Chemistry Reference
In-Depth Information
1.12
Configurational Isomerism
Configuration specifies the relative spatial arrangement of bonds in a molecule
(of given constitution) without regard to the changes in molecular shape that can
arise because of rotations about single bonds. A change in configuration requires
the breaking and reforming of chemical bonds. There are two types of configu-
rational isomerisms in polymers, and these are analogous to geometrical and opti-
cal isomerisms in micromolecular compounds.
1.12.1
Geometrical Isomerism
When conjugated dienes polymerize by 1,4-enchainment, the polymer backbone
contains a carbon
carbon double bond. The two carbon atoms in the double bond
cannot rotate about this linkage and two nonsuperimposable configurations are
therefore possible if the substituents on each carbon differ from each other. For
example, the two monomers maleic acid (1-64, cis) and fumaric acid (1-65, trans)
H
C
COOH
H
C
COOH
1-64
HC
CH
COOH
HOOC
1-65
are geometrical isomers. Natural rubber is the all-cis isomer of 1,4-polyisoprene
and has the structure shown in 1-18.
The molecules in solid trans isomers pack more tightly and crystallize more
readily than cis isomers. (The melting point of fumaric acid is 160
C higher than
that of maleic acid.) These corresponding differences in polymers are also major.
The 1,4-cis-polydienes are rubbers, whereas the trans isomers are relatively low
melting thermoplastics.
Isomerism in diene polymers can be measured by infrared and nuclear mag-
netic resonance spectroscopy. Some of the polymerization methods described in
Chapter 11 allow the production of polydienes with known controlled constitu-
tions and geometrical configurations.
Cellulose (1-11) and amylose starch do not contain carbon
carbon double
bonds but they are also geometrical isomers. Both consist of 1,4-linked D-
glucopyranose rings, and the difference between them is in configuration at car-
bon 1. As a result, cellulose is highly crystalline and is widely applied as a struc-
tural material while the more easily hydrolyzed starch is used primarily as food.
