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Polymer Chemistry. The Basic Concepts - Himenz P.C.

Himenz P.C. Polymer Chemistry. The Basic Concepts - Copyright, 1984. - 736 p.
Download (direct link): polymerchemistry1984.djvu
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involve only hydrogens. This formalism also applies to 1,1-disubstituted
ethylenes in which the substituents are different. With these symbols,
the isotactic, syndiotactic, and atactic structures shown in Fig. 1.2 are
represented by structures [VI]-[VIII], respectively:
[VI]

x [VII]
[VIII]
The carbon atoms carrying the substituents are not truly asymmetric,
since the two chain sections-while generally of different length-are
locally the same on either side of any carbon atom, except near the ends
of the chain. As usual, we ignore any uniqueness associated with chain
ends.
There are several topics pertaining to stereoregularity which we
shall not cover to simplify the presentation:
1.
3.
Stereoregular copolymers. We shall restrict our discussion to
stereoregular homopolymers.
Complications arising from other types of isomerism. Positional and
geometrical isomerism, also described in Sec. 1.6, will be excluded for
simplicity. In actual polymers these are not always so easily ignored.
Polymerization of 1,2-disubstituted ethylenes. Since these introduce two
different "asymmetric" carbons into the polymer backbone (second
substituent Y), they have the potential to display ditacticity. Our
attention to these is limited to the illustration of some terminology
which is derived from carbohydrate nomenclature (structures [IX]-[XII]):
X Y X Y X Y
Y X
Erythro-di-isotactic
[IX]
Y X Y
Erythro-di-syndiotactic
[X]
Y Y Y
Threo-di-isotactic
[XI]
X Y
X Y X Y
Threo-di-syndiotactic
[XII]
Stereoregular Polymers
473
The successive repeat units in strucutres [VI]-[VIII] are of two
different kinds. If they were labeled Mj and M2, we would find that, as
far as microstructure is concerned, isotactic polymers are formally the
same as homopolymers, syndiotactic polymers are formally the same as
alternating copolymers, and atactic polymers are formally the same as
random copolymers. The analog of block copolymers, stereoblock polymers,
also exist. Instead of using and M2 to differentiate between the two
kinds of repeat units, we shall use the letters D and L as we did in
Chap. 1.
The statistical nature of polymers and polymerization reactions has
been illustrated at many points throughout this volume. It continues to
be important in the discussion of stereoregularity. Thus it is generally
more accurate to describe a polymer as, say, predominately isotactic
rather than perfectly isotactic. More quantitatively, we need to be able
to describe a polymer in terms of the percentages of isotactic,
syndiotactic, and atactic sequences.
Certain bulk properties of polymers also reflect differences in
stereoregularity. We saw in Chap. 4 that crystallinity is virtually
impossible unless a high degree of stereoregularity is present in a
polymer. Since crystallinity plays such an important part in determining
the mechanical properties of polymers, stereoregularity manifests itself
in these other behaviors also. These gross, bulk properties provide
qualitative evidence for differences in stereoregularity, but, as with
copolymers, it is the microstructural detail that quantitatively
characterizes the tacticity of a polymer. We shall examine the statistics
of this situation in the next section, and the application of NMR to the
problem in Sec. 7.11.
The comparison between stereoregular polymers and copolymers can be
extended still further. We can write chemical equations for propagation
reactions leading to products which differ in configuration along with
the associated rate laws. We do this without specifying anything-at least
for now-about the mechanism. There are several things that need to be
defined to do this:
1. These are addition polymerizations in which chain growth is
propagated through an active center. The latter could be a free radical
or an ion; we shall see that coordinate intermediates is the more usual
case.
2. The active-center chain end is open to front or rear attack in
general; hence the configuration of a repeat unit is not fixed until the
next unit attaches to the growing chain.
3. The reactivity of a growing chain is assumed to be independent of
chain length. In representing this schematically, we shall designate
active species, of whatever chain length, as either DM* or LM*. The M*
indicates the terminal active center, and the D or L, the penultimate
units of fixed configuration. From a kinetic point of view, we ignore
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