  We investigate the phase behaviour of random copolymers melts via large scale
Monte Carlo simulations. We observe macrophase separation into A and B--rich
phases as predicted by mean field theory only for systems with a very large
correlation lambda of blocks along the polymer chains, far away from the
Lifshitz point. For smaller values of lambda, we find that a locally
segregated, disordered microemulsion--like structure gradually forms as the
temperature decreases. As we increase the number of blocks in the polymers, the
region of macrophase separation further shrinks. The results of our Monte Carlo
simulation are in agreement with a Ginzburg criterium, which suggests that mean
field theory becomes worse as the number of blocks in polymers increases.
