Force ahead: Emerging Applications and Opportunities of Polymer Mechanochemistry

Abstract

T he use of mechanochemical reactions in polymer science is a relatively new concept. Initially, the widespread introduction of polymers as commodity materials required the investigation of their behavior under mechanical action. One of the early experiments in this regard was performed by Staudinger and Heuer in 1934 who observed that the degradation of polystyrene and natural rubber led to a decreasing viscosity and hence attributed this to a decreasing molar mass. They speculated that this was caused by the mechanochemically induced depolymerization of the polymer backbone. 1 Kauzmann and Eyring confirmed Staudinger's hypothesis in 1940 and offered the first kinetic description of the mechanochemical bond scission process in polymers. 2 Besides degradation, it was found that mechanically produced macroradicals could be used for secondary polymerizations thus rendering mechanochemistry a versatile tool in polymer chemistry. 3,4 Continuing this early work on polymer fragmentation and formation, 5 the past decade experienced rapid growth of polymer mechanochemistry employing designer polymers that bear force sensitive functional molecular motifs (mechano-phores). 6,7 These mechanophores were placed in different polymer architectures and their mechanochemical actuation resulted in specific molecular transformations realizing a desired function. Exciting examples are the incorporation of chromogenic molecules as mechanophores that undergo a molecular transformation accompanied by alteration of absorption and/or emission, rendering them optical force probes (OFPs). 8 Thereby, the mechanical failure mechanisms in different architectures ranging from soft matter to high-performance polymers were studied with the aim to use the gained knowledge to design materials with better properties. Moreover, the incorporation of multiple, nonconjugated, fused four-membered carbon rings, reminiscent of the unusual ladderane membrane lipids of anaerobic ammonium-oxidizing bacteria in polymers, led to the formation of polyacetylene with extended π-conjugation along the polymer backbone after mechanical activation. 9 Besides changing the optical or electronic properties of polymer systems under force, transition metals were incorporated into the central region of linear polymers resulting in catalysts that were activated by applying elongational flow as mechanical stimulus. 10 More generally, polymer mechanochemistry was used to dynamically alter potential energy surfaces by force allowing access to products unavailable by traditional reaction pathways. 11 Arguably, these seminal contributions had transformative character for the field and have sparked new developments and unprecedented applications in the field. In this Editorial, we present selected emerging trends, opportunities, and obstacles for the application of mechanochemical methods in polymer science. We highlight the trend toward bond scission quantification using OFPs and its associated difficulties. Moreover, we summarize which solutions polymer mechano-chemistry contributes to a more sustainable usage of polymers. Lastly, we put focus on the emerging field of sonopharmacol-ogy where the principles of polymer mechanochemistry are employed for drug delivery and activation.