Polymer Chemistry Review

8/3/2019 Polymer Chemistry Review

http://slidepdf.com/reader/full/polymer-chemistry-review 1/15

Chemistry 1040

“Sex, Drugs and Organic Chemistry”

Topic 4: POLYMERS“THERE’S A GREAT FUTURE IN PLASTICS”

Introduction

MR. MCGUIRE: Ben!

BEN: Mr. McGuire

MR. MCGUIRE (overwhelmed with pride):

Ben.

BEN: Mr. McGuire.

Mr. McGuire takes Ben's arm and steers him down the hall toward the back of the house and out through

the back door.

EXT. BRADDOCK BACKYARD AND POOL AREA - NIGHT

The pool is eerily lit. There are FOUR PEOPLE standing and TALKING, drinks in their hands, at the back

of the yard.

MR. MCGUIRE: Ben - I just want to say one word to you - just one word -

BEN: Yes, sir.

MR. MCGUIRE: Are you listening?

BEN: Yes I am.

MR. MCGUIRE: (gravely) Plastics.

They look at each other for a moment.

BEN: Exactly how do you mean?

MR. MCGUIRE: There is a great future in plastics.

Think about it. Will you think about it?

BEN: Yes, I will.

MR. MCGUIRE: Okay. Enough said. That's a deal.

Excerpt from the Original Screenplay of The Graduate , by Buck Henry

The scene in The Graduate, in which the earnest family friend confides in newly graduated

Dustin Hoffman that, “...there is a great future in plastics,” was a humorous comment on thehuge gap between the generation represented by Hoffman’s Benjamin Braddock and that of his

parents. With the perspective of thirty+ years, the exchange is even funnier while at the same

time quite prophetic. Already in the 1960s, plastics had become ubiquitous in our society, but

even Mr. McGuire would have been surprised by the sheer diversity of products today that are adirect outgrowth of the original plastics. High-tech camping gear, cell phones, computers and

CDs are just some of the items made of polymers (the more general name for synthetic materials

which include plastics), and new cars include nearly 400 lb of various plastics. This section will



2

provide a brief introduction to the chemistry of some of the most common and well-knownpolymers, with an emphasis on how their molecular structure leads to the properties that make

them valuable materials.

Polymers—The Concept

A polymer is a material made up of small repeating structural units combined to give a very

large, linear structure. These ultra-large molecules are called macromolecules. The small

repeating units found within polymers come from the individual starting molecules, which arecalled monomers. All commercially important polymers are made up of hundreds or thousands

of monomer units joined together to form one large molecule. Long before the first synthetic

polymer was invented, we used natural polymers, such as cotton, silk and wool, for our clothing.

Like synthetic polymers, these natural polymers consist of many of the same small molecules joined together in a long chain to make macromolecules. Regardless of their ultimate source,

what all polymers have in common is that they are made up of very large, linear molecules

containing repeating groups within the chain. The size of the individual polymer molecules and

the nature of the groups ultimately controls what physical properties the bulk material will have.In many cases (e.g., polyethylene, polyvinyl chloride, polystyrene) the monomer molecules

contain C=C double bonds that are broken during the polymerization process. These are

referred to as “chain-growth polymers.” In other cases, the monomer molecules may connect

while forming a molecule of water (e.g., nylon or polyethylene terephthalate [PET]). These arecalled “step-growth polymers.” Generic examples of each type of process are shown below (Fig.

1).

C CW

X

Z

YC

W

X

C

Z

Y

C

W

X

C

Z

Y

C

W

X

C

Z

Y

C

W

X

C

Z

Y

Monomer (alkene) Polymer (n is a large number)repeatingunits (frommonomers)

n

RC C

O

HO

O

OH

HO CH2

CH2

OH

Monomers (alcoholand carboxylic acid)

HO CH2

CH2

O RC C

O O

O CH2

CH2

O RC C

O O

OH

n

Polymer (n is a large number)(a polyester)

H2O

Figure 1. Generic Examples of Polymer Formation From Monomers.

