he 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 θ3 and θ1 of 91 and 714 K for crystalline polypropylene. For the calculation of Cp − Cv, a Lindemann A0 value of 1.5 × 10−3 K 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.