A polyol is an alcohol containing multiple hydroxyl groups. In two different technological disciplines the term
"polyol" has a special meaning: food science and polymer
chemistry.
Detailed description
Food science
Sugar alcohols,
a class of polyols, are commonly added to foods because of their lower caloric
content than sugars; however, they are also, in general, less sweet, and are often
combined with high-intensity sweeteners. They are also
added to chewing gum because they are not
broken down by bacteria in the mouth or metabolized to acids, and thus do not
contribute to tooth decay.[1] Maltitol, sorbitol, xylitol, erythritol, and isomalt are
some of the more common types. Sugar alcohols may be formed under mild reducing conditions from
their analogue sugars.
Polymer chemistry
In polymer chemistry, polyols are
compounds with multiple hydroxyl functional groups available for organic reactions. A molecule with
two hydroxyl groups is a diol, one with three is a triol, one with four is a
tetrol and so on.
Monomeric polyols such as glycerin, pentaerythritol, ethylene glycol and sucrose often serve as the
starting point for polymeric polyols. These materials are often referred to as
the "initiators" and reacted with propylene oxide or ethylene oxide to produce polymeric
polyols. However, they should not be confused with free radical "initiators"
used to promote other polymerization reactions. The functional group used as
the starting point for a polymeric polyol need not be a hydroxyl group; there
are a number of important polyols which are built up from amines. A primary amino group (-NH2) often functions as the
starting point for two polymeric chains, especially in the case of polyether polyols.
Polymeric polyols are generally used to produce
other polymers. They are reacted with isocyanates to
make polyurethanes used to make mattresses, foam insulation
for appliances (refrigerators and freezers), home and
automotive seats, elastomeric shoe soles, fibers
(e.g. Spandex), and adhesives.[2]
Polymeric polyols are usually polyethers or polyesters. Polyether polyols
are made by reacting epoxides like
ethylene oxide or propylene oxide with the multifunctional initiator in the
presence of a catalyst, often a strong
base such as potassium hydroxide or a double metal
cyanide catalyst such as zinc hexacyanocobaltate-t-butanol complex.[3] Common polyether
diols are polyethylene glycol, polypropylene glycol, and poly(tetramethylene
ether) glycol. The examples shown below are fairly low molecular
weight triols based on glycerin (a triol) being
reacted with propylene oxide, ethylene oxide or a combination of
the two. In reality, the chains would not be of equal length in any one
molecule and there would be a distribution of molecular weight polyols within
the material. Polyether polyols account for about 90% of the polymeric polyols
used industrially; the balance is polyester polyols.
Another class of polymeric polyols
is the polyesters. Polyesters are formed by condensation or step-growth
polymerization of diols and dicarboxylic acids (or their derivatives), for example diethylene glycol reacting with phthalic acid.[5] Alternatively, the hydroxyl group and the
carboxylic acid (or their derivatives) may be within the same molecule, as in
the case of caprolactone. The example below is an idealized
structure that could be obtained by reacting pentaerythritol (a tetrol) with gamma-butyrolactone.
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