Polymer Mechanochemistry on Reactive Species Generated from Mechanochemical Reactions

Comprehensive Summary Polymer mechanochemistry on reactive species has attracted more and more attentions over the past 20 years,as the mechanochemical generation of reactive species has a great potential in developing different polymeric materials for various purposes,such as stress detection,self-healing,self-strengthening,controllable degradation and release of small molecules.In this review,we first discuss the recent progress on polymer mechanochemistry of the reactive species that are generated from the mechanochemical reactions of mechanophores.Five types of reactive species,including radical,zwitterion,ionic,carbene and neutral intermediates,and their applications were reviewed in detail.Since mechanochemical reactions are sensitive to the mechanophore structure and polymer framework,we then discuss how mechanophore isomerism,polymer structure,polymer attachment point,and polymer architecture influence the mechanophore activation.At last,we provide our perspectives on the polymer mechanochemistry of reactivespecies.

Emerging Bio-relevant Applications of Polymer Mechanochemistry

Polymer mechanochemistry has rapidly evolved since the mid-20o0s.Recent advancements highlight the development of mechanophore platforms for the controlled release of bioactive payloads and the exploration of biocompatible activation strategies.These platforms,ranging from furan-maleimide Diels-Alder adducts to disulfide motifs withβ-carbonate linkages,demonstrate promising prospects in targeted drug delivery.Additionally,supramolecular assemblies and free radical-generating mechanophores present innovative avenues for potential therapeutic applications.Biocompatible activation methods,notably high-intensity and/or low-intensity focused ultrasound,hold potential for in vivo applications.However,challenges persist in comprehending the fundamental physics of ultrasound and its utilization for activation.

High-Intensity Focused Ultrasound-Induced Disulfide Mechanophore Activation in Polymeric Nanostructures for Molecule Release

Ultrasound(US)activation of mechanophores in polymers that initiates cascade chemical reactions is a promising strategy for on-demand molecule release.However,the typical US frequency used for mechanochemistry is around 20 kHz,producing inertial cavitation that exceeds the tolerance threshold of biological systems.Here,high-intensity focused US(HIFU)as a mechanical stimulus is introduced to drive the activation of disulfide mechanophores in hyperbranched star polymers(HBSPs)and microgels(MGLs).The mechanism of molecular release is attributed to the thiol-disulfide exchange reaction and subsequent intramolecular cyclization.We reveal that HBSPs and MGLs effectively transduce HIFU as mechanical input to chemical output,demonstrated by the quantification of the release of fluorescent umbelliferone(UMB).Moreover,an in vitro study of drug release is carried out using camptothecin as the model drug,which is covalently loaded in MGLs,demonstrating the potential of our system for controlled drug delivery to cancer cells.

Exploring the Nanomechanical Properties of a Coordination-bond Based Supramolecular Polymer

Due to their dynamic nature and strength tunability,metallo-supramolecular polymers have been introduced into various materials.The mechanical strength of the metallo-supramolecular polymers in the system directly influences the mechanical properties(e.g.,the toughness)of the materials.Therefore,it is necessary to explore the mechanical behavior of the metallo-supramolecular polymers.Herein,we present a single-molecule method to systematically explore the chain structure and mechanical properties of metallo-supramolecular polymer by using a loop protected architecture.Notably,we found that the mechanical stability of the individual chain,which is determined by the strength of terpyridine-Fe^(2+) -terpyridine(tpy-Fe^(2+)-tpy)bonds,was about 0.6–1.0 nN,depending on the pulling speed.This value is around three times higher than those measured using old methods.In addition,the unique loop protected structure further reduces the interference of non-specific polymer-AFM tip(or polymer-substrate)interactions on the quantification of the actual strength and kinetic parameter of noncovalent interactions in supramolecular polymers.Furthermore,the single chain flexibility of the metallo-supramolecular polymer was investigated and found to be comparable to the corresponding covalent analogues.Our findings provide a new way to explore the force response of supramolecular polymers composed of metal-ligand interactions and will be useful for the design of metallo-supramolecular polymer-based functional materials with tailored mechanical properties.

Carbon Dots Intensified Mechanochemiluminescence from Waterborne Polyurethanes as Tunable Force Sensing Materials

1,2-Dioxetane is a well-known chemiluminescent mechanophore allowing real-time monitoring of polymer chain scission,but usually suffers from fluorescence quenching in polar environments.Herein,a series of mechanochemiluminescent waterborne polyurethanes/carbon dots composites(WPU-CDs)have been synthesized by incorporating fluorescent CDs to promote the energy transfer process in different environments.The resulting bulk WPUs,and in particular,their swollen films filled with a large amount of polar solvents(water and ionic liquid)emit intense mechanochemiluminescence.Thus force-induced covalent bond scission and stress distribution within these different WPU-CDs films can be sensitively visualized.Furthermore,the ionic liquid containing films exhibited both electrical and luminescent signal changes under stretching,which offer a new kind of force sensor responsive at a broad detecting strain range and for multi-mode strain analysis.This study is expected to stimulate new research endeavors in mechanistic insight on waterborne polyurethanes and the corresponding stretchable sensing devices.

Single-molecule observation of mechanical isomerization of spirothiopyran and subsequent Click addition

Spirothiopyran(STP)is particularly attractive when used as a mechanophore to endow polymers with both damage-signaling and.self-reinforcing capacity.It is,however,not clear the actual force required to induce the cycloreversion of STP into ring-opened thiomerocyanine(TMC),which reacts spontaneously with activated C=C bonds.Here,we used atomic force microscopy(AFM)-based single molecule force spectroscopy(SMFS)to study the mechanochemistry of STP mechanophore.It is found that the ring-opening of STP at room temperature requires forces of-200-400 pN,depending on the pulling speed.In addition,the reversibility of STP to TMC isomerization is demonstrated.Finally,mechanochemically induced intermolecular Click addition is achieved in single'molecule level by pulling STP in the presence of maleimide.

A mechanochemical two for one:Mechanical activation of norborn-2-en-7-one releases carbon monoxide and switches on aggregation-induced emission

Mechanical forces are typically considered to be destructive in polymeric materials,leading to degradation via nonspecific chain scission.However,the emergence of the mechanophore hypothesis has enabled the design of polymers that are able to harness these typically destructive forces and convert them into productive chemical transformations.