Many of the most important and most interesting naturally occuring molecules are polymeric. In general these biopolymers belong to one of five classes of materials:
In this topic we will consider just the first two classes.
Natural rubber, is a polymeric form of isoprene, 2-methyl-1,3-butadiene. Figure 1 presents the structure of a tetrameric fragment of polyisoprene.
The red lines connecting the 4th carbon atom of one isoprene unit with the 1st carbon of the next indicate that latex is an addition polymer that results from the 1,4-addition of one isoprene unit to the next. Note the head-to-tail pattern in which the isoprene units are connected. Note, too, that the stereochemistry is the same at each double bond, namely cis.
The regularity that is evident in the structure of polyisoprene is characteristic of terpenes as well. Terpenes are oligomers of isoprene, typically containing 2, 4, or 6 isoprene units. Whether oligomeric or polymeric, these compounds all arise from a common biosynthetic path.
Exercise 1 Assume that you polymerized a sample of isoprene by treating it with a catalytic amount of sulfuric acid. Use curved arrows to indicate how the polymer fragment shown in Figure 1 would be formed. Draw the structures of the intermediate carbocations and show the resonance interaction between each cation and the double bond that is conjugated to it.
In 1839 Charles Goodyear discovered, literally by accident, that heating natural rubber with elemental sulfur altered the properties of the polymer, most notably making it tougher and more elastic. Goodyear's discovery led to the development of synthetic rubber, a material that found its most profitable application in the manufacture of automobile tires. Investigation of the structure of synthetic rubber revealed that the sulfur had formed disulfide bonds that linked one polyisoprene chain to the next. As Figure 2 demonstrates, these cross-links serve to restore the polymer to its original shape after it has been deformed by the application of a force.
Not surprisingly, the desireable properties of natural as well as synthetic rubber led to investigations of the polymerization of structural analogs of isoprene. One notable success came from the polymerization of 2-chloro-1,3-butadiene, sometimes called chloroprene. Polychloroprene is known commercially as neoprene rubber. It is widely used in the automotive industry for the manufacture of oil-resistant hoses. Neoprene that contains entrapped air has good insulating properties and is used in the production of wet suits.
Exercise 2 Draw a tetrameric fragment of polychlorprene similar to that shown for polyisoprene in Figure 1. Identify the repeat unit.
Some of the structurally most complex natural polymers are the polyphenols that occur in woody plants. These materials are referred to as lignins. Degradation of lignins has revealed that they are composed of three basic building blocks, p-coumaric acid, ferulic acid, and syringyl alcohol. These structures are shown in Figure 3 along with the structure of gallic acid, the principal phenolic component in tannins.
Figure 4 suggests a possible fragment of a lignin co-polymer based on these three monomers. The point here is simply that lignins are structurally very complex and, therefore, very difficult to characterize completely.
Putting the Pieces Together
Exercise 3 Cellulose is a polysaccharide. As such it contains many alcohol OH groups. While there are many alcohol OH groups in lignins, they also contain phenolic OH groups. The pKa values of alcohols and phenols are 16 and 10, respectively. The paper industry exploits this difference in pKa values to separate cellulose from lignins during the pulping process. Can you explain how?
In the presence of a catalytic quantity of acid, a mixture of phenol, C6H5OH, and formaldehyde, H2C=O, condenses to form a polymeric material known as Bakelite. This was one of the first commercial polymers. It was used for many years in the manufacture of cases for radios and televisions as well as electronic circuit boards. The formation of this co-polymer involves several of the types of reactions we have discussed:
- nucleophilic addition
- nucleophilic aliphatic substitution
- electrophilic aromatic substitution
Figure 5 illustrates how the combination of these reactions leads to the formation of polymeric material.
Shake and Bake(lite)
Exercise 4 Which of the steps labeled in Figure 5 involves nucleophilic addition? 1 2 3 4
Exercise 5 Which of the steps labeled in Figure 5 involves nucleophilic aliphatic substitution? 1 2 3 4
Exercise 6 Which of the steps labeled in Figure 5 involves electrophilic aromatic substitution? 1 2 3 4