Polypropylene is one of the most widely used thermoplastics, accounting for one-sixth of total consumption. Many automobiles, textile appliances, and other everyday items contain substantial amounts of these polymers. The ability to manufacture a broad variety of polypropylenes having properties tailored for specific applications is the primary factor in its widespread use. Comonomers, usually ethylene, are often used to provide impact-modified or lower melting polymers. The types of polymers produced, processing methods, and end use are discussed.
Highly stereoselective Ziegler-Natta catalyst systems are the basis for all propylene polymerization processes. The development of these catalysts, from the early TiCl3 systems to the modern, high yield MgCl2-supported systems, including polymerization mechanisms, is discussed. These catalysts enabled the evolution of these polymerization processes from the early slurry process, to the current liquid monomer or gas-phase processes. In the future, metallocene catalysts may provide further commercial advances. The Spheripol and Unipol processes are the most notable of the many processes discussed. The large expansion in polypropylene production in the late 1980s was facilitated by the availability of this newer, less costly technology. This expansion, well in excess of growth in consumption, caused a dramatic reduction in polypropylene prices in the 1990s. This expansion was most dramatic in the Far East, where few polypropylene plants existed outside Japan before 1985.
1. Top of Article
Propylene polymerization processes have undergone a number of revolutionary changes since the first processes for the production of crystalline polypropylene (PP) were commercialized in 1957 by Montecatini in Italy and Hercules in the United States. These first processes were based on Natta's discovery in 1954 that a Ziegler catalyst could be used to produce highly isotactic polypropylene (1). The stereoregular, crystalline polymers produced by this technology had sufficiently attractive economic and property performance that they became significant commercial thermoplastics in a remarkably short period. Consequently, Ziegler and Natta were awarded the Nobel Prize in Chemistry in 1963. Other technologies invented during the same period (2, 3), were incapable of achieving commercially viable performance. The tremendous amount of activity in olefin polymerization at this time led to a massive patent interference in the United States, and the award of a U.S. Patent to Phillips for the composition of matter of crystalline polypropylene (4, 5). Improvements to the basic Ziegler-Natta catalysts made at Solvay increased the activity and stereoregularity of these catalyst systems, extending their commercial application (6). In most cases, however, supported high yield catalyst systems, invented by Montedison and Mitsui Petrochemical (7), are used in plants that have been constructed since the early 1980s (see CATALYSTS, SUPPORTED). The superior performance of these systems allows the use of simplified processes with dramatic reductions in capital and operating costs. This revolution in the economics of polypropylene production spurred an increase in worldwide capacity in the 1980s. Worldwide production of propylene polymers was 14,381,000 t/yr in 1993 (8), an annual increase of 4.7% from 7,147,000 t/yr in 1978 (9).
Interest has been generated by the advances made in the homogeneously catalyzed polymerization of propylene using metallocenes (10). Polymers produced with these catalyst systems have an unusual balance of properties that may become commercially attractive.
2.1. Structure and Crystallinity
The stereochemistry of propylene polymers was first studied by Natta, who defined three possible structures of polypropylene by the location of the pendent methyl groups relative to the polymer backbone (1, 11, 12). Isotactic polypropylene consists of molecules in which all methyl groups have the same stereochemistry as a result of all insertions of propylene monomer being identical. Syndiotactic polypropylene is produced by regular alternating stereochemistry of monomer insertion, resulting in alternating locations of the pendent methyl groups. Atactic polypropylene, which is noncrystalline, is the result of nonstereospecific monomer insertion and random location of the pendent methyl groups (Fig. 1). A polymer structure referred to as stereoblock is an intermediate structure consisting of isotactic and atactic segments (13). When Ziegler-Natta catalysts are used, monomer is inserted in a head-to-tail manner with few head-to-head or tail-to-tail inversions (14). The degree of stereoregularity of polypropylene molecules is determined by the use of 13C nmr of the polymer in solution (15). The percentage of (mmmm) pentads is used to describe the percentage of isotacticity in the polymer molecule. Meso (m) insertions produce a polymer with the same methyl configuration, while racemic (r) insertions produce polymers with alternating methyl configuration. Consequently, (rrrr) pentads are syndiotactic. Random insertions sequences, such as (mrrm) produce hetero- or atactic polymers. Information on the macromolecular structure can also be obtained from infrared spectra, which for isotactic polypropylene are affected by vibrations of the regular spiral chain and not by the polymer crystallinity. Bands are detectable at 1167, 997, and 841 cm–1 (16). Stereoregularity can be inferred from solubility measurements. Historically, these methods have been used to determine stereoregularity. The isotactic index of the polymer is the fraction insoluble in boiling n-heptane (17). Solubility in xylene, or in decalin (decahydronaphthalene), is also commonly used to describe stereoregularity on a commercial basis (18); it is also possible to separate the polymer into fractions of different crystallinity (19). The use of pulsed proton solid-state nmr has been demonstrated as an alternative to solubility measurements of stereoregularity (20).
Crystallinity of polypropylene is usually determined by x-ray diffraction (21). Isotactic polymer consists of helical molecules, with three monomer units per chain unit, resulting in a spacing between units of identical conformation of 0.65 nm (Fig. 2a). These molecules interact with others, or different segments of the same molecule, to form a monoclinic unit cell containing 12 monomer units having a crystallographic density of 0.936 g/cm3 (22). The predominant crystalline form of polypropylene is the -form (23); however, -, -, and smectic forms can also be obtained. Smectic polypropylene, obtained by rapid cooling of the polymer melt at low temperatures, has no wide-range crystalline order (24). The -form is obtained by rapid cooling at temperatures between 100 and 130°C (25). The -form is obtained by crystallization under high pressure (26). Both the - and -forms can be converted to the -form by heating.
Syndiotactic polypropylene also forms helical molecules; however, each chain unit consists of four monomer units having a spacing of 0.74 nm. The unit cell is orthorhombic and contains 48 monomer units having a crystallographic density of 0.91 g/cm3 (27).
Polypropylene molecules repeatedly fold upon themselves to form lamellae, the sizes of which are a function of the crystallization conditions. Higher degrees of order are obtained upon formation of crystalline aggregates, or spherulites. The presence of a central crystallization nucleus from which the lamellae radiate is clearly evident in these structures. Observations using cross-polarized light illustrates the characteristic Maltese cross model (Fig. 2b). The optical and mechanical properties are a function of the size and number of spherulites and can be modified by nucleating agents. Crystallinity can also be inferred from thermal analysis (28) and density measurements (29).