Chemistry Reference
In-Depth Information
7.3
Requirements for Step-Growth Polymerization
A great many step-growth reactions are theoretically available to produce a given
polymer.
Figure 7.2
provides a partial list of reactions of bifunctional monomers
that could be used to produce poly(ethylene terephthalate). Reaction (c) is the
fastest of those listed and proceeds very quickly at room temperature. It can be
carried out by dissolving the diacid chloride in an inert organic solvent and mix-
ing this solution with an aqueous basic solution of the glycol. The polymer will
form quickly at the interfaces between the two phases (cf.
Section 7.5
on interfa-
cial polymerization). It must, however, be freed of residual solvent and salts,
dried, and densified before it can be fed into downstream equipment which will
melt the polymer and shape it into fiber, film, or other products. Densification
normally involves melting, extrusion, and chopping into granules that will flow
efficiently in the hoppers of equipment used for fiber spinning or film extrusion.
This operation and other required postpolymerization processes are expensive, as
are the costs of the diacid chloride monomer and recovery of the organic solvent.
As a result, the net costs of process (c) are higher than those of either (a) or (b).
Process (a) is the preferred synthetic route. It involves a melt polymerization
which is finished at high temperatures (about 275
C) and low pressures (about
1 mm Hg) to strip out the water produced as a condensation product and drive the
equilibrium depicted to the polymer side. The product is a molten polymer which
is suitable for immediate formation of fibers or films or for granulation. The acid
monomer is less expensive than the diester in reaction (b). Thus, while the actual
polymerization of route (a) is slower and more expensive than that of (c), the
overall costs of producing a finished polymer are less.
(a)
( OCH
2
CH
2
0 - C
n HOCH
2
CH
2
OH + n HOOC
COOH
C )
+ 2nH
2
O
n
O
O
(b)
n HOCH
2
CH
2
OH + n CH
3
OC
COCH
3
( OCH
2
CH
2
OC
C )
+ 2nCH
3
OH
n
O
O
O
O
ba
se
(c)
C - Cl
( OCH
2
CH
2
0 - C
C )
n HOCH
2
CH
2
OH + n Cl - C
+ 2n HCl
n
O
O
O
O
(d)
( OCH
2
CH
2
0 - C
C )
n ClCH
2
CH
2
Cl + n NaOC
CONa
+ 2nNaCl
O
O
O
O
(e)
n CH
3
COCH
2
CH
2
OCCH + n HOOC
COOH
( OCH
2
CH
2
0 C
C )
+ 2nCH
3
COOH
n
O
O
O
O
FIGURE 7.2
Some possible syntheses of poly(ethylene terephthalate) by step-growth reactions of
bifunctional monomers.
