Design and Synthesis of Polymer Networks and Branched Polymers for Triggered Deconstruction of Self-Assembly

Abstract

The accumulation of plastic waste is becoming a global crisis. While many plastics can technically be recycled, robust plastics which are held together by permanent covalent crosslinks–thermosets, which make up 20% of all plastics–cannot generally be reprocessed or broken down. Generalizable strategies for the deconstruction and/or reprocessing of thermoset materials are therefore severely lacking and highly desirable.

Here, we report the first generalizable approaches to enable both deconstruction and reprocessing of existing thermosets, using an unrecyclable industrial thermoset, polydicyclopentadiene (pDCPD) as a model material. We demonstrate that statistical incorporation of small amounts of molecular additives into the backbone strands of thermosets enables complete material deconstruction under mild and chemoselective conditions, without adversely affecting the parent material’s original manufacturing workflow, mechanical properties, or appearance. The deconstruction products can be functionalized and re-incorporated into virgin material to produce partially recycled material of equivalent performance to the parent.

Next, we show that structural modification of these molecular additives to include a crosslinking site enables equivalent material deconstruction compared to first generation additives, but with improvements in material thermomechanical performance. Alternative structural modification to include multiple functionality is shown to enable four-fold reductions in the minimum required additive loading for material deconstruction. Theoretical bases for both of these phenomena are provided to corroborate experimental observations. Next, we show that by facilitating bond exchange rather than cleavage, direct reprocessing of permanently crosslinked thermosets is achieved. We report the first example of self-healing pDCPD, in addition to the first reported Si-O exchange of bifunctional silyl ethers, consequently adding a new and accessible reaction to the toolbox of dynamic covalent chemistry.

Finally, we deviate from thermosets and present a new strategy for the orthogonal tuning of thermal and mechanical properties of self-assembled graft copolymer silicones. We show that small structural variations to aliphatic pendant groups along copolymer backbones of free-standing self-assembling graft copolymers underpin enormous changes in material morphology and thermomechanical properties. Notably, we also introduce highly ordered body-centered-cubic morphologies which behave as soft plastic crystals, and lamellar morphologies which behave as robust and transparent, yet unentangled and uncrosslinked thermoplastics.