The Foundations of Chemistry

Neoprene is less affected by gasoline and oil and is more elastic than natural rubber. It
resists abrasion well and is not swollen or dissolved by hydrocarbons. It is widely used to
make hoses for oil and gasoline, electrical insulation, and automobile and refrigerator
parts.
When two different monomers are mixed and then polymerized, copolymersare
formed. Depending on the ratio of the two monomers and the reaction conditions, the
order of the units can range from quite regular (e.g., alternating) to completely random.
In this way, polymers with a wide variety of properties can be produced. The most impor-
tant rubber produced in the largest amount in the United States is styrene butadiene
rubber (SBR), a polymer of styrene with butadiene in a 1
3 molecular ratio.

Dihydric alcohols contain two XOH

groups per molecule. They are often

called glycols.

27-19 Polymerization Reactions 1093

The double bonds in SBR can be cross-linked by vulcanization as described for natural
rubber. SBR is used primarily for making tires. Other copolymers are used to make car
bumpers, body and chassis parts, wire insulation, sporting goods, sealants, and caulking
compounds.
Some addition polymers and their uses are listed in Table 27-14.

Condensation Polymerization

Some polymerization reactions are based on condensation reactions,in which two molecules
combine by splitting out or eliminating a small molecule. For such a polymer to be formed,
each monomer must have two functional groups, one on each end. A polymer formed in
this way is called a condensation polymer.There are many useful condensation poly-
mers, based on a wide variety of bifunctional molecules.
Polyesters(short for “polymeric esters”) are condensation polymers that are formed
when dihydric alcoholsreact with dicarboxylic acids.An ester linkage is formed at each end
of each monomer molecule to build up large molecules. A useful polyester is prepared
from terephthalic acid and ethylene glycol.

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 11.10, Condensation

Polymerization.

H H

H

3CC

CH 2 CH 2

C



H

HH

CC

H H

H

HH

CC polymerization

HH

H

CH 2

HH

CH 2 )

(CH 2 CH 2 CH 2 n

HO C OH HO

O

HHOCH 2 CH 2 O

HOCH 2 CH 2 O

H 2 O

n

C

O

C

O

C

O

C

O

C OCH 2 CH 2 O H

O

OH H OCH 2 CH 2 O H

terephthalic acid

polyethylene terephthalate (also known as PET)

(Dacron, Mylar)

ethylene glycol terephthalic acid ethylene glycol