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POLYMER CHEMISTRY
P.1 Polymers
P.2 Definition of Terms
P.3 Elastomers
P.4 Free Radical Polymerization Reactions
P.5 Ionic Polymerization Reactions
P.6 Coordination Polymerization
P.7 Addition Polymers
P.8 Condensation Polymers
P.9 Properties of Polymers
Chemistry in the World Around Us: The Search for Synthetic Fibers
P.1 POLYMERS
Imagine that one evening you decide to go bowling. Wearing a pair of jeans, a bowling
shirt, and a lightweight jacket, you walk into the local bowling alley. After you change into
an appropriate pair of shoes, you pick up a ball, stride to the line, smoothly release the
ball, watch as it rolls down the lane until it collides with a set of wooden pins, and then
use a pencil to record your score on a sheet of paper.
The fabric of your cotton jeans, your polyester shirt, your nylon jacket, and your leather
shoes have something in common with the rubber bowling ball, the polyurethane coating
on the bowling lane, the wooden pins, the mixture of graphite and clay in the “lead” pencil, and the sheet of paper on which you wrote your score. Each of these substances is a
polymer.
In 1833, Jöns Jakob Berzelius suggested that compounds with the same molecular formula but different structures should be called isomers (literally, “equal parts”). He then
proposed the term polymer (literally, “many parts”) to describe compounds that had the
same empirical formula but different molecular weights. Ethylene (C2H4) and butene
(C4H8) are compounds that Berzelius would classify as polymers. Each compound has the
same empirical formula (CH2) but they have different molecular weights. Acetylene (C2H2)
and benzene (C6H6) are another example of compounds Berzelius would call polymers.
The term polymer eventually came to mean compounds such as cellulose and natural
rubber that have unusually large molecular weights, in the range of 10,000 to 100,000 or
more grams per mole. These molecules are so large they are often called macromolecules—
literally, molecules large enough to be seen with the naked eye. A perfect diamond, for example, can be thought of as a single molecule containing an array of COC bonds arranged
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toward the corners of a tetrahedron around each carbon atom in the crystal. The plastic
case that enclosed one of the radios that your grandparents listened to can be thought of
as a single molecule. So can a 14-pound bowling ball.
The first explanation of why polymer molecules are so heavy was offered by Hermann
Staudinger in 1920. Staudinger argued that polymers contain long chains of relatively simple repeating units, or monomers. Natural rubber, for example, is a polymer that contains
large numbers of OCH2C(CH3)PCHCH2O units. The number of monomers in a polymer can differ from one chain to the next. Rubber, for example, is a mixture of polymer
chains whose mass differs by a factor of 10 or more, but whose average molecular weight
is 100,000 grams per mole.
The term rubber was first used in 1770 by Joseph Priestley to describe the gum from a
South American tree that could be used to “rub out” pencil marks. Because natural rubber is tacky, strong-smelling, perishable, too soft when warm, and too hard when cold, it
had only limited uses. Nathaniel Hayward was the first to note that rubber loses some of
its sticky properties when treated with sulfur. It was Charles Goodyear, however, who accidentally dropped a mixture of rubber and sulfur onto a hot stove and discovered “vulcanized” rubber, which is stable over a wide range of temperatures and is far more durable
than natural rubber.
Cellulose is another example of a polymer that contains many copies of a simple repeating
unit: C6H10O5. Wood is about 50% cellulose by weight; cotton is almost 90% cellulose. Cellulose is used to make paper from wood pulp and cloth from cotton. In the last hundred
years, cellulose has also served as the starting material for the synthesis of the first plastics—
cellulose nitrate and celluloid—and the first synthetic fibers—Chardonnet silk, or rayon.
Each OC6H10O5O repeating unit in cellulose contains three OOH groups that can react with nitric acid to form nitrate esters known as cellulose nitrate. In 1869 John Wesley
Hyatt found that mixtures of cellulose nitrate and camphor dissolve in alcohol to produce
a plastic substance he named celluloid. Cellulose nitrate, or celluloid, was used as a substitute for ivory in the manufacture of a variety of items ranging from billiard balls to movie
film. Because it is extremely flammable, cellulose nitrate has been replaced by other plastics for almost all uses except Ping-Pong balls. No other plastic has been found that has
quite the same “bounce” as celluloid.
The cellulose from wood pulp contains too many impurities to be used to make fibers.
It can be purified, however, by dissolving the polymer in a mixture of NaOH and carbon
disulfide (CS2). When the viscous solution is forced through tiny holes in a nozzle into an
acid bath, the cellulose fiber is regenerated. When this process was introduced in 1885, the
product was the first synthetic fiber. It was originally called Chardonnet silk, but soon become known as rayon. A similar process is still used to make a thin film of regenerated
cellulose known as cellophane.
P.2 DEFINITIONS OF TERMS
Linear, Branched, and Cross-linked Polymers
The term polymer is used to describe compounds with relatively large molecular weights
formed by linking together many small monomers. Polyethylene, for example, is formed
by polymerizing ethylene molecules.
n CH2PCH2
[...(CH2CH2)n...]
Ethylene
Polyethylene
Polyethylene is called a linear or straight-chain polymer because it consists of a long string
of carbon–carbon bonds. Those terms are misleading, however, because the geometry
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around each carbon atom is tetrahedral and the chain is neither linear nor straight, as shown
in Figure P.1.
FIGURE P.1 Small portion of a straight-chain polyethylene
molecule.
As the polymer chain grows, it folds back on itself in a random fashion to form structures
such as the one shown in Figure P.2.
Linear polymer
FIGURE P.2 Random structure formed by a straight-chain polymer as it folds back
on itself.
