We have examined the fracture toughness as well as the deformation and fracture mechanisms of planar interfaces between polystyrene (PS) or blends of PS and poly(2,6-dimethylphenylene oxide) (PPO) and poly(2-vinylpyridine) (PVP) polymers. These interfaces were reinforced with various deuterated polystyrene/poly(2-vinylpyridine) (dPS/PVP) block copolymers, each of which had a short PVP block (shorter than, or just above, the entanglement molecular weight M(e) of PVP); the dPS block in all cases was substantially longer than the M(e) of PS. The fracture mechanism is found to be pure pullout rather than crazing followed by craze breakdown by chain pullout. When the fracture toughness, G(c), was plotted against N(PVP)2SIGMA, where SIGMA represents the areal chain density of a block copolymer, all data points (both for PS/PVP and PS(PPO) blend/PVP interfaces) fell on a single line, indicating that, in the pullout regime, only a single parameter, the friction coefficient between the PVP block segments and the PVP homopolymer, determines G(c). The upper limit N* of the PVP block polymerization index above which chain scission occurs rather than pullout was investigated experimentally using a 420/270 block copolymer; the numbers represent the polymerization indices of the dPS and the PVP block, respectively. The PVP block is slightly longer than the polymerization index of PVP between entanglements, N(e;PVP) (=255). We found the upper limit, N*, to be N* almost-equal-to N(e;PVP), which is smaller than the estimate from the static friction force between PVP segments under the assumption that the friction force depends linearly on N(PVP). This fact indicates that there is an extra friction due to entanglements in the glass.