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Organic Chemistry

31.7 Polymer Structure and Physical Properties

Organic Chemistry31.7 Polymer Structure and Physical Properties

31.7 • Polymer Structure and Physical Properties

Polymers aren’t really that different from other organic molecules. They’re much larger, of course, but their chemistry is similar to that of analogous small molecules. Thus, the alkane chains of polyethylene undergo radical-initiated halogenation, the aromatic rings of polystyrene undergo typical electrophilic aromatic substitution reactions, and the amide linkages of nylon are hydrolyzed by aqueous base.

The major difference between small and large organic molecules is in their physical properties. For instance, their large size means that polymers experience substantially greater van der Waals forces than do small molecules (Section 2.12). But because van der Waals forces operate only at close distances, they are strongest in polymers like high-density polyethylene, in which chains can pack together closely in a regular way. Many polymers, in fact, have regions that are essentially crystalline. These regions, called crystallites, consist of highly ordered portions in which the zigzag polymer chains are held together by van der Waals forces (Figure 31.4).

A representation of crystallites in linear polyethylene. The polymer changes are arranged as parallel lines in crystallites. Dotted ellipses are present surrounding the lines.
Figure 31.4 Crystallites in linear polyethylene. The long polymer chains are arranged in parallel lines in the crystallite regions.

As you might expect, polymer crystallinity is strongly affected by the steric requirements of substituent groups on the chains. Linear polyethylene is highly crystalline, but poly(methyl methacrylate) is noncrystalline because the chains can’t pack closely together in a regular way. Polymers with a high degree of crystallinity are generally hard and durable. When heated, the crystalline regions melt at the melt transition temperature, Tm, to give an amorphous material.

Noncrystalline, amorphous polymers like poly(methyl methacrylate), sold under the trade name Plexiglas, have little or no long-range ordering among chains but can nevertheless be very hard at room temperature. When heated, the hard amorphous polymer becomes soft and flexible at a point called the glass transition temperature, Tg. Much of the art in polymer synthesis lies in finding methods for controlling the degree of crystallinity and the glass transition temperature, thereby imparting useful properties to the polymer.

In general, polymers can be divided into four major categories, depending on their physical behavior: thermoplastics, fibers, elastomers, and thermosetting resins. Thermoplastics are the polymers most people think of when the word plastic is mentioned. These polymers have a high glass transition temperature and are therefore hard at room temperature but become soft and viscous when heated. As a result, they can be molded into toys, beads, telephone housings, or any of a thousand other items. Because thermoplastics have little or no cross-linking, the individual chains can slip past one another in the melt. Some thermoplastic polymers, such as poly(methyl methacrylate) and polystyrene, are amorphous and noncrystalline; others, such as polyethylene and nylon, are partially crystalline. Among the better-known thermoplastics is poly(ethylene terephthalate), or PET, used for making plastic soft-drink bottles.

The structure of poly(ethylene terephthalate) is indicated in parentheses with n as a subscript.

Plasticizers—small organic molecules that act as lubricants between chains—are usually added to thermoplastics to keep them from becoming brittle at room temperature. An example is poly(vinyl chloride), which is brittle when pure but becomes supple and pliable when a plasticizer is added. In fact, most drip bags used in hospitals to deliver intravenous saline solutions are made of poly(vinyl chloride), although replacements such as polypropylene are appearing.

Dialkyl phthalates such as di(2-ethylhexyl) phthalate (generally called dioctyl phthalate) are commonly used as plasticizers, although questions about their safety have been raised. The U.S. Food and Drug Administration (FDA) has advised the use of alternative materials in compromised patients and infants but has found no evidence of toxicity for healthy individuals. In addition, children’s toys that contain phthalates have been banned in the United States.

The structure of di(2-ethylhexyl) phthalate (or dioctyl phthalate) a plasticizer.

Fibers are thin threads produced by extruding a molten polymer through small holes in a die, or spinneret. The fibers are then cooled and drawn out, which orients the crystallite regions along the axis of the fiber and adds considerable tensile strength Figure 31.5. Nylon, Dacron, and polyethylene all have the semicrystalline structure necessary for being drawn into oriented fibers.

A representation of unoriented crystallites in a thermoplastic which is drawn to form oriented crystallites in a fiber.
Figure 31.5 Oriented crystallite regions in a polymer fiber.

Elastomers are amorphous polymers that have the ability to stretch out and spring back to their original shapes. These polymers must have low glass transition temperatures and a small amount of cross-linking to prevent the chains from slipping over one another. In addition, the chains must have an irregular shape to prevent crystallite formation. When stretched, the randomly coiled chains straighten out and orient along the direction of the pull. Van der Waals forces are too weak and too few to maintain this orientation, however, and the elastomer therefore reverts to its random coiled state when the stretching force is released (Figure 31.6).

The unstretched form of an elastomer stretches to form another elastomer. This elastomer relaxes to form the unstretched elastomer.
Figure 31.6 Unstretched and stretched forms of an elastomer.

Natural rubber (Section 14.6) is the most common example of an elastomer. Rubber has the long chains and occasional cross-links needed for elasticity, but its irregular geometry prevents close packing of the chains into crystallites. Gutta-percha, by contrast, is highly crystalline and is not an elastomer (Figure 31.7).

The structure of natural rubber in part a and gutta percha in part b. Both are eighteen-carbon chain with double bonds at different positions. Wavy lines denote the bond extensions.
Figure 31.7 (a) Natural rubber is elastic and noncrystalline because of its cis double-bond geometry, but (b) gutta-percha is nonelastic and crystalline because its geometry allows for better packing together of chains.

Thermosetting resins are polymers that become highly cross-linked and solidify into a hard, insoluble mass when heated. Bakelite, a thermosetting resin first produced in 1907, has been in commercial use longer than any other synthetic polymer. It is widely used for molded parts, adhesives, coatings, and even high-temperature applications such as missile nose cones.

Chemically, Bakelite is a phenolic resin, produced by reaction of phenol and formaldehyde. On heating, water is eliminated, many cross-links form, and the polymer sets into a rocklike mass. The cross-linking in Bakelite and other thermosetting resins is three-dimensional and is so extensive that we can’t really speak of polymer “chains.” A piece of Bakelite is essentially one large molecule.

Phenol reacts with formaldehyde in the presence of heat to form Bakelite. The structure of Bakelite shows phenol rings connected to formaldehyde units. Wavy lines denote the bond extensions.
Problem 31-11
What product would you expect to obtain from catalytic hydrogenation of natural rubber? Would the product be syndiotactic, atactic, or isotactic?
Problem 31-12
Propose a mechanism to account for the formation of Bakelite from acid-catalyzed polymerization of phenol and formaldehyde.
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