Chemistry Reference
In-Depth Information
another polymer.
OROC
O
R
′
C
O
OR
OH
n
OROC
O
R
′
C
O
O
R
n
C
O
C
O
C
O
C
O
C
O
C
O
C
O
C
O
R
OO
R
R
′
O
R
O
R
′
OO
R
R
′
O
m
m
+
HO
R
O
C
O
R
′
C
O
OO
(7-14)
It has been established that random interchange reactions occur with
polyesters and polyamides at the high temperatures at which these
thermoplastics are normally polymerized and subsequently extruded or
molded. Other condensation polymers probably undergo similar shufflings
of repeating units under the proper conditions. This implies that the
equilibrium constant for condensation reactions like that in reaction (a) of
Fig. 7.2
is independent of the sizes of the molecules that are reacting.
Note that interchange reactions do not change the number of molecules
in the system. Therefore, the number average degree of polymerization of
the polymer will not be altered unless condensation products are also being
removed from the system. The weight average degree of polymerization can
change dramatically, however, as the sizes of the macromolecules become
randomized through this process.
Interchange reactions represent an approach to an equilibrium distribution
of molecular sizes. Polymerizations that use very reactive monomers can
proceed at room temperature [as in reaction (c) of
Fig. 7.2
]. They are
kinetically controlled and will not produce random molecular weight
distributions. Polymers from such syntheses will undergo interchange reactions,
however, if they are subsequently heated to temperatures at which the equilibria
such as that of reaction (7-14) can be reached in a reasonable time.
(ii)
All the functional groups in an n-functional reactant are usually assumed to
be equally reactive. This has been demonstrated for dibasic aliphatic acids
but is not true for 2,4-toluene diisocyanate (7-2) where the para isocyanate
reacts more readily than the ortho group. It is also thought that the
secondary hydroxyl in glycerol is less reactive than the primary alcohol
groups. (If such differences are known quantitatively they can be incorporated
into the subsequent calculations by multiplying the concentration of less
reactive groups by a fraction equal to the ratio of its reactivity to that of the
more reactive functional groups.) Further, we take it for granted that the
reactivity of a functional group does not change when other functional groups
