Chemistry Reference
In-Depth Information
regular structures for this to occur. Irregularities like inversions in monomer
placements (head-to-head instead of head-to-tail), branches, and changes in con-
figuration generally inhibit crystallization. Crystalline polymers will be high
melting, rigid, and difficultly soluble compared to amorphous species with the
same constitution. A spectacular difference is observed between isotactic polypro-
pylene, which has a crystal melting point of 176
C, and the atactic polymer,
which is a rubbery amorphous material. Isotactic polypropylene is widely used in
fiber, cordage, and automotive and appliance applications and is one of the
world's major plastics. Atactic polypropylene is used mainly to improve the low-
temperature properties of asphalt.
Isotactic and syndiotactic polymers will not have the same mechanical proper-
ties, because the different configurations affect the crystal structures of the poly-
mers. Most highly stereoregular polymers of current importance are isotactic.
[There are a few exceptions to the general rule that atactic polymers do not crys-
tallize. Poly(vinyl alcohol) (1-8) and poly(vinyl fluoride) are examples. Some mono-
mers with identical 1,1-substituents like ethylene, vinylidene fluoride, and vinylidene
chloride crystallize quite readily, and others like polyisobutene do not. The concepts
of configurational isomerism do not apply in these cases for reasons given above.]
Stereoregularity has relatively little effect on the mechanical properties of
amorphous vinyl polymers in which the chiral carbons are trisubstituted. Some dif-
ferences are noted, however, with polymers in which X and Y in 1-67 differ and
neither is hydrogen. Poly(methyl methacrylate) (
Fig. 1.4
) is an example of the lat-
ter polymer type. The atactic form, which is the commercially available product,
remains rigid at higher temperatures than the amorphous isotactic polymer.
Completely tactic and completely atactic polymers represent extremes of ste-
reoisomerism which are rarely encountered in practice. Many polymers exhibit
intermediate degrees of tacticity, and their characterization requires specification
of the overall type and extent of stereoregularity as well as the lengths of the tac-
tic chain sections. The most powerful method for analyzing the stereochemical
nature of polymers employs nuclear magnetic resonance (NMR) spectroscopy for
which reference should be made to a specialized text
[7]
. Readers who delve into
the NMR literature will be aided by the following brief summary of some of the
terminology that is used
[8]
. It is useful to refer to sequences of two, three, four,
or five monomer residues along a polymer chain as a dyad, triad, tetrad, or pen-
tad, respectively. A dyad is said to be racemic (r) if the two neighboring mono-
mer units have opposite configurations and meso if the configurations are the
same. To illustrate, consider a methylene group in a vinyl polymer. In an isotactic
molecule the methylene lies in a plane of symmetry. This is a meso structure.
R
H
R
C
H
m
1-70
