Organic Chemistry

086 CHAPTER 28 Synthetic Polymers

Ti

Ti

Ti

ZCH CH 2

CHCH 2 R

Z

CHCH 2 CHCH 2 CHCH 2 R

ZZ

CHCH 2 CHCH 2 R

ZZ

CHCH 2 CHCH 2 R

Z ZZ

R Ti

ZCH CH 2

Ti

R Ti

CHCH 2 R

Z

CH 2

CHZ

Ti

a vacant

coordination site

a vacant

coordination site

a vacant

coordination site

a vacant

coordination site

28.4 Polymerization of Dienes •

The Manufacture of Rubber

When the bark of a rubber tree is cut, a sticky white liquid oozes out. This is the same

liquid found inside the stalks of dandelions and milkweed. The sticky material is latex,

a suspension of rubber particles in water. Latex protects the tree after an injury, by

covering the wound like a bandage.

Natural rubber is a polymer of 2-methyl-1,3-butadiene (isoprene; Section 26.6). On

average, a molecule of rubber contains 5000 isoprene units. All the double bonds in

natural rubber are cis. Rubber is a waterproof material because it consists of a tangle

of hydrocarbon chains that have no affinity for water. Charles Macintosh, a Scotsman,

was the first to use rubber as a waterproof coating for raincoats.

Gutta-percha (from the Malaysian words getah, meaning “gum,”and percha, mean-

ing “tree”) is a naturally occurring isomer of rubber in which all the double bonds are

trans. Like rubber, gutta-percha is exuded by certain trees, but it is much less common.

It is also harder and more brittle than rubber. Gutta-percha is the filling material that

dentists use in root canals and the material used for the casing of golf balls.

PROBLEM 13

Draw a short segment of gutta-percha.

By mimicking nature, scientists have learned to make synthetic rubbers with properties

tailored to meet human needs. These materials have some of the properties of natural

rubber, including being waterproof and elastic, but they have some improved properties as

well—they are tougher, more flexible, and more durable than natural rubber.

Synthetic rubbers have been made by polymerizing dienes other than isoprene. One

synthetic rubber is a polymer of 1,3-butadiene in which all the double bonds are cis.

Polymerization is carried out in the presence of a Ziegler–Natta catalyst so that the

configuration of the double bonds in the polymer can be controlled.

isoprene units cis-poly(2-methyl-1,3-butadiene)

natural rubber

n

3-D Molecules:

Isoprene; Natural rubber

Latex being collected from a
rubber tree.

Figure 28.1

The mechanism of the Ziegler–Natta-catalyzed polymerization of a substituted ethylene.

A monomer forms a complex with an open coordination site of titanium and then is

inserted between the titanium and the growing polymer.

p