Chemistry Reference
In-Depth Information
In step-growth polymerizations of commercial linear polymers the
de
gr
ees
of
conversion are close to unity. Thus, for linear nylons and polyesters, M
w
=
M
n
is
close to 2 as indicated by
Eq. (7-36)
with p
1.
Equations (7-24) and (7-26)
describe a random distribution of molecular sizes.
This distribution is also known as a Flory
5
Schulz distribution or a most probable
distribution. Note that it is most probable only for linear step-growth polymers
made under conditions that satisfy the assumptions in
Section 7.4.1
. If such poly-
mers are made under other conditions (cf.
Section 7.5
), the molecular weight dis-
tributions of the products will not conform to the relations we have derived.
However, when such polymers are melted and shaped, these processing conditions
usually facilitate interchange reactions and randomization of the molecular weight
distribution.
For example, while poly(ethylene terephthalate) is made from ethylene glycol
and terephthalic acid according to the reaction of
Fig. 7.2a
, the very viscous
nature of the reaction medium at high conversions limits the molecular weights
that can be attained economically. The chain lengths of such polyesters can be
further enhanced by a subsequent “solid-state polymerization” process in which
finely divided polymer is held under reduced pressure at temperatures just below
its softening point. Under these conditions, glycol is removed and the polymer
molecular weight is increased:
HO
CH
2
CH
2
OO
O
O
O
O
CH
2
CH
2
O
H
n
+
HO
CH
2
CH
2
O
C
O
C
O
O
CH
2
CH
2
O
H
n+m
HO
CH
2
CH
2
OO
O
O
O
O
CH
2
CH
2
O
H
n
+ HOCH
2
CH
2
CH
2
OH
(7-39)
Interchange reactions do not occur readily under such conditions, and the
molecular weight distribution of the product may differ significantly from the
random one derived above. When the polymer is melted subsequently for
spinning or extrusion, the molecular weight distribution will tend to equili-
brate toward the predicted “most probable” type, however. The rate at which
this equilibrium distribution is approached will depend on the time and tem-
perature conditions during melt processing and will be
e
nhanced by the pres-
ence of residual polymerization catalyst or water. The M
n
of the polymer will
not change during this randomization because neither the number of struc-
tural units n
or
the number of molecules can chang
e
in a closed spinning
system. The M
w
will, however, tend to approach 2M
n
in accordance with
Eq. (7-36)
at p
1.
Molecular weight distributions have also been derived for polymerizations of
multifunctional monomers, and the original work
[2]
should be consulted for
details of these calculations which are beyond the scope of this text.
5
