Ochiai E. Chemicals for Life and Living

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5 Clothing and Shelters: Polymeric Material

staff. That is a kind of compound containing SH group (ammonium thioglycolate is

commonly used), which reduces the –S–S– bridges and breaks them down to –SHs.

Now the cross-linkages have been removed, and the hair has become more ﬂexible.

You will have it shaped in any way you want. When this is done, the next step is to

recreate the –S–S– bridges in new locations (new setting). Now the hair shape has

been ﬁxed. This last step is chemically speaking “oxidation” and is usually accom-

plished by oxygen coming from hydrogen peroxide; occasionally an oxidant such as

sodium bromate might be used.

5.2.4 Natural Rubber

Rubber can be obtained from a number of trees, but commercial production is

done with tropical plant Hevea brasiliensis. Rubber is a polymer of a single

monomer isoprene (2-methyl butadiene). Isoprene itself can chemically (i.e.,

artiﬁcially) be polymerized by various means to produce polyisoprene, but this is

not the way the natural rubber is produced in rubber trees. The biological process

to produce rubber (i.e., biological polymerization) is shown in Fig. 5.5. This

process is a kind of condensation with removal of a pyrophosphate group

-C

-C=CH-CH

=C-CH

-CH

-O-PO

-PO

=CH-CH

-CH

-C=CH-CH

-O-PO

-PO

+ HO-PO

-PO

H-(-CH

-C=CH-CH

-O-PO

-PO

=C-CH

-CH

-O-PO

-PO

-CH

-O-PO

-PO

(isoprene unit)

(natural rubber)

HHH

Fig. 5.5 The biological synthesis of polyisoprene

635.3 Synthetic Polymers

5.3 Synthetic Polymers

A large number and quantity of polymers are synthesized and used today. They are

used for a variety of purposes: clothing, wrapping ﬁlm, plastics, bottle for soda and

others, water piping, all kinds of small parts for automobiles and other machinery,

and CD/DVD.

An earlier motivation to produce polymers artiﬁcially was to produce natural

ﬁbers like silk and/or to modify the natural material to desirable ﬁbers. Cotton is a

very good ﬁber, but silk has some appealing characters such as luster and its soft

feel upon touch. But silk was much more expensive. People tried to convert cellu-

lose which was much more abundantly available to silk-like ﬁber. From this effort

came the ﬁrst synthetic ﬁber, “rayon.”

In 1845, a Swiss chemist tried to treat cotton with nitric acid (and sulfuric acid).

The resulting material, nitrocellulose, had several interesting properties. Cotton itself

does not dissolve in water nor alcohol (ethanol), but nitrocellulose does. Solutions of

nitrocellulose were used to turn it into ﬁbrous or plastic form. Plastic form is called

“celluloid.” It is still used for some purposes. An interesting property of nitrocellulose

is its explosiveness. It turned out that if all the three OH groups of glucose units in

cellulose are nitrated, the resulting nitrocellulose is very explosive (see Chap. 8 for

the explanation of explosive). But it is not explosive, though still quite ﬂammable, if

the degree of nitration is less than two. In 1884, nitrocellulose was made into ﬁber and

sold as Chardonnet. Unfortunately, it occasionally burst into ﬂame or even exploded.

Three British chemists discovered that cellulose could be solubilized when it was

treated with sodium hydroxide and carbon disulﬁde. The product is called cellulose

xanthate. The viscous solution was then extruded through a nozzle into an acidic

solution, forming lustrous, silk-like ﬁbers. They patented the process and commer-

cialized the product in 1894. This is “rayon.” It is chemically still cellulose, but its

texture is different from that of cotton. It can be produced from not only from cot-

ton, but also from any pulp.

Almost a century later, scientists at Courtaulds’, a rayon manufacturer, chanced

to discover that cellulose could be dissolved when heated in a solvent, N-methyl

morpholine oxide. A new ﬁber (of cellulose) was produced from this solution. Its

brand name is “Tencel.” It has a luxurious look and feel and yet strong to be made

into clothes like jeans.

Rayon used to be called “art silk,” but the ﬁbers mentioned above are not quite

artiﬁcial. They are modiﬁed products of natural material. Another effort to produce

silk-like ﬁbers led to the invention of “nylon,” a completely synthetic (artiﬁcial) ﬁber.