Polymers with branches at irregular intervals along the polymer chain are called
branched polymers (see Figure P.3). The branches make it difficult for the polymer molecules to pack in a regular array, and therefore make the polymer less crystalline. Crosslinked polymers contain branches that connect polymer chains, as shown in Figure P.4. At
first, adding cross-links between polymer chains makes the polymer more elastic. The vulcanization of rubber, for example, results from the introduction of short chains of sulfur
atoms that link the polymer chains in natural rubber. As the number of cross-links increases, the polymer becomes more rigid.
Branched polymer
FIGURE P.3 Branched polymers contain short side chains that extend
from the backbone of the polymer.
Cross-linked polymer
FIGURE P.4 Cross-linked polymers
have branches that connect chains.
The decision to classify a polymer as branched or cross-linked is based on the extent
to which the side chains on the polymer backbone link adjacent polymer chains. The easiest way to distinguish between these categories is to study the effect of various solvents on
the polymer. Branched polymers are often soluble in one or more solvents because it is
possible to separate the polymer chains. Cross-linked polymers are insoluble in all solvents
because the polymer chains are tied together by strong covalent bonds.
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Linear and branched polymers form a class of materials known as thermoplastics. These
materials flow when heated and can be molded into a variety of shapes which they retain
when they cool. Heavy cross-linking produces materials known as thermoset plastics. Once
the cross-links form, the polymers take on a shape that cannot be changed without destroying the plastic. The polypropylene used in the plastic chairs that fill so many classrooms is a thermoplastic; as you lean back on the chair you can feel it give. The plastic
case in which early radios were placed is an example of a thermoset plastic; it had a tendency to shatter rather than bend if the radio was dropped on the floor.
Exercise P.1
Polyethylene can be obtained in two different forms. High-density polyethylene (0.96 g/cm3)
is a linear polymer. Low-density polyethylene (0.92 g/cm3) is a branched polymer with short
side chains on 3% of the atoms along the polymer chain. Explain how the structures of
these polymers give rise to the difference in their densities.
Solution
Linear polymers are more regular than branched polymers. Linear polymers can therefore
pack more tightly, with less wasted space. As a result, linear polymers are slightly more
dense than branched polymers.
Homopolymers and Copolymers
Polyethylene is an example of a homopolymer that is formed by polymerizing a single
monomer. Copolymers are formed by polymerizing more than one monomer. Ethylene
(CH2PCH2) and propylene (CH2PCHCH3) can be copolymerized, for example, to produce a polymer that has two kinds of repeating units.
x CH2 PCH2 y CH2 PCHCH3
CH3
A
...(CH2CH2)x CH2CH y...
Copolymers are classified on the basis of the way monomers are arranged along the polymer chain, as shown in Figure P.5. Random copolymers contain repeating units arranged in
a purely random fashion. Regular copolymers contain a sequence of regularly alternating repeating units. The repeating units in block copolymers occur in blocks of different lengths.
Graft copolymers have a chain of one repeating unit grafted onto the backbone of another.
[...OAOBOBOAOAOAOBOAOBOBO...]
Random copolymer
[...OAOBOAOBOAOBOAOBOAOBO...]
Regular copolymer
[...OAOAOAOAOBOBOBOBOAOAOBOBOBOBOBOBOAOAOA...]
Block copolymer
...OAOAOAOAOAOAOAOAOAOAO...
A
BOBOBOBOBO...
Graft copolymer
FIGURE P.5 Random, regular, block, and graft copolymers.
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Tacticity
Polymers with regular substituents on the polymer chain possess a property known as tacticity (from the Latin word tacticus, “fit for arranging”). Tacticity results from the different ways in which the substituents can be arranged on the polymer backbone (see Figure P.6). When the substituents are arranged in an irregular, random fashion, the polymer
is atactic (literally, “no arrangement”). When the substituents are all on the same side of
the chain, the polymer is isotactic (literally, “the same arrangement”). If the substituents
alternate regularly from one side of the chain to the other, the polymer is syndiotactic.
CH3
CH3
CH3
A
A
A
... OCH2OCHOCH2OCHOCH2OCHOCH2OCHOCH2OCHO...
A
A
CH3
CH3
Atactic polypropylene
... OCH2OCHOCH2OCHOCH2OCHOCH2OCHOCH2OCHO...
Syndiotactic polystyrene
... OCH2OCHOCH2OCHOCH2OCHOCH2OCHOCH2OCHO...
A
A
A
A
A
Cl
Cl
Cl
Cl
Cl
Isotactic poly(vinyl chloride)
FIGURE P.6 Atactic, isotactic, and syndiotactic polymers.
Exercise P.2
Atactic polypropylene is a soft, rubbery material with no commercial value. The isotactic
polymer is a rigid substance with an excellent resistance to mechanical stress. Explain the
difference between the physical properties of the two forms of polypropylene.
Solution
Isotactic polypropylene is easier to pack in a regular fashion than the atactic form of the
polymer. As a result, isotactic polypropylene is more crystalline, which makes the solid
more rigid.
Addition versus Condensation Polymers
Polyethylene, polypropylene, and poly(vinyl chloride) are addition polymers formed by
adding monomers to a growing polymer chain. Addition polymers can be recognized by
noting that the repeating unit always has the same formula as the monomer from which
the polymer is formed.
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n CH2PCH2
[...(CH2CH2)n...]
Polyethylene
n CH2PCHCH3
CH3
A
... CH2CH n...
Polypropylene
n CH2PCHCl
Cl
A
... CH2CH n...
Poly(vinyl chloride)
To condense means to make something more dense, or compact. Polymers formed when
a small molecule condenses out during the polymerization reaction are therefore called
condensation polymers. Silicone, for example, is a condensation polymer formed by polymerizing (CH3)2Si(OH)2. Each time a monomer is added to the polymer chain, a molecule
of water is condensed out, as shown in Figure P.7. Note that the repeating unit in a condensation polymer is inevitably smaller than the monomer from which it is made.
