Sulfur-Containing Polymers Prepared from Fatty Acid-Derived

Review Sulfur-Containing Polymers Prepared from Fatty Acid-Derived Monomers: Application of Atom-Economical Thiol-ene/Thiol-yne Click Reactions and Inverse Vulcanization Strategies

Ashlyn D. Smith 1,2,*, Andrew G. Tennyson 2,3,* and Rhett C. Smith 2,* 1 Department of Chemistry, Anderson University, Anderson, SC 29621, USA 2 Department of Chemistry, Clemson University, Clemson, SC 29634, USA 3 Department of Materials Science and Engineering, Clemson University, Clemson, SC 29634, USA * Correspondence: [email protected] (A.D.S.); [email protected] (A.G.T.); [email protected] (R.C.S.) Received: 23 July 2020; Accepted: 22 September 2020; Published: 3 October 2020

Abstract: This paper is review with 119 references. Approaches to supplant currently used plastics with materials made from more sustainably-sourced monomers is one of the great contemporary challenges in sustainable chemistry. Fatty acids are attractive candidates as polymer precursors because they can be affordably produced on all inhabited continents, and they are also abundant as underutilized by-products of other industries. In surveying the array of synthetic approaches to convert fatty acids into polymers, those routes that produce organosulfur polymers stand out as being especially attractive from a sustainability standpoint. The first well-explored synthetic approach to fatty acid-derived organosulfur polymers employs the thiol-ene click reaction or the closely-related thiol-yne variation. This approach is high-yielding under mild conditions with up to 100% atom economy and high functional group tolerance. More recently, inverse vulcanization has been employed to access high sulfur-content polymers by the reaction of fatty acid-derived olefins with elemental sulfur. This approach is attractive not only because it is theoretically 100% atom economical but also because elemental sulfur is itself an underutilized by-product of fossil fuel refining. The thiol-ene, inverse vulcanization, and mechanistically-related thiol-yne and classic vulcanization are therefore discussed as promising routes to access polymers and composites from fatty acid-derived precursors.

Keywords: sustainable polymers; fatty acids; thiol-ene; inverse vulcanization

1. Introduction

1.1. Motivation for this Review Plastics pervade modern life [1–5], yet their accumulation in the environment is a threat to ecosystems globally [6–9] and only a small fraction of post-consumer plastics are recycled [10]. Strategies to use bio-derived precursors to prepare polymers and to assure the biodegradability or facile recyclability of polymers are at the forefront of sustainable chemistry and materials science [11–13]. Plant oils and their component fatty acids can be produced in all inhabited regions of the world, an important consideration for resilience and equity in resource distribution as it relates to sustainability. In this review, the focus is on plant oil-derived free fatty acids as precursors to organosulfur polymers. Free fatty acids are of interest because they are found in high percentages in waste products such as acid oil by-product of biodiesel production, used food grease waste, and low value products of animal fat rendering [14–22].

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Sustain.waste products Chem. 2020, such1 as acid oil by-product of biodiesel production, used food grease waste, and low210 value products of animal fat rendering [14–22]. PreparingPreparing modernmodern polymerspolymers from plantplant oil-derivedoil-derived precursors has been ofof interestinterest forfor severalseveral decadesdecades [[23,24],23,24], andand therethere havehave beenbeen severalseveral insightfulinsightful reviewsreviews coveringcovering variousvarious aspectsaspects ofof materialsmaterials derivedderived from triglycerides [25–31] [25–31] and and fatty fatty acids acids [32–35]. [32–35]. The The current current review review centers centers specifically speciﬁcally on onorganosulfur organosulfur polymers polymers derived derived from from fatty fatty acids. acids. These These materials materials are are especially especially attractive attractive from from a asustainability sustainability standpoint standpoint because because of ofthe the high high atom atom economy economy with with which which they they can can be prepared be prepared by the by themethods methods discussed discussed in the in next the nexttwo twosections. sections. A review A review on this on particular this particular niche nicheis timely is timely because because in the inlast the few last years few yearsseveral several important important advances advances have capitalized have capitalized on the on facile the facilereversibility reversibility of S–S of bond S–S bondformation formation to access to access healable healable and and readily-recyclable readily-recyclable materials materials with with high high strength strength and chemicalchemical resistance.resistance. ThisThis new class of recyclable and healable fatty acid-derived materials hashas notnot been covered previouslypreviously inin thethe aforementionedaforementioned reviewsreviews onon fattyfatty acid-derivedacid-derived polymers.polymers.

1.2.1.2. Sources and ClassificationClassification ofof FattyFatty AcidsAcids FattyFatty acidsacids areare generallygenerally obtainedobtained byby thethe hydrolysishydrolysis ofof triglyceridestriglycerides (Figure(Figure1 1),), thethe primary primary constituentsconstituents ofof plantplant oilsoils andand animalanimal fats.fats. The typical fatty acid component breakdown for a varietyvariety ofof familiarfamiliar sourcessources isis summarizedsummarized inin TableTable1 1.. Common Common fatty fatty acids acids provide provide a a plethora plethora of of possibilities possibilities forfor chemicalchemical modification.modification. The The carboxylate carboxylate function functionalityality can can be esterified esterified or undergo amidation, whilewhile unsaturatedunsaturated fatty acids also aaffordfford thethe possibilitypossibility of functionalization at the olefin olefin units. Ricinoleate,Ricinoleate, primarilyprimarily foundfound inin castorcastor oil,oil, isis uniqueunique amongamong thethe structuresstructures shownshown inin FigureFigure1 1 in in that that it it alsoalso featuringfeaturing anan alcoholalcohol moiety,moiety, allowingallowing additionaladditional modificationmodification at at this this site. site.

Figure 1. General structures for fatty acids derivedderived from naturally-occurringnaturally-occurring triglycerides.triglycerides.

WhileWhile thethe relativerelative percentagespercentages ofof eacheach classclass ofof fattyfatty acidsacids variesvaries widelywidely byby species,species, itit isis notablenotable thatthat in a a wide wide range range of of species species ranging ranging from from tree-de tree-derivedrived sources sources (olive (olive oil) oil) to flowers to flowers (sunflower (sunflower and andsafflower safflower oil), oil),to underground to underground legumes legumes (peanut (peanut oil), oil), the the most most common common saturated saturated fatty fatty acid acid (SFA) componentcomponent isis palmiticpalmitic acidacid (16-carbon(16-carbon chain),chain), thoughthough coconutcoconut oiloil featuresfeatures predominantlypredominantly lauriclauric acidacid (12-carbon(12-carbon chain)chain) as as the the SFA SFA in its in triglycerides. its triglyceride Thes. mostThe commonmost common monounsaturated monounsaturated fatty acid fatty (MUFA) acid is(MUFA) generally is generally oleic acid oleic (18-carbon acid (18-carbon chain). Givenchain). that Given the that olefin the functionality olefin functionality plays a plays key role a key in role the reactivityin the reactivity of fatty of acids fatty with acids sulfur with species, sulfur oleicspecies, acid-derived oleic acid-derived monomers monomers are the most are well-studied the most well- for developmentstudied for development of polymers of discussed polymers herein. discussed The herein. most common The most polyunsaturated common polyunsaturated fatty acid (PUFA) fatty acid in common(PUFA) in oils common is diunsaturated oils is diunsaturated linoleic acid (18-carbonlinoleic acid chain (18-carbon with two chain olefins), with though two olefins), linseed though oil is a commonlinseed oil example is a common in which example linolenic in which acid (18-carbon linolenic ac chainid (18-carbon with three chain olefins) with is three the most olefins) common is the PUFA.most common Throughout PUFA. the Throughout course of the the current course review,of the current fatty acids review, available fatty acids from available many of thefrom sources many shownof the sources in Table 1shown, as well in theTable unique 1, as ricinoleate well the unique derivatives ricinoleate from castor derivatives oil, will fr beom discussed. castor oil, will be discussed.

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Sustain. Chem. 2020, 1 Table 1. Fatty acid chain type compositions of some common plant oils. 211

SFA Chains MUFA Chains PUFA Chains Source Content Predominant Content Predominant PUFA Content Predominant Chain (%Table of Total) 1. aFatty Chain acid Length chain (% type of Total) compositions Chain Length of some (% common of Total) plantLength oils. (Unsaturation) b Coconut 92 12 6 18 2 18 (2) Olive SFA 15 Chains16 74 MUFA Chains18 10 PUFA Chains 18 (2) Canola 8 16 62 18 32 18 (2) Predominant SourcePeanut Content18 Predominant16 Content50 Predominant18 32PUFA Content 18 (2) Safflower 9 a 16 14 18 77 18 Chain(2) Length (% of Total) Chain Length (% of Total) Chain Length (% of Total) b Soybean 16 16 24 18 60 18(Unsaturation) (2) CoconutCorn 92 15 1216 28 618 18 57 2 18 (2) 18 (2) Sunflower 13 16 22 18 66 18 (2) Olive 15 16 74 18 10 18 (2) Linseed 10 16 19 18 72 18 (3) Canola 8 16 62 18 32 18 (2) a Peanut some values 18 add up to more 16 or less than 100% 50 due to rounding 18 to the nearest percentage. 32 Numbers 18 (2) Safflowerare reported9 from the published 16 studies cited 14 in the text. b number 18 of olefin units 77in the most prevalent 18 (2) Soybeanfatty acid 16chain of the PUFA 16 component. 24 18 60 18 (2) Corn 15 16 28 18 57 18 (2) Sunflower2. Applications13 of Thiol-ene and 16 Thiol-yne 22Reactions 18 66 18 (2) Linseed 10 16 19 18 72 18 (3) a 2.1.some General values Overview add up to of more Thiol-ene or less and than Thiol-yne 100% due Reactions to rounding to the nearest percentage. Numbers are reported from the published studies cited in the text. b number of olefin units in the most prevalent fatty acid chain of the PUFAThe component. general thiol-ene reaction (Figure 2a) and closely-related thiol-yne reaction (Figure 2b) are 100% atom-economical click reactions, meaning that they can generally be accomplished under mild 2. Applicationsconditions with of Thiol-ene high yield and and Thiol-yne good functional Reactions group tolerance [36–38]. In either of these two reactions, an initially-generated sulfur radical undergoes addition across a C–C pi bond to form a 2.1. Generalnew C–S Overview bond via ofthe Thiol-ene mechanism and shown Thiol-yne in Figure Reactions 2c. The initiation can be affected by the use of a catalyst or by photoinitiation. The general thiol-ene reaction (Figure2a) and closely-related thiol-yne reaction (Figure2b) are In the context of applying these C–S bond forming reactions on the path to accessing fatty acid- 100% atom-economical click reactions, meaning that they can generally be accomplished under mild derived organosulfur polymers, an olefin ultimately derived from an unsaturated fatty acid is conditionsemployed with as high one yieldof the andrequisite good re functionalactive moieties, group while tolerance a variety [36– of38 different]. In either thiols of theseor dithiols two reactions, can an initially-generatedbe employed as the sulfur sulfur radicalsource. The undergoes thiol-ene/th additioniol-yne acrossreactions a C–Cmay pibe bondapplied to as form part aof new the C–S bondmonomer via the mechanism synthesis, in shown post-polymerization in Figure2c. Themodification, initiation or can to befacilitate a ffected crosslinking by the use of ofpolymers. a catalyst or by photoinitiation.Examples for all of these possible variations will be described in the following section.

(a)

(b)

(c)

FigureFigure 2. General2. General net net reaction reaction for for thethe thiol-enethiol-ene ( (aa)) and and thiol-yne thiol-yne (b) ( breactions,) reactions, and anda simplified a simpliﬁed mechanismmechanism for C–Sfor C–S bond bond formation formation (c ().c).

In the context of applying these C–S bond forming reactions on the path to accessing fatty acid-derived organosulfur polymers, an olefin ultimately derived from an unsaturated fatty acid is employed as one of the requisite reactive moieties, while a variety of different thiols or dithiols can be employed as the sulfur source. The thiol-ene/thiol-yne reactions may be applied as part of the monomer synthesis, in post-polymerization modification, or to facilitate crosslinking of polymers. Examples for all of these possible variations will be described in the following section. Sustain. Chem. 2020, 1 212

2.2. ApplicationsSustain. Chem. 2020 of Thiol-ene, 1 Reactions to Fatty Acid-Derived Polymers 212

The2.2. Applications first several of Thiol-ene studies Reac discussedtions to Fatty here Acid-Derived involve the Polymers initial synthesis of α,ω-diols from fatty acids. Such diols are valuable monomers for the production of polyesters and polyurethanes. The first several studies discussed here involve the initial synthesis of α,ω-diols from fatty acids. Türünç, et al., for example [39], recently detailed an optimized synthesis of castor oil-derived diol or Such diols are valuable monomers for the production of polyesters and polyurethanes. Türünç, et al., α,ω-ester-alcoholfor example [39], monomers recently usingdetailed the an thiol-ene optimized addition synthesis reaction of castor (Figure oil-derived3). The diol sulfur-derivatized or α,ω-ester- monomersalcohol were monomers subsequently using the polymerized thiol-ene addition via condensationreaction (Figure polymerization. 3). The sulfur-derivatized One notable monomers advantage of thewere process subsequently described polymerized in this study via overcondensation prior work polymerization. it that monomer One notable synthesis advantage could beof atheffected usingprocess solvent-free described techniques. in this study Once over prepared, prior work the it monomers that monomer were synthesis used to could access be structurally-diverse affected using polymers.solvent-free For example,techniques. polymerization Once prepared, ofthe M1 monome with glycerolrs were used using to TBDaccess (triazabicyclodecene) structurally-diverse as an initiatorpolymers. yielded For example, a hyperbranched polymerization polymer of M1 structure, with glycerol whereas using TBD linear (triazabicyclodecene) polymers were obtained as an by copolymerizationinitiator yielded of a M1 hyperbranched with M2, M3 polymer with M5,structur or M4e, whereas with M5. linear These polymers polymerizations were obtained a ffbyorded copolymerization of M1 with M2, M3 with M5, or M4 with M5. These polymerizations afforded materials with good thermal properties such as melting points (Tm) between 50 and 71 ◦C and thermal materials with good thermal properties such as melting points (Tm) between 50 and 71 °C and thermal stability up to around 300 C as assessed by 5% weight loss observed in thermogravimetric analysis stability up to around 300◦ °C as assessed by 5% weight loss observed in thermogravimetric analysis (TGA).(TGA).

FigureFigure 3. Monomer 3. Monomer preparation preparation fromfrom castor oil-derived oil-derived compound compound 2 [39].2 [39 ].

