Chemistry Reference
In-Depth Information
7.1.4 Techniques of Polymer Preparation
Many step-growth polymerizations are carried out by mass or bulk type polymerization. This is widely
done not only due to convenience, but also because it results in minimum contamination. Few step-
growth reactions are highly exothermic, so thermal control is not hard tomaintain. Because equilibrium
considerations are very important, the reactions are usually carried out in a way that allows continuous
removal of the by-product. Occasionally, the polymerizations are carried out in dispersion in some
convenient carriers. Solution polymerizations are sometimes used as a way of moderating the reactions.
Step-growth polymerizations can also be carried out with certain monomers at low temperature by
a technique known as
interfacial polymerization
or
interfacial polycondensation
[
28
]. The reactions
(applicable only to fast reactions) are conducted at the interface between two immiscible liquids.
Usually one of the liquids is water and the other one an organic solvent. An example may by a
Schotten-Baumann polarization reaction. In such an interfacial polymerization, the diamine would be
in the aqueous phase and the diacid chloride in the organic phase. The strong reactivity of acid
chloride groups with amines allows the reaction to be carried out at room temperature:
O
O
O
O
n
+
H
2
N
+
2n HCl
x
2
y
N
x
N
y
Cl
Cl
n
H
H
Addition of a base to the aqueous phase removes the hydrochloric acid that forms and catalyzes the
reaction. The choice of the organic solvent is important, because it appears that the reaction occurs on
the organic side of the interface [
28
].
There are several important differences between interfacial polymerizations and high temperature
condensations. Much higher molecular weight products form from polymerizations at the interface. This
is probably due to the high speed of the reactions between the diamines that diffuse into the organic phase
and the diacid chloride chain ends [
28
]. Exact stoichiometry is not necessary to attain high molecular
weights in interfacial polycondensation. The opposite is true in high temperature polymerizations.
Interfacial polycondensation is an interesting procedure that is often used in demonstrations in
polymer chemistry courses. Polyamides are prepared rapidly, in front of the class from diacid chlorides
and diamines. The products are removed quickly as they form, by pulling them out as a string from the
interface [
47
]. Polyesters can also be prepared from diacid chlorides and bisphenols. On the other hand,
preparation of polyesters fromglycols and diacid chlorides is usually unsuccessful due to low reactivity
of the dialcohols. The diacid chlorides tend to undergo hydrolysis instead. Commercially, this proce-
dure is so far confined mainly to preparations of polycarbonates (discussed further in this chapter).
7.2 Polyesters
The class of compounds called polyesters consists of all heterochain macromolecular compounds that
possess repeat carboxylate ester groups in the backbones. This excludes all polymers with ester
groups located as pendant groups, like acrylic and methacrylic polymers, poly(vinyl esters), and
esters of cellulose, or starch. What remains, however, is still a large group of polymeric materials that
can be subdivided into saturated and unsaturated polyesters.
7.2.1 Linear Saturated Polyesters
The saturated polyesters that find commercial applications are mostly linear except for some specially
prepared branched polymers used in the preparation of polyurethanes. The linear polyesters became
