Envelope time reversal can be achieved in a phase conjugation experiment. In this work, we study the influence of the various microscopic parameters of the nonlinear medium as well as the effects of rotational diffusion of the dipole moments on the envelope reversal. To this end, we consider a backward degenerate four-wave mixing experiment with pulsed fields and assume that any characteristic times of the envelopes or of the rotational diffusion are larger than the transverse decay time. Propagation and field polarizations effects are introduced. From the propagation equations solved in the slowly varying envelope approximation, we calculate the amplitude of the conjugated wave. Its expression depends on the field polarization as well as on the various overlap integrals of the pump and probe envelopes. In order to obtain a complete time reversal between the amplitudes of the conjugated wave E4(0,t) and probe field E3(0,t), supplementary contributions to E4(0,t) which do not satisfy the proportionality relation between E4(0,t) and E3*(0,-t) must wash-out. Some of these contributions cancel by taking a convenient choice of beam polarizations. It is shown here, that if the dipole moments undergo a rapid rotational diffusion, then complete time reversal is observed.