One of the much debated mysteries in 1H NMR relaxation measurements of bitumen and heavy crude oils is the departure from expected theoretical trends at high viscosities, where traditional theories of 1H–1H dipole–dipole interactions predict an increase in T1 with increasing viscosity. However, previous experiments on bitumen and heavy crude oils clearly show that T1LM (i.e., log-mean of the T1 distribution) becomes independent of viscosity at high viscosities; in other words, T1LM versus viscosity approaches a plateau. We report 1H NMR data at ambient conditions on a set of pure polymers and polymer–heptane mixes spanning a wide range of viscosities (η = 0.39 cP ↔ 334 000 cP) and NMR frequencies (ω0/2π = f0 = 2.3 MHz ↔ 400 MHz) and find that at high viscosities (i.e., in the slow-motion regime) T1LM plateaus to a value T1LM> ∝ ω0 independent of viscosity, similar to bitumen. More specifically, on a frequency-normalized scale, we find that T1LM> × 2.3/f0 ≃ 3 ms (i.e., normalized relative to 2.3 MHz), in g...