Polyfullerene is a basic polymer of the C60 monomer group, in which fullerene segments are connected via covalent bonds into a polymeric chain without side or bridging groups. They are called intrinsic polymeric fullerenes, or more often all C60 polymers.
Fullerene can be part of a polymer chain in many different ways. Fullerene-containing polymers are divided into following structural categories:
Fullerene is a relatively new substance in chemistry sciences. Buckminsterfullerene itself was discovered in 1985 [1] and the first fullerene-containing polymers were reported at least 6 [2] years later.
The main milestones in the use of fullerene in polymer chemistry are listed below:
High content of double bonds in the fullerene molecule (30 double bonds in Buckminsterfullerene) leads to crosslinking and formation of regioisomers. Polymerization without any sophisticated control of forming structure leads to very high randomization of polymer grid. Thus, linking units of second monomer are needed to prepare linear copolymers (see main-chain polymers).
This group includes heteroatomic C60 polymers containing non-carbon atoms in polyfullerene chains. [8]
This section describes most of the main structural types of fullerene-containing polymers.
Polyfullerenes can be prepared via many polymerization mechanisms. Research is mainly focused on photopolymerization, [9] polymerization under high pressure [10] and charge-transfer polymerization. [11]
The most likely connection of fullerene units is [2+2] cycloaddition of two double bonds of the benzene parts of fullerene molecules. Cycloaddition provides a cyclobutane ring connecting two fullerene molecules. [12] [13]
Main-chain polymers are characterized by the presence of fullerene units in the polymer backbone. They are not heteroatomic fullerene homopolymers but linear fullerene copolymers.
The structure can be described as necklace-type. One approach to achieving fullerene main-chain polymers is by copolymerizing fullerene with a difunctional monomer. Second option is polycondensation of bifunctionalized fullerene with monomer bearing compatible functional groups.
Fullerene copolymers can be obtained through standard polymerization techniques used for industrially standard polymers. Examples of first approach are Diels-Alder addition and free radical copolymerization. Fullerene can be copolymerized with methylmethacrylate by initiation with azobisisobutyronitrile (AIBN). [14]
In Diels-Alder copolymerization fullerene acts as a dienophile with diene to form a cyclohexene ring. The figure below shows Diels-Alder reaction with the simplest diene – buta-1,3-diene. Comonomer must contain two pairs of conjugated double bonds in order to react with two fullerene molecules obtaining linear polymeric chain molecules. Used monomers are usually bulkier than conventional monomers in order to compensate the space requirements of fullerene spheres.
Most fullerene polymers fall into this category. [8] Similarly to the previous polymer type, two synthetic approaches are available. First, bonding fullerene spheres onto a polymerized chain or second, polymerizing monomer unit already bearing fullerene.
An example of the second approach is ring-opening metathesis polymerisation (ROMP) of norbornene bearing C60 or copolymerization of pure norbornene and C60 functionalized norbornene [15] .
As mentioned earlier, Buckminsterfullerene is capable of multiple additions and basic polymerization conditions lead to a polymer grid. Fullerene behaves the same way in copolymerization. In free radical copolymerization of styrene and C60 fullerene, the resulting copolymer is cross-linked and heterogeneous. [16]
Easy preparation of cross-linked fullerene polymer is copolymerization with polyurethanes. In this technique, fullerenol bearing up to 44 hydroxyl groups C60(OH)4 – 44 [17] and di- or tri- isocyanate prepolymers are used as initial substances. Successful syntheses were conducted in a mixture of dimethylformamide (DMF) and tetrahydrofuran (THF)(1:3) at 60°C. [18]
Fullerene End-caped polymers
Also incorrectly named “telechelic” polymers, but telechelic polymers have reactive functional end-groups. They can be synthesized by incorporating fullerenes onto the ends of polymerized chains or growth of a polymeric chain from a functionalized fullerene derivative and additionally closure. Introducing fullerene spheres into the end of the macromolecule significantly increases hydrophobicity of the original polymer.
