Organic Chemistry - Fundamentals of Chemistry - page 540

Protein is a product of the polymerization of amino acids. Twenty different nat-
ural amino acids, each with a different R group in the formula, are components
of human proteins.

Amino end Acid end

NH 2 CHCOOH
R

Each amino acid has a
different R group

Amino acid

A polymerization reaction can also take place between molecules contain-
ing two organic acid groups and molecules containing two alcohol groups; a
polyester, widely used in clothing manufacture, is formed.
The reactions described so far are examples of condensation polymeriza-
tion. In such reactions, small molecules (in these cases, water molecules) are
eliminated from the reacting functional groups. Another type of polymerization
is addition polymerization, illustrated by the polymerization of ethylene:

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
C
H

→

H
W

H
W

H
W

H
W

H
W

H
W

H
W

H
W

H
W

H
W

H
W

H
W

UC±C±C±C±C±C±C±C±C±C±C±CU

H H H H H H H H H H H H

A portion of a polyethylene molecule

In this polymerization process, no small molecules are produced. The monomer
molecules add together by shifting electrons from their double bonds to form
new carbon-carbon single bonds.

EXAMPLE 20.17

Show how shifting the electrons of four C 2 H 4 molecules causes reaction lead-

ing to polymerization.

Solution

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
C
H

H
C
H

H
C
H

H
C
H

H
C
H

£

This process continues until a polymer is formed.

Practice Problem 20.17 If one hydrogen on each monomer is
replaced with a group, (a) what difference does this change make to the
electron shifting process? (b) what polymer results?

CH 3 ¬