AnotherAnother study study [40 ][40] reports reports the the preparation preparation ofof thermoplasticthermoplastic polyurethanes. polyurethanes. In this In thiscase casethe the requisiterequisite sulfur-derivatized sulfur-derivatized diol diol monomers monomers were were synthesized synthesized byby thethe thiol-ene reaction reaction of of oleic oleic acid acid or 10-undecenoicor 10-undecenoic acid, which acid, are which major are products major products of castor of oil castor pyrolysis oil pyrolysis and saponification and saponification of sunflower of oil sunflower oil (Figure 4a). These monomers were then readily polymerized with 4,4′- (Figure4a). These monomers were then readily polymerized with 4,4 0-methylenebis(phenylisocyanate) methylenebis(phenylisocyanate) in N,N-dimethyl formamide (DMF) using tin(II) 2-ethylhexanoate in N,N-dimethyl formamide (DMF) using tin(II) 2-ethylhexanoate as a catalyst (Figure4b). Polymers as a catalyst (Figure 4b). Polymers so produced exhibited Mn values in the range of 36–83 kDa. so produced exhibited Mn values in the range of 36–83 kDa. Thermal properties include glass Thermal properties include glass transition temperatures (Tg) ranging from 8 to 56 °C and Tm values transitionof between temperatures 104 and 124 (T °Cg) from ranging differential from 8scanning to 56 ◦ calorimetryC and Tm (DSC)values analysis. of between TGA revealed 104 and high 124 ◦C fromthermal differential stability scanning as well, calorimetry as manifest (DSC)by thermal analysis. decomposition TGA revealed temperatures high thermal (Td) above stability 269 °C. as It well, as manifestshould bybe thermalnoted that decomposition PU1 and PU3, temperatures both derived (T dfrom) above 10-undecenoic 269 ◦C. It shouldacid, showed be noted higher that PU1 and PU3,crystallinity both derivedthan PU2 fromand PU4 10-undecenoic due to the pendant acid, me showedthylene chains higher incrystallinity the oleic acid backbone, than PU2 which and PU4 due toprevent the pendant close packing methylene of the chains. chains Stress–strain in the oleic cu acidrves backbone,were acquired which to evaluate prevent the close mechanical packing of the chains.strength Stress–strainof several of the curves materials were in this acquired study. toPU2 evaluate and PU4 the exhibited mechanical behavior strength similar to of that several of of the materialsamorphous in rubbers, this study. whereas PU2 PU1 and and PU4 PU3 exhibited showed mo behaviorre crystalline similar behavior. to that All of polymers amorphous studied rubbers, were able to withstand ≥300% strain before breaking. These properties make them competitive whereas PU1 and PU3 showed more crystalline behavior. All polymers studied were able to withstand candidates, in terms of mechanical strength, for replacing traditional polyurethane resins. Perhaps 300% strain before breaking. These properties make them competitive candidates, in terms of ≥ the most interesting aspect of this work was the cytotoxicity study. In this case cytotoxicity was mechanicalevaluated strength, by the MTT for replacing assay (MTT traditional = 3-[4,5-dimethylthiazole-2-yl]-2,5-diphenyltetrazolium polyurethane resins. Perhaps the most interesting bromide). aspect of thisThis work assay was revealed the cytotoxicity that after 7 days, study. human In this fibroblast case cytotoxicitycell viability was was not evaluated affected by by the the presence MTT assay (MTT = 3-[4,5-dimethylthiazole-2-yl]-2,5-diphenyltetrazolium bromide). This assay revealed that after

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7 days,of humanthe materials. ﬁbroblast This cell was viability striking, was and not indicates aﬀected good by the preliminary presence of promise the materials. for the Thisuse of was these striking, and indicatesmaterials for good biopolymer preliminary applications. promise for the use of these materials for biopolymer applications.

(a)

(b)

FigureFigure 4. 4.(a ()a) PreparationPreparation of offour four diol diolmonomers. monomers. (b) Utility ( bof) monomers Utility of in monomerspolyurethane in preparation polyurethane preparation[40]. [40].

Desroches,Desroches, et al. et demonstratedal. demonstrated that that transesterification transesterification of of canola canola (rapeseed) (rapeseed) oil oil with with ethylene ethylene glycol is a viableglycol routeis a viable for producing route for producing pseudo-telechelic pseudo-telec diolshelic (Figure diols (Figure5)[ 41]. 5) Quantitative [41]. Quantitative thiol-ene thiol-ene reaction was thenreaction aff ectedwas then between affected the between diols the and diols 2-mercaptoethanol and 2-mercaptoethanol under under mild mild conditions conditions with with UVUV light light initiation. Step-growth polymerization of diols with methylene diphenyl-4,4′-diisocyanate initiation. Step-growth polymerization of diols with methylene diphenyl-4,40-diisocyanate (analogous to the(analogous reaction to shown the reaction in Figure shown4b) in then Figure a ff orded4b) then polyurethanes afforded polyurethanes that boasted that boasted thermal thermal properties properties (Tg = −3 °C) similar to those of commodity polyurethanes made from traditional diols (Tg (Tg = 3 C) similar to those of commodity polyurethanes made from traditional diols (Tg = 8 C) for =− 8 °C)◦ for use as coatings and binders. ◦ use as coatings and binders.

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FigureFigure 5. 5.Routes Routes to to synthesizesynthesize pseudo-telechelic pseudo-telechelic diols. diols.

In additionIn addition to the to preparation the preparation of diol of monomersdiol monome fromrs fattyfrom acidsfatty discussedacids discussed above, above, several several strategies strategies have been developed to access diester or dicarboxylic acid monomers. Diester and have been developed to access diester or dicarboxylic acid monomers. Diester and dicarboxylic acid dicarboxylic acid monomers are attractive candidates for use in trans/esterification routes to monomers are attractive candidates for use in trans/esterification routes to polyesters or in amidation polyesters or in amidation reactions to yield polyamides, for example. A recent example of facile reactionsdiester to production yield polyamides, from fatty for acid example. precursors A recent comes example from Meier, of facile et al. diester who productiondemonstrated from the fatty acid precursorssynthesis of comesan oleic-acid from Meier,derived et dimer al. who 3 that demonstrated can be usedthe without synthesis its further of an purification oleic-acidderived to dimersynthesize3 that can polyamides be used (Figure without 6) [42]. its further The ability purification to use the monomer to synthesize without polyamides purification (Figureis possible6)[ 42]. The abilitybecause to its use synthesis, the monomer affected by without UV-initiated purification thiol-ene is reaction possible of 3 because with 1,2-ethanedithiol its synthesis, at a ffroomected by UV-initiatedtemperature, thiol-ene proceeded reaction cleanly of 3 with with 1,2-ethanedithiol approximately at98% room conversion. temperature, Polyamides proceeded were cleanly then with approximatelyprepared via 98% metal-catalyzed conversion. Polyamides polycondensation were then of preparedhexaminediamine, via metal-catalyzed dimethyl adipate, polycondensation and 3 of hexaminediamine,(Figure 6). One of dimethylthe drawbacks adipate, of some and commercial3 (Figure6 polyamides). One of the is that drawbacks they exhibit of some a sharp commercial drop off in mechanical properties upon water uptake. Nylon-6,6, for example, exhibits 3.5 wt.% water polyamides is that they exhibit a sharp drop off in mechanical properties upon water uptake. Nylon-6,6, uptake at a relative humidity of just 55%, accompanied by a precipitous drop in Tg from 100 °C when for example,dry to 43 exhibits°C and a 3.5drop wt.% in tensile water strength uptake at atyield a relative from 80 humidityMPa to 43 ofMPa just [43]. 55%, Water accompanied uptake tests by a precipitousconducted drop on in polymersTg from shown 100 ◦C in when Figure dry 6 toexhibit 43 ◦C significantly and a drop lower in tensile water strength uptake than at yield commercial from 80 MPa to 43polyamides MPa [43]. at Water both 25 uptake °C and tests 80 °C conducted and also exhibited on polymers faster water shown release in Figure than does6 exhibit nylon-6,6. significantly lower water uptake than commercial polyamides at both 25 ◦C and 80 ◦C and also exhibited faster water release than does nylon-6,6.

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FigureFigure 6. Preparation6. Preparation of of dimer dimer 3 andand subsequent subsequent polymerization. polymerization.

Another study utilized the thiol-ene reaction to produce dicarboxylic acid monomers containing Another study utilized the thiol-ene reaction to produce dicarboxylic acid monomers containing two sulfur atoms per monomer with varying aliphatic chain lengths (2, 6, and 10 methylene groups two sulfur atoms per monomer with varying aliphatic chain lengths (2, 6, and 10 methylene groups two sulfurbetween atoms the sulfur per monomer atoms). These with monomers varying aliphatic then underwent chain lengths polycondensation (2, 6, and 10with methylene diamines groupsto between the sulfur atoms). These monomers then underwent polycondensation with diamines to betweenyield the fatty-acid sulfur derived, atoms). linear These polyamides monomers (Figure then 7)underwent [44]. A host polycondensation of properties were characterized,with diamines to yieldmost fatty-acid notably derived, thermal linearand tensile polyamides properties, (Figure impact 77))[ [44].resistance,44]. A host water of properties absorption, were and characterized,chemical moststability. notably The thermal authors and found tensiletensile that increasing properties,properties, the impact spacing resistance,resistance,between amide water units absorption,absorption, allows attenuation and chemical of stability.inter-/intra-chain The authors hydrogen found that bonding, increasing which, the while spacing imparting between physical amide strength units to allowsthe polymers, attenuation can of inter-inter-/intra-chainalso/intra-chain be accompanied hydrogen by brittleness bonding, which,and lower while impact imparting strength. physical Increased strength H-bonding to the also polymers,polymers, leads to can also behigher accompaniedaccompanied melting temperatures, by brittlenessbrittleness which andand can lowerbelower an impediment impactimpact strength.strength. to processability. Increased More H-bonding amide units also in leadsthe to polymer backbone also increase the propensity for water absorptivity, which can reduce the strength higher melting temperatures, which can be an impediment to processability. More amide units in the and potentially degrade the polymer. The synthesized polymers were thus more readily processible polymer backbone also increase the propensity for water absorptivity, which can reduce the strength than typical commercial polyamides by merit of their lower Tg, Tc, and Tm values. The polymers also and potentially degrade the polymer. The synthesized polymers were thus more readily processible and potentiallyexhibited improvements degrade the polymer.in chemical The andsynthesized impact resistance,polymers wereand thusadvantageous more readily low processiblewater than typical commercial polyamides by merit of their lower T , T , and T values. The polymers also than absorptivitytypical commercial and high polyamidesoxygen and water by merit vapor of permeation their lower compared Tgg, Tcc, and to commercial Tmm values. polyamides.The polymers also exhibited improvementsimprovements in chemicalin chemical and impactand impact resistance, resistance, and advantageous and advantageous low water absorptivitylow water andabsorptivity high oxygen and high and wateroxygen vapor and permeationwater vapor compared permeation to compared commercial to polyamides. commercial polyamides.

Figure 7. Preparation of dicarboxylic acid monomers and subsequent polymerization.

Employing linear bifunctional monomers of the type discussed so far in this section is only one of the approaches that has been surveyed for incorporating fatty acids into materials. Because the Figure 7. Preparation of dicarboxylic acid monomers and subsequent polymerization. thiol-eneFigure reaction 7. Preparation employs an of olefin dicarboxylic substrate, acid se monomersveral strategies and subsequenthave been enumerated polymerization. to prepare monomers with olefinic end caps, side chains or in the backbone. Kempe, et al., for example, explored Employingsome cationic linear ring-opening bifunctional polymerization monomers of cyclic of the fatty type acid-based discussed monomers so far in that this can section be facilitated is only one of theby approachesapproaches microwave thatthatirradiation hashas beenbeen at 100 surveyedsurveyed °C to form forfor incorporatingincorporating DecEnOx (Figure fattyfatty 8) acidsacids [45]. intointoThis materials. materials.process leaves Because an the thiol-ene reaction employs an oleﬁnolefin substrate,substrate, severalseveral strategies have been enumerated to prepare monomers with oleﬁnicolefinic endend caps, side chains oror in the backbone. Kempe, et al., for example, explored some cationic ring-opening polymerization ofof cyclic fatty acid-based monomers that can be facilitated by microwave irradiation at 100 °C to form DecEnOx (Figure 8) [45]. This process leaves an

Sustain. Chem. 2020, 1 216 Sustain. Chem. 2020, 1 216 by microwaveunmodified irradiation terminal at 100 olefin◦C tounit form poised DecEnOx for furt (Figureher modification.8)[ 45]. This processPost-pol leavesymerization an unmodified modifications terminalusing olefin the unitthiol-ene poised reaction for further in green modification. solvents (methyl-THF Post-polymerization or methyl laurate) modifications result in using low PDIs the (1.22– thiol-ene1.26) reaction and respectable in green solvents Mn ranging (methyl-THF from 4900–11,400 or methyl g/mol. laurate) Especially result in notable low PDIs in (1.22–1.26)this study is the and respectableability to Mappendn ranging a saccharide from 4900–11,400 from the gpolymer/mol. Especially backbone, notable a demonstration in this study of is the the functional ability to group appendtolerance a saccharide and frommild theconditions polymer possible backbone, for athe demonstration thiol-ene reaction. of the functionalThe stark groupdisparity tolerance between the and mildpolarity conditions of the possible polymers for modified the thiol-ene to include reaction. alkyl The ch starkains or disparity the saccharide between also the serves polarity to demonstrate of the polymersthe modifiedrange of tomaterials include alkylaccessible chains by or this the techniqu saccharidee. Although also serves their to demonstrate successful synthesis the range has of been materialsdemonstrated, accessible by the this full technique. property Althoughsets afforded their by successful these structurally synthesis-diverse has been materials demonstrated, has yet to be the fullfully property reported. sets afforded by these structurally-diverse materials has yet to be fully reported.

Figure 8.FigurePost-polymerization 8. Post-polymerization modiﬁcations modifications of DecEnOx of DecEnOx using green using solvents green solvents [45]. [45].

De andDe coworkers and coworkers utilized utilized RAFT polymerizationRAFT polymerization of a methacrylate-functionalized of a methacrylate-functionalized oleic acid oleic acid monomermonomer (2-(methacryloyloxy)ethyl (2-(methacryloyloxy)ethyl oleate) (Figure oleate)9), (Figure resulting 9), in resulting polymers in (PMAEO) polymers with (PMAEO) a narrow with a PDI, controllednarrow PDI, molecular controlled weights molecular and defined weights end and groups defined [46]. end Of groups particular [46]. interest Of particular is the ability interest to is the furtherability functionalize to further the functionalize polymer, as thethe alkenepolymer, unit as within the alkene the oleic unit acidwithin side the chain oleic are acid unreacted side chain are followingunreacted polymerization. following polymerization. Further functionalization Further functionalization with a variety with of thiola variety compounds of thiol compounds via the via thiol-enethe reaction, thiol-ene for reaction, example, for example, was affected waswhen affected excess when of excess thiols of was thiols used. was The used. excess The thiolexcess was thiol was requiredrequired in this casein this because case because of the lowof the reactivity low reactivity of the internalof the internal olefins. olefins. Despite Despite this lower this lower reactivity, reactivity, 1 however,however, quantitative quantitative conversion conversion was confirmed was confirmed by H NMRby 1H spectrometricNMR spectrometric analysis analysis for some for of some the of the materialsmaterials surveyed. surveyed. The functionalization The functionalization of the olefinof the sidechainsolefin sidechains provided provided a convenient a convenient avenue avenue for for the installationthe installation of a variety of a variety of additional of additional functionalities functionalities into the into polymer, the polymer, resulting resulting in a wide in a range wide range of accessibleof accessible properties. properties. When 3-mercaptopropanoicWhen 3-mercaptopropanoic acid was acid employed, was employed, for example, for example, the resulting the resulting polymerpolymer was soluble was soluble in aqueous in aqueous media. media. Epoxides Epoxides were also were readily also readily installed installed by reaction by reaction of PMAEO of PMAEO with mwith-CPBA m-CPBA in CH 2inCl 2CHat2 roomCl2 at temperature.room temperature. Facile Facile crosslinking crosslin ofking epoxide of epoxide moieties moieties upon theirupon their reactionreaction with diamines with diamines was also was demonstrated. also demonstrated.

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(a)

(b)

Figure 9.FigureSynthesis 9. Synthesis of PMAEO of PMAEO (a) and (a) and post-polymerization post-polymerization modiﬁcation modification via via the the thiol-ene thiol-ene reaction reaction and epoxidationand epoxidation reaction. (reaction.b) Part ( ofb) (Partb) is of reprinted (b) is reprinted with with permission permission from from reference reference [46]. [46]. Copyright Copyright 2011 Royal Society2011 Royal of Chemistry. Society of Chemistry.

