Fullerene Polymers: a Brief Review

Journal of C Carbon Research

Review Fullerene Polymers: A Brief Review

Peter J. F. Harris

Electron Microscopy Laboratory and Department of Chemistry, J.J. Thomson Building, University of Reading, Whiteknights, Reading RG6 6AF, UK; [email protected]

Received: 18 September 2020; Accepted: 3 November 2020; Published: 5 November 2020

Abstract: This paper reviews the ways in which C60 and other fullerene molecules can be incorporated into polymeric structures. Firstly, polymers in which the fullerenes are incorporated into the structure by covalent or noncovalent bonding are discussed. These include “pearl necklace” structures, “charm bracelet” structures, organometallic polymers, crosslinked polymers, end-capped polymers, star-shaped polymers and supramolecular polymers. Secondly, all-carbon polymers, which are produced by fusing fullerenes together, are covered. The synthesis and properties of each class of fullerene polymer are outlined and the prospects for commercial applications considered.

Keywords: fullerenes; C60; polymers

1. Introduction Fullerenes and their derivatives have many remarkable chemical, electronic and optical properties, but to date, few of these properties have been exploited in commercial products. One reason is that C60 and the other fullerenes do not easily lend themselves to processing. The pure fullerenes are insoluble in water, which is a problem when it comes to potential applications in medicine and other areas. Although water-soluble fullerenes can be produced by functionalisation with groups such as –COOH and –OH, the dissolved fullerenes have a tendency to clump together rather than remain as separate molecules. Moreover, in order to solubilise the fullerenes, it is often necessary to add more than one functional group. Not only can this change the chemical and physical properties of the carbon molecules, but the derivatised fullerenes often contain a mixture of regioisomers, and these are difficult to separate. An alternative approach which avoids some of these problems is to incorporate fullerenes into polymers. In many cases, a single attachment between the fullerene and the polymer is sufficient to create solubility in water. As well as making fullerenes soluble, incorporating them into polymers generally enhances the processibility of these extraordinary molecules, thus facilitating their application in a wide variety of areas. Work began on the preparation of fullerene polymers shortly after the bulk synthesis of C60 in 1990 [1]. Since that time, the study of these polymers has grown into a significant sub-discipline of fullerene science [2–6]. Many different kinds of polymer have been synthesised, including the so-called “pearl necklace”, or main-chain, structures, in which the fullerene molecules are joined together with short bridging groups, as shown in Figure1a, and the “charm bracelet”, or side-chain, polymers, where the fullerenes dangle from the backbone of existing polymers, as in Figure1b. Other structures in which fullerenes are linked to existing polymers include random crosslinked and end-capped ones. Organometallic C60 polymers, which can be one-, two- or three-dimensional, have been quite widely researched, while star-shaped polymers, in which a single fullerene molecule is joined to a number of polymer chains, have also been studied. Supramolecular polymers, where the fullerenes are incorporated into to the structure by weak intermolecular interactions rather than covalent bonds, have been prepared, and some have been shown to be soluble in water.

C 2020, 6, 71; doi:10.3390/c6040071 www.mdpi.com/journal/carbon CC2020 2020C, ,62020 6,, 71 x, FOR6, x FOR PEER PEER REVIEW REVIEW 2 of 112 ofof 1111

Figure 1. Schematic illustration of (a) pearl necklace polymer, (b) charm bracelet polymer. Adapted from [7]. FigureFigure 1.1.Schematic Schematic illustrationillustration ofof ((a)) pearl necklace polymer, (b) charm braceletbracelet polymer.polymer. AdaptedAdapted from [7]. fromA [7]. quite separate class of fullerene polymers is produced by fusing fullerenes together to form all-carbonA quite separate polymers. class These of structures fullerene can polymers have intere is producedsting electrical by fusing properties fullerenes and they together can display to form A quite separate class of fullerene polymers is produced by fusing fullerenes together to form all-carbonextreme polymers. hardness. TheseThe aim structures of the current can have article interesting is to give a electrical brief overview properties of the and various they forms can display of all-carbon polymers. These structures can have interesting electrical properties and they can display extremefullerene hardness. polymer, The with aim references of the current up to 2020. article is to give a brief overview of the various forms of extreme hardness. The aim of the current article is to give a brief overview of the various forms of fullerene polymer, with references up to 2020. fullerene2. Pearl polymer, Necklace with Polymers references up to 2020.

2. Pearl NecklaceAn assembly Polymers consisting of two linked C60 molecules was synthesised by Wudl et al. in 1992 [8]. 2. PearlThis Necklacecould be considered Polymers the first step towards a pearl necklace polymer. The synthesis involved An assembly consisting of two linked C60 molecules was synthesised by Wudl et al. in 1992 [8]. reacting C60 with the diazomethanes derived from m-phenylenebis(benzoyl) and p- This couldAn assembly be considered consisting the ﬁrst of steptwo towardslinked C a60 pearl molecules necklace was polymer. synthesised The synthesis by Wudl involved et al. in reacting1992 [8]. Thisphenylenebis(benzoyl). could be considered Earlythe first attempts step totowards prepare a extended pearl necklace pearl necklace polymer. polymers The synthesis often resulted involved C60 within intractable, the diazomethanes insoluble mixtures, derived e.g., from [9]. m-phenylenebis(benzoyl)In 2000, Geckeler et al. reported and the p-phenylenebis(benzoyl). synthesis of the first reacting C60 with the diazomethanes derived from m-phenylenebis(benzoyl) and p- Earlywater-soluble attempts toC60 prepare main-chain extended polymer pearl [10]. necklaceHere, the polymerslinking groups often were resulted shielded in intractable,inside phenylenebis(benzoyl). Early attempts to prepare extended pearl necklace polymers often resulted insolublecyclodextrin mixtures, cavities e.g., with [9]. Inthe 2000, fullerene, Geckeler as shown et al. in reported Figure 2. the They synthesis showed ofthat the the ﬁrst fullerene water-soluble retained C60 main-chainin intractable,its ability polymer to insoluble cleave [ 10DNA ].mixtures, Here, nucleotides the e.g., linking via[9]. a In groupssingle 2000,t oxygen Geckeler were shielded transfer et al. at insidereported the guanosine cyclodextrin the synthesis base cavitiessequence of the with first thewater-soluble fullerene,when attached as C shown60 to main-chainthis in polymer. Figure 2 polymer. They showed [10]. thatHere, the the fullerene linking retained groups its abilitywere shielded to cleave DNAinside nucleotidescyclodextrin via cavities a singlet with oxygen the fullerene, transfer atas theshown guanosine in Figure base 2. sequenceThey showed when that attached the fullerene to thispolymer. retained its ability to cleave DNA nucleotides via a singlet oxygen transfer at the guanosine base sequence when attached to this polymer.

