Page 337 - Engineering Plastics Handbook

Polyarylethersulfones (PAES) 295

reacting this single difunctional polymer in the presence of potassium
carbonate or potassium hydroxide to form PES [3]. While conceptually
elegant, this route has not gained in commercial significance due to the
complexity of the process and the inability to form the intermediate in
sufficiently high yield and high purity to be of commercial viability.
Separation of the polymer from the solvent can be achieved by several
means. Coagulation of the polymer by contacting the polymer solution
with a nonsolvent can be used, followed by filtration, washing, and drying
of the resin powder, which is subsequently melt-extruded through an
extruder to form pellets. Another recovery method is the freeze-drying of
the reaction medium, followed by extraction of the solvent from the poly-
mer with a suitable low-boiling extraction solvent such as acetone. Both
of these recovery schemes are amenable to commercial-scale production.
Removal of the solvent by direct devolatilization in a vacuum-vented
extruder is also possible, provided the solvent has good thermal stability
and its boiling point is reasonably low.
The aromatic nucleophilic displacement chemistry described above is
the most viable route for production of sulfone polymers on a commer-
cial scale today. Other routes have been developed for the synthesis of
sulfone polymers over the years, and these will be touched on briefly in
this section for the sake of completeness.
A variation on the aromatic nucleophlic displacement chemistry
described above employs the cuprous chloride catalyzed polycondensa-
tion of an aromatic dihydroxy compound with a divalent aromatic
dibromo diaryl compound. This method has the advantage that the
dibromo diaryl compound does not have to be activated by the sulfone
group. This route can therefore be employed for copolymers and ter-
polymers that incorporate the sulfone moiety along with other non-
sulfone-containing repeat units. This synthesis route is commonly
known as the Ullmann synthesis [4].
One of the early and well-documented methods for producing poly-
arylethersulfones involves use of an electrophilic Friedel-Crafts poly-
merization process. First applied to PES, this synthesis route involved
reacting bis(4-chlorosulfonylphenyl)ether in a 1:1 molar ratio with
diphenylether [5–7]. Alternatively, a single-monomer Friedel-Crafts syn-
thesis can be employed starting with 4-chlorosulfonyldiphenyl ether as the
sole monomer. The Friedel-Crafts route to the production of sulfone poly-
mers was used briefly on a commercial basis in the 1960s but was quickly
abandoned due to a number of limitations. In addition to being opera-
tionally complex, this synthesis route yields polymers that are not always
linear or para-linked.
Nickel-catalyzed coupling of aromatic dihalides is a synthesis route that
can be used to make sulfone polymers containing a phenyl-phenyl bond
in their backbone, such as PPSF. This is a two-step synthesis that starts