Chemistry Reference
In-Depth Information
7.5
Interfacial and Solution Polymerizations of Acid
Chlorides and Other Reactive Monomers
If step-growth polymerizations are carried out with very reactive monomers, the
polymer synthesis conditions may be mild enough that interchange reactions
between the polymer formed and low-molecular-weight reaction products do not
take place at a significant rate. Then the distribution of macromolecular sizes
depends on the kinetics of the particular polymerization system. It is not deter-
mined statistically according to the concepts developed in the preceding section,
because no equilibrium is reached between the various species in the reaction
mixture at a given time. An example has been given in
Section 7.3
, in connection
with reaction (c) of
Fig. 7.2
. This particular synthesis would not be used commer-
cially for the reasons given earlier but such reactions are very useful generally in
laboratory-scale polymerizations
and in selected larger
scale
step-growth
polymerizations.
Interfacial polymerizations rely on reactions of diacid chlorides and active
hydrogen compounds. Other examples include production of a polyurethane from
a diamine and a bischloroformate:
H
H
nH
2
N
R
NH
2
+
nCl
C
O
R'
O
C
Cl
N
R
N
C
OO
R'
C
+ 2nHCl
Base
n
O
O
O
O
(7-40)
and reaction of
a diamine
and a disulfonic
acid chloride
to yield a
polysulfonamide:
H
H
O
O
(7-41)
nH
2
N
R
NH
2
+
nCl
SO
2
R'
SO
2
Cl
N
R
N
S
R'
S
+ 2nHCl
Base
n
O
O
The reactants are dissolved in a pair of immiscible liquids, usually water and a
hydrocarbon or chlorinated hydrocarbon. The aqueous phase contains the diol,
diamine, dimercaptan, or other active hydrogen compound, an acid acceptor like
sodium hydroxide, and perhaps a surfactant, while the organic phase dissolves the
acid chloride. Polymerization takes place very rapidly at the interface between the
two phases, and the rate of reaction is controlled by the rate at which the mono-
mers can diffuse to the reaction site. The number of growing polymer chains is
restricted because the incoming monomers find it difficult to diffuse through the
polymer layer at the interface without being captured by a coreactive end.
For this reason, it is possible to obtain higher molecular weight products than
are normally formed in bulk step-growth polymerizations in which the high vis-
cosity of the molten product limits the efficiency of stirring. Since interchange
reactions are not significant under the mild conditions of these reactions there is