FigureFigure 2. Water-soluble 2. Water-soluble main-chain main-chain fullerene fullerene polymerpolymer with supramolecular supramolecular cyclod cyclodextrinextrin shielding shielding of of linkinglinking groups groups [10 ].[10]. Main-chain C polymers have also been considered as components of photovoltaic devices. Main-chain60 C60 polymers have also been considered as components of photovoltaic devices. The The fullerene derivative most frequently employed in such devices is phenyl-C -butyric acid fullereneFigure 2. derivative Water-soluble most main-chain frequently emfullereneployed polymer in such deviceswith supramolecular is phenyl-C61-butyric cyclodextrin acid methyl 61shielding ester of methyl ester (PCBM), but Hiorns and colleagues suggested that copolymers which incorporate (PCBM),linking groups but Hiorns [10]. and colleagues suggested that copolymers which incorporate fullerenes as fullerenesalternating as alternatingrepeated moieties repeated in a conjugated moieties inpolymer a conjugated structure might polymer have structure advantages might in this have application [11–13]. The idea was that nanodomains of well-organised copolymers parallel to the advantagesMain-chain in this C60 polymers application have [11 also–13 ].been The considered idea was as components that nanodomains of photovoltaic of well-organised devices. The electrodes might enhance interface interactions and increase current flow. This group prepared bis- copolymersfullerene derivative parallel most to the frequently electrodes em mightployed enhancein such devices interface is phenyl-C interactions61-butyric and increaseacid methyl current ester benzyl C60 polymers through atom transfer radical addition (ATRA) of 1,4-bis(bromomethyl)benzene flow.(PCBM), This but group Hiorns prepared and bis-benzylcolleagues C 60suggestedpolymers that through copolymers atom transfer which radical incorporate addition fullerenes (ATRA) ofas 1,4-bis(bromomethyl)benzeneintermediate to the fullerene. intermediateA solar cell containing to the fullerene. such a polymer A solar was cell shown containing to have an such efficiency a polymer alternatingof 4.2%. repeated moieties in a conjugated polymer structure might have advantages in this was shown to have an efficiency of 4.2%. application [11–13]. The idea was that nanodomains of well-organised copolymers parallel to the electrodes might enhance interface interactions and increase current flow. This group prepared bis- 3. Charm Bracelet Polymers benzyl C60 polymers through atom transfer radical addition (ATRA) of 1,4-bis(bromomethyl)benzene intermediateIt appears tothat the fullerene. the first charm A solar bracelet cell containing C60 polymer such a was polymer prepared was byshown Olah to et have al. inan 1991efficiency [14]. Thisof 4.2%. group showed that a C60/C70 mixture reacted with polystyrene under AlCl3 catalysis in CS2

C 2020, 6, x FOR PEER REVIEW 3 of 11

3. Charm Bracelet Polymers

It appears that the first charm bracelet C60 polymer was prepared by Olah et al. in 1991 [14]. This groupC 2020, 6showed, 71 that a C60/C70 mixture reacted with polystyrene under AlCl3 catalysis in CS2 to produce3 of 11 a highly crosslinked fullerenated polystyrene. A short time later, Wudl and colleagues attached C60 to polyester and polyurethane and showed that the fullerene molecules retained their electrochemical to produce a highly crosslinked fullerenated polystyrene. A short time later, Wudl and colleagues and non-linear optical properties when joined to the polymers [15]. Since this early work, charm attached C60 to polyester and polyurethane and showed that the fullerene molecules retained their bracelet, or side-chain, structures have become the most widely studied form of C60 polymers. Most electrochemical and non-linear optical properties when joined to the polymers [15]. Since this of the common polymers have been functionalised with C60 in this way, including aminopolymers early work, charm bracelet, or side-chain, structures have become the most widely studied form [16], polycarbonates [17], polysaccharides [18], polysiloxanes [19] and polyesters [20]. A popular of C60 polymers. Most of the common polymers have been functionalised with C60 in this way, method for attaching C60 to polymers has involved cycloaddition to azido groups on the polymer including aminopolymers [16], polycarbonates [17], polysaccharides [18], polysiloxanes [19] and chains. Hawker prepared polystyrene- C60 polymers using this method in 1994 and demonstrated polyesters [20]. A popular method for attaching C to polymers has involved cycloaddition to azido that the copolymer retained both the electronic pr60 operties of the fullerene and the solubility and groups on the polymer chains. Hawker prepared polystyrene- C60 polymers using this method in processability of the polystyrene [21]. Other routes to charm bracelet C60 polymers include 1994 and demonstrated that the copolymer retained both the electronic properties of the fullerene and nucleophilic addition [15] and ATRA [22]. the solubility and processability of the polystyrene [21]. Other routes to charm bracelet C polymers As well as attaching fullerenes to pre-existing polymers, charm bracelet polymers have60 also been include nucleophilic addition [15] and ATRA [22]. prepared by catalysed polymerisation of fullerene-bearing monomers. One method which has As well as attaching fullerenes to pre-existing polymers, charm bracelet polymers have also been become quite popular was introduced by Wudl and co-workers in 1995 [23]. This involved prepared by catalysed polymerisation of fullerene-bearing monomers. One method which has become polymerising norbornene-modified C60, in the presence of an excess of norbornene and a catalyst, quite popular was introduced by Wudl and co-workers in 1995 [23]. This involved polymerising using ring-opening metathesis copolymerisation (ROMP), as shown in Figure 3. Other groups have norbornene-modiﬁed C , in the presence of an excess of norbornene and a catalyst, using ring-opening subsequently used the technique60 [24,25]. metathesis copolymerisation (ROMP), as shown in Figure3. Other groups have subsequently used the technique [24,25].

Figure 3.3.Polymerisation Polymerisation of norbornene-modified of norbornene-modified C60 using ring-opening C60 using metathesis ring-opening polymerisation metathesis [4,23]. polymerisation [4,23]. A number of charm bracelet C60 polymers with water solubility have now been prepared.

In 2006,A number Iwamoto of charm and Yamakoshi bracelet C synthesised60 polymers awith C60 -N-vinylpyrrolidonewater solubility have (NVP) now been copolymer prepared. using In 2006,radical Iwamoto polymerisation and Yamakoshi [26]. This synthesised was found a C to60-N-vinylpyrrolidone have one of the highest (NVP) water-solubilities copolymer using (7.8 radical mM) polymerisationof any fullerene [26]. derivative. This was found In 2014, to have the sameone of authors, the highest with water-solubilities colleagues, prepared (7.8 mM) a solubleof any fullereneC60-polyvinylpyrrolidone derivative. In 2014 (PVP), the copolymer same authors, and investigated with colleagues, its ability toprepared generate a photoinducedsoluble C60- polyvinylpyrrolidonereactive oxygen species (PVP) [27]. copolymer A Russian groupand investigated prepared fullerene-containing its ability to generate copolymers photoinduced using reactive radical oxygenpolymerisation species in [27]. 2017 A [28 Russian]. They suggestedgroup prepared that the copolymersfullerene-containing might ﬁnd copolymers applications using as photo- radical and polymerisationelectroactive materials. in 2017 [28]. They suggested that the copolymers might find applications as photo- and electroactiveIt was noted abovematerials. that main-chain C60 polymers with potential use in solar cells have been prepared. Side-chainIt was polymersnoted above have that also main-chain been prepared C60 withpolymers this application with potential in mind. use in Block solar co-polymers cells have been have prepared.often been Side-chain favoured, owingpolymers to their have tendency also been to self-assembleprepared with into this ordered application structures. in mind. In 2005, Block Drees, co- polymersSariçiftçi andhave colleagues often been described favoured, the owing synthesis to their of tendency the fullerene to self-assemble derivative, C into61-butyric ordered acid structures. glycidol Inester 2005, (PCBG), Drees, which Sariçiftçi they and then colleagues polymerised described [29]. Introducing the synthesis this of into the the fullerene poly(3-hexylthiophene) derivative, C61-butyric (P3HT) acidactive glycidol layer resulted ester (PCBG), in a network which of fullerenethey then molecules polymerised dispersed [29]. throughoutIntroducing the this bulk, into thus the avoiding poly(3- hexylthiophene)the issues with aggregation (P3HT) active that can layer result resulted when individual in a network molecules of offullerene phenyl-C molecules61-butyric aciddispersed methyl throughoutester (PCBM) the are bulk, used inthus such avoiding cells. The the device issues produced with aggregation in this wayhad that an can energy result conversion when individual efficiency moleculesof 2%. Other of phenyl-C groups have61-butyric prepared acid methyl side-chain ester C(PCBM)60 polymers are used for in photovoltaic such cells. The devices, device e.g., produced [30,31]. inPolymerisable this way had C 70anderivatives energy conversion have also efficiency been considered of 2%. forOther this groups application have [ 32prepared]. side-chain C60

