Chemistry Reference
In-Depth Information
1.11
Constitutional Isomerism
As mentioned in
Section 1.6
, polymers exist in a variety of molecular architec-
tures. There are three types of isomerisms that are important in macromolecular
species. These involve constitutional, configurational, and conformational varia-
tions. These terms are defined and illustrated. Their usage in macromolecular sci-
ence is very much the same as in micromolecular chemistry.
The constitution of a molecule specifies which atoms in the molecule are
linked together and with what types of bonds.
Isobutane (1-54) and n-butane (1-55) are familiar examples of constitutional
isomers. Each has the molecular formula C
4
H
10
but the C and H atoms are joined
differently in these two molecules. In polymers the major types of constitutional
differences involve positional isomerism and branching.
CH
3
CH
3
C
H
CH
3
1-54
CH
3
CH
2
CH
2
CH
3
1-55
1.11.1
Positional Isomerism
Vinyl and vinylidene monomers are basically unsymmetrical because the two
ends of the double bond are distinguishable (ethylene and tetrafluorethylene are
exceptions). One C of the double bond can be arbitrarily labeled the head and the
other the tail of the monomer, as shown in the formula for vinyl fluoride (1-56).
H
head
CH
2
C
tail
F
1-56
In principle, the monomer can be enchained by head-to-tail linkages or head-
to-head, tail-to-tail enchainments (1-57). Poly(vinyl fluoride) actually has about
15% of its monomers in the head-to-head, tail-to-tail mode. This is exceptional,
however. Head-to-tail enchainment appears to be the predominant or exclusive
