Chemistry Reference
In-Depth Information
monoglyceride content of the original reaction mixture determines the microgel content of the alkyd
and the dynamic properties of the dried film [
31
]. The finished alkyd resin can be illustrated as follows:
R
R
O
O
O
O
O
O
O
O
O
O
O
O
7.2.4 Polycarbonates
A special group of polyesters of carbonic acid are known as
polycarbonates
. The first polycarbonates
were prepared as early as 1898 by Einhorn by reacting phosgene with hydroquinone and with
resorcinol [
33
]. These materials lack desirable properties and remain laboratory curiosities. During
the fifties, however, new polymers were developed from 4,4
0
-dihydroxydiphenyl alkanes. These
polycarbonates have high melting points and good thermal and hydrolytic stability. Nevertheless,
to date only one polycarbonate has achieved significant commercial importance. It is based on 2,2
0
-
bis(4-hydroxyphenyl)propane.
There are two main methods for preparing polycarbonates, one is by a direct reaction of phosgene
with the diphenol and the other one by an ester interchange. The direct phosgenation is a form of a
Schotten-Baumann reaction that is carried out in the presence of a base:
O
O
+
n
COCl
2
n
HO
OH
n
O
The reaction may be carried out in the presence of pyridine that acts as a catalyst and as an HCl
scavenger. Often a chlorinated solvent is used as a diluent for the pyridine. Phosgene is bubbled
through a solution of the diphenol at 25-35
C. The pyridine hydrochloride precipitates out and after
washing the pyridine solution with dilute HCl and water, the polymer is precipitated with a nonsolvent.
An interfacial polymerization procedure is also employed in direct phosgenation. A caustic
solution of the diphenol is dispersed in an organic chlorinated solvent containing small quantities
of a tertiary amine. Phosgene is bubbled through the reaction mixture at 25
C. When the reaction is
complete, the organic phase contains the polymer. It is separated and the product isolated as above.
The ester interchange method is carried out between the diphenol and diphenyl carbonate:
O
n
+
HO
OH
n
OO
OH
+
O
O
n
2n
O
To obtain high molecular weights by this method, almost complete removal of the phenol is
required. The reaction is carried out with typical basic catalysts, like lithium hydride, zinc oxide, or
antimony oxide under an inert atmosphere. Initial reaction temperature is 150
C. It is raised over 1 h
