Polypropylene (PP) was first produced commercially in the 1950s following the discovery of Ziegler-Natta catalysts based on titanium chlorides and organo aluminium compounds. The incorporation of small amounts of other monomers, such as ethylene, to extend the properties of PP, is being increasingly employed. Polymer processes used for the production of PP can be divided into three main types: slurry or suspension, bulk or mass, and gas phase. PP was made by slurry polymerization using an aluminium alkyl and titanium trioxide catalyst in a hydrocarbon diluent. In the bulk process, liquid PP is polymerized in a tubular reactor using the same catalyst system as employed in the suspension route. PP has improved impact strength, higher softening point, lower density, better stress cracking and more scratch resistance than the polyethylenes. PP is the fastest growing commodity thermoplastic, with 7–9% growth predicted for the period 1990–1995.