It is shown that C60 single crystals can be polymerized under relatively modest pressure-temperature conditions. The resulting single crystals exhibit long-range order and they are made up of 12 orientation variants. The structure is orthorhombic with a short intermolecular distance along the chains (9.14 Å), characteristic of covalent bonding. We propose a structural mechanism for the polymerization of fullerene-based compounds; it involves a shift of one of the (111) cubic planes together with a shortening of the (111) spacing. The configuration of the polymer chains presents interesting relations with that found in the A1C60 polymer compounds.

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We report on high-resolution thermal expansion measurements of high-temperature-pressure treated C 60 ͓one-dimensional ͑1D͒ and ͑2D͒ polymers and ''hard fullerite''͔, as well as of C 60 dimers and single crystal monomer C 60 between 10 and 500 K. Polymerization drastically reduces the thermal expansivity from the values of monomeric C 60 due to the stronger and less anharmonic covalent bonds between molecules. The expansivity of the ''hard'' material approaches that of diamond. The large and irreversible volume change upon depolymerization between 400 and 500 K makes it possible to study the kinetics of depolymerization, which is described excellently by a simple activated process, with activation energies of 1.9Ϯ0.1 eV ͑1D and 2D polymers͒ and 1.75Ϯ0.05 eV ͑dimer͒. Although the activation energies are very similar for the different polymers, the depolymerization rates differ by up to four orders of magnitude at a given temperature, being fastest for the dimers. Preliminary kinetic data of C 70 polymers are presented as well.

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