4. Organometallic Polymers

Organometallic C60 polymers have attracted signiﬁcant interest. The ﬁrst example of such a polymer was a pearl necklace structure synthesised in 1992 by Nagashima and colleagues [33]. By mixing C 2020, 6, x FOR PEER REVIEW 4 of 11 polymers for photovoltaic devices, e.g., [30,31]. Polymerisable C70 derivatives have also been considered for this application [32].

4. Organometallic Polymers

Organometallic C60 polymers have attracted significant interest. The first example of such a

Cpolymer2020, 6, 71 was a pearl necklace structure synthesised in 1992 by Nagashima and colleagues [33].4 of By 11 mixing C60 with Pd2(dba)3, where dba is dibenzylideneacetone), with 1:1 stoichiometry, these workers obtained a linear C60Pd polymer. They also showed that changing the Pd:C60 ratio or heating the one- Cdimensional60 with Pd2(dba) polymer3, where could dba produce is dibenzylideneacetone), three-dimensional with polymers. 1:1 stoichiometry, Since th theseis early workers work, obtained many afurther linear Cpolymers60Pd polymer. with Theyfullerenes also showed covalently that changingbonded to the palladium, Pd:C60 ratio platinum or heating and the one-dimensionalother transition polymermetals have could been produce synthesised, three-dimensional e.g., [34–36]. polymers. The polymers Since can this be early one-, work, two- many or three-dimensional. further polymers withFigure fullerenes 4, from work covalently by Leng bonded et al. [36], to palladium,shows a linear platinum organometallic and other polymer. transition A theoretical metals have analysis been synthesised,of the interactions e.g., [ 34between–36]. The Pd polymersand fullerenes can be in one-, polymers two- orhas three-dimensional. been given by Goclon Figure and4, fromcolleagues, work bywho Leng found et al.that [36 the], shows most stable a linear polymer organometallic structure polymer. is one in A which theoretical the C60 analysiss are bonded of the via interactions the [6,6] betweenposition Pdof andthe fullerenesfullerene in molecules polymers haswith been Pd given in bya Goclondistorted and tetrahedral colleagues, whocoordination found that [37]. the mostOrganometallic stable polymer C60 polymers structure ismay one have in which potential the C 60applicationss are bonded in via catalysis the [6, 6[34,38],] position electrochemistry, of the fullerene moleculese.g., [7,35], with and Pdother in aareas. distorted tetrahedral coordination [37]. Organometallic C60 polymers may have potential applications in catalysis [34,38], electrochemistry, e.g., [7,35], and other areas.

Figure 4. One-dimensional Ru–C60 polymer. Ru atoms are in grey [36]. Figure 4. One-dimensional Ru–C60 polymer. Ru atoms are in grey [36]. 5. Crosslinked Polymers 5. Crosslinked Polymers Many of the early attempts to prepare extended pearl necklace and charm bracelet polymers resultedMany in of intractable, the early crosslinkedattempts to mixtures,prepare exten e.g.,ded [9]. pearl However, necklace a few and groups charm havebracelet succeeded polymers in introducingresulted in intractable, some control crosslinked into the synthesis mixtures, of e.g. crosslinked, [9]. However, polymers a few in order groups to preparehave succeeded potentially in usefulintroducing products. some A control schematic into illustration the synthesis of a of crosslinked crosslinked C 60polymerspolymer in is order shown to in prepare Figure5 potentiallya. In 2016, Sugawarauseful products. et al. A described schematic the illustration preparation of a of crosslinked crosslinked C60 polymer–fullerene polymer is shown networks in Figure by5a. reactingIn 2016, aSugawara tetrazole-functionalised et al. described the polymer preparation with C of60 crosunderslinked UV irradiationpolymer–fullerene [39]. They networks suggested by reacting that the a networkstetrazole-functionalised may have applications polymer in with bulk heterojunctionC60 under UV solar irradiation cells. Chen [39]. and They colleagues suggested synthesised that the a crosslinkednetworks may composite have applications by reacting in C bulk60 with heterojunc an aromatiction solar poly-amide cells. Chen with and pendent colleagues furan synthesised groups [40]. Theya crosslinked demonstrated composite improved by reacting high-temperature C60 with an aromatic shape memory poly-amide behaviour with ofpendent the composite furan groups compared [40]. toThey the puredemonstrated polymer. Asimproved a ﬁnal example,high-temperature Zhou et al. shape incorporated memory C 60behaviourinto the conducting of the composite polymer Cpoly(3,4-ethylenedioxythiophene)compared 2020, 6, x FOR to PEERthe pure REVIEW polymer. As (PEDOT), a final example, with the Zhou aim of et producing al. incorporated photoresponsive C60 into the thermoelectric conducting5 of 11 materialspolymer poly(3,4-ethylenedioxythiophene) [41]. (PEDOT), with the aim of producing photoresponsive thermoelectric materials [41].

Figure 5. Schematic illustrations of (a) crosslinked C60 polymer, (b) end-capped polymers, (c) star-shaped polymer [2]. Figure 5. Schematic illustrations of (a) crosslinked C60 polymer, (b) end-capped polymers, (c) star- 6. End-Cappedshaped polymer Polymers [2]. End-capped polymers are polymers with a fullerene bonded to one or both ends (see Figure5b). 6. End-Capped Polymers An advantage here is that the exact number of fullerene molecules attached to the polymer is known: it is eitherEnd-capped one or two,polymers as shown are polymers in Figure with5b, depending a fullerene on bonded the synthesis to one or method. both ends (see Figure 5b). An advantage here is that the exact number of fullerene molecules attached to the polymer is known: it is either one or two, as shown in Figure 5b, depending on the synthesis method. In 2010, Karsten and co-workers prepared diketopyrrolopyrrole-based small band gap oligomers, end-capped at both ends with C60 [42]. Upon photoexcitation of the oligomer, ultrafast energy transfer to the fullerene occurs, followed by an electron transfer reaction, suggesting that the assembly could be a useful component of photovoltaic devices. The synthesis of poly(ethylene glycol) end-capped with C60 was described by Jung et al. [43]. The polymer was added to the P3HT:PCBM active layer of a solar cell, resulting in enhanced performance and air stability of the cell. Hiorns and colleagues have also explored the application of end-capped fullerene polymers in solar cells [44]. In 2017, Topham and colleagues described a method of linking chains of poly(3,4-dimethoxystyrene), generated by reversible-addition fragmentation chain transfer (RAFT), polymerisation, to single C60 molecules [45]. Their relatively simple approach should be applicable to the preparation of a wide range of different fullerene end-capped polymers.

