Internal catalysis for dynamic covalent chemistry applications and polymer science
Strong covalent chemical bonds that can also be reversed, cleaved or exchanged are the subject of so-called dynamic covalent chemistry (DCC). Applications range from classical protective groups in organic chemistry and cleavable linkers for solid phase synthesis, to more modern applications in dynamic compound libraries and adaptive materials. Interest in dynamic, reversible or responsive chemistries has risen in particular in the last few decades for the design and synthesis of new DCC-based polymer materials. Implementation of DCC in polymers yields materials with unique combinations of properties and in some cases even unprecedented properties for covalent materials, such as self-healing materials, covalent adaptable networks (CANs) and vitrimers. In particular, the incorporation of DCC in polymer materials aims to find a balance between a swift and triggerable reactivity, combined with a high degree of intrinsic robustness and stability. Applying harsh conditions, highly active catalysts or highly reactive bonding groups, as is done in classical DCC, is often not feasible or desirable, as it can damage the polymer's integrity, leading to loss of function and properties. In this context, so-called internally catalysed DCC platforms have started to receive more interest in this area. This approach relies on the relative proximity and orientation of common functional groups, which can influence a chemical exchange reaction in a subtle but significant way. This approach mimicks the strategies found in enzymic reactions, and is known in classical organic chemistry as neighbouring group participation (NGP). The use of internal catalysis or NGP within polymer material science has proven to be a highly attractive strategy. This tutorial review will outline examples showing the scope, advantages and pitfalls of using internal catalysis within different DCC applications, ranging from small molecules to dynamic polymer materials.