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Thiol- and Disulfide-Based Stimulus-Responsive Soft Materials molecules Review Thiol- and Disulfide-Based Stimulus-Responsive Soft Materials and Self-Assembling Systems Danielle M. Beaupre 1 and Richard G. Weiss 1,2,* 1 Department of Chemistry, Georgetown University, Washington, DC 20057, USA; [email protected] 2 Institute for Soft Matter Synthesis and Metrology, Georgetown University, Washington, DC 20057, USA * Correspondence: [email protected]; Tel.: +1-202-687-6013 Abstract: Properties and applications of synthetic thiol- and disulfide-based materials, principally polymers, are reviewed. Emphasis is placed on soft and self-assembling materials in which inter- conversion of the thiol and disulfide groups initiates stimulus-responses and/or self-healing for biomedical and non-biomedical applications. Keywords: stimulus-responsive materials; self-healing; redox-responsive; drug delivery; thiol- disulfide exchange; disulfide-disulfide metathesis 1. Introduction 1.1. Thiols and Disulfides in Materials Chemistry Thiols (RSH) and disulfides (RSSR) are components of many proteins, biopolymers, and biomolecules. In nature, thiols and disulfides contribute to key biological functions related to cell signaling, protein conformation and folding processes, redox homeostasis, Citation: Beaupre, D.M.; Weiss, R.G. and biopolymer secondary structure development. Thiol groups and/or disulfide linkages Thiol- and Disulfide-Based can be incorporated into polymers to produce stimulus-responsive and/or self-healing Stimulus-Responsive Soft Materials materials. They owe their stimulus-responsiveness to covalent disulfide bonds, which can and Self-Assembling Systems. be broken and re-formed under mild conditions. The extent of reactivity, and thus the Molecules 2021, 26, 3332. https:// stimulus-responsive behavior depends on the nature of the groups flanking the disulfide doi.org/10.3390/molecules26113332 bond, as well as interactions with stimuli such as light, heat, mechanical force, and changes in the pH or redox state [1]. The ease with which thiols can be converted to disulfides, and Academic Editor: Feihe Huang vice versa, renders thiol-containing polymers (thiomers) similarly dynamic. This review will focus on the properties and applications of synthetic thiomers and Received: 4 May 2021 disulfide-containing polymers, with an emphasis on soft and self-assembling materials. Accepted: 25 May 2021 Published: 1 June 2021 Common thiol-containing pendant groups, as well as the synthetic methodologies for their incorporation into polymers, have been discussed extensively in a 2019 review of Publisher’s Note: MDPI stays neutral biomedically relevant thiomers, which includes a discourse on thiomer design strategies [2]. with regard to jurisdictional claims in A recent review by Bej and coworkers covered responsive aggregation of both small published maps and institutional affil- molecule and polymer amphiphiles containing disulfide bonds, with an emphasis on iations. biomedical applications [3]. Additionally, thiol and disulfide-based self-healing materials were reviewed by Yang [4], Wang [5], Utrera-Barrios [6], and their coworkers. Here, we discuss polymers in which the thiol and disulfide groups are important to their function. In addition to the types of systems previously reviewed, many examples of disulfide-based, self-healing polymers, many of which are elastomers, will be mentioned. However, to Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. the best of our knowledge, no review devoted specifically to self-healing disulfides has This article is an open access article been published. distributed under the terms and After a brief overview of reactions between (and among) thiols and disulfides, a conditions of the Creative Commons variety of thiol- and disulfide-based stimulus-responsive systems will be reviewed. The Attribution (CC BY) license (https:// examples will be classified by the structures of the materials and the stimuli employed. creativecommons.org/licenses/by/ The processes include thiol-disulfide and disulfide-disulfide exchange reactions, and redox 4.0/). reactions, as well as multiple responses by materials that contain additional functionalities Molecules 2021, 26, 3332. https://doi.org/10.3390/molecules26113332 https://www.mdpi.com/journal/molecules Molecules 2021, 26, x FOR PEER REVIEW 2 of 45 Molecules 2021, 26, 3332 2 of 44 whichas multiple trigger responses “cascade” by materials reactions that (such contai as decomposition).n additional functionalit The connectionsies which trigger between “cas- thecade” applications reactions (such and the as decomposition). structures of the The materials connections and the between nature the of theirapplications reactions and will the bestructures emphasized. of the materials and the nature of their reactions will be emphasized. 