The intentional preparation of polymers began in the mid-19th century. Celluloid, formed

from a mixture of nitrocellulose and camphor, was the first moldable plastic, and was used as a

substitute for ivory in piano keys and billiard balls. Because nitrocellulose is explosive, this

created some excitement during billiard matches or particularly vigorous piano recitals. Not longafter, the first synthetic fiber, rayon, was invented and became used as a silk substitute. As



3

chemists began to discover that many small alkene molecules could be inadvertentlypolymerized, and as certain specific needs arose due to shortages of natural materials, new

polymers were invented with increasing frequency. Another important milestone was the

development of synthetic rubber by the Germans during World War I. In the sections that

follow, we will consider each of the major types of polymers separately, with a discussion of the

monomers and the common uses of each.

Polyethylene

The simplest synthetic polymer is polyethylene. Recall that ethylene is the old name for

ethene, the simplest example of an alkene (Figure 2). Polyethylene is a chain-growth polymer, in

which the double bond of the alkene monomer gets broken as new bonds are formed between

monomer molecules. Can you predict the basic structure of polyethylene? As you mightimagine, it simply consists of –CH2CH2- units repeated again and again. The most common type

of chain-growth polymerization uses “radical initiation,” in which a small amount of a compound

with only seven valence electrons (R•, called a free radical) begins the polymerization by

adding to an alkene monomer and generating a new radical that then adds to another alkene, andso on. The polymer is “terminated” by reaction with another radical, either another initiator or

another growing chain.

C CH

H

H

H

ethene(ethylene)

+ R• R C

H

H

C

H

H

•

radicalinitiator

new radical

C CH

H

H

H

addition of anothermolecule of monomer

C

H

H

C

H

H

R C

H

H

C

H

H

•

C CH

H

H

H

addition of anothermolecule of monomer

C

H

H

C

H

H

R C

H

H

C

H

H

C

H

H

C

H

H

•

repeatingunits (frommonomers)

growing chain

many

moresteps

C

H

H

C

H

H

R C

H

H

C

H

H

C

H

H

C

H

H

R

npolymer molecule

Figure 2. Polymerization of Ethylene to Form Polyethylene.

Polyethylene is used widely in our society. In 1997, over 20 billion lb of varying grades of

polyethylene was produced in the United States. There are two main types of polyethylene, high

density polyethylene (HDPE) and low density polyethylene (LDPE). High density

polyethylene is harder and stronger than LDPE, so it is used for gas tanks, heavy duty pipes,

appliance cabinets, and in other situations requiring great strength. In contrast, LDPE tends to be

much softer and is also usually transparent. Examples of LDPE that you may have encounteredinclude plastic wrap, grocery bags and disposable diaper liners.

What is the difference between low- and high-density polyethylene that leads to the different

physical properties? The same starting material is used in either case. The answer lies in the

nature of the individual polymer molecules from which the material is made. Low-densitypolyethylene tends to consist of relatively short chains of about 500 monomer units, with a

considerable degree of branching (Fig. 3), while HDPE chains are much longer (about 10,000

monomer units) and are not branched. Because they are long and straight, HDPE molecules are



4

able to pack together more closely, which leads to the higher density of this material. Theirstrength derives from both the rigidity of this tightly-packed structure and the overall length of

the molecules. While HDPE is generally a superior material, it is more expensive to make than

LDPE. LDPE is formed by “radical polymerization” as discussed above, while formation of

HDPE requires the use of sophisticated and expensive catalysts. As a result, LDPE is the

polymer of choice for products that do not require a lot of strength or toughness.

C

H

H

C

H

C

C

H

H

C

C

H

C

H

H

C

H

H

H H

CH H

H H

CH C

CH H

CH H

H

H

C

H

H

LDPE: polymer molecules havelots of branching

Molecules unable to packtogether very closely

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

HDPE: polymer molecules havelittle or no branching

Molecules able to arrange themselvesin very close proximity

Figure 3. Shape of Low Density and High Density Polyethylene Polymer Molecules Affects

Their Arrangement

Polypropylene

Polypropylene is quite similar in structure to polyethylene, given the close relationshipbetween the two monomers (Fig. 4). Propylene (or propene, as it is also called) is like ethylene,

except with a methyl group (CH3) replacing one of the hydrogens. So, once the polymer isformed, there will be methyl groups attached to every other carbon on the long chain. You might

imagine that the arrangement of the methyl groups would be completely random, that is, somemight end up on adjacent carbons, while others would be separated by one or two CH2 groups.

