Plastics and Tacticity
Prior to the 1950s propylene was polymerized by a process that produced atactic polypropylene. Since the physical properties of this material offered little advantage over those of polyethylene, there was not much demand for polpropylene since polyethylene was cheaper to manufacture. The situation changed dramatically in 1953 when Karl Ziegler and Giulio Natta published articles describing the development of catalyst systems that lead to the formation of isotactic and syndiotactic polypropylene. These systems all involve a mixture of titanium tetrachloride, TiCl4, and a trialkyl aluminum species such as triethyl aluminum, Al(CH2CH3)3. Ziegler and Natta shared the 1963 Noble Prize in Chemistry for their discovery.
Besides increasing tacticity, Ziegler-Natta catalysts decrease the amount of branching that occurs during polymerization. Since both of these factors produce polymers with greater structural regularity, the individual polymer chains fit together more tightly and the bulk polymers have higher melting points and higher densities than those prepared by traditional methods. These properties are desireable from a commercial perspective. The composition of many commercial polymers is indicated by an abbreviation: PE = polyethylene, HDPE = high density polyethylene, LDPE = low density polyethylene, LLDPE = linear low density polyethylene, PP = polypropylene, HDPE = high density polypropylene, PS = polystyrene, etc. Table 1 provides information about the most common ethylene monomers that are used in the production of commercial polymers. Most of the names should be familiar to you.
Many commercial polymers are prepared from more than one monomer. These materials are called co-polymers. Saran Wrap®, for example, is a co-polymer of vinyl chloride (CH2=CHCl) and vinylidene chloride ( CH2=CCl2). A plastic called ABS is a terpolymer of acrylonitrile ( CH2=CHCN), 1,3-butadiene ( CH2=CH-CH=CH2), and styrene ( CH2=CHC6H5). The monomers from which co-polymers are made may be combined in different ways. The four most common are
Thermoplastics and Thermosets
Commercial polymers may be classified into two broad categories, those that are thermoplastic and those that are thermosetting. Thermoplastic polymers are materials that may be deformed, e.g. bent, by applying a force to the heated polymer. However, the deformation is reversible.
Objects made from thermosetting polymers, on the other hand, retain their shape once they have been manufactured. The structural feature that differentiates these two types of materials is cross linking. In a thermosetting plastic the individual chains that comprise the bulk polymer are joined together by covalent bonds. These cross linking bonds are created after the polymer has been heated and forced into the desired shape. Once formed, these cross links prevent the object from changing shape even when reheated.
Figure 6 presents a comparison of themoplastic and thermosetting polymers. The letter L represents any species that links one polymer chain to another. The disulfide unit, S-S, is a common cross linking species, both in commercial polymers such as automobile tires and in natural polymers such as proteins.