CH3
CH3
CH3
A
A
A
... HOOOSiOOOH HOOOSiOOOH HOOOSiOOOH ...
A
A
A
CH3
CH3
CH3
H2O
CH3
CH3
CH3
A
A
A
...OOSiOOOSiOOOSiOO...
A
A
A
CH3
CH3
CH3
Silicone
FIGURE P.7 Silicone is a condensation polymer produced by eliminating a
molecule of water each time a bond between monomers is formed.
Exercise P.3
Classify the products of the following reactions as either addition or condensation polymers.
(a) poly(methyl methacrylate), sold as Lucite or Plexiglas
CH3
A
n CH2 P CCO2CH3
CH3
A
... CH2C
A
CO2CH3
n...
(b) nylon 6
n H2N(CH2)5CCl
O
P
P
O
... HN(CH2)5C n... n HCl
Solution
(a) Poly(methyl methacrylate) is an addition polymer because every atom in the monomer
ends up in the repeating unit of the polymer.
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(b) Nylon 6 is a condensation polymer because a molecule of HCl is eliminated each time
a monomer is added to the chain.
P.3 ELASTOMERS
Elastomers are polymers that have the characteristic properties of rubber—they are both
flexible and elastic. To be elastic, a polymer must meet the following criteria.
•
•
•
•
It must contain long, flexible molecules that are coiled in the natural state and that
can be stretched without breaking, as shown in Figure P.8.
It must contain a few cross-links between polymer chains so that one chain does not
slip past another when the substance is stretched.
It cannot contain too many cross-links, or else it would be too rigid to be stretched.
The force of attraction between chains must be relatively small, so that the polymer
can curl back into its coiled shape after it has been stretched.
Stretch
Relax
FIGURE P.8 The effect of stretching and relaxing the cross-linked chains
in an elastomer.
We can understand these requirements by taking a closer look at the chemistry of natural rubber, which is a polymer of a C5H8 hydrocarbon known as isoprene.
CH3
A
n CH2 P CCHP CH2
CH3
A
... CH2C P CHCH2 n...
Isoprene
Natural rubber
The double bonds in natural rubber are all in the cis form. The force of attraction between
polymer chains is relatively small, so the polymer can curl back into its original shape after the molecules have been oriented by stretching. By adding sulfur to natural rubber it
is possible to introduce a small number of cross-links between polymer chains that hold
the chains together when the polymer is stretched.
At first glance, it might seem easy to make synthetic rubber. All we have to do is find
a suitable catalyst that can polymerize isoprene. The task is made more difficult by the fact
that the cis isomer of isoprene rearranges into the trans isomer during polymerization, and
the trans isomer of polyisoprene, which is known as gutta percha, is not elastic. It is therefore important to control the geometry around the CPC double bond during polymerization to make sure that as few of the bonds as possible are converted to the trans geometry. Until recently, this wasn’t possible, and other approaches to making synthetic rubber
were necessary.
The first solution to the problem involved polymerizing 2-chloro-1,3-butadiene, or
“chloroprene,” to form the first major synthetic rubber, neoprene.
Cl
A
n CH2 P CCHP CH2
Cl
A
... CH2C P CHCH2 n...
Chloroprene
Neoprene
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This approach is still used to produce a copolymer of 75% butadiene and 25% styrene
known as styrene–butadiene rubber (SBR). Roughly 40% of the rubber used in the world
today is SBR; another 35% is natural rubber that has been treated with sulfur.
The effect of cross-linking on elastomers can be demonstrated with a pair of rubber
balls available from Flinn Scientific. One of the balls is a polybutadiene rubber that contains an unusually large amount of sulfur. Because the polymer chains are extensively crosslinked, this ball dissipates very little energy in the form of heat when it bounces. It is therefore extremely resilient when bounced on the floor.
The other ball is a styrene–butadiene copolymer with much less cross-linking. When
dropped on the floor, the ball seems to “die.” This copolymer is used in applications where
an energy-absorbing medium is desired, such as automobile tires which must absorb some
of the energy associated with the bumps we encounter as we drive down the highway.
P.4 FREE RADICAL POLYMERIZATION REACTIONS
It isn’t difficult to form addition polymers from monomers containing CPC double bonds;
many of these compounds polymerize spontaneously unless polymerization is actively inhibited. One of the problems with early techniques for refining gasoline, for example, was
the polymerization of alkene components when the gasoline was stored. Even with modern gasolines, deposits of “gunk” can form when a car or motorcycle is stored for extended
periods without draining the gas tank.
The simplest way to catalyze the polymerization reaction that leads to an addition polymer is to add a source of a free radical to the monomer. The term free radical is used to
describe a family of very reactive, short-lived components of a reaction that contain one
or more unpaired electrons. In the presence of a free radical, addition polymers form by
a chain reaction mechanism that contains chain initiation, chain propagation, and chain
termination steps.
Chain Initiation
A source of free radicals is needed to initiate the chain reaction. The free radicals are usually produced by decomposing a peroxide such as di-tert-butyl peroxide or benzoyl peroxide, shown below. In the presence of either heat or light, the peroxides decompose to form
a pair of free radicals that contain an unpaired electron.
CH3
CH3
A
A
CH3 O C O O O OO CO CH3
A
A
CH3
CH3
O
O C O O OOO C O
P
O
P
P
O
CH3
A
2 CH3 O C O O
A
CH3
2
O CO O
Chain Propagation
The free radical produced in the chain initiation step adds to an alkene to form a new free
radical.
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CH3
A
CH3 O C O O ⫹ CH2 P CH2
A
CH3
9
CH3
A
CH3 O C O O O CH2O CH2
A
CH3
The product of this reaction can then add additional monomers in a chain reaction.