PolyhydroxyalkanoatesPolyhydroxyalkanoates (PHAs), (PHAs), polyesters polyesters that ar aree produced produced by bycertain certain bacteria bacteria strains, strains, have have recently emerged as an attractive alternative to petroleum-based plastics because they are recentlybiodegradable emerged as anand attractive can be alternativesynthesized tofrom petroleum-based renewable sources. plastics Despite because the promise they are of biodegradable these and canmaterials, be synthesized the lack of fromchemical renewable reactivity in sources. the side chains Despite has thelimited promise derivatization of these to access materials, a wider the lack of chemicalapplication reactivity space. Levine in the and side coworkers chains thus has investigated limited derivatization an engineered strain to access of Escherichia a wider coliapplication that space. Levineallows for and the identity coworkers and quantity thus investigated of repeat units an to engineeredbe controlled, strainopening of theEscherichia door to a panoply coli that of allows for the identitynew materials and [47]. quantity of repeat units to be controlled, opening the door to a panoply of new The focus of this particular study was on a polymers such as polymeric α,ω-diol PHBU (Figure materials [47]. 10) with unsaturated side chains that could be modified using the thiol-ene reaction. Three different Thethiols focus were of thisincorporated, particular including studywas one that on aallowe polymersd for crosslinking such as polymeric of the polymer.α,ω-diol Decomposition PHBU (Figure 10) with unsaturatedtemperatures side of the chains polymers that ranged could from be modified 288 °C for usingthe unmodified the thiol-ene polymer reaction. to 301 °C Three for the di fferent thiols werecrosslinked incorporated, polymer (using including a 1:1 onemole that ratio allowed of crosslinker for crosslinking to alkene). Change of thes in polymer. thermal properties Decomposition among the polymers were unremarkable, with the exception of a change in melting temperatures for temperatures of the polymers ranged from 288 ◦C for the unmodified polymer to 301 ◦C for the crosslinkedmodified polymer (116.4–134.0 (using °C) a 1:1versus mole unmodified ratio of crosslinkerpolymer (143.2 to °C). alkene). However, Changes mechanical in thermal properties properties were decidedly different between the crosslinked (X-PHBU, Figure 10, bottom) and non-crosslinked among the polymers were unremarkable, with the exception of a change in melting temperatures for polymers. Tensile strength, elongation at break, and Young’s modulus were determined for each modifiedmaterial. (116.4–134.0 These mechanical◦C) versus properties unmodified exhibit polymer the greatest (143.2 change◦C). However, upon comparison mechanical of as-prepared properties were decidedly different between the crosslinked (X-PHBU, Figure 10, bottom) and non-crosslinked polymers. Tensilestrength, elongation at break, and Young’s modulus were determined for each material. These mechanical properties exhibit the greatest change upon comparison of as-prepared PHBU with the crosslinked derivative. PHBU had elongation at break of 89% and Young’s modulus of 368 MPa, compared to values of 495% and 335 MPa for the thiol-crosslinked structure. Sustain. Chem. 2020, 1 218

PHBU with the crosslinked derivative. PHBU had elongation at break of 89% and Young’s modulus of 368 MPa, compared to values of 495% and 335 MPa for the thiol-crosslinked structure. The primary mechanism of degradation of PHAs is hydrolytic cleavage of the polyester backbone. One of the goals of post-polymerization of the PHAs was therefore to tune the hydrophilicity of the surfaces of the polymers as a means of attenuation their degradation. This effort Sustain.was moderately Chem. 2020, 1successful, as the water contact angles for PHBU-COOH and PHBU-OH were 6.4°218 and 8.1° lower than for PHBU, respectively.

Figure 10. Post-polymerizationPost-polymerization modification modiﬁcation of PHBU via th thee thiol-ene thiol-ene reaction yields three modified modiﬁed variations [[47].47].

DurandThe primary and mechanismcoworkers sought of degradation to synthesize of PHAs ph isotosensitive hydrolytic cleavagepolycarbonate of the polyesterderived from backbone. fatty acid-basedOne of the goalsstarting of post-polymerization materials. Polymers of thethat PHAs could was be therefore selectively to tune crosslinked the hydrophilicity and partially of the decrosslinkedsurfaces of the upon polymers exposure as a to means UV light of attenuation were targeted their (Figure degradation. 11a) [48]. This Such eff ortmaterials was moderately provide a potentialsuccessful, route as the to wateraddress contact the major angles issue for PHBU-COOHof recycling of andpolycarbonates PHBU-OH were used 6.4in ◦theand biomedical 8.1◦ lower field. than Biomedicalfor PHBU, respectively. polycarbonates are often high strength, highly-crosslinked materials for which there is currentlyDurand no viable and coworkers method to sought reshape to or synthesize recycle. photosensitive polycarbonate derived from fatty acid-basedMethyl starting undecenoate materials. was Polymers thus thatreacted could with be selectively an amine crosslinked functionalized and partially diol, decrosslinkedfollowed by intramolecularupon exposure reaction to UV light of the were resultant targeted amide (Figure to form 11a) a [48 6-membered]. Such materials cyclic providecarbonate, a potential NH-Und-6CC route (Figureto address 11a), the which major issuethen ofunderwent recycling ofring-opening polycarbonates polymerization used in the biomedical(ROP). Following field. Biomedical ROP, the polycarbonatespendant alkene aremoiety often was high furthe strength,r functionalized highly-crosslinked via the materials thiol-ene for reaction which therewith isa currentlycinnamoyl- no viablederivatized method thiol. to reshapeThe percentage or recycle. of cinnamoyl incorporated was controlled by tuning the reaction time.Methyl Up to 100 undecenoate mol% substitution was thus was reactedachievable with in 30 an min. amine functionalized diol, followed by intramolecularThe cinnamoyl reaction group of thecontains resultant an internal amide toalkene form capable a 6-membered of photochemical cyclic carbonate, [2 + 2] NH-Und-6CCcycloaddition (Figurereaction 11 whena), which exposed then to underwent UV light with ring-opening a waveleng polymerizationth of 365 nm. (ROP).The [2 Following+ 2] cycloaddition ROP, the was pendant thus usedalkene to moiety affect wascrosslinking further functionalized to strengthen via and the cu thiol-enere the material. reaction withWhen a cinnamoyl-derivatizedthe crosslinked polymer thiol. is Theirradiated percentage withof 254 cinnamoyl nm light, incorporated the crosslinks was controlledpartially reverse, by tuning which the reaction allowstime. the materials Up to 100 mol%to be reshapedsubstitution and was recycled. achievable Tensile in 30testing min. was done to probe the mechanical properties of the crosslinked materials.The cinnamoyl As the cinnamoyl group contains content an increased, internal alkeneso did capablethe Young’s of photochemical modulus and [2 maximum+ 2] cycloaddition stress at break.reaction At when 10 mol% exposed cinnamoyl to UV content, light with the aYoung wavelength modulus of and 365 maximum nm. The [2stress+ 2] are cycloaddition 1.3 MPa and was 0.5 MPa,thus usedrespectively. to affect crosslinkingWhen the cinnamoyl to strengthen content and cureis increased the material. to 100 When mol%, the those crosslinked values polymerincrease dramatically,is irradiated withto 1266 254 MPa nm light,and 35 the MPa, crosslinks respective partiallyly. As reverse, materials which became allows more the crosslinked, materials to the be percentreshaped elongation and recycled. at break Tensile decreased testing wasfrom done 156% to in probe the 10 the mol% mechanical cinnamoyl properties content of polymer, the crosslinked to 3% inmaterials. the 100 mol% As the cinnamoyl cinnamoyl content content polymer. increased, so did the Young’s modulus and maximum stress at break. At 10 mol% cinnamoyl content, the Young modulus and maximum stress are 1.3 MPa and 0.5 MPa, respectively. When the cinnamoyl content is increased to 100 mol%, those values increase dramatically, to 1266 MPa and 35 MPa, respectively. As materials became more crosslinked, the percent elongation at break decreased from 156% in the 10 mol% cinnamoyl content polymer, to 3% in the 100 mol% cinnamoyl content polymer.

Sustain. Chem. 2020, 1 219 Sustain. Chem. 2020, 1 219

(a)

(b)

Figure 11. ((aa)) Synthesis Synthesis of of polycarbonate polycarbonate deri derivativesvatives containing cinnamate. (b)) Photo Photo demonstrating the flexibility ﬂexibility of low cinnamate-contentcinnamate-content polymers, 30 mol% cinnamate shown. Part Part ( (b)) is reprinted with permission from reference [[48].48]. CopyrightCopyright 2018 American Chemical Society.

Building on the work described above, Durand and coworkers also showed that commercially available castor oil derivatives can be leveraged to synthesizesynthesize biologically-sourcedbiologically-sourced polycarbonate network polymers polymers [49]. [49]. Efforts Efforts in in this this paper paper were were focused focused on onpost-polymerization post-polymerization modification modification of the of danglingthe dangling olefin olefin moieties moieties via viathe the thiol-ene thiol-ene reaction reaction to to produce produce cross-linked cross-linked materials materials exhibiting superior transparency and flexibility. flexibility. The thiol crosslinkers were selected to include examples with aromatic spacersspacers (1,4-benzenedimethanethiol, (1,4-benzenedimethanethiol, BDT) BDT) and and aliphatic aliphatic spacers spacers comprising comprising two chain two lengths chain lengths(1,6-hexanedithiol,Sustain. Chem. (1,6-hexanedithiol, 2020, 1 HDT and HDT 1,9-nonanedithiol, and 1,9-nonanedithiol, NDT) (Figure NDT) 12(Figure). 12). 220 Mechanical properties were investigated by dynamic mechanical analysis (DMA). With increasing flexibility of crosslinking agents, the Tg of the materials increased: The Tg for polymers crosslinked by BDT, HDT and NDT were 8.8 °C, 15.1 °C, and 37.7 °C, respectively. This trend was also observed for the storage modulus and Young’s modulus (1.3, 50, and 240 MPa, respectively), while the inverse was observed for elongation at break (190%, 76%, and 26%, respectively). HDT-crosslinked material was selected for further evaluation of the extent to which crosslink density influences each property of the network. Theoretically, 0.5 equivalents of dithiol crosslinker is sufficient to crosslink all of the olefin units in the linear polymer. The amount of dithiol in the crosslinkingFigure 12.12. reaction Thiol-ene was reaction thus usedvaried to modifymodify from P(NH-Und-6CC)P(NH-U0.1 to 0.8nd-6CC) equivalents. after polymerizationpolymeri The zationgel content using variousvarious increased predictablythiolsthiols [[49].49 from]. 0.1 to 0.5 equivalents, and then slightly decreased with an excess of dithiol (89% gel content with 0.5 equivalents of dithiol compared to 83% gel content with 0.8 equivalents). The Mechanical properties were investigated by dynamic mechanical analysis (DMA). With increasing swellingMoser ratio and in coworkers CH2Cl2 also synthesized generally semicrystallinedecreased as the poly(thioether-ester)s dithiol content was utilizing increased, the withthiol-ene the notableflexibilitypolymerization exception of crosslinking under of 0.8 mild, equivalents agents, solvent-free the whichTg of conditions the significantly materials [50]. increased:raised Monomers the swelling The wereTg for ratioprepared polymers without from crosslinked significantly 9-decenoic by BDT, HDT and NDT were 8.8 C, 15.1 C, and 37.7 C, respectively. This trend was also observed for compromisingacid, which can the be gel produced content, ◦from attributable oleic◦ acid to a [51]. lower ◦Af cross-linkingter esterification density. to yield The terminalTg increased dienes, from they 5.3 tothewere 15.1 storage then °C as reacted modulus the crosslinker with and dithiols Young’s concentration (1,2-ethanedithiol modulus was (1.3, increa 50, andorsed 1,3-propanedithiol) 240 from MPa, 0.1 to respectively), 0.5 equivalents, to yield while decreasinglinear the inverse polyesters to was 7.4 observed for elongation at break (190%, 76%, and 26%, respectively). °C(Figure upon 13).addition The presenceof 0.8 equivalents of both ofa Tcrosslinker.m and a T gThe lead storage to the modulus classification (ranging of thesefrom 0.1–4.5polymers MPa) as andsemicrystalline. Young’sHDT-crosslinked modulus Wide-angle material(ranging X-ray wasfrom scattering selected 0.8–50 MPa) for (W further AXS)likewise evaluationwas progressively thus used of the toincreased extent determine to whichwith the increasing crosslink relative crosslinkdensitycrystallinity. influences density, Polymer with each 6 was propertythe foundsame of observableto the be network.the most trend crystalline, Theoretically, of values followed decreasing 0.5 equivalents by 5 , up3, andon of addition dithiol4. The inclusion crosslinker of excess of crosslinkingis1,3-propanedithiol sufficient toagent. crosslink as the all crosslinking of the olefin agent units in inpolymers the linear 5 and polymer. 6 lead Theto the amount greater of crystallinity, dithiol in the as the additional methylene unit allows for greater chain flexibility when compared to 1,2-ethanedithiol. The Tg, as determined by DMA, ranged from −36.8 (polymer 6) to −25.7 °C (polymer 3), and Tm of the polymers ranged from 60.9 (polymer 5) to 70.5 °C (polymer 4). Mechanical properties were probed using DMA from −60 to +60 °C. Storage and loss moduli, as well as tan δ at Tg and room temperature were reported for all polymers. Polymers 5 and 6 exhibited higher storage moduli values at both Tg and room temperature. These materials also exhibited higher loss moduli at both temperatures, suggesting that they are more efficient at dissipating energy through heat. Thermal and chemical stability as well as solubility were also reported. Using TGA, it was determined that all polymers were thermally stable below 300 °C. All polymers were soluble in chloroform and THF, but insoluble in more polar solvents, likely due to the long aliphatic chains present in the polymer backbone. The polymers were also subjected to acid and base challenge by soaking in acetic acid, sulfuric acid, and sodium hydroxide for seven days. All polymers were stable, meaning undissolved, in acetic acid and sodium hydroxide. However, when exposed to the oxidizing acid, the polymers were completely soluble, suggesting that they would be susceptible to other oxidizing agents.

Figure 13. Preparation of polymers 3–6 [50].

