Two series of polyestercarbonates containing 1,4-cyclohexylene linkages were designed and synthesized to allow investigation of the relationship between local molecular motion and mechanical properties in these materials. Incorporation of increasing amounts of 1,4-cyclohexylene dicarboxylate linkages enhanced the segmental mobility in tetramethyl Bisphenol A polyestercarbonates and in Bisphenol A polyester carbonates. This effect is thought to be due to the cooperative conformational transition of neighboring cyclohexylene rings. The cooperative conformational transition is thought to be able to activate motions of the linked carbonate segments and further facilitate the molecular motions of neighboring segments. This motion is thought to favor shear yielding. The results are found to be consistent with this notion.