Depolymerizable polymers: preparation, applications, and future outlook
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olymers/Soft Matter Prospective Article
Depolymerizable polymers: preparation, applications, and future outlook Joshua A. Kaitz, Olivia P. Lee, and Jeffrey S. Moore, Department of Chemistry and Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA Address all correspondence to Jeffrey S. Moore at [email protected] (Received 18 March 2015; accepted 24 April 2015)
Abstract Depolymerizable polymers are stimuli-responsive materials triggered to depolymerize rapidly and completely into their constituent monomers on command. Applications include triggerable vehicles for controlled release, restructurable materials, disappearing or sacrificial composites, and lithographic resists. Owing to their widespread utility, significant efforts have aimed to prepare and explore depolymerizable polymers and their corresponding triggers. This “Prospective” highlights advances since their discovery over a half-century ago, discusses methods in their preparation, and presents recent developments in triggered depolymerization. It also surveys applications that harness these polymers’ unique properties, while offering insights into research directions that may contribute to progress in this dynamic field.
Introduction Depolymerizable polymers and closely related self-immolative polymers are triggerable materials that respond to a specific stimulus by unzipping into small molecules. They have been developed as triggerable delivery vehicles, recyclable and restructurable materials, disappearing or sacrificial composites, and lithographic resists.[1–6] Specifically, depolymerizable polymers regenerate constituent monomers by chemically reversing propagation,[7] while self-immolative polymers afford small molecular products that differ from parent monomers.[1–3] For either case, the unzipping sequence is initiated by chemically or physically cleaving the polymer backbone, side-chains, or end-groups to expose a reactive intermediate that subsequently liberates molecular products in a stepwise fashion.[2] These steps are spontaneous under certain conditions. Although there are other approaches to polymer chain depolymerization and self-immolation, the focus herein is on polymers that fully and spontaneously unzip to monomer on demand; partial depolymerization that proceeds via radical mechanisms during thermal degradation has been comprehensively reviewed elsewhere.[8] It is important to note that “depolymerization” is a concept distinct from degradation. Many polymers have been prepared with cleavable bonds within their backbone or side-chains; however, cleaving these bonds, by hydrolysis for example, constitutes degradation and not intentionally designed, triggerable depolymerization. This “Prospective” highlights the discovery, preparation, and development of depolymerizable polymers with a major focus on polymers exhibiting low ceiling temperatures (Tc), the critical threshold temperature above which depolymerization and thus monomer formation are thermodynamically favo
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