2.3. Comparison or Combination of Thiol-ene and Olefin Metathesis Reactions Monomers and polymers featuring side chain and endcap olefins described as substrates for thiol-ene modification in the previous section are often also attractive substrates for olefin metathesis

Sustain. Chem. 2020, 1 220

Figure 12. Thiol-ene reaction used to modify P(NH-Und-6CC) after polymerization using various thiols [49]. Sustain. Chem. 2020, 1 220 Moser and coworkers synthesized semicrystalline poly(thioether-ester)s utilizing the thiol-ene polymerization under mild, solvent-free conditions [50]. Monomers were prepared from 9-decenoic crosslinkingacid, which reactioncan be produced was thus variedfrom oleic from acid 0.1 to[51]. 0.8 Af equivalents.ter esterification The gel to content yield terminal increased dienes, predictably they fromwere 0.1then to reacted 0.5 equivalents, with dithiols and then(1,2-ethanedithiol slightly decreased or 1,3-propanedithiol) with an excess of to dithiol yield (89%linear gel polyesters content with(Figure 0.5 equivalents13). The presence of dithiol of comparedboth a Tm toand 83% a gelTg contentlead to withthe classification 0.8 equivalents). of these The swelling polymers ratio as insemicrystalline. CH2Cl2 also generally Wide-angle decreased X-ray asscattering the dithiol (W contentAXS) was was thus increased, used withto determine the notable the exception relative ofcrystallinity. 0.8 equivalents Polymer which 6 was significantly found to be raised the most the swellingcrystalline, ratio followed without by significantly 5, 3, and 4. The compromising inclusion of the1,3-propanedithiol gel content, attributable as the crosslinking to a lower agent cross-linking in polymers density. 5 and The 6 Tleadg increased to the greater from 5.3 crystallinity, to 15.1 ◦C as thethe additional crosslinker methylene concentration unit allows was increased for greater from chai 0.1n flexibility to 0.5 equivalents, when compared decreasing to 1,2-ethanedithiol. to 7.4 ◦C upon additionThe Tg, as of determined 0.8 equivalents by DMA, of crosslinker. ranged from The storage −36.8 (polymer modulus 6 (ranging) to −25.7 from °C (polymer 0.1–4.5 MPa) 3), and and T Young’sm of the moduluspolymers (ranging ranged from from 60.9 0.8–50 (polymer MPa) likewise 5) to 70.5 progressively °C (polymer increased 4). with increasing crosslink density, withMechanical the same observable properties trend were of probed values using decreasing DMA upon from addition−60 to +60 of °C. excess Storage crosslinking and loss agent.moduli, as well Moseras tan δ and at T coworkersg and room synthesized temperature semicrystalline were reported poly(thioether-ester)sfor all polymers. Polymers utilizing 5 and the 6 exhibited thiol-ene polymerizationhigher storage moduli under mild,values solvent-free at both Tg and conditions room temperature. [50]. Monomers These were materials prepared also fromexhibited 9-decenoic higher acid,loss moduli which can at beboth produced temperatures, from oleic suggesting acid [51]. Afterthat they esterification are more to yieldefficient terminal at dissipating dienes, they energy were thenthrough reacted heat. with Thermal dithiols and (1,2-ethanedithiol chemical stability or 1,3-propanedithiol)as well as solubility to were yield also linear reported. polyesters Using (Figure TGA, 13 it). Thewas presencedetermined of boththat all a Tpolymersm and a Twereg lead thermally to the classification stable below of 300 these °C. All polymers polymers as semicrystalline.were soluble in Wide-anglechloroform X-rayand THF, scattering but insoluble (WAXS) in was more thus polar used tosolvents, determine likely the due relative to the crystallinity. long aliphatic Polymer chains6 waspresent found in tothe be polymer the most backbone. crystalline, The followed polymers by 5 ,were3, and also4. Thesubjected inclusion to acid of 1,3-propanedithiol and base challenge as theby crosslinkingsoaking in acetic agent acid, in polymers sulfuric acid,5 and and6 leadsodium to the hydroxide greater crystallinity,for seven days. as theAll additionalpolymers were methylene stable, unitmeaning allows undissolved, for greater chainin acetic flexibility acid and when sodium compared hydroxide. to 1,2-ethanedithiol. However, when The exposedTg, as to determined the oxidizing by DMA, ranged from 36.8 (polymer 6) to 25.7 C (polymer 3), and Tm of the polymers ranged from acid, the polymers −were completely soluble,− ◦suggesting that they would be susceptible to other 60.9oxidizing (polymer agents.5) to 70.5 ◦C (polymer 4).

Figure 13.13. Preparation of polymers 3–6 [[50].50].

2.3. ComparisonMechanical or properties Combination were of Th probediol-ene usingand Olefin DMA Metathesis from 60 Reactions to +60 ◦ C. Storage and loss moduli, − as well as tan δ at Tg and room temperature were reported for all polymers. Polymers 5 and 6 exhibited Monomers and polymers featuring side chain and endcap olefins described as substrates for higher storage moduli values at both Tg and room temperature. These materials also exhibited higher thiol-ene modification in the previous section are often also attractive substrates for olefin metathesis loss moduli at both temperatures, suggesting that they are more eﬃcient at dissipating energy through heat. Thermal and chemical stability as well as solubility were also reported. Using TGA, it was determined that all polymers were thermally stable below 300 ◦C. All polymers were soluble in chloroform and THF, but insoluble in more polar solvents, likely due to the long aliphatic chains present in the polymer backbone. The polymers were also subjected to acid and base challenge by soaking in acetic acid, sulfuric acid, and sodium hydroxide for seven days. All polymers were stable, meaning undissolved, in acetic acid and sodium hydroxide. However, when exposed to the oxidizing acid, the polymers were completely soluble, suggesting that they would be susceptible to other oxidizing agents. Sustain. Chem. 2020, 1 221

2.3. Comparison or Combination of Thiol-ene and Olefin Metathesis Reactions Sustain. Chem. 2020, 1 221 Monomers and polymers featuring side chain and endcap olefins described as substrates for reactions.thiol-ene modification Several studies in the have previous therefore section examined are often the alsoviability attractive of using substrates thiol-ene for click olefin reactions metathesis in tandemreactions. with Several olefin studies metathesis have or therefore have compared examined fatty the acid-derived viability of using polymers thiol-ene prepared click reactionsusing thiol- in enetandem versus with metathesis olefin metathesis routes oras havecomplementary compared fatty approaches. acid-derived In one polymers such preparedstudy [52], using Türünç thiol-ene and versuscoworkers metathesis sought routes to compare as complementary polymerization approaches. by thiol-ene In one reaction such study and ADMET [52], Türünç as mechanisms and coworkers to producesought to biodegradable compare polymerization polymers byfrom thiol-ene renewabl reactione feedstocks and ADMET with aspotential mechanisms application to produce as biomedically-relevantbiodegradable polymers materials. from renewable Polymers feedstocks produced with by potential both techniqu applicationes were as fully biomedically-relevant characterized to assessmaterials. their Polymers chemical producedstability when by both exposed techniques to different were fullysolvents, characterized acidic and to enzymatic assess their conditions. chemical Thestability polymers when exposedsynthesized to di ffviaerent ADMET solvents, proved acidic to and tole enzymaticrate a wider conditions. range of The functional polymers groups synthesized (both viaesters ADMET and anhydrides proved to could tolerate function a wider as range monomers). of functional In contrast, groups the (both thiol-ene esters reaction and anhydrides could tolerate could estersfunction but as not monomers). anhydrides In (Figure contrast, 14). the However, thiol-ene the reaction thiol-ene could reaction tolerate lead esters to polymers but not with anhydrides higher (Figuremolecular 14). weights However, of up the to thiol-ene 11,850 g/mol reaction compared lead to polymersto a maximum with higher of 9000 molecular g/mol for weights those prepared of up to 11,850by ADMET. g/mol compared to a maximum of 9000 g/mol for those prepared by ADMET.

Figure 14. StructuresStructures of dienes and thiols probed, along with a chain stopper (topmost structure) [52]. [52].

In a related related study study [53], [53], the the same same research research group group investigated investigated copolymers copolymers synthesized synthesized from castor oil derivatives by ADMET or the thiol-ene reaction with the goal of elucidating an alternative to polyethylene (Figure 1515).). The The polymers polymers exhibited exhibited desirable desirable melting melting points points ranging ranging from from 40–80 40–80 °C.◦C. All polymers, with the exception of P3, exhibited degrees of crystallization values between 36% and 52%, which which is is higher higher than than that that reported reported for for LLDPE LLDPE [54]. [54 On]. this On thisbasis, basis, it wa its concluded was concluded that the that thiol- the enethiol-ene reaction reaction is a useful is a useful facsimile facsimile towards towards the synthesis the synthesis of rene ofwable renewable polyethylenes. polyethylenes. A study by Unverferth and coworkers [55] also focused on post-polymerization modification, in this case of star-shaped polyesters prepared via ADMET polymerization of castor oil derivatives (Figure 16). Two main cores were used for the star shaped polyesters, one containing four arms and another containing six arms available for polymer growth. These two cores were then functionalized with either 10 or 20 repeat units per arm. Both internal and terminal olefins in the resultant structures were successfully modified using the thiol-ene reaction. Five different thiols were used in the post-polymerization modification of each star-shaped precursor. Modification extent and kinetics were easily quantifiable by 1H NMR spectrometric analysis. Although internal alkenes reacted expectedly more slowly than terminal alkenes, quantitative consumption of all alkene units was achievable in all cases. It was also noted that no degradation of the polymer backbone occurred during the post-polymerization modification manipulations. Functionalization with a variety of different thiols allowed ready tuning of the polarity of the modified polymers. The effective polarity of each construct was thus assessed using RP-HPLC to determine the octanol-water partition coefficient.

Figure 15. Synthesis of bio-derived polymers P1–P6 [54].

Sustain. Chem. 2020, 1 221 reactions. Several studies have therefore examined the viability of using thiol-ene click reactions in tandem with olefin metathesis or have compared fatty acid-derived polymers prepared using thiol- ene versus metathesis routes as complementary approaches. In one such study [52], Türünç and coworkers sought to compare polymerization by thiol-ene reaction and ADMET as mechanisms to produce biodegradable polymers from renewable feedstocks with potential application as biomedically-relevant materials. Polymers produced by both techniques were fully characterized to assess their chemical stability when exposed to different solvents, acidic and enzymatic conditions. The polymers synthesized via ADMET proved to tolerate a wider range of functional groups (both esters and anhydrides could function as monomers). In contrast, the thiol-ene reaction could tolerate esters but not anhydrides (Figure 14). However, the thiol-ene reaction lead to polymers with higher molecular weights of up to 11,850 g/mol compared to a maximum of 9000 g/mol for those prepared by ADMET.

Figure 14. Structures of dienes and thiols probed, along with a chain stopper (topmost structure) [52].

In a related study [53], the same research group investigated copolymers synthesized from castor oil derivatives by ADMET or the thiol-ene reaction with the goal of elucidating an alternative to polyethylene (Figure 15). The polymers exhibited desirable melting points ranging from 40–80 °C. All polymers, with the exception of P3, exhibited degrees of crystallization values between 36% and Sustain.52%, which Chem. 2020is higher, 1 than that reported for LLDPE [54]. On this basis, it was concluded that the thiol-222 ene reaction is a useful facsimile towards the synthesis of renewable polyethylenes.

Sustain. Chem. 2020, 1 222

A study by Unverferth and coworkers [55] also focused on post-polymerization modification, in this case of star-shaped polyesters prepared via ADMET polymerization of castor oil derivatives (Figure 16). Two main cores were used for the star shaped polyesters, one containing four arms and another containing six arms available for polymer growth. These two cores were then functionalized with either 10 or 20 repeat units per arm. Both internal and terminal olefins in the resultant structures were successfully modified using the thiol-ene reaction. Five different thiols were used in the post- polymerization modification of each star-shaped precursor. Modification extent and kinetics were easily quantifiable by 1H NMR spectrometric analysis. Although internal alkenes reacted expectedly more slowly than terminal alkenes, quantitative consumption of all alkene units was achievable in all cases. It was also noted that no degradation of the polymer backbone occurred during the post- polymerization modification manipulations. Functionalization with a variety of different thiols allowed ready tuning ofFigureFigure the 15.polarity15. Synthesis of ofthe bio-derivedbio- modiderivedfied polymers polymers. P1– P6The [[54].54 effective]. polarity of each construct was thus assessed using RP-HPLC to determine the octanol-water partition coefficient.

Figure 16. Dienes and thiols used to prepare star-shaped polymers [[55].55].

Dannecker and coworkers aimed to synthesize sustainable polyethers via ADMET and thiol-ene polymerizations employing employing monomers monomers derived derived from from methyl methyl oleate oleate [56]. [56 In]. some In some previous previous work, work, the theauthors authors had had demonstrated demonstrated synthesis synthesis of ofether ether mo monomersnomers using using the the Williamson Williamson ether ether synthesis; however, that approach was lackinglacking in sustainability. In the updated strategy, thethe authorsauthors useuse GaBrGaBr33 for the catalytic reduction of esters to yieldyield ethersethers for polymerization,polymerization, and the utility of greener polymerization solvents suchsuch asas methylmethyl THFTHF andand polarcleanpolarclean waswas establishedestablished (Figure(Figure 1717).). The thiol-ene polymerization was first attempted solvent-free in a microwave reactor with AIBN initiator at 80 °C. The number average molecular weights obtained by this route were low (Mn = 950 g/mol); this was thought to be due to poor mixing because of the high viscosity of the monomers at this temperature. It was determined that a solvent was necessary to accomplish high molecular weights. Methyl THF was chosen for this purpose on the basis of its sustainable nature, lower toxicity, and higher boiling point than the THF more commonly used in traditional polymer syntheses. The resulting polymers exhibited Mn increased by an order of magnitude, and after optimization, number average molecular weights greater than 15 kDa were achieved. The thiol-ene polymers were then oxidized by hydrogen peroxide solution to form sulfoxides and sulfones to improve their thermal properties. For the ADMET polymerization, three different catalysts were screened for their ability to affect solvent-free polymerization. It was determined that the 2nd generation Hoveyda-Grubbs catalyst tolerated the conditions best, and with the addition of polarclean solvent, produced the highest Mn of 32 kDa. The resulting polymers were then hydrogenated to increase the melting temperatures.

Sustain. Chem. 2020, 1 223 Sustain. Chem. 2020, 1 223

FigureFigure 17.17. GeneralGeneral syntheticsynthetic scheme scheme for for comparison comparison of thiol-eneof thiol-ene and and metathesis metathesis routes routes to polymers to polymers [56]. [56]. The thiol-ene polymerization was first attempted solvent-free in a microwave reactor with AIBN2.4. General initiator Applications at 80 ◦C. of The Thiol-yne number Reactions average to molecularFatty Acid-Derived weights Polymers obtained by this route were low (Mn = 950 g/mol); this was thought to be due to poor mixing because of the high viscosity of the Given the natural occurrence of olefin moieties in a wide range of fatty acids, most of the work monomers at this temperature. It was determined that a solvent was necessary to accomplish high in the field has employed the thiol-ene rather than the thiol-yne reaction. However, there are a few molecular weights. Methyl THF was chosen for this purpose on the basis of its sustainable nature, lower studies in which modified fatty acid precursors have been utilized as substrates for thiol-yne toxicity, and higher boiling point than the THF more commonly used in traditional polymer syntheses. elaboration towards polymer synthesis. In one such study employing the thiol-yne route, products The resulting polymers exhibited Mn increased by an order of magnitude, and after optimization, of common reactions of fatty acids found in sunflower oil and castor oil were used as the initial number average molecular weights greater than 15 kDa were achieved. The thiol-ene polymers starting materials (Figure 18) [57]. After installation of the requisite alkyne unit, the compounds were were then oxidized by hydrogen peroxide solution to form sulfoxides and sulfones to improve their subjected to the thiol-yne reaction with 2-mercaptoethanol to yield diol derivatives. Subsequent thermal properties. reduction of the methyl ester using lithium aluminum hydride led to the target triols. Both diol and For the ADMET polymerization, three different catalysts were screened for their ability to affect triol derivatives were exploited for use in polymer formulations to access target thermoplastic and solvent-free polymerization. It was determined that the 2nd generation Hoveyda-Grubbs catalyst thermosetting polyurethanes upon their reaction with methylenebis(phenylisocyanate) (PU1 and tolerated the conditions best, and with the addition of polarclean solvent, produced the highest Mn of PU2 derived from diols; PU3 and PU4 derived from triols). The resulting polymers exhibited good 32 kDa. The resulting polymers were then hydrogenated to increase the melting temperatures. thermal properties, with Td values between 245 and 261 °C and Tg values ranging from 15–59 °C. PU1 2.4.and GeneralPU2 exhibited Applications high of strain Thiol-yne upon Reactions breaking to Fatty(1693% Acid-Derived and 965%), Polymers but low Young’s moduli (≤0.054 MPa), whereas PU3 and PU4 were much stiffer, exhibiting low strain at breaking of around 10% but Given the natural occurrence of olefin moieties in a wide range of fatty acids, most of the work in high Young’s moduli ranging from 89–143 MPa. the field has employed the thiol-ene rather than the thiol-yne reaction. However, there are a few studies Polyurethanes have utility in tissue engineering and bone grafting, so the biocompatibility of in which modified fatty acid precursors have been utilized as substrates for thiol-yne elaboration the synthesized materials was investigated. Cell viability using an MTS assay (3-(4,5-dimethylthiazol- towards polymer synthesis. In one such study employing the thiol-yne route, products of common 2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium) in an osteoblast cell line was reactions of fatty acids found in sunflower oil and castor oil were used as the initial starting materials monitored as a means of assessing the utility of these new materials in bone-grafting applications (Figure 18)[57]. After installation of the requisite alkyne unit, the compounds were subjected to (Figure 18b). The materials showed no significant cytotoxicity, with all materials providing over 75% the thiol-yne reaction with 2-mercaptoethanol to yield diol derivatives. Subsequent reduction of the cell viability over a 24 h period. No difference in cell viability was observable between materials methyl ester using lithium aluminum hydride led to the target triols. Both diol and triol derivatives prepared from diols versus triols. were exploited for use in polymer formulations to access target thermoplastic and thermosetting polyurethanes upon their reaction with methylenebis(phenylisocyanate) (PU1 and PU2 derived from diols; PU3 and PU4 derived from triols). The resulting polymers exhibited good thermal properties, with Td values between 245 and 261 ◦C and Tg values ranging from 15–59 ◦C. PU1 and PU2 exhibited high strain upon breaking (1693% and 965%), but low Young’s moduli ( 0.054 MPa), whereas PU3 ≤ and PU4 were much stiffer, exhibiting low strain at breaking of around 10% but high Young’s moduli ranging from 89–143 MPa.

