Types of Polymers
|Poly(vinyl Chloride) and Poly(vinylidene Chloride)||Acrylics||Condensation Polymers|
Addition polymers such as polyethylene, polypropylene, poly(vinyl chloride), and polystyrene are linear or branched polymers with little or no cross-linking. As a result, they are thermoplastic materials, which flow easily when heated and can be molded into a variety of shapes. The structures, names, and trade names of some common addition polymers are given in the table below.
Common Addition Polymers
|Structure||Chemical Name||Trade Name or
|poly(methyl methacrylate)||Plexiglas, Lucite|
Low-density polyethylene (LDPE) is produced by free-radical polymerization at high temperatures (200C) and high pressures (above 1000 atm). The high-density polymer (HDPE) is obtained using Ziegler-Natta catalysis at temperatures below 100C and pressures less than 100 atm. More polyethylene is produced each year than any other plastic. About 7800 million pounds of low-density and 4400 million pounds of high-density polyethylene were sold in 1980. Polyethylene has no taste or odor and is lightweight, nontoxic, and relatively inexpensive. It is used as a film for packaging food, clothing, and hardware. Most commercial trash bags, sandwich bags, and plastic wrapping are made from polyethylene films. Polyethylene is also used for everything from seat covers to milk bottles, pails, pans, and dishes.
The isotactic polypropylene from Ziegler-Natta-catalyzed polymerization is a rigid, thermally stable polymer with an excellent resistance to stress, cracking, and chemical reaction. Although it costs more per pound than polyethylene, it is much stronger. Thus, bottles made from poly-propylene can be thinner, contain less polymer, and cost less than conventional polyethylene products. Polypropylene's most important impact on today's college student takes the form of the plastic stackable chairs that abound on college campuses.
Tetrafluoroethylene (CF2=CF2) is a gas that boils at -76C and is therefore stored in cylinders at high pressure. In 1938 Roy Plunkett received a cylinder of tetrafluoroethylene that didn't deliver as much gas as it should have. Instead of returning the cylinder, he cut it open with a hacksaw and discovered a white, waxy powder that was the first polytetrafluoroethylene polymer. After considerable effort, a less fortuitous route to this polymer was discovered, and polytetrafluoro-ethylene, or Teflon, became commercially available.
Teflon is a remarkable substance. It has the best resistance to chemical attack of any polymer, and it can be used at any temperature between -73°C and 260°C with no effect on its properties. It also has a very low coefficient of friction. (In simpler crude terms, it has a waxy or slippery touch.) Even materials as "sticky" as rubber, adhesives, bread dough, and candy won't insects that stick to a Teflon-coated surface. Teflon is so slippery that it has even been sprayed on plants, so that might prey on the plants fall off.
Poly(vinyl Chloride) and Poly(vinylidene Chloride)
Chlorine is one of the top ten industrial chemicals in the US more than 20 billion pounds are produced annually. About 20% of this chlorine is used to make vinyl chloride (CH2=CHCl) for the production of poly(vinyl chloride), or PVC. The chlorine substituents on the polymer chain make PVC more fire-resistant than polyethylene or polypropylene. They also increase the force of attraction between polymer chains, which increases the hardness of the plastic. The properties of PVC can be varied over a wide range by adding plasticizers, stabilizers, fillers, and dyes, making PVC one of the most versatile plastics.
A copolymer of vinyl chloride (CH2=CHCl) and vinylidene chloride (CH2=CCl2) is sold under the trade name Saran. The same increase in the force of attraction between polymer chains that makes PVC harder than polyethylene gives thin films of Saran a tendency to "cling."
Acrylic acid is the common name for 2-propenoic acid: CH2=CHCO2H. Acrylic fibers such as Orlon are made by polymerizing a derivative of acrylic acid known as acrylonitrile.
Other acrylic polymers are formed by polymerizing an ester of this acid, such as methyl acrylate.
One of the most important acrylic polymers is poly(methyl methacrylate), or PMMA, which is sold under the trade names Lucite and Plexiglass.
|Poly(methyl methacrylate), PMMA|
PMMA is a lightweight, crystal-clear, glasslike polymer used in airplane windows, taillight lenses, and light fixtures. Because it is hard, stable to sunlight, and extremely durable, PMMA is also used to make the reflectors embedded between lanes of interstate highways.
