Polymer modification is an essential approach to help advance the agenda of sustainable plastics and eventually circular economy of synthetic polymers. This approach enables preparation of polymers that are otherwise challenging via direct polymerization of monomers, thereby expanding the arsenal of synthetic polymers and the longevity of used plastics. Polymer mechanochemistry is a unique method that uses mechanical force to enable changes of polymer backbones with complementary outcomes to other methods, but the existing paradigm lacks diversity in their reaction profile. By establishing diversified mechanochemical modification strategies, we seek to expand the utility of polymer mechanochemistry in polymer upcycling, enabling polymers with novel structures and properties.