Molecular dynamics simulations of the dissociation of methane, ethylene, and benzene at 50 GPa show that thermally activated carbon-carbon bond breaking produces a stationary n(-5/2) distribution of short-lived n-atom carbon chains. Chain fragmentation rates follow an Arrhenius law in temperature, and the fraction of carbon existing as small chains increases with increasing temperature, ultimately approaching 1 at some critical temperature. Mean-field coagulation theory with added fragmentation terms yields the stationary n(-5/2) distribution and shows that the rate of carbon coagulation to bulk material slows down with increasing temperature, approaching zero at the critical temperature.