Discovering how to selectively polymerize monomer mixtures to give well-controlled copolymers is still a fundamental challenge in polymer synthesis but remains important as an efficient means to deliver reproducible repeat unit sequence and tailored properties. Although such selective polymerization catalysis is still under-explored, it could have the potential to mimic Natural selectivity observed in biopolymer synthesis and to deliver more complex and sophisticated polymer structures. Our research group are investigating means to control block copolymer selectivity using monomer mixtures. Recently, we discovered how to switch single catalysts between distinct polymerization reactions – such switchable catalysis offers an attractive and rapid means to control block sequence. It enables a single catalyst to be used together with mixtures of monomers to deliver specific block copolymer sequences and patterns. The selectivity is controlled by the nature of the chemical bond between the growing polymer chain and the metallic active site. By controlling and switching the type of metal chain end group chemistry it is possible to direct the enchainment of particular monomers even from mixtures.