Plastics make modern life better, with an annual output of more than 300 million tons. However, the vast majority come from non-renewable petrochemicals. There is a growing demand for sustainable polymeric materials derived from renewable monomers or waste. Unfortunately, their practical application has invariably been limited by underwhelming performance. For this issue, copolymerization extends the scope of physical properties accessed by the polymer. Especially, learn from nature and biology, precise control over complex sequences in synthetic polymers can tailor properties and applications, which is highly desirable but also challenging. The evolution of new and versatile catalytic methods for sequence control continues to drive innovation in polymer science. A versatile approach to synthesize sustainable sequencedefined polymers is to conjoin ring-opening polymerization (ROP) of lactones and ringopening copolymerization (ROCOP) of epoxide/heteroallene. As an example, Williams and co-workers developed a state-of-the-art switch catalysis strategy whereby a unimodal catalyst can switch between ROP and ROCOP to deliver block copolymers from mixed monomers. The key is to construct robust catalysts capable of simultaneously adapting to the different mechanisms of ROP and ROCOP.