7. Star-Shaped Polymers

The so-called star-shaped polymers usually consist of a single molecule C60 joined to a number of polymer chains, as shown in Figure 5c [46]. The first such structures were prepared by Samulski et al. in 1992 [47], who attached between 1 and 10 linear polystyrene (PS) chains to C60 to produce what they called “flagellenes”. Their method involved reacting a living PS carbanion with C60. Ederlé and Mathis used a similar method in 1997 to produce polystyrene, polyisoprene and other C60 polymer stars [48]. In 2007, Xu and colleagues synthesised C60-cored star polyfluorenes by modifying the bromo end-groups of C60 hexakisadducts using Suzuki polycondensation [49]. The resulting star- shaped polymers might be used as blue light-emitting materials. Another potential application for fullerene-containing star-shaped polymers might be as components of membranes for gas or water purification [50–52].

8. Supramolecular Polymers In supramolecular polymers, the fullerenes are incorporated into the polymer by weak intermolecular interactions rather than covalent bonds. This has been used as a route to producing water-soluble fullerene materials. For example, in 2004, Liu and co-workers prepared the polymer shown in Figure 6, where C60s are held within dimeric cyclodextrin units with coordinated metal centres [53]. Assemblies of 60–80 units were prepared and imaged by scanning tunnelling spectroscopy and transmission electron microscopy. The polymer was water-soluble and displayed DNA-cleavage ability under light irradiation. As with the work by Geckeler et al. mentioned above, the DNA-cleavage involves the generation of excited singlet molecular oxygen by the fullerene. C 2020, 6, 71 5 of 11

In 2010, Karsten and co-workers prepared diketopyrrolopyrrole-based small band gap oligomers, end-capped at both ends with C60 [42]. Upon photoexcitation of the oligomer, ultrafast energy transfer to the fullerene occurs, followed by an electron transfer reaction, suggesting that the assembly could be a useful component of photovoltaic devices. The synthesis of poly(ethylene glycol) end-capped with C60 was described by Jung et al. [43]. The polymer was added to the P3HT:PCBM active layer of a solar cell, resulting in enhanced performance and air stability of the cell. Hiorns and colleagues have also explored the application of end-capped fullerene polymers in solar cells [44]. In 2017, Topham and colleagues described a method of linking chains of poly(3,4-dimethoxystyrene), generated by reversible-addition fragmentation chain transfer (RAFT), polymerisation, to single C60 molecules [45]. Their relatively simple approach should be applicable to the preparation of a wide range of diﬀerent fullerene end-capped polymers.

7. Star-Shaped Polymers

The so-called star-shaped polymers usually consist of a single molecule C60 joined to a number of polymer chains, as shown in Figure5c [ 46]. The first such structures were prepared by Samulski et al. in 1992 [47], who attached between 1 and 10 linear polystyrene (PS) chains to C60 to produce what they called “flagellenes”. Their method involved reacting a living PS carbanion with C60. Ederlé and Mathis used a similar method in 1997 to produce polystyrene, polyisoprene and other C60 polymer stars [48]. In 2007, Xu and colleagues synthesised C60-cored star polyfluorenes by modifying the bromo end-groups of C60 hexakisadducts using Suzuki polycondensation [49]. The resulting star-shaped polymers might be used as blue light-emitting materials. Another potential application for fullerene-containing star-shaped polymers might be as components of membranes for gas or water purification [50–52]. C 2020, 6, x FOR PEER REVIEW 6 of 11 8. Supramolecular Polymers In 2014, Isla and co-workers prepared supramolecular polymers from monomer units consisting ofIn a supramolecularC60 derivative and polymers, a 9,10-di(1,3-dithiol-2-yliden the fullerenes aree)-9,10-dihydroanthracen incorporated intoe the(exTTF) polymer macrocycle by weak intermolecular[54]. They reported interactions a high rather degree than of polymerisation, covalent bonds. reaching This has molecular been used weights as a greater route tothan producing 150 water-solublekDa in solution, fullerene which materials. was believed For to example, be higher in than 2004, any Liu fullerene-based and co-workers supramolecular prepared thepolymer polymer reported at that time. The authors suggest that these materials might find use in the construction of shown in Figure6, where C 60s are held within dimeric cyclodextrin units with coordinated metal self-organised optoelectronic devices. Haino and colleagues have recently synthesised fullerene- centres [53]. Assemblies of 60–80 units were prepared and imaged by scanning tunnelling spectroscopy terminated polymethyl methacrylates (PMMAs) and demonstrated that calix [5] arene hosts can bind and transmission electron microscopy. The polymer was water-soluble and displayed DNA-cleavage to the fullerene moieties of the PMMAs, inducing interesting shape transformations in the polymers ability[55]. under light irradiation. As with the work by Geckeler et al. mentioned above, the DNA-cleavage involves the generation of excited singlet molecular oxygen by the fullerene.

Figure 6. Supramolecular fullerene polymer mediated by metallobridged β-cyclodextrins, from Liu et al. [53 ]. Figure 6. Supramolecular fullerene polymer mediated by metallobridged β-cyclodextrins, from Liu In 2014,et al. Isla[53]. and co-workers prepared supramolecular polymers from monomer units consisting of a C60 derivative and a 9,10-di(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (exTTF) macrocycle [54]. They9. reported All-Carbon a high Polymers degree of polymerisation, reaching molecular weights greater than 150 kDa in solution,As which well was as incorporating believed to be fullerenes higher than into any a polymer fullerene-based chain, or supramolecularattaching them to polymer a polymer reported via at that time.functional The authorsgroups, suggestit is also that possible these materialsto prepare might all-carbon ﬁnd use fullerene in the constructionpolymers. This of self-organisedwas first optoelectronicdemonstrated devices. by Rao, Eklund Haino and and co-workers colleagues in 1993 have [56]. recently They showed synthesised that irradiating fullerene-terminated fcc C60 films, polymethylin the absence methacrylates of oxygen, (PMMAs)with visibleand or ultravio demonstratedlet light produced that calix a phase [5] arene transformation. hosts can Analysis bind to the fullereneof the moietiesnew structure of the showed PMMAs, that inducingthe fullerene interesting molecules shape within transformations the film had been in photopolymerised. the polymers [55 ]. Work published in the following year showed that polymerisation could also be induced by the application of pressures above 1 GPa at moderate temperatures [57]. A theoretical analysis of this process by a group from Grenoble suggested that it involves [2 + 2] cycloadditions between two 6-6 double bonds of neighbouring C60s to form cyclobutene rings connecting the fullerenes, to form a one-dimensional chain, as shown in Figure 7a [58]. It is now established that a number of different polymeric phases, including two-dimensional and three-dimensional as well as one-dimensional, can be produced under different pressure and temperature conditions. Examples are shown in Figure 7. Davydov and colleagues have shown that there are two distinct one-dimensional phases, both orthorhombic: a phase formed at low pressure [59], which could be transformed to a new phase through the simultaneous action of pressure and X-rays [60]. At higher temperatures and pressures, two-dimensional phases form. There are two different phases of these: tetragonal and rhombohedral. The application of still higher pressures, above 8 GPa, can result in the formation of 3D polymers, and these have been very widely studied, both experimentally and theoretically, e.g., [61–64]. In many cases, the poor crystallinity of the 3D polymers means that X-ray crystal structures have not been obtained. One structure which has been established is shown in Figure 7d. This was prepared by a group led by Yamanaka in 2006 by applying a pressure of 15 GPa to 2D C60 polymer single crystals at 600 °C [65]. Single crystal X-ray diffraction revealed that the 3D polymer crystallised in a body-centred orthorhombic structure, with the C60 monomer units substantially deformed to rectangular cuboidal shapes, each unit being bonded to eight cuboidal C60 neighbours via [3 + 3] cycloaddition. The structure contains both sp3 and sp2 carbon atoms. The polymer shown in Figure 7e is taken from work by Marques and colleagues [66,67]. This is an fcc polymeric C60 phase obtained at 9.5 GPa and 550 °C. This phase was reported previously by Talyzin et al., but not structurally characterised [63]. Marques et al. carried out synchrotron diffraction studies of the polymer, coupled with Density Functional Theory (DFT) investigations, and found that whether or not neighbouring C60s bonded with each other depended on their orientation. As a result, it is not possible to describe the polymer in terms of a defined crystallographic structure. Instead, they proposed a “frustrated” C 2020, 6, 71 6 of 11