Sustain. Chem. 2020, 1 224 Sustain. Chem. 2020, 1 224

O 1. Br2/Et2O O 2. KOH OH R O n 3. CH OH n R 3 reflux/Amberlyst 10-undecynoic acid (UDY) UDYM/OLYM R=H,n=6 9-octadecenoic acid (OLY) SH HO R= -(CH2)7CH3,n=5 DMPA UV HO HO

S LiAlH4/THF S O R R OH O n n S S (a) UDYM-triol UDYM-diol OH OLYM-triol OH OLYM-diol

MDI CH2 N C O O 2 MDI O n

Hyperbranched polyurethane H H PU3 (from UDYM-triol) S O N N HO S PU4 (from OLYM-triol) m R O O

PU1 (from UDYM-diol) PU2 (from OLYM-diol)

(b)

Figure 18.18.( a()a Synthesis) Synthesis of polyolsof polyols from sustainablefrom sustainable starting starting materials. materials. (b) Scanning (b) electronScanning micrograph electron imagemicrograph of MG63 image cells of culturedMG63 cells on cultured PU3 demonstrates on PU3 demonstrates the viability the of viability human cellsof human on this cells substrate. on this Partsubstrate. (b) is reprintedPart (b) is withreprinted permission with permission from reference from [ 57refe]. Copyrightrence [57]. 2012Copyright Royal 2012 Society Royal of Chemistry. Society of Chemistry.

Sustain. Chem. 2020, 1 225

Polyurethanes have utility in tissue engineering and bone grafting, so the biocompatibility of the synthesized materials was investigated. Cell viability using an MTS assay (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium)Sustain. Chem. 2020, 1 225 in an osteoblast cell line was monitored as a means of assessing the utility of these new materials in bone-graftingBeyazkilic applications and coworkers (Figure likewise 18b). Thedescribed materials the modification showed no of significanta fatty acid with cytotoxicity, a terminal with all alkyne, thus allowing for subsequent reaction with 2-mercaptoethanol via the thiol-yne addition to materialsgive providing monomer VSHA over 75%(Figure cell 19) viability [58]. Polyesters over awere 24 hthen period. synthesized No di byfference combination in cell of viabilityVSHA was observablewith betweenmonomers materials such as 1,4-butanediol prepared from and ε diols-caprolactone versus triols.in varying feed ratios to yield di- and tri- Beyazkilicblock copolymers. and coworkers Two likewisedifferent describedpolymerization the modification methods—traditional of a fatty metal-catalyzed acid with a terminal alkyne,polycondensation thus allowing for and subsequent enzymatic polycondensation reaction with—were 2-mercaptoethanol employed in this via pursuit. the thiol-yne While metal- addition to give monomercatalyzed VSHA reactions (Figure are the 19 predominant)[58]. Polyesters method were used thento synthesize synthesized polyesters by combination in industrial settings, of VSHA with the inclusion of metals can have deleterious effects in the eventual use of the polymer. Even when a monomers such as 1,4-butanediol and ε-caprolactone in varying feed ratios to yield di- and tri-block high loading (10–25 wt%) of metal catalyst was employed, the lipase catalyzed reaction still exhibited copolymers.better kinetics Two di andfferent higher polymerization ultimate molecular methods—traditional weights. metal-catalyzed polycondensation and enzymaticThe polycondensation—werealkene unit remaining in the employedpolymer chain in thisallowed pursuit. for post-polymerization While metal-catalyzed modification reactions are the predominantto be performed. method Using used 1,4-butane-dithiol to synthesize and polyesters UV irradiation in industrial at room temperature, settings, thethe polyester inclusion was of metals can havesuccessfully deleterious crosslinked effects in within the eventual one hour, use as confir of themed polymer. by monitoring Even whenthe disappearance a high loading of the (10–25C=C wt%) 1 of metalstretch catalyst via wasIR spectroscopy employed, and the lipaseH NMR catalyzed spectrometry. reaction Surface still modification exhibited of better films kinetics prepared and by higher solution-casting the triblock polyesters was also affected by reacting it with a fluorescent label ultimate molecular weights. (7-mercapto-4-methylcoumarin) as a convenient means of quantifying the extent of modification.

FigureFigure 19. Synthesis 19. Synthesis of of random random polymerpolymer and and triblock triblock copolymer copolymer [58]. [58].

3. Applications of Vulcanization and Inverse Vulcanization The alkene unit remaining in the polymer chain allowed for post-polymerization modification to be performed.3.1. Inverse Vulcanization Using 1,4-butane-dithiol versus Classical Vulcanization and UV irradiation at room temperature, the polyester was successfully crosslinked within one hour, as confirmed by monitoring the disappearance of the Vulcanization is a term that most commonly refers to the process discovered by Charles 1 C=C stretchGoodyear via IRin 1844 spectroscopy whereby olefin and unitsH NMRin natural spectrometry. rubber are crosslinked Surface modificationwith a small percentage of films preparedof by solution-castingelemental sulfur the (Figure triblock 20a) [59]. polyesters The result was of this also process affected is the by durable reacting vulcanized it with rubber a fluorescent familiar label (7-mercapto-4-methylcoumarin)in modern life as the black material as a convenient of which most means automobile of quantifying tires are made. the extentMore recently, of modification. Pyun’s group coined the term inverse vulcanization [60] to refer to a process in which crosslinking of olefins 3. Applicationsis affected ofby their Vulcanization reaction with and a majority Inverse component Vulcanization sulfur (Figure 20b), rather than as the minority component (<5 wt%) originally used in classic Goodyear vulcanization. Because they are comprised 3.1. Inversein large Vulcanization part by sulfur, versus materials Classical prepared Vulcanization by the inverse vulcanization process are generally formulated as incorporating polymeric sulfur catenates, generally unstable at STP, stabilized by their Vulcanizationentrapment iswithin a term a thatcrosslinked most commonly network. refersMechanistically, to the process both discoveredvulcanization by and Charles inverse Goodyear in 1844vulcanization whereby olefin employ units the in addition natural of rubber a sulfur are radical crosslinked across a withC–C pi a smallbond, percentagemuch like the of first elemental sulfur (Figure 20a) [59]. The result of this process is the durable vulcanized rubber familiar in modern life as the black material of which most automobile tires are made. More recently, Pyun’s group coined Sustain. Chem. 2020, 1 226

Sustain. Chem. 2020, 1 226 the term inverse vulcanization [60] to refer to a process in which crosslinking of olefins is affected by their reactionpropagation with astep majority of the componentthiol-ene reaction sulfur disc (Figureussed 20inb), the rather previous than section. as the minorityIn the inverse component vulcanization and vulcanization reactions, however, sulfur radicals are generated by heating (<5 wt%) originally used in classic Goodyear vulcanization. Because they are comprised in large elemental sulfur above its floor temperature of 159 °C to induce thermal homolysis of S8 rings (the part bytypical sulfur, sulfur materials allotrope prepared at STP) byand the ultimate inverse formation vulcanization of polymeric process sulfur areradicals. generally Like the formulated thiol- as incorporatingene reaction, polymeric inverse sulfurvulcanization catenates, is also generally theoretically unstable 100% atom at STP, economical, stabilized although by their some entrapment side within areactions crosslinked are sometimes network. observed. Mechanistically, Elemental bothsulfur vulcanization is also an attractive and inversecomonomer vulcanization because it isemploy the additionproduced of a sulfuras a fossil radical fuel refining across by-product a C–C pi bond, in a large much excess like for the which first there propagation is currently step no practical of the thiol-ene reactionuse. discussed Another innotable the previousfeature of these section. reactions In the is inversethat vulcanization vulcanization is used andon large vulcanization industrial scale reactions, in the rubber industry and, though not yet commercialized on industrial scale, inverse vulcanization however,has sulfur been demonstrated radicals are generatedon kilogram by scales heating [61]. elemental sulfur above its floor temperature of 159 ◦C to induce thermalSince the homolysis first articulation of S8 ringsof inverse (the vulcanization typical sulfur in its allotrope modern atembodiment STP) and in ultimate 2013 [60], formation this of polymericmechanism sulfur radicals. has proven Like broadly the thiol-eneapplicable reaction,to producing inverse high sulfur vulcanization content materials is also (HSMs) theoretically from 100% atom economical,a wide range although of olefins for some a plethora side reactions of applications are sometimes [62–76]. In addition observed. to petrochemically-derived Elemental sulfur is also an attractiveolefins, comonomer many biologically-derived because it is produced olefins have as al aso fossil been fuelsuccessfully refining exploited by-product as comonomers in a large with excess for sulfur, including terpenoids [77–80]. Triglycerides [81–88], fatty acids [89–92], sorbitan esters [93], which there is currently no practical use. Another notable feature of these reactions is that vulcanization amino acid derivatives [94], guaiacol derivatives [95], and cellulose/lignin derivatives or is used onlignocellulosic large industrial biomass scale [11,96–100]. in the rubber industry and, though not yet commercialized on industrial scale, inverseHSMs vulcanization resulting from has inverse been vulcanization demonstrated are onprimarily kilogram comprised scales by [61 S–S]. bonds, allowing for Sincetheir the facile first recycling articulation and thermal of inverse healability vulcanization by merit of inthe its thermal modern reversibility embodiment of S–S inbond 2013 [60], this mechanismformation. has The proven high sulfur broadly content applicable materials to producing produced by high the sulfur inverse content vulcanization materials of (HSMs)some from a wide rangetriglycerides of olefins have also for abeen plethora established of applications not only to be [biodegradable,62–76]. In addition but to even topetrochemically-derived enhance the growth of food crops as timed-release sulfur fertilizer sources. All of the aforementioned properties make olefins, many biologically-derived olefins have also been successfully exploited as comonomers with HSMs derived from fatty acids tantalizing targets from a sustainability standpoint. In the last few sulfur, includingyears, several terpenoids examples [77of– 80such]. Triglyceridesmaterials, as well [81– 88as], materials fatty acids crosslinked [89–92], by sorbitan traditional esters [93], amino acidvulcanization derivatives processes, [94], guaiacol have been derivatives reported. These [95], ma andterials cellulose are the/ ligninsubject derivativesof discussion orin the lignocellulosic final biomassportion [11,96 of–100 this]. review.

(a)

(b)

Figure 20.FigureVulcanization 20. Vulcanization (a) and(a) and inverse inverse vulcanization vulcanization ( (bb) )processes. processes. Reproduced Reproduced from [63] from (part [63 (]a)) (part (a)) and [94]and (part [94] (b (part)) under (b)) under a Creative a Creative Commons Commons 3 License 3 License (https: (https://creativecommons.org/licenses/by-//creativecommons.org/licenses/by-nc/3. 0/). nc/3.0/).

HSMs resulting from inverse vulcanization are primarily comprised by S–S bonds, allowing for their facile recycling and thermal healability by merit of the thermal reversibility of S–S bond formation. The high sulfur content materials produced by the inverse vulcanization of some triglycerides have also been established not only to be biodegradable, but to even enhance the growth of food crops as timed-release sulfur fertilizer sources. All of the aforementioned properties make HSMs derived from Sustain. Chem. 2020, 1 227 fatty acids tantalizing targets from a sustainability standpoint. In the last few years, several examples of such materials, as well as materials crosslinked by traditional vulcanization processes, have been reported. These materials are the subject of discussion in the final portion of this review. Sustain. Chem. 2020, 1 227 3.2. Applications of Vulcanization/Inverse Vulcanization to Fatty Acid-Derived Polymers 3.2. Applications of Vulcanization/Inverse Vulcanization to Fatty Acid-Derived Polymers Estolides are employed in many commercial applications, including cosmetics, inks, Estolides are employed in many commercial applications, including cosmetics, inks, and and emulsifiers in margarine, and they are also biodegradable. Dworakowska and coworkers emulsifiers in margarine, and they are also biodegradable. Dworakowska and coworkers thus thus investigatedinvestigated a a one-pot one-pot polyesterification polyesterification of fatty of acid fatty derivatives acid derivatives to form estolides to form as part estolides of a broader as part of a broaderstudy study on the on theextent extent to which to which their theirproperties properties could be could improved be improved through crosslinking through crosslinking by by vulcanizationvulcanization or or otherother radical radical routes routes [101]. [101 The]. estolides The estolides proved readily proved crosslinkable readily using crosslinkable dicumyl using dicumylperoxide peroxide or by or their by theirvulcanization vulcanization to yield bio-based to yield elastomers bio-based (Figure elastomers 21). Thermal (Figure and 21mechanical). Thermal and mechanicalproperties properties of the crosslinked of the crosslinked networks networks were evalua wereted. evaluated.TGA of the materials TGA of indicated the materials high thermal indicated high stability, with a 5% decomposition temperature between 205 and 318 °C. Low glass transition thermaltemperatures stability, with were a 5%also decomposition reported (−69 to temperature−54 °C), coupled between with good 205 tensile and 318 strength◦C. Low (0.11 glass to 0.40 transition temperatures were also reported ( 69 to 54 C), coupled with good tensile strength (0.11 to 0.40 MPa). MPa). − − ◦

(a)

(b)

Figure 21.Figure(a) Formation21. (a) Formation of estolide of estolide from methylfrom methyl ricinoleate ricinoleate containing containing oleﬁnic olefinic linkages linkages in in the the backbone that canbackbone subsequently that can undergo subsequently radical undergo crosslinked radical crosslinked by vulcanization by vulcanization (analogous (analogous to Figure to Figure 20 a) or by action of20a) dicumyl or by action peroxide of dicumyl (b). peroxide (b).

In a similarIn a similar study, study, Ebata Ebata and and coworkers coworkers investigatedinvestigated castor castor oil oilderivatives derivatives as monomers as monomers for for eventual inclusion into sulfur-vulcanized thermosetting elastomers (Figure 22) [102]. Such bio- eventualderived inclusion thermosets into sulfur-vulcanized were envisioned as thermosetting potential replacements elastomers for conventional (Figure 22 )[rubber102]. monomers. Such bio-derived thermosetsThe initial were envisionedstep of the process as potential was to replacementssubject methyl forricinoleate conventional to lipase-catalyzed rubber monomers. condensation The initial step of thepolymerization. process was This to subjectprocess methylafforded ricinoleatereadily-proc toessable lipase-catalyzed polymers according condensation to Figure polymerization.22. This This processmethod a ffallowedorded some readily-processable control over molecular polymers weight, according and polymers to having Figure M 22w .of This 52 and method 101 kDa allowed were prepared to assess the influence of molecular weight on properties. Following their synthesis, some control over molecular weight, and polymers having Mw of 52 and 101 kDa were prepared to these polymers were crosslinked with sulfur by addition of stearic acid, zinc oxide, carbon black, and assess the influence of molecular weight on properties. Following their synthesis, these polymers were other sulfur-based crosslinking agents (Figure 22). The cured materials could then be fabricated into crosslinkedfilms withusing sulfura thermal by press. addition The resulting of stearic films acid, exhibited zinc oxide, good hardness carbon black,(up to 48 and A), other an ultimate sulfur-based crosslinking agents (Figure 22). The cured materials could then be fabricated into films using a thermal press. The resulting films exhibited good hardness (up to 48 A), an ultimate tensile strength of up to Sustain. Chem. 2020, 1 228

tensile strength of up to 6.91 MPa, and elongation at break of up to 350%. The best physical properties were observed with the higher molecular weight polymer. Thus far, the studies discussed in this section have illustrated the toughening of fatty acid- derived polymers by traditional vulcanization by addition of small percentages of sulfur (or other sulfur-containing curatives). More recently, the utility of inverse vulcanization of fatty acids with Sustain. Chem.majority2020 ,component1 sulfur has been explored. For example, Smith and coworkers reported the 228 inverse vulcanization of oleic acid and sulfur in varying ratios [92]. The inverse vulcanization of oleic acid and sulfur led to copolymers ZOSx, where x is the weight percent of sulfur in the monomer feed, 6.91 MPa,varied and from elongation 8 to 99 (Figure at break 23a). of Heating up to 350%. sulfur Thewith bestorganics physical at high properties temperatures were can observedrelease H2S with the gas, which is highly toxic even at low concentrations. In this study, the authors found that doing the higher molecular weight polymer. reaction in the presence of ZnO effectively suppressed H2S generation.

