  We study the dynamics of one electron wave packet in a chain with a
non-adiabatic electron-phonon interaction. The electron-phonon coupling is
taken into account in the time-dependent Schr\"odinger equation by a delayed
cubic nonlinearity. In the limit of an adiabatic coupling, the self-trapping
phenomenon occurs when the nonlinearity parameter exceeds a critical value of
the order of the band width. We show that a weaker nonlinearity is required to
produce self-trapping in the regime of short delay times. However, this trend
is reversed for slow nonlinear responses, resulting in a reentrant
phase-diagram. In slowly responding media, self-trapping only takes place for
very strong nonlinearities.