Star fullerene polymers can be prepared by two major approaches.
Reported star fullerene polymers were prepared by anionic copolymerization with polystyrene to form C60(CH2CH(C6H5))x)n, where n stands for the number of polystyrene star “arms” from 2 to 6. [19] [20] Second approach is growing polymer chains directly from fullerene derivative C60Cln (n = 16–20) by atom transfer radical polymerization. The chlorine fullerene derivative virtually works as an ATRP initiator. Countless polymers can be used for star arms.
Polyphenylakyne polymers can be used as an example since they give photoemitting macromolecules when grafted onto fullerene. C60-poly(1-phenyl-1-propyne) can be prepared via wolfram-catalyzed metathesis reaction connecting prepared poly(1-phenyl-1-propyne) onto the fullerene by carbene addition resulting in cyclopropane connecting ring. [21] Fullerene acts as a cocatalyst since tungsten catalyst (WCl6-Ph4Sn) is not able to polymerize 1-phenyl-1-propyne itself.
Polyfullerenes are currently in an early research phase and real-world applications or even industrial production solutions are yet to be found. The main reasons for this are the novelty of combining fullerene chemistry with polymer chemistry and the fact that fullerene can be currently synthesized on a scale of a few grams. All-C60 polymers exhibit practically no solubility, thus preventing proper testing of processability and chemical properties.
Upcoming text only refers to potential applications of fullerene polymers according to founded properties of particular macromolecules.
Fullerene itself stands out in the class of organic compounds because of its electronic properties. Current research studies utilization of fullerene by bonding it onto an optimal polymeric substrate. Practical reasons are easy processability of polymers and low price in comparison to pure C60 fullerene.
Polymer backbones bearing fullerene spheres exhibit good or great photoconductivity and even generate photocurrent when exposed to white light. [22] [23]
C60-polyvinylcarbazole (C60–PVK) exhibits photoinduced electron transfer within the polymer, which could be used for digital rewritable memory electronic parts. Prototype of such part of indium tin oxide, fullerene polymer and aluminum (ITO/ C60–PVK /Al) was capable to read, write and erase information for about 100 million times. [24]
Polyvinylcabazole polymer grown from fullerene polychloride (C60Cln) was observed to increase the intensity of radiated light of an electroluminescent device. This star polymer with three arms is acting as a hole-transporting layer for semiconductor parts of a device. [25]
On the other hand, hole-trapping materials affect electroluminescence the same way. Double-cable polymers are also candidates for functional layers for OLED displays. Adding 1 wt. % into basic OLED material increased luminescence of the diode. [26] Very promising hole-trapping materials are polyacetylene backbone polymers with fullerene in combination with different electron-accepting groups in branches. [27]
Star copolymer (PS)xC60(PMMA)y (polystyrene and polymethylmethacrylate being different star “arms”) acted as an active electroluminescence layer. It improved emitting of a semiconductor electroluminescence device by up to 20 times. [28] C60-poly(1-phenyl-1-propyne) is also reported to exhibit light emission. [29] Fullerene moiety increased emission of blue light two times in comparison to pure poly(phenyl propyne). Stability and processability of such polymer is very good.
Fullerene polymers are widely studied in organic solar cells for active layers of new-generation photovoltaic panels. Examples are homopolymers of C60-polystyrene [30] and C60-polyethyleneglycols [31] or C60 copolymers prepared by ROMP polymerization. [32] [33] The current efficiency of converting incoming sun radiation to electricity is about 3%. [32]
Another polymer type with intrinsic properties are “Double-Cable” polymers. They are brush-like structures consisting of 𝜋-electron conjugated backbone (P-type part) bearing electron-accepting branches (N-type part). [34] [35]
Particular fullerene (co)polymers exhibit an optical limiting property, meaning they block intense light flux passing through them. Low intensity light flux is not affected. It is useful for light control parts in optics and as sensor or eye protection. [36] [37] [38] [39]
Currently, fullerene copolymerized with palladium showed some practical aspects, particularly (C60Pd3)n due to the content of palladium on its surface, exhibits catalytic effect for hydrogenation of alkenes [40] and can lead to the development of new catalytic systems and products.
