We apply field cycling NMR to study segmental reorientation dynamics in melts of linear 1,4-polybutadiene (PB) in the entanglement regime (M >= M-e). Dispersion data of the spin-lattice relaxation time T-1(omega) are transformed to the susceptibility representation chi ''(omega) = omega/T-1(omega), and using frequency temperature Superposition master Curves chi ''(omega tau(s)) are constructed which reflect spectral contributions from glassy as well as polymer specific dynamics. The correlation time tau(s) is determined by glassy dynamic. chi ''(omega tau(s)) Transforming into the time domain and Studying the crossover from Rouse to entanglement regime, the full dipolar or segmental reorientational correlation function F-2(t/tau(s)) is presented covering six decades in amplitude and 8 decades in time. Assuming F-2(t) congruent to < u(b)(t)u(b)(0)>(2) the bond vector correlation function phi(b)(t) = < u(b)(t)u(b)(0)> is obtained. Reaching Z = M/M-e <= 9, comparison with theoretical predictions by the tube-reptation model as well as renormalized Rouse theory reveals significant discrepancies whereas good agreement is found with simulations. The crossover to entanglement dynamics appears to be very protracted.