  We derive an expression for the local transverse polarization of a
boost-invariant expanding system of massive particles, which involves a set of
dynamical spin moments. Starting from spin kinetic theory, we obtain a closed
set of equations of motion for these spin moments. These equations are valid
during the full evolution of the system, from free streaming to local
equilibrium, and can be used to study polarization phenomena in relativistic
heavy-ion collisions.
