A high-pressure-type stopped-flow polymerization system was developed for the production of a real block copolymer, polypropene-block-poly(ethene-co-propene), having a variable molecular weight. The preliminary homopolymerization experiments of propene at 1-6 atm for 0.1 s with an MgCl2-supported Ziegler catalyst and TEA indicated that the polymer yield and molecular weight of the resulting polypropene were proportional to the monomer concentration in the system. This indicates that the catalyst activity is constant and the unfavorable side reactions can be negligible, i.e., independent of the monomer pressure. The higher propene and ethene concentrations, which could be regulated by the pressures of the vessels in the apparatus, were found to induce a higher molecular weight of the resulting block copolymer without a significant change in the molecular weight distribution and microstructure. The CFC, DSC, and AFM analyses results indicated that the increased molecular weight had an influence only on the lamellar thickness but had no effect on the crystallinity and its distribution of the block copolymer.