However, polypropylene is made only using catalysts like the ones used for HDPE, and these

conditions lead to a very regular arrangement of the methyl groups on alternating carbon atoms

as shown.Polypropylene is a fairly versatile polymer, used as both as a plastic (i.e., moldable material)

and as fiber (i.e., spun into long strands that can be used in textiles, etc.). In 1997, over 13

billion pounds of it was manufactured. Because it melts at a relatively high temperature, it is



5

used to make dishwasher-safe containers. Both HDPE and LDPE melt at considerably lowertemperatures, so they are seldom used in this way. (Try putting a milk jug, often made of HDPE,

in your dishwasher. On second thought, don’t try it.) One of the most common uses of

polypropylene fibers is in indoor-outdoor carpeting. It is also used in thermal underwear.

C CH

H

H

H

ethene(ethylene)

vs. C CH

H

CH3

H

propene(propylene)

polymerization

catalystC

H

H

C

CH3

H

R C

H

H

C

CH3

H

C

H

H

C

CH3

H

R

n

polypropylene(note CH

3groups on

every other carbon)

Monomer units come togetherin regular fashion:

C CH

H

CH3

H C CH

H

CH3

HC C

H

H

CH3

H

Not in a random way:

C CH

H

CH3

H CC H

H

H3

C

HC C

H

H

CH3

H

X

C

CH3

H

C

H

H

R C

H

H

C

CH3

H

C

H

H

C

CH3

H

irregular arrangement of CH3groups not seen

Figure 4. Formation of Polypropylene from Propene.

Additional Work:

1. Although polypropylene forms only with the methyl groups on every other carbon,there is still a possibility for different arrangements of the methyl groups. Can you

see it? (Hint: are there any chiral centers in the polymer molecule?)

2. Polyisobutylene is a useful material that is completely impermeable to gases.

Because of this property, it is used for the inner liner of tires and basketballs. It wasoriginally developed by the Germans as a rubber substitute, and is sometime known

as “butyl rubber.” Given the structure of the monomer below, can you predict what

the polymer would look like?

C CH

H

CH3

CH3

isobutylene

3. Apply a similar analysis to polyisobutylene as you did to polypropylene in problem 1.

(I.e., is there more than one possible way the methyl groups can be arranged on thepolymer chain?)



6

Polyvinyl Chloride

Polyvinyl chloride (often abbreviated as PVC), as you might guess, comes from the

monomer vinyl chloride (Fig. 5). It is a commercially important product, with over 14 billion

pounds produced in 1997. Most of you are familiar with PVC as a material for making pipes,

suggesting that it is a fairly stiff material. It is also the main component of linoleum flooring and“vinyl” siding,* and the material of choice for electrical insulation. However, it can also be

formulated as a highly flexible material, either opaque or transparent, to make upholstery

(“vinyl” covering), garden hoses and shower curtains. This is accomplished by includingadditional compounds, called plasticizers, that keep it soft and flexible. However, some of the

most common plasticizers have received quite a bit of attention over the last ten years because

they are suspected of being “environmental estrogens” that act on the estrogen receptor and

affect sexual development in higher animals (including us). This will be discussed further in thesection on sex hormones.

C C

H

H

Cl

HC

H

HC

Cl

HR C

H

HC

Cl

HC

H

HC

Cl

HR

radical

polymerization

vinyl chloride n

polyvinyl chloride

Figure 5. Formation of Polyvinyl Chloride from Vinyl Chloride.