5.3.1 Nylon

In 1928, an organic chemist at Harvard, Wallace Carothers, was appointed to the

director of organic research lab at the DuPont Chemical Company. He led a team to

study polymers, the chemistry of which was just emerging. In 1938, he and his team

5 Clothing and Shelters: Polymeric Material

The repeating unit in nylon 66 is the combination of adipic acid and hexameth-

ylenediamine, both of which consist of six carbon atoms. Hence, the qualiﬁcation

“66.” In each step as two molecules are connected, one molecule of water is removed.

This process is a typical condensation polymerization. Because an amide group on

one polymer chain can hydrogen-bond to that of the adjacent polymer chain as in

silk (see Fig. 5.4), nylon forms a structure similar to the b-pleated sheet. The forma-

tion of nylon 66 can readily be demonstrated as some of you might have seen

(Fig. 5.6). As you see, you can combine any compounds as long as one has amine

invented “nylon 66.” This is an entirely new, synthetic polymer, and the only

resemblance (in chemical terms) to a natural ﬁber, silk, is that the two monomers are

connected by a peptide bond as in proteins. The same type of bond, peptide, is

called “amide” in synthetic polymer chemistry, and hence nylon 66 is an example of

polyamides. Kevlar is another example of polyamide.

Nylon 66 is synthesized by the following condensation reaction between adipic

acid and hexamethylenediamine:

Fig. 5.6 Formation

of nylon 66 [from Atkins P

and Jones L, “Chemistry

Principles, 3rd ed.”

(W. H. Freeman and Co.,

2005)]

HO C

N-CH

-NH

adipic acid

hexamethylenediamine

- H

N-CH

-N

)

(

nylon 66

amide (bond)

655.3 Synthetic Polymers

groups (NH

) at both ends and the other carboxylic acid group (COOH). Indeed,

many types of nylon have been synthesized, but only a few are still produced in

quantities.

One of these is nylon 66, and the other is nylon 6, which can be synthesized by

polymerizing a compound called caprolactam. This compound is ﬁrst hydrolyzed

by water to form amino caproic acid. It is a kind of amino acid. Whereas natural

amino acids should be characterized as a-amino acid, as the amine group and the

carboxylic acid group bind to the same carbon atom, the amino caproic acid is said

to be e-amino acid, because an amine group is situated at e (5th) position from the

carboxylic group. The amine and carboxylic groups in two molecules of e-amino

caproic acid can condense (with removal of water) to form a dimer. Its continua-

tion will result in the formation of nylon 6 (see the reaction equation below).

Nylons are cheaper and much stronger than the natural ﬁber silk and yet give the

feel and touch of silk. Nylon made an instant success, particularly as hosiery

material.

5.3.2 Polyesters

Polyesters are produced by a chemically similar means; i.e., condensation polymer-

ization. For example, methyl ester of terephthalic acid and ethylene glycol are

polymerized by removing methanol (instead of water), and the poly(ethylene

terephthalate) results. This polymer can be made into a unique ﬁber; crease-resistant

ﬁber. As the hexagonal unit (benzene ring) is rather rigid, the ﬁber made of this

C=O

()

nylon 6

caprolactam

+ H

NCH

HOH

-amino caproic acid

COOCH

COOC

+ HOCH

COOC

COO CH

- CH

COOC COO COO

OOC(

)

polyethylene terephthalate (PET)

5 Clothing and Shelters: Polymeric Material

polymer can resist shape change. Once it has been set in a certain shape, it can retain

its shape, no matter how you treat it. Hence, it has been used for shirts that require

no ironing after washing: a convenience for travelers.

Today, however, the polymer is more often used as the material to make PET

bottle. In this case, the polymer is produced by heating terephthalic acid and

ethylene glycol with removal of water molecule. The reaction is similar to that

shown above, except that the CH

groups in methyl terephthalate should be

replaced by Hs.

