Mechanical recycling of postconsumer plastics, especially incompatible mixed plastics, is often a downcycling process, resulting in brittle materials. Here we introduce a multi-arm universal dynamic crosslinker (UDC) platform to enable mechanical upcycling of incompatible mixed plastics. The topological UDC platform is based on readily accessible diazomalonate carbene precursors joined by dynamic siloxane linkages. It creates in situ star-like multiblock copolymer compatibilizing architectures from mixed plastics, irrespective of polymer types, and is compatible with reactive extrusion re/processing. Such architectures are credited for not only drastically enhancing creep resistance by two to four orders of magnitude but also improving the extensibility of polymer blends by up to 100-fold. By adjusting the topological UDC architecture and loading, broad property tunability between thermoset- and thermoplastic-like materials is achieved from mechanical upcycling of mixed apolar/polar plastics. This work provides a viable approach to improve the circularity of plastics.