Propylene oxide may be polymerized by methods similar to those described
in the preceding section for ethylene oxide. Similarly, polymers of low
molecular weight and of high molecular weight are of commercial interest.
(a) Low molecular weight polymers
Poly(propylene oxide)s of low molecular weight, i.e. in the range 500-3500,
are important commercial materials principally on account of their extensive
use in the production of both flexible and rigid polyurethane foams.
At first, the most common polyether used in flexible polyurethane foams
was a linear poly(propylene glycol) with a molecular weight of about 2000.
This is prepared by polymerizing the oxide at about 160C in the presence of
propylene glycol and sodium hydroxide. The resulting
polymer has the following general form:
The majority of the hydroxyl groups in the polymer are secondary groups
and are rather unreactive in the urethane reaction. Initially, this limitation
was overcome by the preparation of pre-polymers and by the
use of block copolymers with ethylene oxide. The latter products are 'tipped'
with poly(ethylene oxide) and are terminated with primary hydroxyl groups
of enhanced reactivity:
(It may be noted that straight poly(ethylene glycol) is not satisfactory for
foam production owing to its water sensitivity and tendency to crystallize.)
The advent of more effective catalyst systems, however, now makes it
possible for poly(propylene oxide)s to be used in the preparation of flexible
polyurethane foams without recourse to the above mentioned procedures.
Also, it is now common practice to use polyethers which are triols rather than
diols; these lead to slightly cross-linked flexible foams with improved load
bearing characteristics. The triols are produced by polymerizing propylene
oxide in the presence of a trihydroxy compound such as glycerol, 1,1,1-
trimethylolpropane or 1,2,6-hexane triol; the use of, for example, trimethylolpropane leads to the following polyether triol:
Polyethers of molecular weights in the range 3000-3500 are normally used.
For the production of rigid polyurethane foams, polyether triols of lower
molecular weight (about 500) are used so that the degree of cross-linking is
increased. Alternatively, polyethers of higher functionality may be used; these
are prepared by polymerizing propylene oxide in the presence of hydroxy
compounds such as pentaerythritol and sorbitol.
(b) High molecular weight polymers
Poly(propylene oxide)s of high molecular weight, i.e., greater than 100000
have been prepared by the use of initiators similar to those employed in the preparation of high molecular weight poly(ethylene oxide)s.
The most extensively investigated initiators have been organoaluminium and
organozinc compounds, generally with added co-initiators. As has been
noted previously, the structural unit of poly(propylene oxide)
contains an asymmetric carbon atom and the polymer can exhibit tacticity.
Both atactic and isotactic poly(propylene oxide) have been prepared. As
normally obtained, i.e. from D-L-propylene oxide, the isotactic polymer is
optically inactive but optically active isotactic polymer has been produced
from L-propylene oxide. Except for their optical activity, both forms of
isotactic polymer are very similar in properties; they are both crystalline and
have a melting point of 74C. Isotactic poly(propylene oxide)s have not yet
found commercial application. Atactic poly(propylene oxide) has been investigated as a rubber but does not appear to have been produced in any
quantity.
