Polymers are substances whose molecules have high molar
masses and are composed of a large number of repeating units. There are both
naturally occurring and synthetic polymers. Among naturally occurring polymers
are proteins, starches, cellulose, and latex. Synthetic polymers are produced
commercially on a very large scale and have a wide range of properties and uses.
The materials commonly called plastics are all synthetic polymers.
Polymers
are formed by chemical reactions in which a large number of molecules called
monomers are joined sequentially, forming a chain. In many polymers, only one
monomer is used. In others, two or three different monomers may be combined.
Polymers are classified by the characteristics of the reactions by which they
are formed. If all atoms in the monomers are incorporated into the polymer, the
polymer is called an addition polymer.
If some of the atoms of the monomers are released into small molecules, such as
water, the polymer is called a condensation
polymer. Most addition polymers are made from monomers containing a
double bond between carbon atoms. Such monomers are called olefins, and most
commercial addition polymers are polyolefins. Condensation polymers are made
from monomers that have two different groups of atoms which can join together to
form, for example, ester or amide links. Polyesters are an important class of
commercial polymers, as are polyamides (nylon).
Polyethylene terephthalate (PET), or polyethylene
terephthalic ester (PETE), is a condensation polymer produced from the monomers
ethylene glycol, HOCH2CH2OH, a dialcohol, and dimethyl
terephthalate, CH3O2C–C6H4–CO2CH3,
a diester. By the process of transesterification, these monomers form ester
linkages between them, yielding a polyester. PETE fibers are manufactured under
the trade names of Dacron and Fortrel. Pleats and creases can be permanently
heat set in fabrics containing polyester fibers, so-called permanent press
fabrics. PETE can also be formed into transparent sheets and castings. Mylar is
a trade name for a PETE film. Transparent 2-liter carbonated beverage bottles
are made from PETE. (The opaque base on some bottles is generally made of HDPE.)
One form of PETE is the hardest known polymer and is used in eyeglass lenses.
POLYETHYLENE
Polyethylene is perhaps the simplest polymer, composed of
chains of repeating –CH2– units. It is produced by the addition
polymerization of ethylene, CH2=CH2
(ethene). The properties of polyethylene depend on the manner in which ethylene
is polymerized. When catalyzed by organometallic com pounds at moderate pressure
(15 to 30 atm), the product is high density polyethylene, HDPE. Under these
conditions, the polymer chains grow to very great length, and molar masses
average many hundred thousands. HDPE is hard, tough, and resilient.
Most HDPE is used in the manufacture of containers, such as milk bottles and
laundry detergent jugs. When ethylene is polymerized at high pressure (1000–2000
atm), elevated temperatures (190–210°C), and catalyzed by peroxides, the product
is low density polyethylene, LDPE. This form of polyethylene has molar masses of
20,000 to 40,000 grams. LDPE is relatively soft, and most of it is used in the
production of plastic films, such as those used in sandwich bags.
POLYVINYL CHLORIDE
Polymerization
of vinyl chloride, CH2=CHCl (chloroethene), produces a polymer
similar to polyethylene, but having chlorine atoms at alternate carbon atoms on
the chain. Polyvinyl chloride (PVC) is rigid and somewhat brittle. About
two-thirds of the PVC produced annually is used in the manufacture of pipe. It
is also used in the production of “vinyl” siding for houses and clear plastic
bottles. When it is blended with a plasticizer such as a phthalate ester, PVC
becomes pliable and is used to form flexible articles such as raincoats and
shower curtains.
POLYPROPYLENE
This polymer is produced by the addition polymerization of
propylene, CH2=CHCH3
(propene). Its molecular structure is similar to that of polyethylene, but has a
methyl group (–CH3) on alternate carbon atoms of the chain. Its molar
masses falls in the range 50,000 to 200,000 grams. Polypropylene (PP) is
slightly more brittle than polyethylene, but softens at a temperature about 40°C
higher. Polypropylene is used extensively in the automotive industry for
interior trim, such as instrument panels, and in food packaging, such as yogurt
containers. It is formed into fibers of very low absorbance and high stain
resistance, used in clothing and home furnishings, especially carpeting.
POLYSTYRENE
Styrene, CH2=CH–C6H5,
polymerizes readily to form polystyrene (PS), a hard, highly transparent
polymer. The molecular structure is similar to that of polypropylene, but with
the methyl groups of polypropylene replaced by phenyl groups (–C6H5).
A large portion of production goes into packaging. The thin, rigid, transparent
containers in which fresh foods, such as salads, are packaged are made from
polystyrene. Polystyrene is readily foamed or formed into beads. These foams and
beads are excellent thermal insulators and are used to produce home insulation
and containers for hot foods. Styrofoam is a trade name for foamed polystyrene.