(C60Pd)n polymers can adsorb gases, making them useful as adsorbents for volatile and toxic species. For example, a great affinity to toluene was proved. [41] The palladium atoms in the backbone are partially positive and thus attract 𝜋-electrons of aromatic core of toluene.
Introducing correct amount of fullerene as side groups onto poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) increases permeability of gas separation membranes by 80 % in comparison with pure PPO. Bulky fullerene probably increases the free volume of PPO. [42]
Materials originating from polyurethane synthesis exhibit improved thermal mechanical stability. [43] Fullerene-containing polyurethanes also exhibit strong optical response and are potentially applicable for optical signal processing. [38]
Linear polymer chains containing fullerene undergo crosslinking. Resulting material exhibits elastomeric behavior with 10 times higher tensile strength and 17 times higher elongation at break than the same material without fullerene. [8]
Blending of fullerene end-capped polymers (polyethylene glycols for example) with H-donating polymers (polyvinylchloride, poly(p-vinyl phenol), polymethylmethacrylate, etc.) leads to the enhancement of mechanical properties of H-donating polymers.
Fullerene end-caped poly(N-isopropylacrylamide) is a water-soluble polymer with the tendency to form clusters. [44] It is a very good scavenger of free radicals, and it can be used for controlling radical polymerizations.
Fullerene polymers are potential candidates for establishing polymer circular economy.
Depolymerizeable polymers are the hope of polymer recycling. C60 fullerene copolymerized with [4,4′-bithiazole]-2,2′-bis(diazonium)chloride (see Magnetic behavior) was observed to depolymerize in a temperature range of 60-75°C. Polymerization and depolymerization can be done several times before degradation of initial components. [45] The depolymerization temperature and the difference between polymerization and depolymerization temperatures must be increased.
Basic fullerene polymers without polar functional groups are strongly hydrophobic, thus incompatible for medicinal use in the human body.
An example of water-soluble derivatives are polyfullerocyclodextrins. They are prepared by reaction of 𝛽-cyclodextrin complexes with fullerene. They exhibit excellent DNA-cleaving activity [46] (in the presence of visible light, they cleave DNA quantitatively). This phenomenon can be used for eliminating cancer cells.
The introduction of hydrophilic groups into the macromolecule is the principle of preparing water-soluble polymers. Examples of backbones for water-soluble fullerene side-chain polymers are for example poly(maleic anhydride-co-vinyl acetate) (52) or pullulan. [47]
Polymers with C60-backbone with ferromagnetic properties were reported in literature, [48] although fullerene itself is antiferromagnetic. An example of a successful synthesis of ferromagnetic C60–polymer uses [4,4′-bithiazole]-2,2′-bis(diazonium)dichloride, C60 and FeSO4.
Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per unit weight. It is a poor barrier to air and water vapor and has a relatively low melting point. Polystyrene is one of the most widely used plastics, with the scale of its production being several million tonnes per year. Polystyrene is naturally transparent, but can be colored with colorants. Uses include protective packaging, containers, lids, bottles, trays, tumblers, disposable cutlery, in the making of models, and as an alternative material for phonograph records.
In polymer chemistry, living polymerization is a form of chain growth polymerization where the ability of a growing polymer chain to terminate has been removed. This can be accomplished in a variety of ways. Chain termination and chain transfer reactions are absent and the rate of chain initiation is also much larger than the rate of chain propagation. The result is that the polymer chains grow at a more constant rate than seen in traditional chain polymerization and their lengths remain very similar. Living polymerization is a popular method for synthesizing block copolymers since the polymer can be synthesized in stages, each stage containing a different monomer. Additional advantages are predetermined molar mass and control over end-groups.
