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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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increases the melting point according to Eq. (4.5). This same stiffness
enhances the crystallizability and increases the various deformation
moduli both through the chain stiffness and the crystallinity. The
improved interchain attraction in the stiffer chains decreases the
solubility of these materials, and this in itself contributes to their
diminished susceptibility to chemical attack. In addition, the backbone
rings effectively provide multiple strands along the polymer so that
there is a possibility for the polymer to remain intact even if one
strand is cleaved.
Once the potential associated with this aspect of molecular
architecture is recognized, the principles of the last section coupled
with the richness of organic (and inorganic) chemistry suggest numerous
synthetic possibilities. We shall not attempt to be comprehensive in
discussing this facet of polymer chemistry; instead we cite only a few
examples of step-growth polymers which incorporate
More Rings
335
successively greater proportions of ring character into the chain
backbone. These examples will also remind us that polyesters, polyamides,
and formaldehyde polymers are not the only kinds of polymers possible
from monomers with two or more functional groups.
Aromatic polyimides are the first example we shall consider of
polymers with a rather high degree of backbone ring character. This
polymer is exemplified by the condensation product of pyromellitic
dianhydride [VII] and p-amino-aniline [VIII]:
/
n
\
\

+ nHjN-NH1
[VII]
H
[VIII]

COOH
COOH
[IX]
- - N
[X]

(5.II)
This polymerization is carried out in the two stages indicated above
precisely because of the insolubility and infusibility of the final
product. The first-stage polyamide, structure [IX], is prepared in polar
solvents and at relatively low temperatures, say, 70C or less. The
intermediate is then introduced to the intended application-for example,
a coating or lamination-then the second-stage cyclization is carried out
at temperatures in the range 150-300C. Note the formation of five-
membered rings in the formation of the polyimide, structure [X] , and
also that the proportion of acid to amine groups is 2:1 for reaction
(5H).
336
Condensation or Step-Growth Polymerization
When the proportion of acid to amine groups is reversed-namely,
1:2-a process rather similar to reaction (5.II) yields a polymer which
ultimately contains the five-membered imidazole ring. This reaction is
also carried out in the stages listed below and illustrated by reaction
(5.JJ):
1. The diphenyl ester of the diacid [XI] is used to prevent side
reactions such as decarboxylation.
2. The condensation of the monomers with the elimination of water
and the
formation of a polyimine [XII] occurs at temperatures around
250C.
3. This first-stage polymer is then introduced into the application
environment, where the final cyclization reaction occurs.
4. The polyimidazole [XIII] is formed by heating to 350-400C, with
the
elimination of phenol and ring closure :
Another interesting structure with a high degree of ring
character along the backbone is the product obtained by the reaction of
1,4-cyclohexanedione
[XIV] and pentaerythritol [XV]:

[XI]
[XII]
[XIII]
(5.JJ)
n
(5.KK)
[XIV]
[XV]
The backbone here is made up of six-membered rings in which the two rings
share a common atom. These are called spiro polymers.
More Rings
337
Lastly, we consider a class of compounds called ladder polymers,
which are made up of a double-stranded backbone that is linked at regular
intervals into rings so that the schematic structure is
Spiro polymers are also sometimes classified as ladder polymers, and
molecules in which the ladder structure is interrupted by periodic single
bonds are called semiladders. Consisting entirely of fused ring
structures, ladder polymers possess very rigid chains with excellent
thermal stability.
Discussion of ladder polymers also enables us to introduce a step-
growth polymerization that deviates from the simple condensation
reactions which we have described almost exclusively in this chapter. The
Diels-Alder reaction is widely used in the synthesis of both ladder and
semiladder polymers. In general, the Diels-Alder reaction occurs between
a diene [XVI] and a dienophile [XVII] and yields an adduct with a ring
structure [XVIII]:
/

1 + II


(5.LL)
[XVI] [XVII] [XVIII]
Since the six carbons shown above have 10 additional bonds, the variety
of substituents they carry or the structures they can be a part of is
quite varied, making the Diels-Alder reaction a powerful synthetic tool
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