FigureFigure 22. Production22. Production of of polyricinoleate polyricinoleate thermosetting thermosetting elastomer. elastomer.

Infrared spectroscopy and 1H NMR spectrometry were used to confirm the consumption of Thus far, the studies discussed in this section have illustrated the toughening of fatty alkene units in the oleic acid starting materials, and thermal techniques, such as TGA and DSC, were acid-derivedused to polymersshow the presence by traditional of polymeric vulcanization sulfur in the materials, by addition as indicated of small by a percentages Tg in the DSC ofat sulfur (or other sulfur-containing−40 °C. Perhaps the curatives).most interesting More results recently, of the study the utility are indicated of inverse in the vulcanization mechanical properties. of fatty acids with majorityTemperature component scanning sulfur DMA in has single been cantilever explored. mode For was example, undertaken Smith from and−60 °C coworkers to +60 °C. It reported was the inversefound vulcanization that with ofthe oleic addition acid of and one sulfur percent in of varying oleic acid ratios into [ 92the]. materials The inverse (ZOS vulcanization99), the storage of oleic modulus of the material increased almost 8-fold when compared to elemental sulfur. These materials acid and sulfur led to copolymers ZOSx, where x is the weight percent of sulfur in the monomer feed, also showed thermal healing ability; when the surfaces of the materials were scratched, the gentle varied fromheating 8 toof 99the (Figurematerials 23 ata). 100 Heating °C for a sulfur short ti withme allowed organics the at scratch high temperaturesto heal (Figure 23b). can releaseThe H2S gas, whichmaterials is highly also showed toxic even the ability at low to concentrations. be recycled at least In four this times study, without the authors the lossfound of strength, that doingas the reactiondetermined in the presence by the storage of ZnO modulus. eﬀectively suppressed H2S generation. Infrared spectroscopy and 1H NMR spectrometry were used to conﬁrm the consumption of alkene units in the oleic acid starting materials, and thermal techniques, such as TGA and DSC, were used to show the presence of polymeric sulfur in the materials, as indicated by a Tg in the DSC at 40 C. Perhaps the most interesting results of the study are indicated in the mechanical properties. − ◦ Temperature scanning DMA in single cantilever mode was undertaken from 60 C to +60 C. It was − ◦ ◦ found that with the addition of one percent of oleic acid into the materials (ZOS99), the storage modulus of the material increased almost 8-fold when compared to elemental sulfur. These materials also showed thermal healing ability; when the surfaces of the materials were scratched, the gentle heating of the materials at 100 ◦C for a short time allowed the scratch to heal (Figure 23b). The materials also showed the ability to be recycled at least four times without the loss of strength, as determined by the storage modulus.

The inverse vulcanization of technical grade and pure linoleic acid (ZLSx and ZPLSx, respectively, where x = weight percentage of feedstock sulfur) was similarly probed by Smith and coworkers (Figure 24)[91]. Technical grade linoleic is a blend of many fatty acids, with the majority component being linoleic acid, followed by primarily oleic acid and a small percentage of short and long chain, saturated, and unsaturated fatty acids. A noteworthy aspect of this work was thus to compare how the blend of fatty acids performed compared to a pure linoleic acid sample as a monomer for inverse vulcanization. Realizing useful polymers from fatty acid mixtures would improve their aﬀordability. The ZLSx and ZPLSx polymers were thus evaluated for physical and mechanical properties. The polymers were thermally stable at high temperatures (ranging from 231 to 273 ◦C), and the technical grade-derived materials boasted higher Td than the analogous pure linoleic acid-derived materials. Some of the materials with higher organic content were crosslinked to such a degree that thermosets were formed, precluding their recyclability or thermal healing by the methods described. Sustain. Chem. 2020, 1 229 Sustain. Chem. 2020, 1 229

(a)

(b)

FigureFigure 23. (a )23. Synthesis (a) Synthesis of ZOS of ZOSx materials.x materials. ((b) Optical Optical micrographs micrographs of ZOS of ZOS96 before96 before and after and thermal after thermal annealing at 100 °C for 5 min. Reprinted with permission from reference [92]. Copyright 2019 John annealing at 100 ◦C for 5 min. Reprinted with permission from reference [92]. Copyright 2019 John Wiley andWiley Sons. and Sons.

The inverse vulcanization of technical grade and pure linoleic acid (ZLSx and ZPLSx, The ZLSx and ZPLSx polymers that were remeltable were subjected to DMA from 60 to +60 C. respectively, where x = weight percentage of feedstock sulfur) was similarly probed by Smith− and ◦ As the sulfurcoworkers content (Figure increased 24) [91]. inTechnical the materials, grade linoleic the storage is a blend and of lossmany moduli fatty acids, decreased. with the Thismajority is expected, as elementalcomponent sulfur being isa linoleic weak acid, and followed brittle material. by primaril Flexuraly oleic acid stress–strain and a small analysispercentage was of short also and performed on theselong materials chain, saturated, at room and temperature. unsaturated Whilefatty acids. these A noteworthy materials exhibitedaspect of this moderate work was ﬂexural thus to moduli and strength,compare most how interestingthe blend of was fatty that acids the performed materials compared could withstand to a pure overlinoleic 20% acid of sample strain. as In a contrast, similarmonomer materials for made inverse from vulcanization. lignocellulosic Realizing waste useful can only polymers be deformed from fatty by

In the work discussed above, Smith and coworkers noted that the synthesis of polymers using oleic acid and elemental sulfur could be carried out in the presence of zinc oxide to suppress H2S gas evolution. Zinc oxide is not sustainably sourced, however, and thus an alternative was sought to function as the base/H2S-supressing agent in this reaction [90]. For this purpose, Portland cement or Pozzalanic cements were considered to supplant the ZnO. Portland cement (PC), along with fly ash (FA), ground granulated blast furnace slag (GGFBS), metakaolin (MK), and silica fume (SF) were reacted with oleic acid and elemental sulfur to form YOS90, (where Y is the cementitious material and all materials are comprised by 90 weight percent of feedstock sulfur). Each of the materials synthesized was remeltable and subjected to compressive strength and acid resistance testing. The compressive strengths of ZOS90, FAOS90, and PCOS90 were similar to that of ordinary Portland cement (OPC), with PCOS90 exceeding the strength of OPC. The other materials were less than 70% of the strength of OPC. Flexural strength and moduli were also evaluated using DMA, but these values were notable lower than that of OPC. One of the interesting findings in this study is that the materials are resistant to oxidizing acids like sulfuric acid, as assessed by strain-strain analysis of these materials before and after soaking in Sustain. Chem. 2020 1 0.5 M H2SO,4 for 30 min. Two of the materials (GGBFSOS90 and MKOS90) experienced no loss in 230 flexural modulus after the acid challenge (Figure 25). For comparison, after soaking OPC in the same acidic conditions it completely deteriorates, and its flexural strength cannot be measured. The sulfur- and itsbearing ﬂexural materials strength also cannot experience be measured. enhanced flexibil The sulfur-bearingity after the acid materials challenge, also with experience some able enhanced to ﬂexibilityexperience after up the to acid ~30% challenge, strain before with breaking. some able to experience up to ~30% strain before breaking.

(a)

Sustain. Chem. 2020, 1 231

(b) (b)

FigureFigure 24. Preparation24. Preparation of ofZLS ZLSx andx andZPLS ZPLSxx ((aa)) andand photos ( (bb) )of of the the materials. materials. Reprinted Reprinted with with permissionpermissionFigure from 24. from referencePreparation reference [91 of ].[91]. ZLS Copyright Copyrightx and ZPLS 2020 2020x (a John )John and Wiley Wileyphotos and (b )Sons. Sons.of the materials. Reprinted with permission from reference [91]. Copyright 2020 John Wiley and Sons.

FigureFigure 25.FigureCompressive 25. 25. Compressive Compressive strength strength strength of ofof sulfur sulfur cement cement materials, materials, normalized normalized normalized to to the tothe strength the strength strength of ordinaryof ordinary of ordinary PortlandPortlandPortland cement. cement. cement. Reprinted Reprinted Reprinted with with with permission permissionpermission from fromfrom referencereference [90]. [90]. [90 Copyright]. Copyright Copyright 2020 2020 2020Elsevier. Elsevier. Elsevier.

4. Recycling4. Recycling and and Environmental Environmental StabilityStability of OrganosulfurOrganosulfur Polymers Polymers Approaches to recycle organosulfur polymers are in their nascent stages of development as Approaches to recycle organosulfur polymers are in their nascent stages of development as compared to the well-studied routes to more widely-used petrochemical plastics. We have recently compared to the well-studied routes to more widely-used petrochemical plastics. We have recently reviewed the chemical recycling routes to polyolefins and polyethylene terephthalate with reviewed the chemical recycling routes to polyolefins and polyethylene terephthalate with comparison to some of the emerging, thermally recyclable high sulfur-content materials discussed comparisonherein [10]. to Many some organosulfurof the emerging, polymers thermally can be re thermallycyclable high processed sulfur-content or healed materials by merit discussedof the hereinthermal [10]. reversibility Many organosulfur of S–S bond polymers formation can be[103–107]. thermally The processed polymers or preparedhealed by by merit inverse of the thermalvulcanization, reversibility for example, of S–S can bondsometimes formation be recycled [103–107]. over a dozen The cyclespolymers without prepared loss in mechanical by inverse vulcanization,properties by for simply example, melting can thesometimes material be down recycled and overrecasting a dozen the materialcycles without into new loss molds in mechanical or by propertiesextrusion by [63,91,92]. simply Themelting ability the of materialthese high down sulfur-content and recasting materials the tomaterial retain their into strength new molds after orso by extrusionmany thermal [63,91,92]. processing The ability cycles of is these a notable high advantage, sulfur-content as it standsmaterials in sharp to retain contrast their to strength the behavior after so manyof common thermal theprocessing existing cyclespetrochemi is a notablecal plastics. advantage, Polyethylene as it stands tereph inthalate, sharp forcontrast example, to the exhibits behavior of initialcommon strain the at breakexisting of 42%petrochemi that plummetscal plastics. to only Polyethylene 0.7% after just tereph the fifththalate, thermal for processingexample, cycleexhibits initial[108]. strain at break of 42% that plummets to only 0.7% after just the fifth thermal processing cycle [108].

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4. Recycling and Environmental Stability of Organosulfur Polymers Approaches to recycle organosulfur polymers are in their nascent stages of development as compared to the well-studied routes to more widely-used petrochemical plastics. We have recently reviewed the chemical recycling routes to polyolefins and polyethylene terephthalate with comparison to some of the emerging, thermally recyclable high sulfur-content materials discussed herein [10]. Many organosulfur polymers can be thermally processed or healed by merit of the thermal reversibility of S–S bond formation [103–107]. The polymers prepared by inverse vulcanization, for example, can sometimes be recycled over a dozen cycles without loss in mechanical properties by simply melting the material down and recasting the material into new molds or by extrusion [63,91,92]. The ability of these high sulfur-content materials to retain their strength after so many thermal processing cycles is a notable advantage, as it stands in sharp contrast to the behavior of common the existing petrochemical plastics. Polyethylene terephthalate, for example, exhibits initial strain at break of 42% that plummets to only 0.7% after just the fifth thermal processing cycle [108]. However, the simple melt processing route to recycling organosulfur polymers is limited to the thermoplastics. Quite recently, however, thermal compression molding techniques have been developed whereby additive manufacturing and recycling of ground powders was demonstrated for thermosetting organosulfur polymers comprising large proportions of triglyceride comonomers [74]. A chemical approach that relies on amine- or phosphine-induced S–S bond metathesis has also proven highly effective for recycling organosulfur polymers that are not remeltable [73]. In addition to recycling of organosulfur polymers into new items, some efforts have been undertaken to reclaim organic and sulfur small molecular materials through pyrolytic disintegration of the polymers. Along with typical organic pyrolysis products (benzene derivatives, oligoolefins, ethylene, and propylene, for example), the ultimate fate of the sulfur from these species is generally SO2, with some contribution of CS2 [109,110]. Although SO2 is toxic, it can be reclaimed as elemental sulfur through catalytic processes or it can be further oxidized to sulfuric acid [111,112]. In terms of the environmental stability and fate of organosulfur polymers, there is good evidence that higher sulfur content materials are quite stable to oxidation, as many have proven impervious to oxidizing acid [88,94,96]. Organosulfur coatings and treatments have even been used to protect metal and mineral cement installations in factories where oxidative damage from oxidizing acids are otherwise problematic [113–119]. However, little work has been done to elucidate the ultimate fate of any of the materials described herein via biodegradation mechanisms. This is clearly an area that deserves additional investigation if these materials are to have a place in green building technologies. Early studies have proven that using some organosulfur polymers as elements of time-release fertilizers could enhance the growth of food crops such as tomatoes that benefit from acidic soil [82]. This growth enhancement is attributable to the slow production of sulfuric acid by sulfur-metabolizing bacteria, providing one environmental fate of these materials. Much work has yet to be done in this area.

5. Conclusions and Outlook The examples delineated in this review demonstrate the wide range of synthetic methodologies available for the modification of alcohol, alkene, alkyne, ester, and carboxylic acid sites in fatty acids. One of the critical hindrances to commercial applications for fatty acid-derived polymer production is that the internal alkenes that are present in most of the naturally-occurring unsaturated fatty acids have lower reactivity when compared to traditional olefin monomers such as ethylene, acrylate, and styrene derivatives. The development of new catalysts that are more active and that exhibit high tolerance for polar functional groups must be an area of continued development to access readily-prepared fatty acid polymers. Existing high activity metathesis catalysts with good functional group tolerance have proven to be some of the more successful in this regard, as illustrated in some of the examples discussed herein. The use of radical routes and click chemistry, as illustrated by the thiol-ene and Sustain. Chem. 2020, 1 232 thiol-yne reactions herein, is another pathway to fatty acid polymers that holds great promise for ongoing development that does not require the development of new transition metal catalysts. In addition to the development of protocols and catalysts noted above, and by carefully considering the foregoing examples in light of the current need for more sustainable and affordable materials having mechanical strength profiles on par with current commercial goods, several specific areas can be identified for further development. First, many of the fatty acid-derived polymers that have been reported to date contain olefinic linkages in their backbones or as side chains. All of these polymers that can be affordably produced should be exposed to vulcanization under different conditions (amount of curative and vulcanization time) to assess the potential improvements in strength. This approach has the potential to yield high toughness materials that will almost certainly be competitive with synthetic rubber based on the early studies discussed here. A drawback to this strategy is that, like traditional tire rubber, the materials may be difficult to recycle at their initial end of service. More easily-recyclable materials obtained by the inverse vulcanization of the olefin-bearing polymers with varying percentage of sulfur should thus also be targeted in future work. Such high sulfur-content materials may be considerably more environmentally benign given initial work on their use as fertilizers [82]. Perhaps the most obvious drawback of the majority of materials discussed herein is that they by and large require an isolated, pure fatty acid for their preparation, with the exception of some of the preliminary work described for technical grade fatty acid mixtures. Clearly, ethical and practical aspects must be weighed when considering the ramification of using food-grade precursors to prepare polymers when there is also a growing challenge to feed a growing global population. Tremendous advances in both economy and ecology of fatty acid derived polymeric material production could be achieved if crude waste materials that are high in free fatty acid content but that are no longer high value food sources—waste cooking grease, brown grease, trap grease, and acid oil, for example—could be leveraged as precursors to materials having useful property profiles.

