Synthesis of brush block copolymers (brush BCPs) enables the generation of BCPs with various block compositions and chain architectures, which determine the morphologies and functionalities of the polymeric materials. Here, we report a synthetic strategy for preparing structurally diverse brush BCPs via combined synthesis of the grafting-through and grafting-to approach. The polystyrene-based brush copolymer with an azide functionality is synthesized by sequential ring-opening metathesis polymerization of norbornene-based macromonomers, followed by post-polymerization modification. The final brush BCPs are then prepared by grafting various side chain polymers [poly(2-vinylpyridine), poly(4-vinylpyridine), or poly(dimethylsiloxane)] onto the precursor BCPs with the desired ratio between the azido group on the backbone and alkynyl side chain polymer. This synthetic method is versatile in producing brush BCPs with various side chain types, backbone lengths, and grafting densities. In particular, it is observed that the morphologies of the brush BCPs can be controlled from lamella to cylinders to spheres by tuning the grafting densities. Furthermore, this interesting transition of self-assembled morphologies of the brush BCPs at different grafting densities is elucidated by the simulation results.