Like LDPE, PVC is generally made using a radical initiator, and like polypropylene, the

monomer units come together so that the non-hydrogen group (the chlorine atom in this case)appears on every other carbon atom of the polymer chain. Polyvinyl chloride was originally

discovered in 1912 by a German chemist, Fritz Klatte. He had been looking for useful things to

do with excess acetylene that his company had acquired, and had converted it to vinyl chloride.

He stored it on the shelf, and later found that it had polymerized. His company patented it, butnever found a use for it and the patent expired in 1925. A year later, an American chemist,

Waldo Semon independently invented PVC. His company, B. F. Goodrich, determined that it

was waterproof and decided to market it for shower curtains. They filed a U.S. patent(apparently Klatte’s company had obtained only a German patent) and made millions out of this

material.

Polystyrene

Polystyrene is found in many places in our society. It is best known as the main ingredient

of “styrofoam” cups and dishes, though it is also used to form the jewel case for CDs. These are

two very different types of material—how can they come from the same polymer? It turns outthat the difference does not come from the way it is formed, as was the case with LDPE and

HDPE, but rather from the way it is fabricated after it is formed. The hard, transparent material

of jewel boxes is made by melting polystyrene and pouring it into molds. For styrofoam, small

beads of polystyrene are prepared, containing a low boiling liquid inside. Nowadays the liquid is

* The term “vinyl” is well known in our culture, as in vinyl siding, vinyl tops to cars and vinyl records (remember

those?). In all cases, this is referring to a plastic material whose main component is PVC. In organic chemistry, a

“vinyl group” is a C=C bond with three hydrogens that is attached to something else (CH 2=CH-X), hence vinyl

chloride.



7

usually pentane (C5H12), which boils at about 36 °C (97 °F), though until recentlychlorofluorocarbons (CFCs) were used. This use of CFCs was curtailed due to their role in the

destruction of stratospheric ozone. The beads are placed in a mold and heated using steam,

which causes the pentane to vaporize and greatly expand the beads. The expanded beads fuse

together to form the shape of the mold. The bubbles left behind make the material very light

weight and also a good thermal insulator.Like LDPE and PVC, polystyrene is formed by radical polymerization (Fig. 6). The starting

monomer, styrene, consists of an ethylene molecule in which one of the hydrogen atoms has

been replaced by an aromatic ring. When these monomers come together, they do so in such away that the aromatic rings are attached to alternating carbon atoms of the chain.

C C

H

H H

C

H

H

C

H

R C

H

H

C

H

C

H

H

C

H

Rradical

polymerizationstyrene n

polystyrene

Figure 6. Polymerization of Styrene to Give Polystyrene.

Additional Work:

4. Polyvinyl acetate is used in wood glue, as a coating to make paper shiny, and in latex

paint. Given the structure of vinyl acetate below, suggest what polyvinyl acetatelooks like.

5. Suppose there were a polymer with the structure shown below (there is). Can youinfer the structure of the likely monomer? [By the way, this is one of the common

ingredients of “superglues.” Can you guess why a monomer would be used in aglue?]

C

H

H

C

C

C

R C

H

H

C

C

C

C

H

H

C

C

C

R

n

OO O O OO

CH3 CH3 CH3

N N N

Teflon

Teflon, more correctly named polytetrfluoroethylene, is a highly useful polymer due to its

unique properties. It was discovered by accident in the 1930s when a chemist at Dupont was

experimenting with tetrafluoroethylene (see Fig. 7) as a possible refrigerant. When he openedthe tank, nothing came out, yet it was heavy, as if it were full. Eventually, he sawed the tank

open and found that it was full of a white powder, which he correctly surmised to be polymerized



8

tetrafluoroethylene. Although it was not clear what caused the polymerization within the tank, itwas found that radical polymerization works well for the formation of this material.

C CF

F

F

F

C

F

F

C

F

F

R C

F

F

C

F

F

C

F

F

C

F

F

Rradical

polymerization

tetrafluoroethylenen

polytetrafluoroethylene(Teflon)

Figure 7. Formation of Teflon from Tetrafluoroethylene.

