Phase selection in the 205-500 K range and the kinetics of phase trans
formations are studied by differential scanning calorimetry in KC60. T
he long-time phase selection is compatible with the Gibbs free energie
s determined previously. The Avrami exponent (n=1.9+/-0.1) found for p
olymerization is consistent with two-dimensional polymorphic growth at
a fixed number of sites. In the 320-380 K temperature range, polymeri
zation is coupled with phase separation of a volume fraction (0 to sim
ilar to 40%) increasing with temperature. Between similar to 380 and 4
40 K, the phase separation takes place by diffusion-controlled three-d
imensional growth accompanied with heterogeneous nucleation approachin
g site saturation.