Small-amplitude oscillatory shear rheology was used to investigate relaxation dynamics for two linear polybutadienes with low-vinyl (7% 1,2-addition; PBd-L) and high-vinyl (94% 1,2-addition; PBd-H) microstructures. The M,, values were equal to 38.7 and 80.1 kg/mol for PBd-L and PBd-H, respectively, and both polymers were near-monodisperse (M-w/M-n < 1.05). These molecular weights were selected in order to keep the number of entanglements per chain, and hence the plateau length, fairly equivalent for these two polymers. Well-defined terminal flow and segmental relaxation (alpha-relaxation) loss modulus peaks in the isothermal linear viscoelastic data were used to determine relaxation times, thus the necessity of time-temperature superposition was avoided. These results are interpreted in the context of the Ngai coupling model (CM). We apply the model to both relaxation processes in a unified approach by adopting the CM framework which contends that the primitive relaxation which underlies the cooperative a-relaxation process also directly controls the temperature dependence of the monomeric friction coefficient for the Rouse relaxation, which, in turn, is the primitive process for entangled terminal flow. Thermorheological complexity is a direct consequence of the model, and the viscoelastic data support this prediction for both PBd-L and PBd-H. The common discrepancy between the Vogel temperature evaluated from the temperature dependence of terminal flow and its a-relaxation counterpart is explained and resolved using this coupling model approach.