CH3
A
CH3 O C O O O CH2O CH2 ⫹ CH2 P CH2
A
CH3
CH3
A
CH3 O C O O O CH2O CH2 O CH2O CH2
A
CH3
Chain Termination
Whenever pairs of radicals combine to form a covalent bond, the chain reactions carried
by these radicals are terminated.
CH3
CH3
A
A
CH3 O C O O O (CH2CH2)n ⫹ (CH2CH2)m O O O C O CH3
A
A
CH3
CH3
CH3
CH3
A
A
CH3OC O O O (CH2CH2)n⫹m O O O C O CH3
A
A
CH3
CH3
The Formation of Branched Polymers
We might expect the product of the free radical polymerization of ethylene to be a straightchain polymer. As the chain grows, however, it begins to fold back on itself. This allows an
intramolecular reaction to occur in which the site at which polymerization occurs is transferred from the end of the chain to a carbon atom along the backbone.
CH2H
CH2 E
CH2
A
CH
(CH3)3C O O O CH2 H
E 2
CH2
CH2H
CH3 E
CH2
A
CH
(CH3)3C O O O CH H
E 2
CH2
When this happens, branches are introduced onto the polymer chain. Free radical polymerization of ethylene produces a polymer that contains branches on between 1% and 5%
of the carbon atoms. Of these branches, 10% contain two carbon atoms, 50% contain four
carbon atoms, and 40% are longer side chains.
P.5 IONIC POLYMERIZATION REACTIONS
Addition polymers can also be made by chain reactions that proceed through intermediates that carry either a negative or positive charge.
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Anionic Polymerization
When the chain reaction is initiated and carried by negatively charged intermediates, the
reaction is known as anionic polymerization. Like free radical polymerizations, these chain
reactions take place via chain initiation, chain propagation, and chain termination steps.
The reaction is initiated by a Grignard reagent or alkyllithium reagent, which can be
thought of as a source of a negatively charged CH3 or CH3CH2 ion.
H H
A A
HO C O C OLi
A A
H H
H H
A A
HO C O C
A A
H H
Li
The CH3 or CH3CH2 ion from one of these metal alkyls can attack an alkene to
form a carbon–carbon bond.
H H
A A
HO C O C
A A
H H
H
H
D
G
CPC
D
G
H
X
H H H H
A A A A
HO C O C O C O C
A A A A
H H H X
The product of the chain initiation reaction is a new carbanion that can attack another
alkene in a chain propagation step.
H H H H
A A A A
HO C O C O C O C
A A A A
H H H H
H
H
D
G
CPC
D
G
H
X
H H H H H H
A A A A A A
HO C O C O C O C O C O C
A A A A A A
H H H X H X
The chain reaction is terminated when the carbanion reacts with traces of water in the solvent in which the reaction is run.
CH3CH2(CH2CH)nCH2CH
A
A
X
X
H2O
CH3CH2(CH2CH)nCH2CH2 OH
A
A
X
X
Cationic Polymerization
The intermediate that carries the chain reaction during polymerization can also be a positive ion, or cation. In this case, the cationic polymerization reaction is initiated by adding
a strong acid to an alkene to form a carbocation.
CH2P CH H
A
X
CH3CH
A
X
The ion produced in this reaction adds monomers to produce a growing polymer chain.
CH3CH CH2P CH
A
A
X
X
CH3CHCH2CH
A
A
X
X
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The chain reaction is terminated when the carbonium ion reacts with water that contaminates the solvent in which the polymerization is run.
CH3CH(CH2CH)nCH2CH H2O
A
A
A
X
X
X
CH3CH(CH2CH)nCH2CHOH H
A
A
A
X
X
X
Advantages of Free-Radical versus Ionic Polymerization
The initiation step of ionic polymerization reactions has a much smaller activation energy
than the equivalent step for free radical polymerizations. As a result, ionic polymerization
reactions are relatively insensitive to temperature, and they can be run at temperatures as
low as 70°C. Ionic polymerization therefore tends to produce a more regular polymer,
with less branching along the backbone, and more controlled tacticity.
Because the intermediates involved in ionic polymerization reactions can’t combine
with one another, chain termination occurs only when the growing chain reacts with impurities or with reagents that can be specifically added to control the rate of chain growth.
It is therefore easier to control the average molecular weight of the product of ionic polymerization reactions.
Ionic polymerizations are more difficult to carry out on an industrial scale than free radical polymerizations. Ionic polymerization is therefore only used for monomers that don’t
polymerize by the free radical mechanism or to prepare polymers with a regular structure.
P.6 COORDINATION POLYMERIZATION
In 1963 Karl Ziegler and Giulio Natta received the Nobel Prize in chemistry for their discovery of coordination compound catalysts for addition polymerization reactions.
Ziegler–Natta catalysts provide the opportunity to control both the linearity and tacticity
of the polymer.
Free radical polymerization of ethylene produces a low-density, branched polymer with
side chains of one to five carbon atoms on up to 3% of the atoms along the polymer chain.
Ziegler–Natta catalysts produce a more linear polymer, which is more rigid, with a higher
density and a higher tensile strength. Polypropylene produced by free radical reactions, for
example, is a soft, rubbery, atactic polymer with no commercial value. Ziegler–Natta catalysts provide an isotactic polypropylene, which is harder, tougher, and more crystalline.
A typical Ziegler–Natta catalyst can be produced by mixing solutions of titanium(IV)
chloride (TiCl4) and triethylaluminum [Al(CH2CH3)3] dissolved in a hydrocarbon solvent
from which both oxygen and water have been rigorously excluded. The product of the reaction is an insoluble olive-colored complex in which the titanium has been reduced to the
Ti(III) oxidation state.
