The dynamics of different blocks in block copolymers are expected to affect the other blocks due to compositional heterogeneity, thus influencing their bulk structure and performance. It is significant to understand the effect of one block on the mobility of another block at the molecular level. Here, we investigated the glass transition temperatures of PMMA blocks in poly(methyl methacrylate)-b-poly(butyl methacrylate) (PMMA-b-PBMA) (T-g(M(b - B))) and poly(methyl methacrylate)-b-poly(n-octadecyl methacrylate) (PMMA-b-PODMA) (T-g(M(b - D))) brushes on an oxide-covered silicon substrate (SiO2-Si) in which the PMMA block was tethered to the substrate by covalent bonds, and PBMA and PODMA were the lower-T-g blocks connected to the PMMA chain end. It was observed that both T-g(M(b - B)) and T-g(M(b - D)) decreased as the thickness of the PBMA or PODMA block increased. When the thickness of PBMA was above 45 nm, T-g(M(b - B)) was approximately 40 K lower than the glass transition temperatures of PMMA brushes (T-g(M)), while T-g(M(b - B)) showed no dependence on the PBMA and PODMA thickness. The results indicated that the connectivity with the lower-T-g block at the PMMA chain end could effectively enhance the mobility of the higher-T-g PMMA chain.