9. All-Carbon Polymers As well as incorporating fullerenes into a polymer chain, or attaching them to a polymer via functional groups, it is also possible to prepare all-carbon fullerene polymers. This was first demonstrated by Rao, Eklund and co-workers in 1993 [56]. They showed that irradiating fcc C60 films, in the absence of oxygen, with visible or ultraviolet light produced a phase transformation. Analysis of the new structure showed that the fullerene molecules within the film had been photopolymerised. Work published in the following year showed that polymerisation could also be induced by the application of pressures above 1 GPa at moderate temperatures [57]. A theoretical analysis of this process by a group from Grenoble suggested that it involves [2 + 2] cycloadditions between two 6-6 double bonds of neighbouring C60s to form cyclobutene rings connecting the fullerenes, to form a one-dimensional chain, as shown in Figure7a [ 58]. It is now established that a number of different polymeric phases, including two-dimensional and three-dimensional as well as one-dimensional, can be produced under different pressure and temperature conditions. Examples are shown in Figure7. Davydov and colleagues have shown that there are two distinct one-dimensional phases, both orthorhombic: a phase formed at low pressure [59], which could be transformed to a new phase through the simultaneous action of pressure and X-rays [60]. At higher temperatures and pressures, two-dimensional phases form. There are two different phases of these: tetragonal and rhombohedral. The application of still higher pressures, above 8 GPa, can result in the formation of 3D polymers, and these have been very widely studied, both experimentally and theoretically, e.g., [61–64]. In many cases, the poor crystallinity of the 3D polymers means that X-ray crystal structures have not been obtained. One structure which has been established is shown in Figure7d. This was prepared by a group led by Yamanaka in 2006 by applying a pressure of 15 GPa to 2D C60 polymer single crystals at 600 ◦C[65]. Single crystal X-ray diffraction revealed that the 3D polymer crystallised in a body-centred orthorhombic structure, with the C60 monomer units substantially deformed to rectangular cuboidal shapes, each unit being bonded to eight cuboidal C60 neighbours via [3 + 3] cycloaddition. The structure contains both sp3 and sp2 carbon atoms. The polymer shown in Figure7e is taken from work by Marques and colleagues [66,67]. This is an fcc polymeric C60 phase obtained at 9.5 GPa and 550 ◦C. This phase was reported previously by Talyzin et al., but not structurally characterised [63]. Marques et al. carried out synchrotron diffraction studies of the polymer, coupled with Density Functional Theory (DFT) investigations, and found that whether or not neighbouring C60s bonded with each other depended on their orientation. As a result, it is not possible to describe the polymer in terms of a defined crystallographic structure. Instead, they proposed a “frustrated” structure, where there is random bonding within the lattice while maintaining the cubic symmetry. Three-dimensional C60 polymers have been shown to have a range of interesting properties. In contrast with the nonconductive behaviour of 2D polymers, 3D C60 polymers are electrically conductive [65], and they can possess extreme hardness [63,68]. Research on C60 polymers and on the effect of high pressures on fullerenes has been reviewed [67,69]. C 2020, 6, x FOR PEER REVIEW 7 of 11 structure, where there is random bonding within the lattice while maintaining the cubic symmetry. Three-dimensional C60 polymers have been shown to have a range of interesting properties. In contrast with the nonconductive behaviour of 2D polymers, 3D C60 polymers are electrically conductive [65], and they can possess extreme hardness [63,68]. Research on C60 polymers and on the effectC 2020 ,of6, 71high pressures on fullerenes has been reviewed [67,69]. 7 of 11

Figure 7. All carbon polymerised C60 structures. (a) 1D, (b) 2D-tetragonal, (c) 2D-rhombohedral, Figure 7. All carbon polymerised C60 structures. (a) 1D, (b) 2D-tetragonal, (c) 2D-rhombohedral, (d) (d) 3D cuboidal, (e) 3D AuCuI-type. Adapted from [67]. 3D cuboidal, (e) 3D AuCuI-type. Adapted from [67]. 10. Discussion 10. Discussion The fullerene polymers described in this paper could be divided into two broad classes. In the firstThe class, fullerene covered polymers in Sections described2–8, the fullerenesin this paper are co incorporateduld be divided into into the two polymers broad byclasses. covalent In the or firstnoncovalent class, covered bonding. in Sections In the second 2–8, the category, fullerenes covered are incorporated in Section9, the into fullerenes the polymers are fused by covalent together inor noncovalentall-carbon structures. bonding. GreatIn the ingenuity second category, has been covered shown by in synthetic Section 9, chemists the fullerenes in finding are waysfused to together prepare inthe all-carbon first class ofstructures. polymeric Great structures, ingenuity and manyhas been diff erentshown varieties by synthetic have been chemists synthesised. in finding The ways simplest to prepareto prepare, the first and class most of widely polymeric studied, structures, are the and charm many bracelet, different or side-chain,varieties have polymers, been synthesised. where the Thefullerenes simplest are to attached prepare, to and an existingmost widely polymer studied, backbone. are the Many charm of bracelet, the most or common side-chain, polymers polymers, have wherebeen modified the fullerenes in this way,are attached since the to pioneering an existing work polymer by Olah backbone. et al. in 1991Many [14 of]. Athe number most common of charm polymersbracelet polymers have been have modified been shown in this to way, be water-soluble; since the pioneering indeed, the work system by Olah prepared et al. by in Iwamoto1991 [14]. and A numberYamakoshi of charm [27] was bracelet found topolymers have one have of the been highest shown water-solubilities to be water-soluble; of any fullereneindeed, the derivative. system preparedAmong the by potential Iwamoto applications and Yamakoshi of charm [27] braceletwas found polymers to have has one been of the in solarhighest cells, water-solubilities where the use of of a anypolymer fullerene enables derivative. the fullerene Among derivatives the potential to be dispersedapplications throughout of charm the bracelet bulk, thus polymers avoiding has the been issues in solarwith aggregationcells, where thatthe use can resultof a polymer when individual enables the fullerene-based fullerene derivatives molecules to arebe used.dispersed Pearl throughout necklace or themain-chain bulk, thus polymers, avoiding in the which issues the with fullerenes aggregation are joined that togethercan result by when linking individual groups, havefullerene-based not been as moleculesextensively are studied used. asPearl charm necklace bracelet or polymers, main-chain as theypolymers, are more in which difficult the to fullerenes synthesise. are However, joined togetherseveral pearl by linking necklace groups, polymers have have not been prepared,as extensively and, studied like charm as charm bracelet brac polymers,elet polymers, they have as they been areconsidered more difficult as components to synthesise. of photovoltaic However, devices,several aspearl well necklace as for other polymers applications. have been The prepared, other members and, likeof the charm first typebracelet offullerene polymers, polymers they have all been have consid potentiallyered as useful components applications. of photovoltaic Thus, organometallic devices, as well as for other applications. The other members of the first type of fullerene polymers all have C60 polymers may be useful in catalysis and electrochemistry, while star-shaped polymers might be potentiallyvaluable as useful components applications. of membranes Thus, organometallic for gas or water C60 purification.polymers may Despite be useful these in many catalysis possible and electrochemistry,applications, however, while itstar-shaped appears that polymers there are migh not currently be valuable available as components commercial of productsmembranes which for gasincorporate or water fullerene purification. polymers. Despite The these reason many may possible be the relatively applications, high costhowever, of these it materials.appears that The there price are no currently available commercial products which incorporate fullerene polymers. The reason of pure C60 is currently around USD 100 per gram, and the preparation of polymeric structures would add considerably to this cost. Until these prices can be reduced, the potential of fullerene polymers may continue to be unrealised. Concerning all-carbon fullerene polymers, these are fascinating materials which continue to be the subject of intensive research. One-, two- and three-dimensional structures have been synthesised, and many more potential all-carbon polymers are possible. The polymers which have C 2020, 6, 71 8 of 11 been produced display a wide range of electronic and of elastic properties, but, as with the other forms of polymer discussed here, the high cost of fullerenes remains a serious problem when considering commercial applications.