The catalyst formed in the reaction can be described as coordinately unsaturated because there is an open coordination site on the titanium atom. This allows an alkene to act
as a Lewis base toward the titanium atom, donating a pair of electrons to form a transition metal complex.
H
H
G D
C
B
C
D G
H
H
CH CH3
G D 2
Ti
D G
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The alkene is then inserted into a TiOCH2CH3 bond to form a growing polymer chain and
a site at which another alkene can bond.
H
H
G D
C
B
C
D G
H
H
CH CH3
G D 2
Ti
D G
CH CH2CH2CH3
G D 2
Ti
D G
Thus, the titanium atom provides a template on which a linear polymer with carefully controlled stereochemistry can grow.
P.7 ADDITION 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 Table P.1.
Polyethylene
Low-density polyethylene (LDPE) is produced by free-radical polymerization at high temperatures (200°C) and high pressures (above 1000 atm). The high-density polymer (HDPE)
is obtained using Ziegler–Natta catalysis at temperatures below 100°C 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.
Polypropylene
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 polypropylene 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.
Poly(tetrafluoroethylene)
Tetrafluoroethylene (CF2PCF2) is a gas that boils at 76°C 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 the polymer was
discovered, and polytetrafluoroethylene, or Teflon, became commercially available.
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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 little effect on its properties. It also has a very low coefficient of friction. (In simpler terms, it has
a waxy or slippery touch.) Even materials as “sticky” as crude rubber, adhesives, bread
dough, and candy won’t stick to a Teflon-coated surface. Teflon is so slippery that it has
even been sprayed on plants, so that insects that might prey on the plants fall off.
TABLE P.I
Common Addition Polymers
Structure
Chemical Name
Trade Name or
Common Name
(OCH2OCH2O)n
(OCF2OCF2O)n
(OCH2OCHO)n
A
CH3
CH3
A
(OCH2OCO)n
A
CH3
(OCH2OCHO)n
polyethylene
poly(tetrafluoroethylene)
polypropylene
Teflon
Herculon
polyisobutylene
butyl rubber
(OCH2OCHO)n
A
CN
(OCH2OCHO)n
A
Cl
(OCH2OCHO)n
A
CO2CH3
CH3
A
(OCH2OCO)n
A
CO2CH3
H H
A A
(OCH2OCPCOCH2O)n
Cl
A
(OCH2OCPCHOCH2O)n
H CH3
A A
(OCH2OCPCOCH2O)n
H
A
(OCH2OCPCOCH2O)n
A
CH3
polyacrylonitrile
Orlon
poly(vinyl chloride)
PVC
polystyrene
poly(methyl acrylate)
poly(methyl methacrylate)
Plexiglas, Lucite
polybutadiene
polychloroprene
neoprene
poly(cis-1,4-isoprene)
natural rubber
poly(trans-1,4-isoprene)
gutta percha
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Poly(vinyl Chloride) and Poly(vinylidene Chloride)
Chlorine is one of the top ten industrial chemicals in the United States—more than 20 billion
pounds are produced annually. About 20% of the chlorine is used to make vinyl chloride
(CH2PCHCl) 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 (CH2PCHCl) and vinylidene chloride (CH2PCCl2) 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.”
Acrylics
Acrylic acid is the common name for 2-propenoic acid: CH2PCHCO2H. Acrylic fibers
such as Orlon are made by polymerizing a derivative of acrylic acid known as acrylonitrile.
n CH2PCHCN
CN
A
... CH2CH n...
Polyacrylonitrile
Other acrylic polymers are formed by polymerizing an ester of 2-propenoic acid, such as
methyl acrylate.
O
B
n CH2PCHCOCH3
CO2CH3
A
... CH2CH
n...
Poly(methyl acrylate)
One of the most important acrylic polymers is poly(methyl methacrylate), or PMMA, which
is sold under the trade names Lucite and Plexiglas.
O
B
n CH2 P CCOCH3
A
CH3
CO2CH3
A
... CH2C
n...
A
CH3
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 the 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 cross-link poly(2-hydroxyethyl methacrylate).
CO2CH2CH2OH
A
... CH2C
n...
A
CH3
Poly(2-hydroxyethyl methacrylate)
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O
O
B
B
CH2 PCCOCH2CH2OCC P CH2
A
A
CH3
CH3
15
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 the reaction has the following structure.
CO2H
A
... CH2CH
x
CO2Na
A
CH2CH
y...
Sodium polyacrylate
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. The polymer can absorb 800 times its own weight of distilled water, but only 300 times
its weight of tap water. Because the ionic strength of urine is equivalent to a 0.1 M NaCl
solution, the superabsorbent polymer, which can be found in disposable diapers, can absorb up to 60 times its weight of urine.
P.8 CONDENSATION POLYMERS
The first plastic (celluloid) and the first artificial fiber (rayon) were produced from cellulose, as noted in Section P.1. 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 100°C, the phenols condense to form a
polymer in which the aromatic rings are bridged by either OCH2OCH2O or OCH2O linkages. The cross-linking in the polymer is so extensive that it is a thermoset plastic. Once a
piece is formed, any attempt to change the shape of the plastic is doomed to failure.
Research started by Wallace Carothers and co-workers at Du Pont 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 a diamine.
O
O
B
B
n H2N(CH2)xNH2 n ClC(CH2)yCCl
Diamine
Diacyl chloride
O
O
B
B
... NH(CH2)xNHC(CH2)yC n... 2n HCl
Polyamide
The polyesters were made by reacting the diacyl chloride with a dialcohol.
O
O
B
B
n HO(CH2)xOH n ClC(CH2)yCCl
Dialcohol
Diacyl chloride
O
O
B
B
... O(C H2)xOC(CH2)yC n... 2n HCl
Polyester
While studying polyesters, Julian Hill found that he could wind a small amount of the
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
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they could make by stretching a sample of the 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.