Polythiophenes (PTs) are polymerized thiophenes, a sulfur heterocycle. The parent PT is an insoluble colored solid with the formula (C4H2S)n. The rings are linked through the 2- and 5-positions. Poly(alkylthiophene)s have alkyl substituents at the 3- or 4-position(s). They are also colored solids, but tend to be soluble in organic solvents.
In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are sometimes called bipolymers. Those obtained from three and four monomers are called terpolymers and quaterpolymers, respectively. Copolymers can be characterized by a variety of techniques such as NMR spectroscopy and size-exclusion chromatography to determine the molecular size, weight, properties, and composition of the material.
In polymer chemistry, free-radical polymerization (FRP) is a method of polymerization by which a polymer forms by the successive addition of free-radical building blocks. Free radicals can be formed by a number of different mechanisms, usually involving separate initiator molecules. Following its generation, the initiating free radical adds (nonradical) monomer units, thereby growing the polymer chain.
A polyolefin is a type of polymer with the general formula (CH2CHR)n where R is an alkyl group. They are usually derived from a small set of simple olefins (alkenes). Dominant in a commercial sense are polyethylene and polypropylene. More specialized polyolefins include polyisobutylene and polymethylpentene. They are all colorless or white oils or solids. Many copolymers are known, such as polybutene, which derives from a mixture of different butene isomers. The name of each polyolefin indicates the olefin from which it is prepared; for example, polyethylene is derived from ethylene, and polymethylpentene is derived from 4-methyl-1-pentene. Polyolefins are not olefins themselves because the double bond of each olefin monomer is opened in order to form the polymer. Monomers having more than one double bond such as butadiene and isoprene yield polymers that contain double bonds (polybutadiene and polyisoprene) and are usually not considered polyolefins. Polyolefins are the foundations of many chemical industries.
Reversible addition−fragmentation chain-transfer or RAFT polymerization is one of several kinds of reversible-deactivation radical polymerization. It makes use of a chain-transfer agent (CTA) in the form of a thiocarbonylthio compound to afford control over the generated molecular weight and polydispersity during a free-radical polymerization. Discovered at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) of Australia in 1998, RAFT polymerization is one of several living or controlled radical polymerization techniques, others being atom transfer radical polymerization (ATRP) and nitroxide-mediated polymerization (NMP), etc. RAFT polymerization uses thiocarbonylthio compounds, such as dithioesters, thiocarbamates, and xanthates, to mediate the polymerization via a reversible chain-transfer process. As with other controlled radical polymerization techniques, RAFT polymerizations can be performed under conditions that favor low dispersity and a pre-chosen molecular weight. RAFT polymerization can be used to design polymers of complex architectures, such as linear block copolymers, comb-like, star, brush polymers, dendrimers and cross-linked networks.
Polyphosphazenes include a wide range of hybrid inorganic-organic polymers with a number of different skeletal architectures with the backbone P-N-P-N-P-N-. In nearly all of these materials two organic side groups are attached to each phosphorus center. Linear polymers have the formula (N=PR1R2)n, where R1 and R2 are organic (see graphic). Other architectures are cyclolinear and cyclomatrix polymers in which small phosphazene rings are connected together by organic chain units. Other architectures are available, such as block copolymer, star, dendritic, or comb-type structures. More than 700 different polyphosphazenes are known, with different side groups (R) and different molecular architectures. Many of these polymers were first synthesized and studied in the research group of Harry R. Allcock.
In polymer chemistry, gradient copolymers are copolymers in which the change in monomer composition is gradual from predominantly one species to predominantly the other, unlike with block copolymers, which have an abrupt change in composition, and random copolymers, which have no continuous change in composition . In the gradient copolymer, as a result of the gradual compositional change along the length of the polymer chain less intrachain and interchain repulsion are observed.
Poly(N-isopropylacrylamide) (variously abbreviated PNIPA, PNIPAM, PNIPAAm, NIPA, PNIPAA or PNIPAm) is a temperature-responsive polymer that was first synthesized in the 1950s. It can be synthesized from N-isopropylacrylamide which is commercially available. It is synthesized via free-radical polymerization and is readily functionalized making it useful in a variety of applications.
