Chemistry Reference
In-Depth Information
In a syndiotactic region, the methylene group is in a racemic structure
RH
C
HR
r
1-71
In a triad, the focus is on the central methine between two neighboring mono-
mer residues. An isotactic triad (mm) is produced by two successive meso
placements:
R
R
R
C
H
m
m
1-72
A syndiotactic triad (rr) results from two successive racemic additions:
R
H
R
C
R
r
r
1-73
Similarly, an atactic triad is produced by opposite monomer placements, i.e.,
(mr) or (rm). The two atactic triads are indistinguishable in an NMR analysis.
The dyads in commercial poly(vinyl chloride) (PVC) are about 0.55% race-
mic, indicating short runs of syndiotactic monomer placements. The absence of a
completely atactic configuration is reflected in the low levels of crystallinity in
this polymer, which have a particular influence on the processes used to shape it
into useful articles.
1.13
Polymer Conformation
The conformation of a macromolecule of given constitution and configuration
specifies the spatial arrangements of the various atoms in the molecule that may