Funding: This research received no external funding. Acknowledgments: I wish to thank Yamakoshi Yoko, Yingfeng Tu, Leonel Marques and Shoji Yamanaka for helpful advice and comments. Conﬂicts of Interest: The author declares no conﬂict of interest.

References

1. Krätschmer, W.; Lamb, L.D.; Fostiropoulos, K.; Huﬀman, D.R. Solid C60: A new form of carbon. Nature 1990, 347, 354–358. [CrossRef] 2. Wang, C.; Guo, Z.-X.; Fu, S.; Wu, W.; Zhu, D. Polymers containing fullerene or carbon nanotube structures. Prog. Polym. Sci. 2004, 29, 1079–1141. [CrossRef] 3. Giacalone, F.; Martín, N. Fullerene polymers: Synthesis and properties. Chem. Rev. 2006, 106, 5136–5190. [CrossRef][PubMed] 4. Giacalone, F. Fullerene-containing polymers. In Fullerenes: Principles and Applications, 2nd ed.; Puente, F.L.D.L., Nierengarten, J.-F., Eds.; Royal Society of Chemistry: Cambridge, UK, 2011; pp. 125–161. 5. Gr ˛adzka,E.; Wysocka-Zołopa,˙ M.; Winkler, K. Fullerene-based conducting polymers: n-Dopable materials for charge storage application. Adv. Energy Mater. 2020, 10, 2001443. [CrossRef] 6. Ravi, P.; Dai, S.; Wang, C.; Tam, K.C. Fullerene containing polymers: A review on their synthesis and supramolecular behavior in solution. J. Nanosci. Nanotechnol. 2007, 7, 1176–1196. [CrossRef][PubMed] 7. Winkler, K.; Balch, A.L. Electrochemically formed two-component ﬁlms comprised of fullerene and transition-metal components. C. R. Chim. 2006, 9, 928–943. [CrossRef] 8. Suzuki, T.; Li, Q.; Khemani, K.C.; Wudl, F. Synthesis of m-phenylene- and p-phenylenebis(phenylfulleroids): Two-pearl sections of pearl necklace polymers. J. Am. Chem. Soc. 1992, 114, 7300–7301. [CrossRef]

9. Loy, D.A.; Assink, R.A. Synthesis of a fullerene C60-p-xylylene copolymer. J. Am. Chem. Soc. 1992, 114, 3977–3978. [CrossRef] 10. Samal, S.; Choi, B.J.; Geckeler, K.E. The ﬁrst water-soluble main-chain polyfullerene. Chem. Commun. 2000, 1373–1374. [CrossRef] 11. Raissi, M.; Erothu, H.; Ibarboure, E.; Bejbouji, H.; Cramail, H.; Cloutet, E.; Vignau, L.; Hiorns, R.C. Main-chain poly(fullerene) multiblock copolymers as organic photovoltaic donor-acceptors and stabilizers. J. Mater. Chem. A 2017, 5, 7533–7544. [CrossRef] 12. Stephen, M.; Dowland, S.; Gregori, A.; Ramanitra, H.H.; Santos Silva, H.; Combe, C.M.S.; Bégué, D.; Dagron-Lartigau, C.; Morse, G.E.; Geneviˇcius,K.; et al. Main-chain fullerene and dye oligomers: Towards alternating fullerene polymers for organic photovoltaics. Polym. Int. 2017, 66, 388–398. [CrossRef] 13. Silva, H.S.; Ramanitra, H.H.; Bregadiolli, B.A.; Tournebize, A.; Bégué, D.; Dowland, S.A.; Chassé, T. In situ generation of fullerene from a poly(fullerene). J. Polym. Sci. Part B Polym. Phys. 2019, 57, 1434–1452. [CrossRef] 14. Olah, G.A.; Bucsi, I.; Lambert, C.; Aniszfeld, R.; Trivedi, N.J.; Sensharma, D.K.; Prakash, G.K.S.

Polyarenefullerenes, C60(H-Ar)n, obtained by acid-catalyzed fullerenation of aromatics. J. Am. Chem. Soc. 1991, 113, 9387–9388. [CrossRef]

15. Shi, S.; Khemani, K.C.; Chan, L.Q.; Wudl, F. A polyester and polyurethane of diphenyl C61: Retention of fulleroid properties in a polymer. J. Am. Chem. Soc. 1992, 114, 10656–10657. [CrossRef]

16. Geckeler, K.E.; Hirsch, A.J. Polymer-bound C60. J. Am. Chem. Soc. 1993, 115, 3850–3851. [CrossRef] 17. Wu, H.; Li, F.; Lin, Y.; Cai, R.; Wu, H.; Tong, R.; Qian, S. Fullerene-functionalized polycarbonate: Synthesis under microwave irradiation and nonlinear optical property. Polym. Eng. Sci. 2006, 46, 399–405. [CrossRef] 18. Okamura, H.; Miyazono, K.; Minoda, M.; Miyamoto, T. Preparation of water-soluble pullulans bearing

pendant C60 and their aqueous solubility. Macromol. Rapid Commun. 1999, 20, 41–45. [CrossRef] 19. Li, Z.; Qin, J. A new postfunctional method to synthesize C60-containing polysiloxanes. J. Appl. Polym. Sci. 2003, 89, 2068–2071. [CrossRef] C 2020, 6, 71 9 of 11