When rubber is dissolved in styrene before it is polymerized, the polystyrene
produced is much more impact resistant. This type of polystyrene is used
extensively in home appliances, such as the interior of refrigerators and air
conditioner housing. [For more information about this polymer, see Chemical
Demonstrations: A Handbook for Teachers of Chemistry, by Bassam Z.
Shakhashiri, Volume 1 (1983), page 241.]
POLYTETRAFLUOROETHYLENE
Teflon is a trade name of polytetrafluoroethylene, PTFE. It
is formed by the addition polymerization of tetrafluoroethylene, CF2=CF2
(tetrafluoroethene). PTFE is distinguished by its complete resistance to attack
by virtually all chemicals and by its slippery surface. It maintains its
physical properties over a large temperature range, -270° to 385°C. These
properties make it especially useful for components that must operate under
harsh chemical conditions and at temperature extremes. Its most familiar
household use is as a coating on cooking utensils.
POLYURETHANE
This important class of polymers is formed by the addition
polymerization of an diisocyanate (whose molecules contain two –NCO groups) and
a dialcohol (two –OH groups). The polymer chain is linked by urethane groups
(–O–CO–NH–). The –NH– portion of the urethane group can react similarly to an
–OH group, producing cross-linking between polymer chains. Polyurethane is spun
into elastic fibers, called spandex, and sold under the trade name Lycra.
Polyurethane can also be foamed. Soft polyurethane foams are used in upholstery,
and hard foams are used structurally in light aircraft wings and sail boards.
The formation of some polyurethane (and polystyrene) foams exploits the
exothermic nature of the polymerization reaction. A liquid with a low boiling
point, called a blowing agent, is added to the monomers before the
polymerization starts. As the polymerization proceeds, it releases enough heat
to boil the liquid. The boiling liquid produces bubbles that create a foam. In
the past, the most commonly used low-boiling liquids were chlorofluorocarbons.
However, the damaging effect of chlorofluorocarbons on the stratospheric ozone
layer has eliminated their use. Other low-boiling liquids have other
disadvantages, such as flammability. Therefore, most polyurethane and
polystyrene foams are manufactured by forcing a pressurized gas, such as
nitrogen or carbon dioxide, into the polymerizing mixture. [For more information
about this polymer, see Ibid., Volume 1, page 216.]
POLYAMIDE
Polyamides are a group of condensation polymers commonly
known as nylon. Nylon is made from two monomers, one a dichloride and the other
a diamine. One particular nylon is made from 1,6-diaminohexane, NH2(CH2)6NH2
and sebacoyl chloride, ClCO(CH2)8COCl. When these
polymerize, the resulting molecules contain repeating units of –NH(CH2)6NH–CO(CH2)8CO–.
Molecules of HCl are released during the polymerization. This particular polymer
is called nylon 6-10 because it contains alternating chains of 6 and 10 carbon
atoms between nitrogen atoms. Nylon can be readily formed into fibers that are
strong and long wearing, making them well suited for use in carpeting,
upholstery fabric, tire cords, brushes, and turf for athletic fields. Nylon is
also formed into rods, bars, and sheets that are easily formed and machined. In
this form, nylon is used for gears and for automobile fuel tanks. [For more
information about this polymer, see Ibid., Volume 1, page 213.]
POLYACRYLAMIDE
Polyacrylamide is a condensation polymer with an unusual and
useful property. The structure of polyacrylamide is similar to that of
polyethylene, but having a hydrogen on every other carbon replace by an amide
group, –CONH2. The molecule is composed of repeating –CH2–CH(CONH2)–
units. The amide groups allow for linking between polymer strands. The –CONH2
group from one molecule can react with the same group of another molecule,
forming a link between them with the structure –CONHCO–. This produces a network
of polymer chains, rather like a tiny sponge. The free, unlinked amide groups,
because they contain –NH2 groups, can form hydrogen bonds with water.
This gives the tiny cross linked sponges a great affinity for water.
Polyacrylamide can absorb many times its mass in water. This property is useful
in a variety of applications, such as in diapers and in potting soil. The
polyacrylamide will release the absorbed water if a substance that interferes
with hydrogen bonding is added. Ionic substances, such as salt, cause
polyacrylamide to release its absorbed water. [For more information about this
polymer, see Ibid., Volume 3 (1989), page 368.]
Proteins
and Amino Acids
Most HDPE is used in the manufacture of containers, such as milk bottles and
laundry detergent jugs. When ethylene is polymerized at high pressure (1000–2000
atm), elevated temperatures (190–210°C), and catalyzed by peroxides, the product
is low density polyethylene, LDPE. This form of polyethylene has molar masses of
20,000 to 40,000 grams. LDPE is relatively soft, and most of it is used in the
production of plastic films, such as those used in sandwich bags.