The polymerization process can be demonstrated by carefully pouring a solution of
hexamethylenediamine in water on top of a solution of adipoyl chloride in CH2Cl2.
O
O
B
B
n H2N(CH2 )6NH2 n ClC(CH2)4CCl
Hexamethylene
diamine
O
O
B
B
... NH(CH2)6NHC(CH2)4C n ... 2n HCl
Adipoyl
chloride
Nylon 6,6
A thin film of polymer forms at the interface between the two phases. By grasping the film
with a pair of tweezers, we can draw a continuous string of nylon from the solution. The
product of the 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 Figure P.9.
H
A
CON
B
O
Y
H
A
NOC
B
O
Y
H
A
NOC
B
O
Y
FIGURE P.9 The hydrogen bonds that form between the polymer
chains when polyamides and polyesters are stretched help to keep
the chains oriented in a two-dimensional fiber.
Exercise P.4
A synthetic fiber known as Nylon 6 has the following structure.
O
B
... NH(CH2)5 C n ...
Explain how the polymer is made.
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17
Solution
The polyamide must be made from a monomer that contains both an ONH2 and a OCOCl
functional group. The polymer is therefore made by the following condensation reaction.
O
B
n H2N(CH2)5 CCl
O
B
... NH(CH2)5 C n ... ⫹ n HCl
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).
O
B
n HOCH2CH2OH ⫹ n CH3OC
O
B
... OCH2CH2OC
O
B
COCH3
O
B
C n ... ⫹ 2 n CH3OH
Phosgene (COCl2) reacts with alcohols to form esters that are analogous to those
formed when acyl chlorides react with alcohols.
O
B
ClCCl ⫹ 2 HOR
O
B
ROCOR ⫹ 2 HCl
The product of the reaction is called a carbonate ester because it is the diester of carbonic
acid, H2CO3. Polycarbonates are produced when one of the esters reacts with an appropriate alcohol, as shown in Figure P.10. The polycarbonate shown in Figure P.10 is known
as Lexan. It has a very high resistance to impact and is used in safety glass, bulletproof
windows, and motorcycle helmets.
O
B
OOCOO
n
... O
⫹ n HO
CH3
A
C
A
CH3
CH3
A
C
A
CH3
O
B
OOC n... ⫹ 2n
OH
OH
FIGURE P.10 Polycarbonates are condensation polymers formed by reacting a diester
of carbonic acid with a dialcohol.
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The structures and names of some common condensation polymers are given in Table
P.2.
TABLE P.2 Common Condensation Polymers
Trade Name or
Common Name
Structure
Polyamides
O
O
B
B
(ONHO(CH2)6ONHOCO(CH2)4OCO)n
O
O
B
B
(ONHO(CH2)6ONHOCO(CH2)3OCO)n
O
B
(ONHO(CH2)5OCO)n
O
O
B
B
(ONH
CH2
NHOCO(CH2)10OCO)n
Nylon 6, 6
Nylon 6, 10
Nylon 6
Qiana
Polyaramides
O
B
CO)n
(ONH
Kevlar
Polyesters
O
B
(OOOCH2CH2OOOC
O
B
CO)n
O
B
CH2OOOC
(OOOCH2
Dacron, Mylar
O
B
CO)n
Kodel
Polycarbonates
CH3
A
C
A
CH3
(OO
O
B
OOCO)n
Lexan
Silicones
CH3
A
(OOOSiO)n
A
CH3
silicone rubber
P.9 PROPERTIES OF POLYMERS
The following variables can be controlled when producing a polymer.
•
•
•
•
•
The
The
The
The
The
monomer polymerized or the monomers copolymerized.
reagent used to initiate the polymerization reaction.
identity and amount of the reagent used to cross-link the polymer chains.
temperature and pressure at which the polymerization occurs.
solvent in which the monomer is polymerized.
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•
19
The way the polymer is collected, which can produce either a more or less random
alignment of the polymer chains or a fabric in which the chains are aligned in one
direction.
Changing one or more of these parameters can affect the linearity of the polymer, its average molecular weight, the tacticity of side chains on the polymer backbone, and the density of the product.
It is also possible to change the properties of a polymer by adding either stabilizers or
plasticizers. Stabilizers are used to increase the ability of a plastic to resist oxidation, to
make it less sensitive to either heat or light, or to make the material flame retardant. Plasticizers increase the flexibility of a plastic by acting as a lubricant, decreasing the friction
between molecules as one polymer chain moves past another. They also increase the amount
of empty space—the so-called free volume—within the polymer by opening up space between the polymer chains to increase the ease with which the chain ends, the side chains,
and the main chain can move.
The result of all of these manipulations can be a polymer as strong as Kevlar, which is
used to make bulletproof vests, or a material as easy to rip as a piece of paper. It can be
as hard as a bowling ball or as soft as a piece of tissue paper. It can be as brittle as the disposable polystyrene glasses used at parties or as elastic as a Styrofoam coffee cup.
The following list describes some of the important properties of a polymer.
Heat capacity/heat conductivity: The extent to which the plastic or polymer acts as an
effective insulator against the flow of heat. (The polystyrene in disposable plastic
glasses isn’t a very good insulator. However, blowing air through styrene while it is
being polymerized gives the Styrofoam used for disposable coffee cups, which is a
much better insulator.)
Thermal expansion: The extent to which the polymer expands or contracts when heated
or cooled. (Silicone is often used to seal glass windows to their frames because it has
a very low coefficient of thermal expansion.) Thermal expansion is also concerned
with the question of whether the polymer expands or contracts by the same amount
in all directions. (Polymers are usually anisotropic. They contain strong covalent
bonds along the polymer chain and much weaker dispersive forces between the polymer chains. As a result, polymers can expand by differing amounts in different
directions.)
