The development of more sustainable materials with a prolonged useful lifetime is a key requirement fora transition towards a more circular economy. However, polymer materials that are long-lasting andhighly durable also tend to have a limited application potential for re-use. This is because such materialsderive their durable properties from a high degree of chemical connectivity, resulting in rigid meshes ornetworks of polymer chains with a high intrinsic resistance to deformation. Once such polymers are fullysynthesised, thermal (re)processing becomes hard (or impossible) to achieve without damaging thedegree of chemical connectivity, and most recycling options quickly lead to a drop or even loss ofmaterial properties. In this context, both academic and industrial researchers have taken a keen interestin materials design that combines high degrees of chemical connectivity with an improved thermal (re)processability, mediated through a dynamic exchange reaction of covalent bonds. In particular vitrimer materials offer a promising concept because they completely maintain their degree of chemical connectivity at all times, yet can show a clear thermally driven plasticity and liquid behavior, enabled through rapid bond rearrangement reactions within the network. In the past decade, many suitable dynamic covalent chemistries were developed to create vitrimer materials, and are now applicable toa wide range of polymer matrices. The material properties of vitrimers, however, do not solely rely on the chemical structure of the polymer matrix, but also on the chemical reactivity of the dynamic bonds.Thus, chemical reactivity considerations become an integral part of material design, which has to take into account for example catalytic and cross-reactivity effects. This mini-review will aim to provide anoverview of recent efforts aimed at understanding and controlling dynamic cross-linking reactions within vitrimers, and how directing this chemical reactivity can be used as a handle to steer material properties.Hence, it is shown how a focus on a fundamental chemical understanding can pave the way towards new sustainable materials and applications