The two major families of polyether polyols are polytetramethylene ether glycols (PTMEG) and polypropylene glycols (PPG). PTMEG is the premier polyol used in high-performance polyurethane elastomers. PTMEG-based polyurethanes exhibit superior resistance to hydrolytic cleavage, good mechanical property retention at low temperature, high resiliency, good processing characteristics, and excellent mechanical and dynamic properties. Strain-induced crystallization of the PTMEG soft segments, exact di-functionality, and low acid values are all contributing factors to the good mechanical properties of the associated polyurethane elastomers. PPG polyols have excellent hydrolysis resistance and low temperature properties as well. However, when compared to PTMEG polyols, the PPG polyols have lower mechanical properties and are more prone to thermo-oxidative degradation.