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Bridging lo lorm a two 10 three dimensional polymer CMPd„ (n « 2-3)

Bridging lo lorm a two 10 three dimensional polymer CMPd„ (n « 2-3)

Thermal recombination (-CM)

Three dimensional C60Pd:

Three dimensional C60Pd:

Deposit ol Pd atoms on the surlace ol C60Pd3

Fig. 10.26. Proposed mechanism for the formation of QoPd,, showing pearl necklace formation at low Pd concentrations, interchain cross-links up to n = 3, and the deposition of Pd on the Qo surface for n > 3 [10.68].

Fig. 10.27. Molecular structure of the "2+2" cycloadduct (Qo),, polyanion in K^Qo. The K+ ions (not shown) are near the bridge sites between adjacent fullerenes [10.69].

and CS2. In the second report [10.72], the amino polymer precursor was ethylene propylene terpolymer (EPDM-amine) (c). Copolymerization with (c) was inferred from viscosity measurements. Similar to the copolymer in Fig. 10.28(b), the copolymer in Fig. 10.28(e) was found to be soluble in common solvents.

The synthesis of a block copolymer of C60 through free radical intermediates has been reported using the diradical xylylene »CH2—C6H4—CH2*. The resulting block copolymer (-[C60]p [xylylene]9-)„ has a ratio of q/p = 3.4 [10.73]. The structure of this copolymer is not known, although it is expected to be a side chain or pendant C60 polymer. 13C NMR data were interpreted to indicate that the C60 attachment to the xylylene units was through benzylated C atoms on the Cjq. The xylylene monomer was prepared by the flash thermolysis of paracyclophane (C16H20) at 650°C. This monomer can react with itself to form the polymer poly(p-xylylene) (Fig. 10.29). In the synthesis of the copolymer, the xylylene was swept from the furnace in which it was produced into a cooled solution of C60 in toluene. Upon warming to room temperature, a brown precipitate (polymer) formed, which

Fig. 10.26. Proposed mechanism for the formation of QoPd,, showing pearl necklace formation at low Pd concentrations, interchain cross-links up to n = 3, and the deposition of Pd on the Qo surface for n > 3 [10.68].

n

Fig. 10.28. Precursor polymers used in the preparation of CM amino polymers, (a) poly(ethyleneimine), (b) poly{4-[(2-aminoethyl)imino]methyl}styrene, (c) ethylene propylene terpolymer (EPDM-amine). Each of these precursors can be substituted for R in Fig. 10.9 [10.71,72]. The straight line segments represent C-C single bonds and double bonds are indicated in the aromatic ring, with C atoms at each unmarked vertex.

Fig. 10.29. Mechanism for formation of the xylylene diradical at 650° C, followed by its copolymerization with Cm [10.73].

was found to be unstable in air. C60 polymers, in which direct linkages between fullerenes are formed, may have also been produced. This proposal stemmed from a series of studies involving the phototransformation of thin solid films of C60 with visible-UV radiation [10.74],

The photopolymerization of C60 has been suggested to proceed by a photochemical "2+2" cycloaddition process (see §10.7) in which double bonds on adjacent C60 molecules are broken to form a four-membered ring between molecules (see §7.5.1) [10.12,13]. The phototransformation reaction takes place in C60 films only above ~260 K, where the C60 molecules rotate relatively freely and can position themselves so that a double bond on one C60 molecule is parallel to a double bond on an adjacent CM molecule [10.75], satisfying a topological requirement for "2+2" cycloaddition. The stability of this four-membered ring is measured by the thermal barrier (Eb ~ 1.25 eV) which must be overcome to break the bond in the four-membered bridge ring. At higher temperatures (above ~400 K), a rapid decrease in the population of oligomers is observed by Raman scattering

Fig. 10.28. Precursor polymers used in the preparation of CM amino polymers, (a) poly(ethyleneimine), (b) poly{4-[(2-aminoethyl)imino]methyl}styrene, (c) ethylene propylene terpolymer (EPDM-amine). Each of these precursors can be substituted for R in Fig. 10.9 [10.71,72]. The straight line segments represent C-C single bonds and double bonds are indicated in the aromatic ring, with C atoms at each unmarked vertex.

Fig. 10.29. Mechanism for formation of the xylylene diradical at 650° C, followed by its copolymerization with Cm [10.73].

Fig. 10.30. Two fulleroids (C61) joined by a phenyl group to form a bifulleroid. This bifulleroid dimer shows the pearl necklace conformation [10.7], measurements [10.76]. Another route for preparing C60 polymers is by extending the synthetic route for the preparation of fulleroids of the type shown in Fig. 10.11.

Bifulleroid formation has also been demonstrated chemically. For example, Fig. 10.30 shows the formation of such a bifulleroid dimer through the bridging of two fulleroids through a phenyl group, suggesting the variety of polymeric structures that might be formed [10.7]. The coupling between the two fulleroids is confirmed by cyclic voltammetry studies (§10.3.2) showing two close doublet peaks for each wave on a diagram such as in Fig. 10.6 [10.27,77-80]. Of course, bifulleroids can be formed by other aromatic hydrocarbon groups. The synthesis of higher polymers has been hindered by poor solubility of the bifulleroid molecules. Nevertheless, this difficulty has been overcome by the attachment of solubilizing groups on the phenylene moiety.

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