  This paper is devoted to the study of propagation of chaos and mean-field
limits for systems of indistinguable particles, undergoing collision processes.
The prime examples we will consider are the many-particle jump processes of Kac
and McKean \cite{Kac1956,McKean1967} giving rise to the Boltzmann equation. We
solve the conjecture raised by Kac \cite{Kac1956}, motivating his program, on
the rigorous connection between the long-time behavior of a collisional
many-particle system and the one of its mean-field limit, for bounded as well
as unbounded collision rates. Motivated by the inspirative paper by Gr\"unbaum
\cite{Grunbaum}, we develop an abstract method that reduces the question of
propagation of chaos to that of proving a purely functional estimate on
generator operators ({\em consistency estimates}), along with differentiability
estimates on the flow of the nonlinear limit equation ({\em stability
estimates}). This allows us to exploit dissipativity at the level of the
mean-field limit equation rather than the level of the particle system (as
proposed by Kac). Using this method we show: (1) Quantitative estimates, that
are uniform in time, on the chaoticity of a family of states. (2) Propagation
of {\it entropic chaoticity}, as defined in \cite{CCLLV}. (3) Estimates on the
time of relaxation to equilibrium, that are \emph{independent of the number of
particles in the system}. Our results cover the two main Boltzmann physical
collision processes with unbounded collision rates: hard spheres and
\emph{true} Maxwell molecules interactions. The proof of the \emph{stability
estimates} for these models requires significant analytic efforts and new
estimates.