5.3.3 Polymers Obtained by Addition Polymerization

Carothers of DuPont company also invented a synthetic rubber, chloroprene. This

polymer is produced by addition-polymerization unlike the polymers mentioned so

far. Let us see how it is done with an example of polyethylene. Polyethylene is used

in everyday life as a wrap, the thin plastic ﬁlm. It is a polymer of ethylene, ethene in

technical terms. Ethylene has the chemical formula of CH

=CH

. Addition polym-

erization can be conducted with a compound having one or two double bonds; such

a compound is often called “oleﬁn,” Ethylene is the simplest oleﬁn. One of the two

bonds in a double bond is a s-bond and the other is a p-bond, which is weaker and

more readily splits than the s-bond. Alternatively, the electron in the p-bond easily

binds to an entity called a “free radical.” A free radical has an unpaired (single)

electron, which seeks another electron to pair up. The electrons in the p-bond pro-

vide such an electron. So what happens is this:

( )

2 2 22

R free radical, indicates an electron CH CH R-CH -CH

+= →

In this reaction, the entity R˙ is artiﬁcially produced or added and is called an

“initiator” (of polymerization). The resulting entity RCH

–CH

˙ now has an

unpaired electron, which can react with another molecule of ethylene:

22 2 2 2222

R-CH -CH CH CH R-CH -CH -CH -CH

+= →

This process can go on and on, making a long chain of –CH

–CH

– units; hence,

the product of such a polymerization is called “polyethylene.” Polyethylene is

essentially the same as the molecules in gasoline or wax; only the chain length is

enormous, often tens of thousands of ethylene units. [You might have noticed that

you smell wax when you burn polyethylene plastic wrap]. In this polymerization

process, you will note, no small molecule like water is removed as in condensation

polymerization, and a monomer is simply added to the growing end of a polymer.

Hence, polymerization of this type is called “addition” polymerization.

Derivatives of ethylene in which one of the hydrogen atoms is replaced by

another entity are called “vinyls.” For example, vinyl chloride is H

C=CHCl,

and vinyl acetate is H

C=CH(OCOCH

). The polymer of vinyl chloride is

poly(vinylchloride), PVC, which has been widely used for plastic cover and water pipe.

675.3 Synthetic Polymers

Other compounds of vinyl type that are used for polymers include acrylonitrile

(vinyl cyanide), H

C=CH(CN), and styrene, H

C=CH(C

). Vinyl compounds are

all polymerized by addition process. Tetraﬂuoroethylene, F

C=CF

, in which all the

hydrogen atoms of ethylene are replaced by ﬂuorine atoms, polymerizes similarly

to form a polymer of unique character, teﬂon.

Another kind of monomer (repeating units) of interest is what is called “diene.”

Ethylene and vinyl compounds are so-called monoene, compounds that have in them

only one double bond, as you saw. Diene molecules have two double bonds in them.

Butadiene CH

=CH–CH=CH

, isoprene CH

=C(CH

)–CH=CH

and chloroprene

=CCl–CH=CH

are important examples of diene. We have seen isoprene before;

it is the repeating unit of natural rubber latex.

A monomer such as XCH=CH

can polymerize in different ways as illustrated in

Fig. 5.7. In the ﬁrst example, the groups Xs are arranged in the same direction when

the monomers polymerize. In the second case, the directions of Xs alternate, whereas

they are random in the third case. The resulting polymers of different speciﬁc struc-

tures (stereospeciﬁc) may have different properties. For example, the ﬁrst two types

of structure tend to crystallize well, as compared with the random one; hence, they

are more suitable for use as a solid plastic. The modern process of polymerization

can control the stereospeciﬁcity of polymer to a large extent.

In the case of a diene CH

=CX–CH=CH

polymerization, there are two orienta-

tions of polymer chain across the central C=C double bond of each monomer unit,

as seen in Fig. 5.8. In one, the polymer chain orients itself in the cis-direction (the

same direction) across the C=C double bond; this is called cis-polymer. In the other,

it orients in the trans-direction (the opposite direction); trans-polymer. The cis-

polymer usually behaves like a rubber, while the trans-polymer is more like

plastic.

