Design of depolymerizable polymers toward a circular economy

Abstract

Transition from the traditional linear economy to a circular economy is the key to reducing the amount of plastic that ends up in the environment and recovering value from the waste. However, energy-efficient, environmentally friendly, and economical technologies to recycle the mounting volume of low-grade plastic waste are lacking when compared to the low cost of primary fossil-based plastic production. This led to extensive efforts to identify methods that mitigate the waste that already exists and develop new polymers to eliminate the poor recyclability problem caused by current materials. Polymers capable of depolymerizing back to their own monomers provide a promising solution. This chemical recycling process has the potential to eliminate contaminants, selectively depolymerize a particular polymer from a waste stream, and recover high-purity monomers from contaminated wastes; however, the process must be energy-efficient and economical. This review discusses recent advances in chemical design of polymers that can depolymerize back to their monomers with an emphasis on strategies that shift the monomer–polymer equilibrium to enable reversible depolymerization and polymerization. Novel approaches developed for ring-opening polymerization and ring-closing depolymerization, reversing free radical polymerization, and reversible reactions that enable depolymerization of highly crosslinked thermosets to small molecular monomers are reviewed. Self-immolative polymers, which have a long history of chemistry innovation to realize the balance between stability and rapid depolymerization, are briefly discussed to serve as an inspiration for new polymer designs. Any new materials designed should have at least matching properties and stabilities compared to existing materials. Thus, this review reflects the status of current research that not only demonstrates the reversible polymerization chemistry but also achieves outstanding material properties.