Crystallinity: The extent to which the polymer chains are arranged in a regular structure instead of a random fashion. (Some polymers, such as Silly Putty and Play Dough,
are too amorphous and lack the rigidity needed to make a useful product. Polymers
that are too crystalline often are also too brittle.)
Permeability: The tendency of a polymer to pass extraneous materials. (Polyethylene is used to wrap foods because it is 4000 times less permeable to oxygen than
polystyrene.)
Elastic modulus: The force it takes to stretch the plastic in one direction.
Tensile strength: The strength of the plastic (i.e., the force that must be applied in one
direction to stretch the plastic until it breaks).
Resilience: The ability of the plastic to resist abrasion and wear.
Refractive index: The extent to which the plastic affects light as it passes through the
polymer. (Does it pass light the way PMMA does, or does it absorb light like
PVC?)
Resistance to electric current: Is the material an insulator, like most polymers, or does
it conduct an electric current? (There is a growing interest in conducting polymers,
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POLYMERS
which can be charged and discharged, and photoconducting polymers that can pick
up an electric charge when exposed to light.)
Chemistry in the World Around Us
The Search for Synthetic Fibers
Synthetic fibers can be traced back to 1885, when the French Count Hilaire de Chardonnet
received a patent for a synthetic silk. The first step in making Chardonnet silk, as it was
known, involved dissolving cellulose from wood pulp in nitric acid to form cellulose
nitrate. Anyone who has watched what happens when a flame is held close to a PingPong ball (see Section P.1) should understand that cellulose nitrate, by itself, is far too
flammable to be used as a fiber. The next step in the production of Chardonnet silk
therefore involved decomposing cellulose nitrate back to cellulose under conditions that
generated a continuous fiber. This was achieved by extruding a viscous solution of cellulose nitrate in alcohol through small holes into water.
In 1903, a slightly different process was patented in the United Kingdom. It involved
dissolving cellulose from wood pulp in a mixture of caustic soda and carbon disulfide
(CS2). The resulting viscous material was then extruded through small holes into a solution of sulfuric acid to form a fiber that was sold under the trade name of Rayon. By
1980, the demand for cellulose-based fibers had reached the point where more than
3,250,000 metric tonnes were produced each year.
More than 100 years after the development of the first synthetic fiber, some people
take comfort from the fact that the most important fiber is still cotton, which accounts
for almost 50% of the total world production of textile fibers. Advances in synthetic
fibers, however, have generated materials that not only compete with, but surpass, natural fibers.
Until recently, the standard against which all fibers were compared as potential insulators was the soft, fluffy down obtained from ducks and geese. Down is simultaneously
lightweight and an excellent thermal insulator. Unfortunately, it also readily absorbs water, and, when wet, loses much of its ability to act as an insulator. Thus, even if the supply of down were plentiful—which it is not—and even if down were inexpensive—which
it is not—there would be a potential demand for a synthetic fiber that had the insulating
properties of down but did not “wet.”
Several years ago, a synthetic insulator known as primaloft was prepared under a
contract issued by the U.S. Army Research, Development and Engineering Center at
Natick [Chemical and Engineering News, Oct. 16, 1989, p. 25]. Scanning electron microscopy has shown that natural down is a mixture of relatively large-diameter fibers,
which make it stiff, and very thin fibers that ensure the presence of many small pockets
of air. The result is a lightweight but stiff material that is a good insulator. Primaloft
achieves the same effect by combining a small number of large-diameter polyester
fibers with many more small fibers. The small fibers have a diameter of 7 m, roughly
one-fourth the diameter of a human hair.
Primaloft is just as “warm” as down and even has the same feel when enclosed in a
jacket or parka. More importantly, primaloft absorbs much less water when wet, and
therefore it doesn’t lose its insulating capacity in the rain. Furthermore, it is considerably
less expensive than down.
A variety of esoteric measurements are done to compare different textile fibers.
Questions that are asked include the following: Does the fabric swell when the fibers
absorb water? Does the fabric have a tendency to build up static electric charge? Is the
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21
fiber even, or does the yarn vary in diameter as it is spun? Does the fiber shrink on exposure to water? How well does the fiber “breathe?” (Is it permeable to air?) What is
the bursting strength of the fiber? What is its tear strength? How well does it resist
snag? How well does it stand up to abrasion and other forms of wear? How well does it
conduct heat? (Is it a good insulator, like natural wool?) How well does it pass moisture in the form of water vapor? Is it water repellent, or, at least, can it be made water
repellent? How well does it stretch? How well does it accept various dyes? Is it colorfast, once dyed? And, of course, perhaps as important as any other property—is it
flammable?
Research in the 1990s will continue the search for new fabrics, such as those recently
formulated by the Hoechst Celanese Corporation for athletic clothes that have the remarkable ability to pass water vapor but not liquid water through the fabric. These fabrics allow perspiration to evaporate and still provide the protection against rain or snow
expected for water-repellant fabrics.
KEY TERMS
Acrylic
Addition polymer
Anionic polymerization
Atactic
Block copolymer
Branched polymer
Cationic polymerization
Condensation polymer
Coordination
polymerization
Copolymer
Cross-linked polymer
Elastomer
Free radical
Free radical
polymerization
Graft copolymer
Homopolymer
Isomer
Isotactic
Linear polymer
Macromolecule
Monomer
Nylon
Polyamide
Polycarbonate
Polyester
Polyethylene
Polymer
Polypropylene
Poly(vinyl chloride),
PVC
Random copolymer
Regular copolymer
Straight-chain polymer
Syndiotactic
Tacticity
Teflon
Ziegler–Natta catalyst
PROBLEMS
Polymers
1. Use Berzelius’ definition of a polymer to sort the following compounds into groups of
polymers.
