  We give general topological rules which very accurately predict the chemical
trends in glass transition temperature $T_g$ variation as a function of
cross-linking. In multicomponent glasses, these chemical trends permit to
distinguish homogeneous compositions (random network) from inhomogeneous ones
(local phase separation). The stochastic origin of the Gibbs-Di Marzio equation
is predicted at low connectivity and the analytical expression of its parameter
emerges naturally from the calculation.
