Chemistry Reference
In-Depth Information
1.1.4
Repeating Unit
The repeating unit of a linear polymer (which is defined below) is a portion of the
macromolecule such that the complete polymer (except for the ends) might be pro-
duced by linking a sufficiently large number of these units through bonds between
specified atoms.
The repeating unit may comprise a single identifiable precursor as in polysty-
rene (1-1), polyacrylonitrile (1-2), polyethylene (1-3), or poly(vinyl chloride):
H
H
H
H
~ CH
2
C
CH
2
C
CH
2
C
CH
2
C ~
Cl
Cl
Cl
Cl
1-4
A repeating unit may also be composed of the residues of several smaller
molecules, as in poly(ethylene terephthalate):
~ OCH
2
CH
2
OC
COCH
2
CH
2
OC
O
COCH
2
CH
2
O ~
O
O
O
1-5
or poly(hexamethylene adipamide), nylon-6,6:
H
H
H
H
~ N
(CH
2
)
6
N
C
(CH
2
)
4
C
N
(CH
2
)
6
N
C
(CH
2
)
4
C ~
O
O
O
O
1-6
The polymers that have been mentioned to this point are actually synthesized
from molecules whose structures are essentially those of the repeating units shown.
It is not necessary for the definition of the term repeating unit, however, that such
a synthesis be possible. For example,
H
CH
2
C
OH
1-7