What uses do you associate with Teflon? Probably the first thing that you thought of was

nonstick pans. It is also put on carpet and fabrics to make them stain resistant (e.g., DupontStainmasterTM), used to seal metal pipes (Teflon tape) and in artificial body parts. The common

feature of all of these uses is the chemical resistance of this material. The fluorine atoms that

surround the carbon chain of the polymer protect it from chemical attack, and also repel mostother materials. As a result, it is very inert (i.e., chemicals won’t react with it) and resistant toanything sticking to it.

Polyethylene Terephthalate

Polyethylene terephthalate (the second word is pronounced terra•thalate) is the most

important example of the class of polymers known as “chain-growth” or condensation

polymers. This group also includes Nylon and virtually all of the naturally occurring polymers

except rubber (e.g., cellulose, silk, etc.). As you saw in Figure 1, these polymers are not formedfrom alkene monomers. Instead, two reactive groups come together to join two monomer

molecules with the simultaneous formation of a small molecule such as water. Polyethyleneterephthalate is typically abbreviated as “PETE” or “PET,” although the latter is sometimes

discouraged due to trademark infringement with a brand of condensed milk.In the lab, PET is very easy to make by simply heating two different kinds of molecules

together: phthalic acid and ethylene glycol (Fig. 8). Each of these monomers has two of the

same functional group; phthalic acid has two carboxylic acid groups, while ethylene glycol hastwo alcohol groups. Another functional group that you have seen, called esters, is usually

formed from an alcohol and a carboxylic acid, as shown in the figure. If this process is carried

out using “double-barreled” monomers such as phthalic acid and ethylene glycol, the product

that is left still has one carboxylic acid and one alcohol left. Therefore, each of these can reactagain to form two more esters, and once again, there are still two reactive functional groups at

each end. As you can imagine, this process can continue for a very long time, giving largepolymer molecules made up of repeating ester units. That is why this particular class of

polymers is also called polyesters. Since PET is by far the most common example, you shouldassume that any item of clothing that contains a certain percentage of “polyester” probably has

PET in it.

PET is a very important commercial product. As already noted, it can be spun into fibers for

clothing, and this form is known by the trade name Dacron. Interestingly, Dacron is also used ina variety of medical applications, such as patching damaged blood vessels and even the septum

in the heart. The great strength of Dacron fibers is a result of the ability of the ester groups and

the aromatic rings of individual polymer molecules to line up with each other in long strands.



9

You have probably also encountered PET in other places, such as drink bottles. Interestingly, theuse of PET in bottles was invented by Nathaniel Wyeth, brother of the famous American painter,

Andrew Wyeth. Another important use is as thin ribbons, called Mylar, which are typically

coated with metal oxide for use as recording tape.

R1 C

O

OH

O R2

H

carboxylicacid

alcohol

R1 C

O

O R2 + H2O

ester

C C

O

OH

O

HO HO CH2

CH2

OH+

terephthalic acid(2 carboxylic acids)

ethylene glycol(2 alcohols)

C C

OO

HO O CH2

CH2

OH

H2

O

ester product(1 carboxylic acid + 1 alcohol)

C C

O

OH

O

HO

HO CH2

CH2

OH

C C

OO

O O CH2

CH2

OHO CH2

CH2

C C

O

OH

O

C C

OO

O O CH2

CH2

OHO CH2

CH2

C C

O

OH

O

2 H2

O

n

polyethylene terephthalate (PET or PETE)

Figure 8. Polyethylene Terephthalate from Phthalic Acid and Ethylene Glycol.

Nylon

Nylon was one of the first big success stories in the field of polymer chemistry. Nylon is an

example of a polyamide. You know what a polyester looks like from the discussion above; if you refer to your table of functional groups, can you predict what a polyamide should look like?