The unusual transparency of PMMA makes this polymer ideal for hard contact lenses. Unfortunately, PMMA is impermeable to oxygen and water. Oxygen must therefore be transported to the cornea of the eye in the tears and then passed under the contact lens each time the eye blinks. Soft plastic lenses that pass both oxygen and water are made by using ethylene glycol dimethacrylate to crosslink poly(2-hydroxyethyl methacrylate).
|Ethylene glycol dimethacrylate|
An interesting polymer can be prepared by copolymerizing a mixture of acrylic acid and the sodium salt of acrylic acid. The product of this reaction has the following structure.
The difference between the Na+ ion concentration inside the polymer network and in the solution in which the polymer is immersed generates an osmotic pressure that draws water into the polymer. The amount of liquid that can be absorbed depends on the ionic strength of the solution the total concentration of positive and negative ions in the solution. This polymer can absorb 800 times it own weight of distilled water, but only 300 times its weight of tap water. Because the ionic strength of urine is equivalent to an 0.1 M NaCl solution, this superabsorbant polymer, which can be found in disposable diapers, can absorb up to 60 times its weight in urine.
The first plastic (Celluloid) and the first artificial fiber (Rayon) were produced from cellulose. The first truly synthetic plastic was bakelite, developed by Leo Baekland between 1905 and 1914. The synthesis of bakelite starts with the reaction between formaldehyde (H2CO) and phenol (C6H5OH) to form a mixture of ortho- and para-substituted phenols. At temperatures above 100C, these phenols condense to form a polymer in which the aromatic rings are bridged by either -CH2OCH2- or -CH2- linkages. The cross-linking in this polymer is so extensive that it is a thermoset plastic. Once it is formed, any attempt to change the shape of this plastic is doomed to failure.
Research started by Wallace Carothers and coworkers at DuPont in the 1920s and 1930s eventually led to the discovery of the families of condensation polymers known as polyamides and polyesters. The polyamides were obtained by reacting a diacyl chloride with an diamine.
The polyesters were made by reacting the diacyl chloride with a dialcohol.
While studying polyesters, Julian Hill found that he could wind a small amount of this polymer on the end of a stirring rod and draw it slowly out of solution as a silky fiber. One day, when Carothers wasn't in the lab, Hill and his colleagues tried to see how long a fiber they could make by stretching a sample of this polymer as they ran down the hall. They soon realized that this playful exercise had oriented the polymer molecules in two dimensions and produced a new material with superior properties. They then tried the same thing with one of the polyamides and produced a sample of what became the first synthetic fiber: Nylon.
This process can be demonstrated by carefully pouring a solution of hexamethylenediamine in water on top of a solution of adipoyl chloride in CH2Cl2.
A thin film of polymer forms at the interface between these two phases. By grasping this film with a pair of tweezers, we can draw a continuous string of nylon from the solution. The product of this reaction is known as Nylon 6,6 because the polymer is formed from a diamine that has six carbon atoms and a derivative of a dicarboxylic acid that has six carbon atoms.
The effect of pulling on the polymer with the tweezers is much like that of stretching an elastomer the polymer molecules become oriented in two dimensions. Why don't the polymer molecules return to their original shape when we stop pulling? Section P.3 suggested that polymers are elastic when there is no strong force of attraction between the polymer chains. Polyamides and polyesters form strong hydrogen bonds between the polymer chains that keep the polymer molecules oriented, as shown in the figure below.
|Practice Problem 4:
A synthetic fiber known as Nylon 6 has the following structure.
Explain how this polymer is made.
The first polyester fibers were produced by reacting ethylene glycol and either terephthalic acid or one of its esters to give poly(ethylene terephthalate). This polymer is still used to make thin films (Mylar) and textile fibers (Dacron and Fortrel).
Phosgene (COCl2) reacts with alcohols to form esters that are analogous to those formed when acyl chlorides react with alcohols.
The product of this reaction is called a carbonate ester because it is the diester of carbonic acid, H2CO3. Polycarbonates are produced when one of these esters reacts with an appropriate alcohol, as shown in the figure below. The polycarbonate shown in this figure is known as Lexan. It has a very high resistance to impact and is used in safety glass, bullet-proof windows, and motorcycle helmets.
The structures and names of some common condensation polymers are given in the table below.
Common Condensation Polymers
|Structure||Trade Name or