Chemistry Reference
In-Depth Information
monomers such as phthalic anhydride and by a higher crosslinking density. Softer,
more flexible, and elastomeric materials result from incorporation of oligo(eth-
ylene oxide)s (see Formula
9.1
), and from a lower crosslinking density usually
combined with longer blocks of polystyrene or other vinyl polymers. Furthermore,
maleic anhydride may partially be replaced by the more bulky tetrahydrophthalic
anhydride, and trimellitic anhydride may be incorporated, when branching is
desired. The numerous applications and variants of UPs were described and dis-
cussed in many topic chapters and review articles, a selection of which is given in
Refs. [
4
-
12
].
The preparation of UPs from maleic anhydride is usually performed at tem-
peratures from 140-180 C The liberated water is removed in vacuo until the
desired viscosity is achieved. A more precise control of molecular weights and
viscosity may be achieved by addition off monofunctional reagent, such as satu-
rated or unsaturated fatty acids. These polycondensations involve two (mostly)
undesired side reactions. The most frequent side reaction is isomerization of
maleic acid units to fumaric acid moieties (see Formula
9.1
). The fumarate units
are thermodynamically more stable, and isomerisation is favored by longer times
and higher temperatures Technical products typically contain 40-70 % fumarate
units. Addition of a catalyst, such as morpholin, may raise the extent of isomer-
ization up to 99 %. High contents of fumarate groups are desired because they
react about 20 times faster with styrene than th maleate units, and they are less
reactive toward the addition of alcohols. The addition of OH end groups of diols or
oligomers onto maleate groups is the second side reaction (see Formula
9.1
). This
addition reaction transforms double bonds into saturated groups and is called after
its discoverer the ''Ordelt Saturation'' [
13
,
14
].
Quite recently, the author and coworkers demonstrated [
15
,
16
] that polycon-
densation of alkanediols with maleic anhydride or substituted maleic anhydrides
proceeds without isomerisation or other side reactions, when the temperature is
limited to 100 C Such low temperatures require metal triflates as catalysts, and
the highest molar masses (Mns up to 12 kDa) were obtained with Sn triflate in the
case of maleic anhydride or with Bi triflate in the case of citraconic anhydride.
Unfortunately, this approach is not suited for technical production of UPs, because
relatively large amounts of expensive catalysts are required.
In various areas of step-growth polymerization, the past 20 years have seen
increasing interest in monomers from renewable resources. An early example,
namely a patent of 1962 [
17
] describes syntheses of UPs from maleic anhydride
and mixtures of isosorbide and other diols. Isosorbide is synthesized from glucose
and can technically be produced at relatively low costs, when quantities of 50,000
tons or more are needed.
Another monomer which can be produced from renewable resources is succinic
acid or its anhydride. Aliphatic polyesters of succinic acid are crystalline and
biodegradable. Particularly interesting and commercial is poly(1,4-butanediol
succinate), because its melting temperature (Tm = 120 C) is higher than that of
all other polyesters from a,x-alkanediols. For most syntheses of poly(alkane
succinate)s described in the literature succinic acid was used as monomer, but