20. Yan, H.; Chen, S.; Lu, M.; Zhu, X.; Li, Y.; Wu, D.; Tu, Y.; Zhu, X. Side-chain fullerene polyesters: A new class of high refractive index polymers. Mater. Horiz. 2014, 1, 247–250. [CrossRef]

21. Hawker, C.J. A simple and versatile method for the synthesis of C60 copolymers. Macromolecules 1994, 27, 4836–4837. [CrossRef] 22. Chen, X.; Gholamkhass, B.; Han, X.; Vamvounis, G.; Holdcroft, S. Polythiophene-graft-styrene and

polythiophene-graft-(styrene-graft-C60) copolymers. Macromol. Rapid Commun. 2007, 28, 1792–1796. [CrossRef] 23. Zhang, N.; Schricker, S.R.; Wudl, F.; Prato, M.; Maggini, M.; Scorrano, G. A new C60 polymer via ring-opening metathesis polymerization. Chem. Mater. 1995, 7, 441–442. [CrossRef] 24. Ball, Z.T.; Sivula, K.; Fréchet, J.M.J. Well-defined fullerene-containing homopolymers and diblock copolymers with high fullerene content and their use for solution-phase and bulk organization. Macromolecules 2006, 39, 70–72. [CrossRef] 25. Biglova, Y.N.; Mustafin, A.G.; Torosyan, S.A.; Biglova, R.Z.; Miftakhov, M.S. Ring-opening metathesis polymerization (ROMP) of fullerene-containing monomers in the presence of a first-generation Grubbs catalyst. Kinet. Catal. 2017, 58, 111–121. [CrossRef]

26. Iwamoto, Y.; Yamakoshi, Y. A highly water-soluble C60-NVP copolymer: A potential material for photodynamic therapy. Chem. Commun. 2006, 4805–4807. [CrossRef][PubMed] 27. Yamakoshi, Y.; Aroua, S.; Nguyen, T.M.D.; Iwamoto, Y.; Ohnishi, T. Water-soluble fullerene materials for bioapplications: Photoinduced reactive oxygen species generation. Faraday Discuss. 2014, 173, 287–296. [CrossRef][PubMed] 28. Biglova, Y.N.; Mustafin, A.G.; Miftakhov, M.S. Physicochemical characteristics of the radical copolymerization of fullerene-containing methacrylates with vinyl monomers. Russ. J. Phys. Chem. B 2017, 11, 324–329. [CrossRef] 29. Drees, M.; Hoppe, H.; Winder, C.; Neugebauer, H.; Sariciftci, N.S.; Schwinger, W.; Schaffler, F.; Topf, C.; Scharber, M.C.; Zhu, Z.; et al. Stabilization of the nanomorphology of polymer–fullerene “bulk heterojunction” blends using a novel polymerizable fullerene derivative. J. Mater. Chem. 2005, 15, 5158–5163. [CrossRef] 30. Sivula, K.; Ball, Z.T.; Watanabe, N.; Frechet, J.M.J. Amphiphilic diblock copolymer compatibilizers and their effect on the morphology and performance of polythiophene: Fullerene solar cells. Adv. Mater. 2006, 18, 206–210. [CrossRef] 31. Miyanishi, S.; Zhang, Y.; Hashimoto, K.; Tajima, K. Controlled synthesis of fullerene-attached poly(3-alkylthiophene)-based copolymers for rational morphological design in polymer photovoltaic devices. Macromolecules 2012, 45, 6424–6437. [CrossRef]

32. Bai, Y.; Yao, X.; Wang, J.; Wang, J.L.; Wu, S.C.; Yang, S.P.; Li, W.S. Polymerizable C70 derivatives with acrylate functionality for eﬃcient and stable solar cells. Tetrahedron 2019, 75, 4676–4685. [CrossRef]

33. Nagashima, H.; Nakaoka, A.; Saito, Y.; Kato, M.; Kawanishi, T.; Itoh, K. C60Pd: The ﬁrst organometallic polymer of buckminsterfullerene. Chem. Commun. 1992, 377–379. [CrossRef] 34. Nagashima, H.; Nahaoka, A.; Tajima, S.; Saito, Y.; Itoh, K. Catalytic-hydrogenation of oleﬁns and acetylenes

over C60Pdn. Chem. Lett. 1992, 21, 1361–1364. [CrossRef] 35. Brancewicz, E.; Gr ˛adzka,E.; Winkler, K. Comparison of electrochemical properties of two-component C60-Pd polymers formed under electrochemical conditions and by chemical synthesis. J. Solid State Electrochem. 2013, 17, 1233–1245. [CrossRef] 36. Leng, F.; Gerber, I.C.; Lecante, P.; Bacsa, W.; Miller, J.; Gallagher, J.R.; Moldovan, S.; Girleanu, M.; Axet, M.R.; Serp, P. Synthesis and structure of ruthenium-fullerides. RSC Adv. 2016, 6, 69135–69148. [CrossRef] 37. Goclon, J.; Winkler, K.; Margraf, J.T. Theoretical investigation of interactions between palladium and fullerene in polymer. RSC Adv. 2017, 7, 2202–2210. [CrossRef]

38. Zheng, D.Y.; Zhou, X.M.; Mutyala, S.; Huang, X.C. High catalytic activity of C60Pdn encapsulated in metal-organic framework UiO-67, for tandem hydrogenation reaction. Chem. A Eur. J. 2018, 24, 19141–19145. [CrossRef] 39. Sugawara, Y.; Hiltebrandt, K.; Blasco, E.; Barner-Kowollik, C. Polymer-fullerene network formation via light-induced crosslinking. Macromol. Rapid Commun. 2016, 37, 1466–1471. [CrossRef] 40. Chen, J.; Luo, K.; Zhu, J.; Yu, J.; Wang, Y.; Hu, Z. Reversibly cross-linked fullerene/polyamide composites based on Diels-Alder reaction. Compos. Sci. Technol. 2019, 176, 9–16. [CrossRef] 41. Zhou, H.; Zheng, Y.; Wang, X.; Tan, H.R.; Xu, J. Photoresponsive thermoelectric materials derived from

fullerene-C60 PEDOT hybrid polymers. ACS Appl. Energy Mater. 2020, 3, 6726–6734. [CrossRef] C 2020, 6, 71 10 of 11

42. Karsten, B.P.; Bouwer, R.K.M.; Hummelen, J.C.; Williams, R.M.; Janssen, R.A.J. Charge separation and (triplet) recombination in diketopyrrolopyrrole–fullerene triads. Photochem. Photobiol. Sci. 2010, 9, 1055–1065. [CrossRef] 43. Jung, J.W.; Jo, J.W.; Jo, W.H. Enhanced performance and air stability of polymer solar cells by formation of a self-assembled buﬀer layer from fullerene-end-capped poly(ethylene glycol). Adv. Mater. 2011, 23, 1782–1787. [CrossRef] 44. Raïssi, M.; Erothu, H.; Ibarboure, E.; Cramail, H.; Vignau, L.; Cloutet, E.; Hiorns, R.C. Fullerene-capped copolymers for bulk heterojunctions: Device stability and eﬃciency improvements. J. Mater. Chem. A 2015, 3, 18207–18221. [CrossRef] 45. Isakova, A.; Burton, C.; Nowakowski, D.J.; Topham, P.D. Diels–Alder cycloaddition and raft chain end functionality: An elegant route to fullerene end-capped polymers with control over molecular mass and architecture. Polym. Chem. 2017, 8, 2796–2805. [CrossRef]

