Chemistry Reference
In-Depth Information
Fig. 6.5
Adiabatic tower
for mass polymerization of
styrene
German patent, polymerization is completed in a plate and frame filter press [
157
]. Water circulating
through the press removes the heat of the reaction, and the solid polymer is formed inside the frames.
This process is still used in some places [
158
].
Another approach is to use adiabatic towers. Styrene is first partially polymerized in two agitated
reaction kettles at 80-100
C. The syrup solution of the polymer in the monomer is then fed
continually into the towers from the top. The temperatures in the towers are gradually increased
from 100-110
C at the top to 180-200
C at the bottom. By the time the material reaches the bottom,
in about 3 h, the polymerization is 92-98% complete [
159
]. The unreacted monomer is removed
and recycled. A modification of the process is to remove the monomer vapor at the top of the tower
for reuse.
An improvement in the above procedure is the use of agitated towers [
160
]. To avoid channeling
inside the towers and for better heat transfer, three towers are arranged in series. They are equipped
with slow agitators and with grids of pipes for cooling and heating [
160
]. Polymeric melt is heated
from 95 to 225
C to reduce viscosity and help heat transfer. A solvent like ethyl benzene may be
added. A vacuum devolatalizer removes both monomer and solvent from the product (Fig.
6.5
).
6.12.2 Polystyrene Prepared by Ionic Chain-Growth Polymerization
Much research was devoted to both cationic and anionic polymerizations. An investigation of cationic
polymerization of styrene with Al(C
2
H
5
)
2
Cl/RCl (R
alkyl or aryl) catalyst/cocatalyst system was
reported by Kennedy [
161
,
162
]. The efficiency (polymerization initiation) is determined by the
relative stability and/or concentration of the initiating carbocations that are provided by the cocatalyst
RCl.
ΒΌ
-butyl chlorides exhibit low cocatalytic efficiencies because of low
tendency for ion formation. Triphenylmethyl chloride is also a poor cocatalyst because the
triphenylmethyl ion that forms is more stable than the propagating styryl ion. Initiation of styrene
polymerizations by carbocations is now well established [
163
].
N
-butyl, isopropyl, and
sec
