Polypropylene (PP) - This polyolefin is readily formed by polymerizing propylene with suitable catalysts, generally aluminum alkyl and titanium tetrachloride. Polypropylene properties vary according to molecular weight, method of production, and the copolymers involved. Generally polypropylene has demonstrated certain advantages in improved strength, stiffness and higher temperature capability over polyethylene. Polypropylene has been very successfully applied to the forming of fibers due to its good specific strength which is why it is the single largest use of polypropylene. Polypropylene also happens to be one of the lightest plastics available with a density of 0.905 g/cm².