1.2.1.2. SomeSome ImportantImportant ReactionsReactionsof ofThiols Thiolsand and Disulfides Disulfides Thiol-disulfideThiol-disulfide exchange exchange reactions reactions can can occur occurbetween betweendeprotonated deprotonated thiols thiols(thiolates) (thiolates) andand disulfides. disulfides. Thiol-disulfide Thiol-disulfide exchange exchange proceeds proceeds via via substitution-type substitution-type pathways, pathways, with with the thiolatethe thiolate anion anion (RS− (RS) functioning-) functioning as the as nucleophilethe nucleophile (1 in (1 Figure in Figure1). In 1). addition, In addition, thiols thiols can becan oxidized be oxidized to disulfides to disulfides (2 through (2 through 10 in Figure10 in Figure1), and 1), disulfides and disulfides can be reducedcan be reduced to thiols. to Althoughthiols. Although the products the products of oxidation of oxidation or reduction or reduction reactions reactions are often are the often same the asthose sameof as thiol-disulfidethose of thiol-disulfide exchange exchange reactions, reactions, the pathways the pathways are mechanistically are mechanistically different (as different shown in(as Figureshown1). in There Figure are 1). well-documented There are well-documented multi-step examples multi-step of examples both one electronof both one (7 through electron 10(7 inthrough Figure1 10) and in Figure two electron 1) and (2 two through electron 6 in (2 Figure through1) oxidation 6 in Figure pathways 1) oxidation within pathways cells [ 7]. Thiylwithin radicals cells [7]. (RS Thiyl·) can radicals be generated (RS⋅) fromcan be thiols generated or disulfides from thiols using or a varietydisulfides of reagents using a va- or stimuli.riety of Thiylreagents radicals or stimuli. play a Thiyl crucial radicals role in play biological a crucial systems role in and biological are important systems reagents and are inimportant organic synthesis.reagents in Thiyl organic radicals synthesis. are intermediates Thiyl radicals in are disulfide-disulfide intermediates in disulfide-di- metathesis reactions,sulfide metathesis also known reactions, as disulfide-disulfide also known as disulfide-disulfide exchange reactions, exchange which reactions, proceed which via a radicalproceed pathway via a radical (11 in Figurepathway1). Because(11 in Figure disulfide 1). Because linkages disulfide are often thelinkages weakest are covalentoften the bondsweakest in ancovalent organic bonds molecule, in an theyorganic provide molecule, an avenue they provide to tune thean avenue properties to tune of a the material prop- undererties mildof a material reaction under or stimulus mild reaction conditions. or st Pleaseimulus note conditions. that the Please product note distributions that the prod- of bothuct distributions thiol-disulfide of andboth disulfide-disulfide thiol-disulfide and exchangedisulfide-disulfide depend upon exchange the ratios depend of reactants upon the andratios the of nature reactants of the and R groups,the nature which of the determine R groups, product which stabilities.determine product stabilities. Figure 1. Reactions of thiols (RSH) and disulfides (RSSR): 1.1 SN2-mediated thiol-disulfide exchange; Figure 1. Reactions of thiols (RSH) and disulfides (RSSR): 1.1 SN2-mediated thiol-disulfide exchange; 1.2–1.6. Thiol oxidation with two-electron oxidants, where RSX is a sulfenyl halide, RSOH is a sul- 1.2–1.6. Thiol oxidation with two-electron oxidants, where RSX is a sulfenyl halide, RSOH is a sulfenic fenic acid, HOX is a halohydrin, and HOx is the reduced oxidizing agent; 1.7–1.10 Thiol oxidation acid, HOX is a halohydrin, and HOx is the reduced oxidizing agent; 1.7–1.10 Thiol oxidation with with one-electron oxidizing agents; 1.11 disulfide-disulfide metathesis. one-electron oxidizing agents; 1.11 disulfide-disulfide metathesis. MoleculesMolecules2021 2021, 26, 26, 3332, x FOR PEER REVIEW 33 of of 44 45 1.3.1.3. Biological Biological Relavance Relavance of of Thiols Thiols and and Disulfides Disulfides SomeSome molecules molecules of of biological biological relevance relevance that that exploit exploit the the disulfide disulfide bond bond are are shown shown in in FigureFigure2. 2. For For example, example, processes processes involving involving cystine cystine (Cys (Cys2)2) and and cysteine cysteine (Cys) (Cys) interconver- interconver- sionssions have have structural structural and and functional functional consequences consequences on on the the macroscopic macroscopic and and microscopic microscopic scale.scale. On On a a macroscopic macroscopic level, level, disulfide disulfide bonds bonds in in Cys Cys2 stabilize2 stabilize the the secondary secondary structure structure of of keratin-basedkeratin-based materials, materials, such such as as hair, hair, wool, wool, and
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