Block junctions are the topological constraints of connecting two homopolymers of different species to form diblock copolymers, which make the phase transition behaviors of diblock copolymers deviate from that of parallel polymer blends. We performed dynamic Monte Carlo simulations of these two parallel polymeric systems to compare their behaviors in the interplay of phase transitions. The results showed the lowered melting point of one component in symmetric diblock copolymers, as well as their enhanced critical segregation strength for demixing prior to crystallization. Furthermore, prior microphase separation in symmetric diblock copolymers produces a template for parallel stacking of lamellar crystals, which is of practical importance in the barrier properties of polymeric materials. Our observations may facilitate a better understanding to the phase transition behaviors of copolymer systems.