Actually, the most important polyamides are the proteins, and it was from proteins that Wallace

Carrothers, a chemist at Dupont, took his inspiration. He found that mixing two monomers,adipic acid (a molecule with two carboxylic acids as we saw with terephthalic acid) and

hexamethylene diamine (a molecule with two amine groups, similar to ethylene glycol), gave the

desired polymer (Fig. 9). As with polyesters, a molecule of water is generated each time a

monomer adds to the growing chain.Nylon’s importance quickly became apparent when it was discovered how easily it could be

spun into strong, smooth fibers. It was first used in the bristles of toothbrushes. The

resemblance of this material to silk led to the notion of making stockings from Nylon, and on the

first day that Nylon stockings were offered for sale in New York City in 1940, four million pairswere bought. Unfortunately for the avid consumers, the supply of Nylon quickly dried up after

the United States entered World War II. It found many uses, including clothing, ropes and

parachutes. It is still used for material that requires great strength and durability.



10

R1C

O

OH

N R2

H

R1C

O

N R2

CH2

CH2

C C

O

OH

O

HO CH2

CH2

CH2

CH2

carboxylic

acid

H2O

amide

+

H2O

+

adipic acid(2 carboxylic acids)

amide product(1 carboxylic acid + 1 amine)

hexamethylene diamine(2 amines)

amine

H H

CH2

CH2

CH2

H2N CH2

NH2

CH2 CH2C C

O

N

O

HO CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 NH2

H

(CH2

)4C

O

HO C

O

N

H

(CH2)6 CN

H O

(CH2)4 C

O

N

H

(CH2)6 NH2n

+ n+1 H2O

Nylon 6,6

6 carbons 6 carbons

Figure 9. Nylon 6,6 from Condensation Polymerization of Adipic Acid and Hexamethylene

Diamine.

The particular type of nylon invented by Carrothers and Dupont was called nylon 6,6 due to

the number of carbons in each of the monomer pieces, and was such a commercial hit that otherchemical companies tried to develop alternative products that did not infringe on the Dupont

patent. A way around this was found with the closely related polymer, nylon 6 (Fig. 10). Unlike

nylon 6,6, nylon 6 is made from a single monomer that contains an amide functional group

within a ring. When this amide, caprolactam, reacts with water and acid (H+), it is convertedinto a molecule with one carboxylic acid and one amine. This molecule can then react with more

of the caprolactam to give a new amide that has a carboxylic acid and an amine at the two ends,

and eventually a polymer that looks very much like nylon 6,6. However, since there is only one

monomer, having six carbon atoms, it is called nylon 6.One of the main reasons for the high tensile strength of the nylons (the property that allows

very strong fibers to be made from them) is the ability of the individual polymer chains to form

connections between each other (Fig. 11). Amides such as those found in the nylons can formwhat is called hydrogen bonds. These connections are weaker than a full fledged covalent

bond, but formation of several of them between two chains will hold them very tightly together.

The same principle is seen in the chemistry of proteins, which are also polymers made up of

repeating amide bonds. As we will see later this semester, hydrogen bonds between amide



11

groups hold proteins into unique shapes that are necessary for their biological activity. They alsoplay an important role in the binding together of two protein molecules, the attraction of a

substrate for an enzyme’s active site, or the affinity of a drug for a particular receptor. Hydrogen

bonds are also an important part of the chemistry of the genetic code, although in this case it

does not involve amide groups.

(CH2)5C

O

HO N

H

C

O

(CH2)5 CN

H O

(CH2)5 NH2

n

Nylon 66 carbons

NC

O

H

caprolactam

amide

+ H2ON

OHC H

H

O

= C

O

HO (CH2)5 NH2

1 carboxylic acid + 1 amine

H+

C

O

HO (CH2)5 NH2

N

C

O

H

+ C

O

N (CH2)5 NH2C

O

HO (CH2)5H


break here

Figure 10. Formation of Nylon 6 from Caprolactam.

(CH2)4C

O

HO C

O

N

H

(CH2)6 CN

H O

(CH2

)4 C

O

N

H

(CH2

)6 NH2

n

(CH2

)4 C

O

OHC

O

N

H

(CH2

)6C N

HO

(CH2)4C

O

N

H

(CH2)6H2

N

n

(CH2

)5C

O

HO N

H

C

O

(CH2)5 CN

H O

(CH2

)5 NH2

n

(CH2)5 C

O

OHN

H

C

O

(CH2

)5C N

HO

(CH2

)5H2N

n

Nylon 6,6:

Nylon 6:

hydrogen bonds holdingchains together

Figure 11. Hydrogen Bonds Between Chains of Nylon 6,6 or Nylon 6.



