The chain dynamics in high molecular weight polymer melts observed from proton magnetic relaxation generates a reference frequency, Delta approximate to 10(3) rad.s(-1), closely related to the chain entanglement interaction. Observing polybutadiene, we report on the slight molecular weight dependence of the segmental correlation time tau(s)(I) associated with the time scale 1/Delta approximate to 10(-3) s, (41 x 10(3) less than or equal to M less than or equal to 375 x 10(3)); tau(s)(I) is shown to exhibit an apparent power law M-0.31. Correspondingly, the reptation time is expected to go as M3+(0.31+/-0.03) within the intermediate range 41 x 10(3) less than or equal to M less than or equal to 375 x 10(3). This semilocal property and the chain self-diffusion M-beta (beta = 2.3+/-0.1 instead of 2) dependence, recently reported for many polymers and solutions, suggest a chain dynamics governed by a semilocal friction effect varying as an apparent M-0.3 power law instead of M-0. A thorough analysis of the pattern of proton relaxation curves is proposed; end-submolecules ( 60 monomeric units) are distinguished from inner ones from their magnetization dynamics; these are described as nonexponential time functions in accordance with the observation of pseudo-solid echoes (PSOSES). The molecular weight dependent pattern of four species of curves (two relaxation types and two PSOSE types) corresponding to 40 recorded curves is quantitatively analyzed using only four independent adjustable parameters.