(a) formaldehyde, H2CO (b) ethylene, C2H4 (c) glyceraldehyde, C3H6O3
(d) 2-butene, C4H8 (e) cyclohexane, C6H12 (f ) glucose, C6H12O6
2. Which of the following substances are polymers?
(a) Teflon (b) propane (c) acetic acid (d) poly(vinyl chloride) (e) polyethylene
Definition of Terms
3. Describe the difference between linear, branched, and cross-linked polymers.
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4. Explain the following observations.
(a) Linear polymers, such as the polyethylene in garbage bags, tear when stretched.
(b) Lightly cross-linked polymers, such as rubber, return to their original shape when
stretched.
(c) Highly cross-linked polymers, such as Bakelite, break when “stretched.”
5. Describe the difference between a homopolymer (such as polystyrene) and a copolymer (such as styrene–butadiene rubber).
6. Describe the difference between random, regular, block, and graft copolymers. Give
an example of the sequence of monomer units that would be found in a short length
of each of the polymers.
7. Describe the differences in the structures of atactic, isotactic, and syndiotactic forms
of poly(vinyl chloride). Which of those would you expect to be the easiest to make?
Which might be the most useful?
8. Calculate the range of molecular weights of polyethylene molecules in a sample in
which individual chains contain between 500 and 50,000 (OCH2CH2O) units.
9. Individual chains in polystyrene polymers typically weigh between 200,000 and 300,000
amu. If the formula for the monomer is C6H5CHPCH2 and the polymer is an example of an addition polymer, how many monomers does the typical chain contain?
10. Calculate the average molecular weight of the polymer chains in the cellulose nitrate
in Ping-Pong balls if the formula for the monomer is C6H9NO7 and the average polymer chain contains 1500 monomers.
11. Explain why it is possible to measure the average molecular weight of linear polymers
such as polyethylene, but not cross-linked polymers such as those found in rubber.
12. Thermoplastic polymers flow when heated and can be molded into shapes they retain
on cooling. Explain how increasing the amount of cross-linking between polymer chains
can transform a thermoplastic polymer into a rigid thermoset polymer.
13. Describe the difference between addition and condensation polymers. Give an example of each.
14. Classify the following as either addition or condensation polymers.
(a) polyethylene, [...(CH2CH2)n...]
(b) poly(vinyl chloride), [...(CH2CHCl)n...]
(c) Nylon 6,6, [...(CO(CH2)4CONH(CH2)6NH)n...]
(d) polyester, [...(OCH2CH2OCOC6H4CO)n...]
(e) silicone, [...(OSi(CH3)2)n...]
(f ) Plexiglas, [...(CH2C(CH3)(CO2CH3))n...]
Elastomers
15. What are the characteristic properties of an elastomer?
16. Explain why elastomers must contain long, flexible molecules that are coiled in the
natural state. Explain why polymers have to have some cross-links to be elastomers
but can’t have too many.
17. Natural rubber becomes too soft when heated and too hard when cooled to be of much
use. Explain what happens when natural rubber is treated with sulfur that turns the
material into a commercially useful product.
Free Radical Polymerization Reactions
18. Describe what happens during the chain initiation, chain propagation, and chain termination steps when propylene (CH3CHPCH2) is polymerized by a free radical mechanism.
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19. Write the Lewis structures of the intermediates in the free radical polymerization of
vinyl chloride, CH2PCHCl.
Ionic and Coordination Polymerization Reactions
20. Describe what happens during the chain initiation, chain propagation, and chain termination steps in the anionic polymerization of vinyl chloride (CH2PCHCl) when
methyllithium (CH3Li) is used as the chain initiator. Write the Lewis structures of all
intermediates.
21. Describe what happens during the chain initiation, chain propagation, and chain termination steps in the cationic polymerization of vinylidene chloride (CH2PCCl2) when
hydrobromic acid (HBr) is used to initiate the reaction. Write the Lewis structures of
all intermediates.
22. A typical Ziegler–Natta catalyst consists of a mixture of TiCl3 and Al(C2H5)3. Explain
how the formation of a complex between a molecule of ethylene and the titanium atom
in the catalyst can be thought of as an example of a Lewis acid-base reaction.
Addition Polymers
23. Polypropylene can be made in both atactic and isotactic forms. The atactic form is soft
and rubbery, with no commercial value. The isotactic form is much more crystalline—
it is hard enough, for example, to be used for furniture. Explain the difference between
the physical properties of the two forms of the polymer.
24. Polyethylene can be made in both high-density and low-density forms. One of the polymers has a linear structure; the other is branched, with short side chains of up to five
carbon atoms attached to the polymer backbone. Which structure would you expect
to give the denser polymer? Which structure would give the more crystalline polymer?
25. Teflon, (CF2CF2)n, is a waxy polymer to which practically nothing sticks. It is also the
most chemically inert of all polymers. Describe at least five ways in which a polymer
with these properties can be used.
26. Use the concept of van der Waals forces to explain why plastic wrap made from Saran
clings to itself, whereas plastic wrap made from polyethylene does not.
Condensation Polymers
27. Predict the formula of the repeating unit in the condensation polymers formed by the
following reactions.
O
O
B
B
HCl ⫹ ...
(a) Dacron: HOCH2CH2OH ⫹ ClC
CCl
O
O
B
B
HCl ⫹ ...
(b) Nylon 6,6: H2N(CH2 ) 6NH2 ⫹ ClC(CH2 ) 4CCl
CH3OH ⫹ ...
(c) polycarbonate: HOCH2CH2OH ⫹ (CH3O)2C P O
O
B
HCl ⫹ ...
CCl
(d) polyaramide: H2N
(e) silicone rubber: (CH3)2Si(OH)2
HCl ⫹ ...
28. Explain the difference between the reactions used to prepare Nylon 6,6 and Nylon 6.