12

One final point concerning polyamides such as the nylons concerns a structurally similarpolymer, Kevlar. Kevlar is an amazingly strong material when spun into fibers; in fact, its

tensile strength is greater than that of steel. Yet, it is much lighter than steel or other metals

known for their strength. As a result, it is used in bulletproof vests, Army helmets and protective

clothing worn by firefighters. One of the monomers used to make Kevlar should be familiar to

you, terephthalic acid (remember PET?). The other is a compound with two amine groups, likethe compound hexamethylene diamine used in nylon 6,6. However, in this case the two amine

groups are attached directly to an aromatic ring (Fig. 12). The two monomers come together in

the same way as we saw before with nylon 6,6, forming a long chain in which the individualmonomers are connected by amide linkages. The aromatic rings in both pieces make the

polymer molecule very rigid, but the individual polymer chains can still bind to each other with

hydrogen bonds, as we saw with the nylons. The result is a very stiff, strong material. In fact,

the biggest challenge with Kevlar is how to fabricate it—it does not melt until it is heated above500 °C (912 °F).

C C

O

OH

O

HO C C

OO

HO N

H2

+

terephthalic acid(2 carboxylic acids)


1,4-diaminobenzene(aka phenylene diamine;

2 amines)

Kevlar

H2N NH

2NH

2

H

C C

OO

N NH2

H

C C

OO

HO N N

H H

n

Figure 12. Formation of Kevlar from Terephthalic Acid and 1,4-Diaminobenzene.

Other Important Polymers

There are many other important polymers aside from those described above. However, space

and time limitations prevent us from covering them in the same level of detail. They will eachbe described briefly, in terms of the molecular structures of the monomers and polymers, their

properties, and their uses.

Another natural polymer that has been widely used is rubber. Rubber is obtained from the

rubber tree as latex, and can be made synthetically from a simple, five-carbon monomer calledisoprene (Fig. 13). Because of its pure hydrocarbon structure, rubber is quite waterproof. When

it is vulcanized by treatment with sulfur, rubber becomes much harder, but still flexible. This

observation was made by Charles Goodyear, and led to the modern tire industry. A close relativeof rubber, also obtained from trees, is gutta-percha. Can you tell the difference between the two

polymers? They are alkene isomers. The cis double bonds in rubber make it very elastic, while

the trans double bonds of gutta-percha make it stiffer and more brittle. Gutta percha is used for

packing root canals and as a covering for golf balls.

Additional Work:

6. Two close relatives of rubber and gutta-percha are polybutadiene and neoprene, made

from the monomers butadiene and chloroprene. Given the structures of the

monomers (next page), can you predict the structures of the polymer products?



13

butadiene chloroprene

Cl

isoprene

H3C H3C H3C H3C

nnatural rubber(polyisoprene)

"cis" double bonds

CH3 CH3 CH3n

"trans" double bonds

gutta-percha

Figure 13. Rubber and Gutta-Percha.

Polyurethanes arealso elastomers, like rubber and its relatives. A urethane is like a crossbetween an amide and an ester—it contains a carbonyl (C=O) with both a nitrogen and an

oxygen attached to the carbonyl carbon (Fig. 14). Polyurethanes are typically made from two

different monomers, one containing two isocyanate groups and one containing two alcohol

groups. There are many examples of polyurethanes, and one is shown in the figure. They areused to make “foam rubber,” and also make up one of the parts of the unusual material known as

spandex (Lycra).

N C

O

OR1R2

H

a urethane

N C ONCO CH2

isocyanate groups

+

O CH2

CH2

ON C

O

N CH2

C

H

N C

O

N CH2

C

O H H

O CH2

CH2

OH

HO CH2

CH2

OH

O

n

a polyurethane

Figure 14. Polyurethane.

