundamental principles of DCvC, detailing its inception and core concepts and noting its key role in reversible covalent bond formation. Then the reprocessability of covalent network polymers enabled by DCvC is thoroughly discussed, starting from the significant milestones that marked the evolution of these polymers and progressing to their current trends and applications. The influence of DCvC on the crystallinity of covalent network polymers is then reviewed, covering their bond diversity, synthesis techniques, and functionalities. In the concluding section, we address the current challenges faced in the field of covalent network polymers and speculates on potential future directions.

The Dynamic Chemistry of Orthoesters and Trialkoxysilanes: Making Supramolecular Hosts Adaptive, Fluxional, and Degradable

ConspectusThe encapsulation of ions into macro(bi)cyclic hosts lies at the core of supramolecular chemistry. While chemically inert hosts such as crown ethers (synthesis) and cyclodextrins (Febreze) have enabled real-world applications, there is a wider and accelerating trend toward functional molecules and materials that are stimuli-responsive, degradable, or recyclable. To endow supramolecular hosts with these properties, a deviation from ether C-O bonds is required, and functional groups that engage in equilibrium reactions under relatively mild conditions are needed.In this Account, we describe our group's work on supramolecular hosts that comprise orthoester and trialkoxysilane bridgeheads. In their simplest structural realization, these compounds resemble both Cram's crown ethers (macrocycles with oxygen donor atoms) and Lehn's cryptands (macrobicycles with 3-fold symmetry). It is therefore not surprising that these new hosts were found to have a natural propensity to bind cations relatively strongly. In recent work, we were also able to create anion-binding hosts by placing disubstituted urea motifs at the center of the tripodal architecture. Structural modifications of either the terminal substituents (e.g., H vs CH3 on the bridgehead), the diol (e.g., chiral), or the bridgehead atom itself (Si vs C) were found to have profound implications on the guest-binding properties.What makes orthoester/trialkoxysilane hosts truly unique is their dynamic covalent chemistry. The ability to conduct exchange reactions with alcohols at the bridgehead carbon or silicon atom is first and foremost an opportunity to develop highly efficient syntheses. Indeed, all hosts presented in this Account were prepared via templated self-assembly in yields of up to 90%. This efficiency is remarkable because the macrobicyclic architecture is established in one single step from at least five components. A second opportunity presented by dynamic bridgeheads is that suitable mixtures of orthoester hosts or their subcomponents can be adaptive, i.e. they respond to the presence of guests such that the addition of a certain guest can dictate the formation of a preferred host. In an extreme example of dynamic adaptivity, we found that ammonium ions can fulfill the dual role of catalyst for orthoester exchange and cationic template for efficient host formation, representing an unprecedented example of a fluxional supramolecular complex. The third implication of dynamic bridgeheads is due to the reaction of orthoesters and trialkoxysilanes with water instead of alcohols. We describe in detail how the hydrolysis rate differs strongly between O,O,O-orthoesters, S,S,S-trithioorthoesters, and trialkoxysilanes and how it is tunable by the choice of substituents and pH.We expect that the fundamental insights into exchange and degradation kinetics described in this Account will be useful far beyond supramolecular chemistry.