to-head and head-to-tail connections, which are calledthe sequence isomerism. The
small amount of head-to-head connections can be regarded as defects if most of
connections are head-to-tail. Some unsaturated repeating units such as 1,4,-
polybutadine havecis-ortrans-configurations in the sequences, as demonstrated
in Fig.2.12, referred tothe structural isomerism.
2.6.3 Spatial Irregularities
Each carbon atom on the backbone chain may contain four substitutes different
from each other, two of which are the rest parts of the chain at both sides with
different chain lengths. These carbon atoms exhibit chiral asymmetry, giving rise to
the optical activity labeled with R for the left-handed direction and with S for the
right-handed direction. Irregularities of chain sequences in optical activities are
calledoptical- or stereo-isomerism. A central mirror exists at the middle of the
sequence-regular chain (RRRRRRRR......|......SSS SSSSSSSS). Therefore, the
whole polymers are in the meso-form and will not show any optical activity.
However, in polypropylene, for example, if we stretch the backbone chain into
fullytrans-conformations, we can see varying sequence regularities from the
orientations of the side methyl groups, as illustrated in Fig.2.13. Three typical
cases are the isotactic, syndiotactic and atactic sequences. The examples of regular
Fig. 2.12Illustration of the structural isomerism of double bonds on polymer chains withcis- and
trans-configurations
Fig. 2.13 Illustration of
sequence regularities of
stereo-isomers along polymer
chains with three typical
cases for isotactic,
syndiotactic and atactic
sequences
2.6 Sequence Irregularities 31