Graphene nanoribbons (GNRs) are considered promising candidates for next-generation nanoelectronics. In particular, GNR heterojunctions have received considerable attention due to their exotic topological electronic phases at the heterointerface. However, strategies for their precision synthesis remain at a nascent stage. Here, we report a novel chain-growth polymerization strategy that allows for constructing GNR heterojunction with N=9 armchair and chevron GNRs segments (9-AGNR/cGNR). The synthesis involves a controlled Suzuki–Miyaura catalyst-transfer polymerization (SCTP) between 2-(6′-bromo-4,4′′-ditetradecyl-[1,1′:2′,1′′-terphenyl]-3′-yl) boronic ester (M1) and 2-(7-bromo-9,12-diphenyl-10,11-bis(4-tetradecylphenyl)-triphenylene-2-yl) boronic ester (M2), followed by the Scholl reaction of the obtained block copolymer (poly-M1/M2) with controlled M_n (18 kDa) and narrow Đ (1.45). NMR and SEC analysis of poly-M1/M2 confirm the successful block copolymerization. The solution-mediated cyclodehydrogenation of poly-M1/M2 toward 9-AGNR/cGNR is unambiguously validated by FT-IR, Raman, and UV/Vis spectroscopies. Moreover, we also demonstrate the on-surface formation of pristine 9-AGNR/cGNR from the unsubstituted copolymer precursor, which is unambiguously characterized by scanning tunneling microscopy (STM).