In the animation shown above, the polymerization of ethylene begins with the electrophilic addition of a proton to the double bond of one ethylene molecule. The carbocation produced then reacts with the best available nucleophile, which under the reaction conditions is another molecule of ethylene. This generates a second carbocation, which reacts with a third ethylene to produce a third carbocation, etc., etc. until, finally, one of the carbocations encounters an anion, at which point the reaction comes to an end. Note that the polymerization involves the pi system of each ethylene. This means that the process is essentially independent of the substituents attached to the doubly bonded carbons. Hence it is a very general process.
Figure 1 summarizes the general features of addition polymerizations.
The first stage is called initiation. In this stage an initiator reacts with the pi system of the alkene to produce an intermediate. If the initiator is a cation, as shown in the figure, the intermediate is a carbocation. In this case the addition that occurs during the initiation stage in Figure 1 follows Markownikov's Rule: in unsymmetrical alkenes, the more stable intermediate is formed preferentially. If the initiator is an anion, such as amide ion, the intermediate is a carbanion. If the initiator is a free radical, the intermediate is a free radical.
Regardless of the charge type of the intermediate, it is propagated in the second stage, during which the length of the polymer chain continues to grow until the concentration of the alkene is reduced to such a level that the final stage, termination, becomes likely.
Exercise 2 Amide ion may be used to initiate anionic polymerizations. Which of the following equations more accurately describes the initiation step in the anionic polymerization of propylene? Hint- We have seen that a methyl group is an activating group in electrophilic aromatic substitution reactions. A B
There are several features of the structures of these addition polymers that deserve comment. First, in the propagation step, the unsbustituted carbon of one monomer adds to the substituted carbon of another. This pattern of propagation is called head-to-tail polymerization. Figure 2 compares the substitution pattern that results from head-to-tail addition to those that would be observed if the addition were to occur in a head-to-head (or tail-to-tail) or a random manner, using polypropylene as an example.
Second, despite the linear nature of the polymer backbone implied in Figures 1 and 2, there is a significant amount of branching that occurs as the polymer chain grows. As Figure 1 makes clear, a "linear" polyethylene is anything but linear. As the polymer chain length increases, free rotation about the C-C bonds produces a large number of different conformations. Inevitably a conformation arises where a hydrogen atom may be transferred intramolecularly from one carbon to another. As a result of this intramolecular hydrogen atom transfer the polymer develops a branched structure as animated in Figure 3.
Ignoring branching for the moment, note that the formation of each new C-C bond is accompanied by the creadtion of a new chiral center at the carbon bearing the R group. This gives rise to three classes of polymers depending upon the relative stereochemistries at each new chiral center. If the stereochemistry at each chiral center is the same, either RRRRRR.... or SSSSSS...., the polymer is called isotactic. If the stereochemistry alternates from one chiral center to the next, RSRSRS.... or SRSRSR...., the polymer is said to be syndiotactic. If the stereochemistry is random, the polymer is atactic. Figure 4 illustrates these three possibilities for the case of polypropylene.
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
Figure 5 depicts each of these possibilities for two monomers, R and B.
a. ethylene/propylene b. ethylene/styrene c. propylene/styrene
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 cartoon 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.
