We have Studied the dynamics of a morphology transition of domains in a reactive system composed of A-B block copolymer and B monomer, where polymerization occurs at the end of the B subchain of the block copolymer. We propose a new dynamical model for reactive phase separation, where the chain architecture., the chain conformation, and the spatial distribution of the reaction points (reactive chain ends) are taken into account by the use of random phase approximation. Our model is essentially equivalent to the dynamical self-consistent field theory if the system is close to the critical point, but it has an enormous improvement in the computational efficiency. Simulation results show that our modeling is quite useful partially because the inhomogeneity of the spatial distribution of the reactants affects the polymerization and partially because we can perform large-scale and long time simulations because of the high efficiency of our model. We found that as a result of the competition between the phase separation and the polymerization the system shows complex nonequilibrium domain structures that depend on the kinetic pathway of the phase separation and the polymerization.