CCC

CC CCCC

CCCC

CC C

CC CC CC CC C

CCC

CCCCCCCCCC

XXXXX

HHHHH

(head-to-tail connection)

(head-to-head connection)

(occasionally

(X’s are in the same direction)

(X’s in the aiternate direction)

(X’s direction: random)

HHHH H

HH HHH

HHH

HH HH

HH HH HH

HH HH HH HH H

(X=CH

C1 etc)

Fig. 5.7 Stereospeciﬁc polymerization of a vinyl compound

5 Clothing and Shelters: Polymeric Material

Addition polymerization can be accomplished not only through a free radical

initiator as mentioned above, but also by some other means. The most important

polymerization catalyst is of the type known as Ziegler–Natta catalyst. These two

chemists discovered that a combination of chemicals titanium tetrachloride and tri-

ethyl aluminum is an excellent catalyst for polymerizing a number of oleﬁns. They

were awarded Nobel Prize in 1963 for this discovery. Subsequent research by others

found that similar combinations of chemicals: a transition element compound and

triethyl aluminum or similar alkylating agent do catalyze polymerization of oleﬁns.

Speciﬁc combination of such chemicals allow formation of polymers of speciﬁc

stereochemistry.

Ethylene is the simplest oleﬁn. Propylene (propene) is the next simplest. The

polypropylene produced by a Ziegler–Natta catalyst turned out to be of the structure

of a special stereochemistry, and to form a fairly crystalline plastic. It was too hard

for most of applications and people who developed this polymer had to invent some

use for it. One of the uses they came up with was “hoola hoop.” This is an example

in which a product had been produced before they knew what they could use it for.

Is this a rare exception? Would not people know the uses for a product before they

decide to produce it? It is usually the case. However, there have been a number of

cases in which a product had been produced ﬁrst for no good reason or other pur-

poses and only afterward they discovered its good application.

5.3.4 A Story of Vinyl Chloride

Let us talk about such an example. Paper industry needs a chemical called sodium

hydroxide (caustic soda) to clean the pulp. The most convenient way to produce

sodium hydroxide of a good quality is to use “electrolysis” of a brine solution con-

taining sodium chloride. That is, if you put electric current through the solution, you

can get a sodium metal which can easily be converted to sodium hydroxide by react-

ing the metal with a steam. The other product of the electrolysis of the brine solution

Fig. 5.8 Cis- or trans-polymerization

C=C-CH=CH

polymerize

CC CC

Cis-polymer

Trans-polymer

695.3 Synthetic Polymers

is “chlorine.” Chlorine (Cl

) is used for bleaching the pulp and to disinfect drinking

water. It is also used for the production of chlorine-containing organic compounds

such as described in Chap. 16, but the production of these compounds has declined

for the reason of environmental concerns in recent decades. People did not know

what to do with the excess chlorine. One of the uses they came up with was to pro-

duce vinyl chloride. Hydrogen chloride that can be produced from chlorine easily is

made to react with acetylene; the resulting compound is vinyl chloride. Today, how-

ever, vinyl chloride is made from ethylene and chlorine.

Vinyl chloride is polymerized to produce polyvinyl chloride (PVC) as mentioned

earlier. It is cheap, and fairly stable and sturdy, and hence has been used very widely

for such purposes as plastic ﬁlm, plastic cover, fake surface material (in any shade

like walnut) for wall and furniture, and water pipe.

It has some problems. The plastic loses plasticity and becomes brittle as it is

exposed to sunlight. This is due to a complicated series of chemical reactions ini-

tially caused by sunlight. Sunlight breaks one of the bonds, likely C–Cl, forming a

Cl-free radical, which abstracts hydrogen atom on the nearby carbon atom, likely

resulting in the formation of C=C double bond. The double bond then breaks, prob-

ably reacting with oxygen (O

) in the air under the inﬂuence of sunlight. Otherwise,

PVC is fairly stable and persists long in the environment.

Another problem is the formation of HCl when it is burned. It is argued that it

even forms the toxic dioxin when it is burned at high temperatures in the incinerator.

The biggest problem, however, seems to be that some toxic substances including

dioxin are produced as by-products in the process of producing vinyl chloride itself.

How to treat them and contain them is the problem.

Essential chemicals outlined in Part I are not sufﬁcient to maintain health of living

organisms. A number of the so-called inorganic elements (chemicals) are indeed

required for proper workings of living systems; they are often called “mineral nutri-

tion.” Human bodies are often subject to health-disrupting causes, and remedying

such ill conditions of health requires often chemicals (as medicine) in addition to a

number of endogenous mechanisms to combat such disruptions present in the

human body.

Part II

Enhancing Human Health