46. Vinogradova, L.V. Star-shaped polymers with fullerene C60 branching center. Russ. Chem. Bull. 2012, 61, 907–925. [CrossRef] 47. Samulski, E.T.; DeSimone, J.M.; Hunt, M.O.; Menceloglu, Y.Z.; Jarnagin, R.C.; York, G.A.; Labat, K.B.; Wang, H. Flagellenes nanophase-separated, polymer-substituted fullerenes. Chem. Mater. 1992, 4, 1153–1157. [CrossRef]

48. Ederlé, Y.; Mathis, C. Grafting of anionic polymers onto C60 in polar and nonpolar solvents. Macromolecules 1997, 30, 2546–2555. [CrossRef]

49. Xu, G.; Han, Y.; Sun, M.; Bo, Z.; Chen, C.J. Synthesis and characterization of star polyﬂuorenes with a C60 core. Polym. Sci. Part A Polym. Chem. 2007, 45, 4696–4706. [CrossRef] 50. Pulyalina, A.Y.; Rostovtseva, V.A.; Pientka, Z.; Vinogradova, L.V.; Polotskaya, G.A. Hybrid gas separation

membranes containing star-shaped polystyrene with the fullerene (C60) core. Petrol. Chem. 2018, 58, 296–303. [CrossRef] 51. Pulyalina, A.Y.; Shugurov, S.M.; Larkina, A.A.; Faikov, I.I.; Tataurov, M.V.; Rostovtseva, V.A. Eﬀect of star-shaped modiﬁers on the transport properties of polymer composites in the butan-1-ol dehydration process. Russ. J. Gen. Chem. 2019, 89, 2082–2091. [CrossRef] 52. Pulyalina, A.Y.; Larkina, A.A.; Tataurov, M.V.; Vinogradova, L.V.; Polotskaya, G.A. Hybrid macromolecular

stars with fullerene(C60) core included in polyphenyleneisophthalamide membranes for n-butanol dehydration. Fuller. Nanotub. Carbon Nanostruct. 2020, 28, 54–60. [CrossRef] 53. Liu, Y.; Wang, H.; Liang, P.; Zhang, H.-Y. Water-soluble supramolecular fullerene assembly mediated by metallobridged β-cyclodextrins. Angew. Chem. Int. Ed. 2004, 43, 2690–2694. [CrossRef] 54. Isla, H.; Pérez, E.M.; Martín, N. High degree of polymerization in a fullerene-containing supramolecular polymer. Angew. Chem. 2014, 126, 5735–5739. [CrossRef] 55. Hirao, T.; Fukuta, K.; Haino, T. Supramolecular approach to polymer-shape transformation via calixarene–fullerene complexation. Macromolecules 2020, 53, 3563–3570. [CrossRef] 56. Rao, A.M.; Zhou, P.; Wang, K.-A.; Hager, G.T.; Holden, J.M.; Wang, Y.; Lee, W.T.; Bi, X.-X.; Eklund, P.C.;

Cornett, D.S.; et al. Photoinduced polymerization of solid C60 ﬁlms. Science 1993, 259, 955–957. [CrossRef] 57. Iwasa, Y.; Arima, T.; Fleming, R.M.; Siegrist, T.; Zhou, O.; Haddon, R.C.; Rothberg, L.J.; Lyons, K.B.;

Carter, H.L.; Hebard, A.F.; et al. New phases of C60 synthesized at high pressure. Science 1994, 264, 1570–1574. [CrossRef] 58. Nuñez-Regueiro, M.; Marques, L.; Hodeau, J.-L.; Berthoux, O.; Perroux, M. Polymerized fullerite structures. Phys. Rev. Lett. 1995, 74, 278–281. [CrossRef] 59. Agafonov, V.; Davydov, V.A.; Kashevarova, L.S.; Rakhmanina, A.V.; Kahn-Harari, A.; Dubois, P.; Céolin, R.;

Szwarc, H. ‘Low-pressure’ orthorhombic phase formed from pressure-treated C60. Chem. Phys. Lett. 1997, 267, 193–198. [CrossRef] 60. Le Parc, R.; Levelut, C.; Haines, J.; Davydov, V.A.; Rakhmanina, A.V.; Papoular, R.J.; Agafonov, V. In situ

X-ray powder diﬀraction study of one-dimensional polymeric C60 phase transformation under high-pressure. Chem. Phys. Lett. 2007, 438, 63–66. [CrossRef] 61. Blank, V.D.; Buga, S.G.; Dubitsky, G.A.; Serebryanaya, N.R.; Popov, M.Y.; Sundqvist, B. High pressure

polymerized phases of C60. Carbon 1998, 36, 319–343. [CrossRef] 62. Chernozatonskii, L.A.; Serebryanaya, N.R.; Mavrin, B.N. The superhard crystalline three-dimensional

polymerized C60 phase. Chem. Phys. Lett. 2000, 316, 199–204. [CrossRef] C 2020, 6, 71 11 of 11

63. Talyzin, A.; Langenhorst, F.; Dubrovinskaia, N.; Dub, S.; Dubrovinsky, L. Structural characterization of the hard fullerite phase obtained at 13 GPa and 830 K. Phys. Rev. B 2005, 71, 115424. [CrossRef] 64. Pei, C.Y.; Feng, M.N.; Yang, Z.X.; Yao, M.G.; Yuan, Y.; Li, X.; Hu, B.W.; Shen, M.; Chen, B.; Sundqvist, B.; et al. Quasi 3D polymerization in C60 bilayers in a fullerene solvate. Carbon 2017, 124, 490–505. [CrossRef] 65. Yamanaka, S.; Kubo, A.; Inumaru, K.; Komaguchi, K.; Kini, N.S.; Inoue, T.; Irifune, T. Electron conductive

three-dimensional polymer of cuboidal C60. Phys. Rev. Lett. 2006, 96, 076602. [CrossRef] 66. Laranjeira, J.; Marques, L.; Mezouar, M.; Melle-Franco, M.; Struty´nski,K. Bonding frustration in the 9.5 GPa

fcc polymeric C60. Phys. Stat. Sol. Rapid Res. Lett. 2017, 11, 1700343. [CrossRef] 67. Laranjeira, J.; Marques, L. C60 structures: Structural, electronic and elastic properties. Mater. Today Commun. 2020, 23, 100906. [CrossRef] 68. Brazhkin, V.V.; Lyapin, A.G.; Popova, S.V.; Klyuev, Y.A.; Naletov, A.M. Mechanical properties of the 3D polymerized, sp2-sp3 amorphous, and diamond-plus-graphite nanocomposite carbon phases prepared from

C60 under high pressure. J. Appl. Phys. 1998, 84, 219–226. [CrossRef] 69. Pei, C.; Wang, L. Recent progress on high-pressure and high-temperature studies of fullerenes and related materials. Matter Radiat. Extrem. 2019, 4, 028201. [CrossRef]

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional aﬃliations.

© 2020 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).