Author Contributions: Conceptualization, A.D.S.; resources, A.G.T.; writing—original draft preparation, A.D.S.; writing—review and editing, all authors. All authors have read and agreed to the published version of the manuscript. Funding: The authors would like to thank the NSF for funding this work (CHE-1708844) and a grant from ACREC. Acknowledgments: The authors would like to thank the NSF for funding this work (CHE-1708844). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. World Per-Capita Consumption of PE, PP & PVC Resins. 2014. Available online: https://www.plasticsinsight. com/world-per-capita-consumption-pe-pp-pvc-resins-2014/ (accessed on 7 April 2019). 2. Malik, N.; Kumar, P.; Shrivastava, S.; Ghosh, S.B. An overview on PET waste recycling for application in packaging. Int. J. Plast. Technol. 2016, 21, 1–24. [CrossRef] 3. Kaufmann, J. New materials for sports equipment made of anisotropic fiber-reinforced plastics with stiffness related coupling effect. Procedia Eng. 2015, 112, 140–145. [CrossRef] 4. McKeen, L.W. Plastics Used in Medical Devices. In Handbook of Polymer Applications in Medicine and Medical Devices; Elsevier: Amsterdam, The Netherlands, 2014; pp. 21–53. 5. Day, M.; Cooney, J.; Touchette-Barrette, C.; Sheehan, S. Pyrolysis of mixed plastics used in the electronics industry. J. Anal. Appl. Pyrolysis 1999, 52, 199–224. [CrossRef] 6. Jeong, J.; Choi, J. Adverse outcome pathways potentially related to hazard identification of microplastics based on toxicity mechanisms. Chemosphere 2019, 231, 249–255. [CrossRef][PubMed] 7. Cole, M.; Lindeque, P.; Halsband, C.; Galloway, T.S. Microplastics as contaminants in the marine environment: A review. Mar. Pollut. Bull. 2011, 62, 2588–2597. [CrossRef][PubMed] 8. Hidalgo-Ruz, V.; Gutow, L.; Thompson, R.C.; Thiel, M. Microplastics in the marine environment: A review of the methods used for identification and quantification. Environ. Sci. Technol. 2012, 46, 3060–3075. [CrossRef] [PubMed] Sustain. Chem. 2020, 1 233

9. Gestoso, I.; Cacabelos, E.; Ramalhosa, P.; Canning-Clode, J. Plasticrusts: A new potential threat in the Anthropocene’s rocky shores. Sci. Total Environ. 2019, 687, 413–415. [CrossRef] 10. Thiounn, T.; Smith, R.C. Advances and approaches for chemical recycling of plastic waste. J. Appl. Polym. Sci. 2020, 58, 1347–1364. [CrossRef] 11. Karunarathna, M.S.; Smith, R.C. Valorization of lignin as a sustainable component of structural materials and composites: Advances from 2011 to 2019. Sustainability 2020, 12, 734. [CrossRef] 12. Kristufek, S.L.; Wacker, K.T.; Tsao, Y.-Y.T.; Su, L.; Wooley, K.L. Monomer design strategies to create natural product-based polymer materials. Nat. Prod. Rep. 2017, 34, 433–459. [CrossRef] 13. Yao, K.; Tang, C. Controlled polymerization of next-generation renewable monomers and beyond. Macromolecules 2013, 46, 1689–1712. [CrossRef] 14. Leung, D.Y.; Wu, X.; Leung, M. A review on biodiesel production using catalyzed transesterification. Appl. Energy 2010, 87, 1083–1095. [CrossRef] 15. Ma, F.; Hanna, M.A. Biodiesel production: A review. Bioresour. Technol. 1999, 70, 1–15. [CrossRef] 16. Demirba¸s,A. Biodiesel fuels from vegetable oils via catalytic and non-catalytic supercritical alcohol transesterifications and other methods: A survey. Energy Convers. Manag. 2003, 44, 2093–2109. [CrossRef] 17. Ramadhas, A.S.; Jayaraj, S.; Muraleedharan, C. Biodiesel production from high FFA rubber seed oil. Fuel 2005, 84, 335–340. [CrossRef] 18. Canakci, M.; Van Gerpen, J. Biodiesel production from oils and fats with high free fatty acids. Trans. ASAE 2001, 44, 1429–1436. [CrossRef] 19. Mekonnen, T.; Mussone, P.; Bressler, D. Valorization of rendering industry wastes and co-products for industrial chemicals, materials and energy: Review. Crit. Rev. Biotechnol. 2016, 36, 120–131. [CrossRef] 20. Pastore, C.; Lopez, A.; Mascolo, G. Efficient conversion of brown grease produced by municipal wastewater treatment plant into biofuel using aluminium chloride hexahydrate under very mild conditions. Bioresour. Technol. 2014, 155, 91–97. [CrossRef] 21. Sim, Y.-L.; Meyappan, N.; Yen, N.S.; A/p, S.S.K.; Khoo, C.H.; Cheah, W.L.; Hilaire, D.S.; Pinnock, T.; Bacolod, B.; Cai, Z.B.; et al. Chemical reactions in the pyrolysis of brown grease. Fuel 2017, 207, 274–282. [CrossRef] 22. Ward, P.M.L. Brown and black grease suitability for incorporation into feeds and suitability for biofuels. J. Food Prot. 2012, 75, 731–737. [CrossRef] 23. Mol, J. Metathesis of unsaturated fatty acid esters and fatty oils. J. Mol. Catal. 1994, 90, 185–199. [CrossRef] 24. Biermann, U.; Friedt, W.; Lang, S.; Lühs, W.; Machmüller, G.; Metzger, J.O.; Klaas, M.R.G.; Schäfer, H.J.; Schneider, M.P. New syntheses with oils and fats as renewable raw materials for the chemical industry. Angew. Chem. Int. Ed. 2000, 39, 2206–2224. [CrossRef] 25. Kaminsky, W.; Fernandez, M. New polymers by copolymerization of olefins with bio oil components. Eur. J. Lipid Sci. Technol. 2008, 110, 841–845. [CrossRef] 26. Ronda, J.C.; Lligadas, G.; Galià, M.; Cádiz, V. Vegetable oils as platform chemicals for polymer synthesis. Eur. J. Lipid Sci. Technol. 2011, 113, 46–58. [CrossRef] 27. De Espinosa, L.M.; Meier, M.A.R. Plant oils: The perfect renewable resource for polymer science?! Eur. Polym. J. 2011, 47, 837–852. [CrossRef] 28. Lligadas, G.; Ronda, J.C.; Galià, M.; Cádiz, V. Monomers and polymers from plant oils via click chemistry reactions. J. Polym. Sci. Part A: Polym. Chem. 2013, 51, 2111–2124. [CrossRef] 29. Adekunle, K.F. A review of vegetable oil-based polymers: Synthesis and applications. Open J. Polym. Chem. 2015, 5, 34–40. [CrossRef] 30. Kunduru, K.R.; Basu, A.; Zada, M.H.; Domb, A. Castor oil-based biodegradable polyesters. Biomacromolecules 2015, 16, 2572–2587. [CrossRef] 31. Doll, K.M.; Moser, B.R.; Liu, Z.; Murray, R.E.; Sharma, B.K.; Biresaw, G. Producing monomers and polymers from plant oils. Environ. Frindly Biobased Lubr. 2016, 79–98. [CrossRef] 32. Jain, J.P.; Sokolsky-Papkov, M.; Kumar, N.; Domb, A. Fatty acid based biodegradable polymer. Polym. Rev. 2008, 48, 156–191. [CrossRef] 33. Maisonneuve, L.; Lebarbé, T.; Grau, E.; Cramail, H. Structure–properties relationship of fatty acid-based thermoplastics as synthetic polymer mimics. Polym. Chem. 2013, 4, 5472–5517. [CrossRef] 34. Yelchuri, V.; Srikanth, K.; Prasad, R.B.N.; Lakshmi, K.M.S. Olefin metathesis of fatty acids and vegetable oils. J. Chem. Sci. 2019, 131, 39. [CrossRef] Sustain. Chem. 2020, 1 234

35. Lomege, J.; Lapinte, V.; Negrell, C.; Robin, J.-J.; Caillol, S. Fatty acid-based radically polymerizable monomers: From novel poly(meth)acrylates to cutting-edge properties. Biomacromolecules 2018, 20, 4–26. [CrossRef] [PubMed] 36. Hoyle, C.E.; Bowman, C.N. Thiol-ene click chemistry. Angew. Chem. Int. Ed. 2010, 49, 1540–1573. [CrossRef] 37. Cramer, N.B.; Bowman, C.N. Thiol-ene chemistry. Chemosel. Bioorthogonal Lig. React. 2017, 1, 117–145. 38. Cramer, N.B.; Bowman, C.N. Thiol-ene and thiol-yne chemistry in ideal network synthesis. RSC Polym. Chem. Ser. 2013, 6, 1–27. 39. Türünç, O.; Meier, M.A.R. Fatty acid derived monomers and related polymers via thiol-ene (Click) additions. Macromol. Rapid Commun. 2010, 31, 1822–1826. [CrossRef] 40. González-Paz, R.J.; Lluch, C.; Lligadas, G.; Ronda, J.C.; Galià, M.; Cádiz, V. A green approach toward oleic- and undecylenic acid-derived polyurethanes. J. Polym. Sci. Part A: Polym. Chem. 2011, 49, 2407–2416. [CrossRef] 41. Desroches, M.; Caillol, S.; Auvergne, R.; Boutevin, B. Synthesis of pseudo-telechelic diols by transesterification and thiol-ene coupling. Eur. J. Lipid Sci. Technol. 2011, 114, 84–91. [CrossRef] 42. Unverferth, M.; Meier, M.A.R. Selective formation of C36-dimer fatty acids via thiol-ene addition for copolyamide synthesis. Eur. J. Lipid Sci. Technol. 2016, 118, 1470–1474. [CrossRef] 43. Pai, C.-C.; Jeng, R.-J.; Grossman, S.J.; Huang, J.-C. Effects of moisture on thermal and mechanical properties of nylon-6,6. Adv. Polym. Technol. 1989, 9, 157–163. [CrossRef] 44. Nguyen, P.H.; Spoljaric, S.; Seppälä, J. Renewable polyamides via thiol-ene ‘click’ chemistry and long-chain aliphatic segments. Polymer 2018, 153, 183–192. [CrossRef] 45. Kempe, K.; Hoogenboom, R.; Schubert, U.S. A green approach for the synthesis and thiol-ene modification of alkene functio1489lized poly(2-oxazoline)s. Macromol. Rapid Commun. 2011, 32, 1484–1489. [CrossRef] [PubMed] 46. Maiti, B.; Kumar, S.; De, P. Controlled RAFT synthesis of side-chain oleic acid containing polymers and their post-polymerization functionalization. RSC Adv. 2014, 4, 56415–56423. [CrossRef] 47. Levine, A.C.; Heberlig, G.W.; Nomura, C.T. Use of thiol-ene click chemistry to modify mechanical and thermal properties of polyhydroxyalkanoates (PHAs). Int. J. Boil. Macromol. 2016, 83, 358–365. [CrossRef] 48. Durand, P.-L.;Brège, A.; Chollet, G.; Grau, E.; Cramail, H. Simple and efficient approach toward photosensitive biobased aliphatic polycarbonate materials. ACS Macro Lett. 2018, 7, 250–254. [CrossRef] 49. Durand, P.-L.; Chollet, G.; Grau, E.; Cramail, H. Versatile cross-linked fatty acid-based polycarbonate networks obtained by thiol–ene coupling reaction. RSC Adv. 2019, 9, 145–150. [CrossRef] 50. Moser, B.R.; Doll, K.M.; Peterson, S.C. Renewable poly(Thioether-Ester)s from fatty acid derivatives via thiol-ene photopolymerization. J. Am. Oil Chem. Soc. 2019, 96, 825–837. [CrossRef] 51. Moser, B.R. Preparation and evaluation of multifunctional branched diesters as fuel property enhancers for biodiesel and petroleum diesel fuels. Energy Fuels 2014, 28, 3262–3270. [CrossRef] 52. Türünç, O.; Meier, M.A.R. Thiol-ene vs. ADMET: A complementary approach to fatty acid-based biodegradable polymers. Green Chem. 2011, 13, 314–320. [CrossRef] 53. Türünç, O.; De Espinosa, L.M.; Meier, M.A.R. Renewable polyethylene mimics derived from castor oil. Macromol. Rapid Commun. 2011, 32, 1357–1361. [CrossRef][PubMed] 54. Huang, L.-P.; Zhou, X.-P.; Cui, W.; Xie, X.; Tong, S.-Y. Toughening effect of maleic anhydride grafted linear low density polyethylene on linear low density polyethylene. J. Mater. Sci. 2008, 43, 4290–4296. [CrossRef] 55. Unverferth, M.; Meier, M.A.R. Tuning the polarity of ADMET derived star-shaped polymers via thiol-ene chemistry. Polymer 2014, 55, 5571–5575. [CrossRef] 56. Dannecker, P.-K.; Biermann, U.; Sink, A.; Bloesser, F.R.; Metzger, J.O.; Meier, M.A.R. Fatty acid–derived aliphatic long chain polyethers by a combination of catalytic ester reduction and ADMET or thiol-ene polymerization. Macromol. Chem. Phys. 2018, 220, 1800440. [CrossRef] 57. González-Paz, R.J.; Lligadas, G.; Ronda, J.C.; Galià, M.; Cádiz, V. Thiol–yne reaction of alkyne-derivatized fatty acids: Biobased polyols and cytocompatibility of derived polyurethanes. Polym. Chem. 2012, 3, 2471–2478. [CrossRef] 58. Beyazkilic, Z.; Lligadas, G.; Ronda, J.C.; Galià, M.; Cádiz, V. Vinylsulfide-containing polyesters and copolyesters from fatty acids: Thiol-yne monomer synthesis and thiol-ene functionalization. Macromol. Chem. Phys. 2014, 215, 2248–2259. [CrossRef] 59. Goodyear, C. Improvement in India—Rubber Fabrics. US3633A, 9 March 1844. Sustain. Chem. 2020, 1 235