Dendronized polymers are linear polymers to every repeat unit of which dendrons are attached. Dendrons are regularly branched, tree-like fragments and for larger ones the polymer backbone is wrapped to give sausage-like, cylindrical molecular objects. Figure 1 shows a cartoon representation with the backbone in red and the dendrons like cake slices in green. It also provides a concrete chemical structure showing a polymethylmethacrylate (PMMA) backbone, the methyl group of which is replaced by a dendron of the third generation.
Living free radical polymerization is a type of living polymerization where the active polymer chain end is a free radical. Several methods exist. IUPAC recommends to use the term "reversible-deactivation radical polymerization" instead of "living free radical polymerization", though the two terms are not synonymous.
Polyfluorene is a polymer with formula (C13H8)n, consisting of fluorene units linked in a linear chain — specifically, at carbon atoms 2 and 7 in the standard fluorene numbering. It can also be described as a chain of benzene rings linked in para positions with an extra methylene bridge connecting every pair of rings.
Conjugated microporous polymers (CMPs) are a sub-class of porous materials that are related to structures such as zeolites, metal-organic frameworks, and covalent organic frameworks, but are amorphous in nature, rather than crystalline. CMPs are also a sub-class of conjugated polymers and possess many of the same properties such as conductivity, mechanical rigidity, and insolubility. CMPs are created through the linking of building blocks in a π-conjugated fashion and possess 3-D networks. Conjugation extends through the system of CMPs and lends conductive properties to CMPs. Building blocks of CMPs are attractive in that the blocks possess broad diversity in the π units that can be used and allow for tuning and optimization of the skeleton and subsequently the properties of CMPs. Most building blocks have rigid components such as alkynes that cause the microporosity. CMPs have applications in gas storage, heterogeneous catalysis, light emitting, light harvesting, and electric energy storage.
IUPAC Polymer Nomenclature are standardized naming conventions for polymers set by the International Union of Pure and Applied Chemistry (IUPAC) and described in their publication "Compendium of Polymer Terminology and Nomenclature", which is also known as the "Purple Book". Both the IUPAC and Chemical Abstracts Service (CAS) make similar naming recommendations for the naming of polymers.
In polymer chemistry, reversible-deactivation radical polymerizations (RDRPs) are members of the class of reversible-deactivation polymerizations which exhibit much of the character of living polymerizations, but cannot be categorized as such as they are not without chain transfer or chain termination reactions. Several different names have been used in literature, which are:
In polymer chemistry, graft polymers are segmented copolymers with a linear backbone of one composite and randomly distributed branches of another composite. The picture labeled "graft polymer" shows how grafted chains of species B are covalently bonded to polymer species A. Although the side chains are structurally distinct from the main chain, the individual grafted chains may be homopolymers or copolymers. Graft polymers have been synthesized for many decades and are especially used as impact resistant materials, thermoplastic elastomers, compatibilizers, or emulsifiers for the preparation of stable blends or alloys. One of the better-known examples of a graft polymer is a component used in high impact polystyrene, consisting of a polystyrene backbone with polybutadiene grafted chains.
Allyl glycidyl ether is an organic compound used in adhesives and sealants and as a monomer for polymerization reactions. It is formally the condensation product of allyl alcohol and glycidol via an ether linkage. Because it contains both an alkene and an epoxide group, either group can be reacted selectively to yield a product where the other functional group remains intact for future reactions.
Functionalized polyolefins are olefin polymers with polar and nonpolar functionalities attached onto the polymer backbone. There has been an increased interest in functionalizing polyolefins due to their increased usage in everyday life. Polyolefins are virtually ubiquitous in everyday life, from consumer food packaging to biomedical applications; therefore, efforts must be made to study catalytic pathways towards the attachment of various functional groups onto polyolefins in order to affect the material's physical properties.
Yves Gnanou is a French chemist, academic and author. He is the Ibn Alhaytham Distinguished Professor of Chemistry at King Abdullah University of Science and Technology.