To control molecular mobility and study its effects on mechanical properties, we synthesized two series of poly(ester carbonate) and polycarbonate copolymers with different linkages: (B(x)t)(n) (x = 3, 5, 7, 9) and (BxT)(n) (x: = 1, 3, 5, 7, 9), where t represents the terephthalate, T represents the tetramethyl bisphenol A carbonate linkages, and B is the conventional bisphenol-A (BPA) carbonate. These two series of materials have distinct differences in their relaxation behaviors and chain mobility, as indicated by the pi -flip motion of the phenylene rings in the B-x blocks. Uniaxial tensile tests of the copolymers indicate that the brittle-ductile transition (BDT) temperatures of the copolymers are correlated to whether the gamma -relaxation peaks due to the B-x sequence is fully established. The materials possessing more fully established low-temperature gamma peaks give rise to a lower BDT. Also, the locations of the gamma peaks are correlated to the ring flips of the B-x blocks of polymer chains.