Chemistry Reference
In-Depth Information
This representation of polymer structures implies that the whole molecule is
made up of a sequence of such repeating units by linking the left-hand atom
shown to the right-hand atom, and so on. Thus, the following structures are all
equivalent to 1-5.
CH
2
CH
2
O
C
O
(a)
C
O
CH
2
O
C
C
O
O
CH
2
C
C
O
CH
2
CH
2
O
n
n
n
O
O
O
O
(b)
(c)
1-5
Formula (b) would not normally be written because the nominal break is in
the middle of a —OCH
2
CH
2
O— unit, which is the residue of one of the precur-
sors (HOCH
2
CH
2
OH) actually used in syntheses of this polymer by the reaction:
n HOCH
2
CH
2
OH + n HOOC
COOH
O
CH
2
CH
2
O
C
O
C
O
+ 2n H
2
O
(1-1)
n
1.1.6
End Groups
The exact nature of the end groups is frequently not known and the polymer struc-
ture is therefore written only in terms of the repeating unit, as in the foregoing
structural representations. End groups usually have negligible effect on polymer
properties of major interest. For example, most commercial polystyrenes used to
make cups, containers, housings for electrical equipment, and so on have molecu-
lar weights of about 100,000. An average polymer molecule will contain 1000 or
more styrene residues, compared to two end units.
1.2
Degree of Polymerization
The term degree of polymerization refers to the number of repeating units in the
polymer molecule. The degree of polymerization of polyacrylonitrile is y. The
definition given here is evidently useful only for polymers which have regular
identifiable repeating units.
H
CH
2
C
y
CN
1-2a
