Closed-loop Recycling of Topochemical Polymer Single Crystals

Plastics play crucial rules in almost every aspect of life. Unique properties of plastics like chemical and light resistant, strong, moldable, and low cost make plastic materials useful in many aspects of our global society. However, largely relying on feedstock resources like fossil fuels, plastics production is not sustainable. Thus, plastic recycling could be an efficient alternative to save feedstock resources as well as to reduce production cost.<br/>Recently, a series of polymer materials synthesized via topochemical polymerization are considered as strong candidates for next generation recyclable plastics. It is well-known that topochemical polymerization has high efficiency and environment-friendly features, such as solvent-free and catalyst-free reaction conditions, high reaction yield without side reactions, and atom economy. Yet, there exist few studies on depolymerizing and recycling those polymers. In 2014, Dou et al synthesized a topochemically polymerizable polyindenedione derivative [2,2'-Bi-1H-indene]-1,1'-dione-3,3'-diyl dialkylcarboxylate (BIT) system. In this study, high-quality polymer single crystals can be obtained by shining visible light and the polymerization yield is quantitative. Interestingly, in the polymer single crystal structures, a newly formed elongated carbon-carbon bond with length of 1.59Å was discovered, which is longer than normal carbon-carbon single bond (~1.54 Å). Inspired by this discovery, we demonstrate a topochemical approach for creating elongated C–C bonds with a bond length of 1.57∼1.63 Å between repeating units in the solid state with decreased bond dissociation energies. 12 distinct polymers from three different classes exhibit rapid depolymerization via breakage of the elongated bond within a desirable temperature range (140∼260 °C) while otherwise remaining remarkably stable under harsh conditions. Additionally, one-dimensional polymer single crystals can be processed into two-dimensional thin films and can be melt processed for 3D printing. Processed polymers can maintain a high depolymerization yield with tunable mechanical properties. This work provides a unique strategy of solving plastic recycle issue by novel molecular design, careful single crystal analysis and innovative material processing.