C-60 polymerization was studied in situ by Raman spectroscopy in the pressure range 10-27 GPa and temperatures up to 830 K. Quenched samples were studied by XRD using synchrotron radiation. Below 18 GPa only one- and two-dimensional polymers were found to form during heating. During slow cooling and pressure release, only the relative amounts of the different phases were changing. The structure of the 10-18 GPa samples after quenching is rhombohedral. The observed polymerization pathway suggests a gradual increase of the polymerization which occurs in the following sequence monomers-->dimers-->chains-->tetra-coordinated polymer-->hexa-coordinated polymer. Above 18 GPa the in situ Raman spectra during heating remained almost unchanged. XRD of the quenched samples revealed that they have a cubic structure with cell parameter above 13 Angstrom. The sample heated at 27 GPa showed elliptical Debye-Scherrer diffraction ring due to the strong anisotropic deformation. No very dense phases reported in previous studies (so called "superhard fullerite") were observed.