Another type of elastic polymer that you may have heard of is silicone. It is interesting to

note that the silicones (there are a number of them in use) do not have any carbon atoms in the

polymer chain. Instead, they are made up of a repeating –O-Si-O-Si- pattern. Using yourknowledge of valence, you should realize that the silicon atoms need to have four bonds, just like

carbon atoms. The remaining two bonds to each Si are connected to organic pieces, such as

methyl (CH3) or phenyl (C6H5) groups (Fig. 15). The reason that silicones are so flexible is that

the polymer backbone (the O-Si chain) can easily bend. Because they are waterproof and easily



14

molded, silicones are used for caulking. They are also chemically quite inert, so they are used invarious prosthetic body parts. Finally, an interesting side note: if silicone is mixed with boric

acid (B(OH)3), the product is an extremely stetchable, yet bouncy material—silly putty.

SiCH3

CH3

O SiCH3

CH3

O SiCH3

CH3

O

n

Si O Si O Si O

n

Figure 15. Structures of Typical Silicone Rubbers.

Those of you who wear glasses may be familiar with another type of polymer, the

polycarbonates. Polycarbonates are made from two pieces, a compound called “bisphenol A”

and phosgene, a very dangerous compound used as a chemical weapon in World War I (Fig. 16).

The two pieces combine to form a carbonate linkage in the repeating chain. Carbonatesresemble the urethane, except that they have two oxygens attached to the C=O instead of an

oxygen and a nitrogen. The polycarbonate that is used for eyeglass lenses is different. One

starts with a monomer containing a C=C bond at each end, and this is polymerized in the sameway as polyethylene or polypropylene. However, in this case the polymer chains are joined

together by carbonate links, which makes the material very strong and hard, yet light—ideal for

glasses.

OHHO C

CH3

CH3

+ C

O

Cl Cl

bisphenol A phosgene

NaOH

OO C

CH3

CH3

OO C

CH3

CH3

C

O

C

O

n

O C

O

OR R

a carbonate

O OO

O

O

O

carbonates

alkenes

O OO

O

O

O

O OO

O

O

O

Figure 16. Two Types of Polycarbonate.

Three final polymers that we will briefly mention are polymethyl methacrylate,polyacrylonitrile, and polyvinyl pyrrolidone (Fig. 17). These are all made by chain-growth

polymerization of alkene monomers, as we saw already with other polymers such as



15

polyethylene, polypropylene, polystryene, polyvinyl chloride and teflon. Polymethylmethacrylate is best known as Plexiglass, an extremely tough (almost unbreakable) and

transparent material, and as Lucite, which is used in bathtubs, sinks and other fixtures.

Polyacrylonitrile itself is used only as a starting material for making carbon fibers. However,

when a mixed polymer made from a mixture of acrylonitrile and methyl methacrylate is made, it

can form the fibers that we know as “acrylic.” Finally, polyvinyl pyrrolidone is interesting,because it is used in hairspray (along with silicone). The B-52’s would not have made it big

without this polymer.

C C

H

H

C

CH3

O

O-CH3

methyl methacrylate

radical

polymerizationR C

H

H

C

CO OCH3

CH3

C

H

H

C

CO OCH3

CH3

C

H

H

C

CO OCH3

CH3

R

npolymethyl methacrylate (Plexiglass, Lucite)

C C

H

H

C

H

acrylonitrile

radical

polymerizationR C

H

H

C

C

H

C

H

H

C

C

H

C

H

H

C

C

H

R

npolyacrylonitrile

NN N N

acrylonitrile

+

methyl methacrylate

radical

polymerizationR C

H

H

C

CO OCH3

CH3

C

H

H

C

C

N

CH3

C

H

H

C

CO OCH3

CH3n

C

H

H

C

C

N

CH3

R

"acrylate" fiber

N

CC

O

H

H

H

radical

polymerizationR C

H

H

C

H

C

H

H

C

H

C

H

H

C

H

R

n

N N NO O O

vinyl pyrrolidone polyvinyl pyrrolidone

Figure 17. Polymethyl Methacrylate, Acrylic, and Polyvinyl Pyrrolidone.

Documents

Polymer Chemistry Review