Polyfullerene

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:

Intrinsic polymeric fullerene (homopolymer), Main-chain polymers, Side-chain polymers, Star polymers, Crosslinked polymers, End-caped polymers.

History Fullerene is a relatively new substance in chemistry sciences. Buckminsterfullerene itself was discovered in 1985 and the first fullerene-containing polymers were reported at least 6 years later. The main milestones in the use of fullerene in polymer chemistry are listed below:

1992 – Synthesis of organometallic C60 polymer (C60Pd3)n 1995 – Synthesis of C60 containing polyurethane and C60-styrene copolymer 1996 – Synthesis of fullerene side-chain polymer 1997 – Synthesis of fullerene polymer with C60 in the backbone by Diels-Alder reaction 2001 – Synthesis of star-shaped C60 (co)polymers

Fullerene polymers 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.

Preparations This section describes most of the main structural types of fullerene-containing polymers.

Homopolymer Polyfullerenes can be prepared via many polymerization mechanisms. Research is mainly focused on photopolymerization, polymerization under high pressure and charge-transfer polymerization. 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.

Main-chain polymers 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).

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.

Side-chain polymers Most fullerene polymers fall into this category. 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.

Cross-linked polymers 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. 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 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. 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-shaped polymers 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. 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. Fullerene acts as a cocatalyst since tungsten catalyst (WCl6-Ph4Sn) is not able to polymerize 1-phenyl-1-propyne itself.

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