AbstractThe thermodynamic properties of isotactic polypropylene in the fully crystalline, glassy, and molten state are established from 0 to 500 K based on data bank heat capacities and the vibrational frequency spectrum. The seven skeletal vibrations are described by θ3and θ1of 91 and 714 K for crystalline polypropylene. For the calculation ofCp− Cv, a LindemannA0value of 1.5 × 10−3K mol/J was found. Calculated and experimental heat capacity data show agreement within an average error of −0.3 ± 1.6%. Glassy polypropylene was similarly treated up to the glass transition temperature at 260 K (θ3= 78 K, θ1= 633 K, average heat capacity error −0.6 ± 2.8%). The residual 0 K entropy of glassy polypropylene is 1.9 J/K mol. Partially crystalline polypropylene is shown through high‐sensitivity DSC measurements to have a glass transition range from 260 to 380 K. Similarly, high‐sensitivity DSC has been used to characterize the condis phase of polypropylene (conformationally disordered polypropylene) which was prior called “smectic mesophase” or “paracrystalline.” Its heat of transformation to the stable