60. Chung, W.J.; Griebel, J.J.; Kim, E.T.; Yoon, H.; Simmonds, A.G.; Ji, H.J.; Dirlam, P.T.; Glass, R.S.; Wie, J.J.; Nguyen, N.A.; et al. The use of elemental sulfur as an alternative feedstock for polymeric materials. Nat. Chem. 2013, 5, 518–524. [CrossRef] 61. Griebel, J.J.; Li, G.; Glass, R.S.; Char, K.; Pyun, J. Kilogram scale inverse vulcanization of elemental sulfur to prepare high capacity polymer electrodes for Li-S batteries. J. Polym. Sci. Part A Polym. Chem. 2014, 53, 173–177. [CrossRef] 62. Karunarathna, M.S.; Lauer, M.K.; Tennyson, A.G.; Smith, R.C. Copolymerization of an aryl halide and elemental sulfur as a route to high sulfur content materials. Polym. Chem. 2020, 11, 1621–1628. [CrossRef] 63. Thiounn, T.; Lauer, M.K.; Bedford, M.S.; Smith, R.C.; Tennyson, A.G. Thermally-healable network solids of sulfur-crosslinked poly(4-allyloxystyrene). RSC Adv. 2018, 8, 39074–39082. [CrossRef] 64. Lopez, C.V.; Maladeniya, C.P.; Smith, R.C. Lithium-sulfur batteries: Advances and trends. Electrochem 2020, 1, 16. [CrossRef] 65. Worthington, M.J.H.; Kucera, R.L.; Chalker, J.M. Green chemistry and polymers made from sulfur. Green Chem. 2017, 19, 2748–2761. [CrossRef] 66. Chalker, J.M.; Worthington, M.J.H.; Lundquist, N.A.; Esdaile, L.J. Synthesis and applications of polymers made by inverse vulcanization. Top. Curr. Chem. 2019, 377, 16. [CrossRef][PubMed] 67. Wadi, V.S.; Jena, K.K.; Khawaja, S.Z.; Ranagraj, V.M.; Alhassan, S.M. Preparation and processing of porous sulfur foams having low thermal conductivity. RSC Adv. 2019, 9, 4397–4403. [CrossRef] 68. Abraham, A.M.; Kumar, S.V.; Alhassan, S.M. Porous sulphur copolymer for gas-phase mercury removal and thermal insulation. Chem. Eng. J. 2018, 332, 1–7. [CrossRef] 69. Zhang, B.; Petcher, S.; Hasell, T. A ternary system for delayed curing inverse vulcanisation. Chem. Commun. 2019, 55, 10681–10684. [CrossRef] 70. Westerman, C.R.; Jenkins, C.L. Dynamic sulfur bonds initiate polymerization of vinyl and allyl ethers at mild temperatures. Macromolecules 2018, 51, 7233–7238. [CrossRef] 71. Zhang, B.; Gao, H.; Yan, P.; Petcher, S.; Hasell, T. Inverse vulcanization below the melting point of sulfur. Mater. Chem. Front. 2020, 4, 669–675. [CrossRef] 72. Wu, X.; Smith, J.A.; Petcher, S.; Zhang, B.; Parker, D.J.; Griﬃn, J.M.; Hasell, T. Catalytic inverse vulcanization. Nat. Commun. 2019, 10, 1–9. [CrossRef] 73. Tonkin, S.J.; Gibson, C.T.; Campbell, J.A.; Lewis, D.A.; Karton, A.; Hasell, T.; Chalker, J.M. Chemically induced repair, adhesion, and recycling of polymers made by inverse vulcanization. Chem. Sci. 2020, 11, 5537–5546. [CrossRef] 74. Lundquist, N.; Tikoalu, A.; Worthington, M.; Shapter, R.; Tonkin, S.; Stojcevski, F.; Mann, M.; Gibson, C.; Gascooke, J.; Karton, A.; et al. Reactive compression molding post-inverse vulcanization: A method to assemble, recycle, and repurpose sulfur polymers and composites. Chem. Eur. J. 2020, 26, 10035–10044. [CrossRef][PubMed] 75. Smith, J.A.; Green, S.J.; Petcher, S.; Parker, D.J.; Zhang, B.; Worthington, M.J.H.; Wu, X.; Kelly, C.A.; Baker, T.; Gibson, C.T.; et al. Crosslinker copolymerization for property control in inverse vulcanization. Chem. A Eur. J. 2019, 25, 10433–10440. [CrossRef][PubMed] 76. Thiounn, T.; Karunarathna, M.S.; Slann, L.M.; Lauer, M.K.; Smith, R.C. Sequential crosslinking for mechanical property development in high sulfur content composites. J. Poly. Sci. 2020. Ahead of print. [CrossRef] 77. Oishi, S.; Oi, K.; Kuwabara, J.; Omoda, R.; Aihara, Y.; Fukuda, T.; Takahashi, T.; Choi, J.-C.; Watanabe, M.; Kanbara, T. Synthesis and characterization of sulfur-based polymers from elemental sulfur and algae oil. ACS Appl. Polym. Mater. 2019, 1, 1195–1202. [CrossRef] 78. Parker, D.J.; Jones, H.A.; Petcher, S.; Cervini, L.; Griﬃn, J.M.; Akhtar, R.; Hasell, T. Low cost and renewable sulfur-polymers by inverse vulcanization, and their potential for mercury capture. J. Mater. Chem. A 2017, 5, 11682–11692. [CrossRef] 79. Gomez, I.; Leonet, O.; Blazquez, J.A.; Mecerreyes, D.; Blazquez, A. Inverse vulcanization of sulfur using natural dienes as sustainable materials for lithium-sulfur batteries. ChemSusChem 2016, 9, 3419–3425. [CrossRef] 80. Maladeniya, C.P.; Karunarathna, M.S.; Lauer, M.K.; Lopez, C.V.; Thiounn, T.; Smith, R.C. A role for terpenoid cyclization in the atom economical polymerization of terpenoids with sulfur to yield durable composites. Mater. Adv. 2020, 1, 1665–1674. [CrossRef] Sustain. Chem. 2020, 1 236

81. Tikoalu, A.D.; Lundquist, N.A.; Chalker, J.M. Mercury sorbents made by inverse vulcanization of sustainable triglycerides: The plant oil structure influences the rate of mercury removal from water. Adv. Sustain. Syst. 2020, 4, 1900111. [CrossRef] 82. Mann, M.; Kruger, J.E.; Andari, F.; McErlean, J.; Gascooke, J.R.; Smith, J.A.; Worthington, M.J.H.; McKinley, C.C.C.; Campbell, J.A.; Lewis, D.A.; et al. Sulfur polymer composites as controlled-release fertilizers. Org. Biomol. Chem. 2019, 17, 1929–1936. [CrossRef] 83. Worthington, M.J.H.; Shearer, C.J.; Esdaile, L.J.; Campbell, J.A.; Gibson, C.T.; Legg, S.K.; Yin, Y.; Lundquist, N.A.; Gascooke, J.R.; Albuquerque, I.S.; et al. Sustainable polysulfides for oil spill remediation: Repurposing industrial waste for environmental benefit. Adv. Sustain. Syst. 2018, 2, 1800024. [CrossRef] 84. Lundquist, N.A.; Worthington, M.J.H.; Adamson, N.; Gibson, C.T.; Johnston, A.M.R.; Ellis, A.V.; Chalker, J.M. Polysulfides made from re-purposed waste are sustainable materials for removing iron from water. RSC Adv. 2018, 8, 1232–1236. [CrossRef] 85. Esdaile, L.J.; Chalker, J.M. The mercury problem in artisanal and small-scale gold mining. Chem. A Eur. J. 2018, 24, 6905–6916. [CrossRef] 86. Worthington, M.J.H.; Kucera, R.L.; Albuquerque, I.S.; Gibson, C.T.; Sibley, A.; Slattery, A.; Campbell, J.A.; Alboaiji, S.F.K.; Muller, K.A.; Young, J.; et al. Laying waste to mercury: Inexpensive sorbents made from sulfur and recycled cooking oils. Chem. A Eur. J. 2017, 23, 16219–16230. [CrossRef][PubMed] 87. Hoefling, A.; Lee, Y.J.; Theato, P. Sulfur-based polymer composites from vegetable oils and elemental sulfur: A sustainable active material for Li-S batteries. Macromol. Chem. Phys. 2016, 218, 1600303. [CrossRef] 88. Lopez, C.V.; Karunarathna, M.S.; Lauer, M.K.; Maladeniya, C.P.; Thiounn, T.; Ackley, E.D.; Smith, R.C. High strength, acid-resistant composites from canola, sunflower, or linseed oils: Influence of triglyceride unsaturation on material properties. J. Appl. Polym. Sci. 2020, 58, 2259–2266. [CrossRef] 89. Duarte, M.E.; Huber, B.; Theato, P.; Mutlu, H. The unrevealed potential of elemental sulfur for the synthesis of high sulfur content bio-based aliphatic polyesters. Polym. Chem. 2020, 11, 241–248. [CrossRef] 90. Smith, A.D.; Smith, R.C.; Tennyson, A.G. Carbon-negative polymer cements by copolymerization of waste sulfur, oleic acid, and pozzolan cements. Sust. Chem. Pharm. 2020, 16, 100249. 91. Smith, A.D.; McMillen, C.D.; Smith, R.C.; Tennyson, A.G. Copolymers by inverse vulcanization of sulfur with pure or technical-grade unsaturated fatty acids. J. Appl. Polym. Sci. 2020, 58, 438–445. [CrossRef] 92. Smith, A.D.; Thiounn, T.; Lyles, E.W.; Kibler, E.K.; Smith, R.C.; Tennyson, A.G. Combining agriculture and energy industry waste products to yield recyclable, thermally healable copolymers of elemental sulfur and oleic acid. J. Polym. Sci. Part A: Polym. Chem. 2019, 57, 1704–1710. [CrossRef] 93. Hasell, T.; Yan, P.; Zhao, W.; Zhang, B.; Petcher, S.; Smith Jessica, A.; Parker Douglas, J.; Cooper Andrew, I.; Jiang, L.; Lei, J. Inverse vulcanized polymers with shape memory, enhanced mechanical properties, and vitrimer behavior. Angew. Chem. 2020, 59, 2–10. 94. Thiounn, T.; Tennyson, A.G.; Smith, R.C. Durable, acid-resistant copolymers from industrial by-product sulfur and microbially-produced tyrosine. RSC Adv. 2019, 9, 31460–31465. [CrossRef] 95. Hoefling, A.; Nguyen, D.T.; Lee, Y.J.; Song, S.-W.; Theato, P. A sulfur–eugenol allyl ether copolymer: A material synthesized via inverse vulcanization from renewable resources and its application in Li–S batteries. Mater. Chem. Front. 2017, 1, 1818–1822. [CrossRef] 96. Lauer, M.K.; Estrada-Mendoza, T.A.; McMillen, C.D.; Chumanov, G.; Tennyson, A.G.; Smith, R.C. Durable cellulose–sulfur composites derived from agricultural and petrochemical waste. Adv. Sustain. Syst. 2019, 3, 1900062. [CrossRef] 97. Karunarathna, M.S.; Tennyson, A.G.; Smith, R.C. Facile new approach to high sulfur-content materials and preparation of sulfur–lignin copolymers. J. Mater. Chem. A 2020, 8, 548–553. [CrossRef] 98. Karunarathna, M.S.; Lauer, M.K.; Thiounn, T.; Smith, R.C.; Tennyson, A.G. Valorization of waste to yield recyclable composites of elemental sulfur and lignin. J. Mater. Chem. A 2019, 7, 15683–15690. [CrossRef] 99. Lauer, M.K.; Karunarathna, M.S.; Tennyson, A.G.; Smith, R.C. Recyclable, sustainable, and stronger than portland cement: A composite from unseparated biomass and fossil fuel waste. Mater. Adv. 2020, 1, 590–594. [CrossRef] 100. Lauer, M.K.; Tennyson, A.G.; Smith, R.C. Green synthesis of thermoplastic composites from a terpenoid-cellulose ester. ACS Appl. Polym. Mater. 2020, 2, 3761–3765. [CrossRef] 101. Dworakowska, S.; Le Coz, C.; Chollet, G.; Grau, E.; Cramail, H. Cross-linking of polyesters based on fatty acids. Eur. J. Lipid Sci. Technol. 2019, 121, 1900264. [CrossRef] Sustain. Chem. 2020, 1 237

102. Ebata, H.; Yasuda, M.; Toshima, K.; Matsumura, S. Poly (ricinoleic acid) based novel thermosetting elastomer. J. Oleo Sci. 2008, 57, 315–320. [CrossRef] 103. Hernández, M.; Grande, A.M.; Dierkes, W.; Bijleveld, J.; Van Der Zwaag, S.; Garcia, S. Turning vulcanized natural rubber into a self-healing polymer: Effect of the disulfide/polysulfide ratio. ACS Sustain. Chem. Eng. 2016, 4, 5776–5784. [CrossRef] 104. Xiang, H.P.; Rong, M.Z.; Zhang, M.Q. Self-healing, reshaping, and recycling of vulcanized chloroprene rubber: A case study of multitask cyclic utilization of cross-linked polymer. ACS Sustain. Chem. Eng. 2016, 4, 2715–2724. [CrossRef] 105. Gordon, M.B.; French, J.M.; Wagner, N.J.; Kloxin, C.J. Dynamic bonds in covalently crosslinked polymer networks for photoactivated strengthening and healing. Adv. Mater. 2015, 27, 8007–8010. [CrossRef] [PubMed] 106. Gwon, S.-W.; Ahn, E.; Shin, M. Self-healing of modified sulfur composites with calcium sulfoaluminate cement and superabsorbent polymer. Compos. Part B Eng. 2019, 162, 469–483. [CrossRef] 107. Azcune, I.; Odriozola, I. Aromatic disulfide crosslinks in polymer systems: Self-healing, reprocessability, recyclability and more. Eur. Polym. J. 2016, 84, 147–160. [CrossRef] 108. López, M.D.M.C.; Pernas, A.A.; Latorre, A.L.; López-Vilariño, J.; González-Rodríguez, M.V. Assessing changes on poly(ethylene terephthalate) properties after recycling: Mechanical recycling in laboratory versus postconsumer recycled material. Mater. Chem. Phys. 2014, 147, 884–894. [CrossRef] 109. Rao, M.R.; Radhakrishnan, T.S. Thermal degradation of liquid polysulfide polymers: Pyrolysis–GC–MS and thermogravimetric studies. J. Appl. Polym. Sci. 1985, 30, 855–873. [CrossRef] 110. Sundarrajan, S.; Surianarayanan, M.; Srinivasan, K.S.V.; Kishore, K. Thermal degradation processes in polysulfide copolymers investigated by direct pyrolysis mass spectrometry and flash pyrolysis-gas chromatography/mass spectrometry. Macromolecules 2002, 35, 3331–3337. [CrossRef] 111. Zhang, X.; Tang, Y.; Qu, S.; Da, J.; Hao, Z. H2S-selective catalytic oxidation: Catalysts and processes. ACS Catal. 2015, 5, 1053–1067. [CrossRef] 112. Demirbas, A.; Alidrisi, H.; Balubaid, M.A. API gravity, sulfur content, and desulfurization of crude oil. Pet. Sci. Technol. 2014, 33, 93–101. [CrossRef] 113. Yusupova, A.; Khatsrinov, A.I.; Akhmetova, R.T. Activating effect of aluminum chloride in the preparation of sulfur concrete from sulfur and silica. Inorg. Mater. 2018, 54, 809–814. [CrossRef] 114. Mohamed, A.-M.O.; Gamal, M.E. Sulfur Concrete for the Construction Industry; J. Ross Publishing: Fort Lauderdale, FL, USA, 2010; p. 424. 115. Abdel-Jawad, Y.; Al-Qudah, M. The combined effect of water and temperature on the strength of sulfur concrete. Cem. Concr. Res. 1994, 24, 165–175. [CrossRef] 116. Pickard, S.S. Sulfur concrete for acid resistance. Chem. Eng. 1985, 92, 80. 117. Platou, J.S. Corrosion resistant sulfur concrete—A new construction material for the chemical industry. CeerChem. Econ. Eng. Rev. 1984, 16, 19–22. 118. Gregor, R.; Hackl, A. A New Approach to Sulfur Concrete. In New Uses of Sulfur—II; American Chemical Society: Washington, DC, USA, 1978; pp. 54–78. [CrossRef] 119. Meltzer, R.; Lieb, K.; Horstman, R.; Moore, I.; Shrive, N.; Gillott, J.; Jordaan, I.; Loov, R. A study of durability in temperature cycles and water resistance of sulfur concretes and mortars. J. Test. Eval. 1977, 5, 484–493. [CrossRef]

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