Two series of polycarbonate block copolymers were synthesized, and their mechanical properties were evaluated by uniaxial tension experiments. One series which consists of the block copolymers of Bisphenol A and tetramethyl-bisphenol A polycarbonates was used to find correlations between their ability to shear yield and their secondary relaxations. The other series of such copolymers in which the intrachain motional correlation distance was limited by inserting flexible linkages between the block units was used to test the effect of the extent of the scale of in-chain secondary relaxation on ductility. We found that the activation of cooperative secondary relaxation at the temperature and time scale of the stress-strain experiment is necessary for the copolymers to be able to yield. Our results also show that the availability of large-scale main-chain cooperative motion is beneficial for shear yielding. When the scale of in-chain cooperative motion is small, a higher temperature is needed in order to have the copolymer yield at moderate strain rates. When the scale of in-chain cooperative motion is large, the brittle to ductile transition temperature shifted to a much lower temperature.