The dielectric properties of poly (α-dimethyl β-propiolactone), also called polypivalolactone (PPL), have been investigated over a frequency range of 0.01 to 100 kHz and from 230 to 490 K. A limited number of dynamic mechanical measurements were also performed using a Rheovibron viscoelastometer and a torsion pendulum apparatus. Two relaxation regions, called β and γ in order of decreasing temperatures, were observed at about 410 K and 275 K. The γ relaxation process is associated with the crystalline phase of the polymer, and the β relaxation is attributed to its glass transition temperature. The analysis of the dielectric data leads to the conclusion that, despite the high degree of crystallinity of PPL, the frequency-temperature superposition principle is applicable to its γ relaxation peak, in agreement with earlier stress relaxation measurements. However, it could not be applied to its β relaxation peak except at low temperatures and high frequencies.