PROPYLENE

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Accelerated production and use of polypropylene began in the late 1950s when the discovery of Ziegler-Natta catalysts made large-scale polymerization of propylene economically feasible. The polymerization of propylene leads to several different structures that vary in their properties based on their tacticity. Tacticity, derived from the Greek word tactos meaning ordered, refers to how groups are arranged in a polymer. The general structure of the polypropylene molecule can be pictured as polyethylene in which a methyl (CH3) group has replaced a hydrogen atom in each monomer. Three general structures for polypropylene are termed isotactic, syndiotactic, and atactic.The different forms of polyethylene, such as low-density polyethylene (LDPE) and highdensity polyethylene (HDPE), dictate its physical properties. In the polymerization process, the isostatic structure forms helical coils that allow tight packing, resulting in a highly ordered crystalline structure and producing a hard, strong, stiff plastic with a high-melting-point. Conversely, the random atactic configuration prevents a tight structure resulting in an amorphous, soft substance. Polypropylene production accelerated around 1960 with the advent of Ziegler-Natta catalyst to control the polymerization process. In the last 20 years, a new group of catalysts called metallocene catalysts have resulted in signifi cant advances in the propylene industry. Metallocene catalysts consist of a transition metal, such as titanium or zirconium, sandwiched between carbon rings. Metallocene catalysts have allowed greater control and advances in polymerization. The polymerization of isostatic propylene up to about 1995 resulted in a structure with approximately 5% atactic polypropylene. Metallocene catalysts made it possible to produce 100% isostatic or syndiotactic polypropylenes. The catalysts have also allowed chemists to control the chain length of polypropylene tacticities in polymers to produce various polypropylenes with a range of physical and chemical characteristics. For example, rubbery elastomer polypropylene results by producing atactic polyethylene chains with regions of isostatic polypropylene interspersed along the chain. The isotactic regions are areas along the chain where greater attraction and packing between molecules takes place, resulting in cross-linking of the chains. This is similar to the vulcanization process in rubber. Th us a soft flexible polypropylene is produced. Polypropylene is also co-polymerized with polyethylene to expand its applications.