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Preparation

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 160°C 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 74°C. 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.

Reactivity Profile

Poly(propylene glycol) is an alcohol. Flammable and/or toxic gases are generated by the combination of alcohols with alkali metals, nitrides, and strong reducing agents. They react with oxoacids and carboxylic acids to form esters plus water. Oxidizing agents convert them to aldehydes or ketones. Alcohols exhibit both weak acid and weak base behavior. They may initiate the polymerization of isocyanates and epoxides.

Health Hazard

The compound has a very low toxicity; few, if any, symptoms will be observed. Contact of liquid with eyes causes slight transient pain and irritation similar to that caused by a mild soap.

Flammability and Explosibility

Nonflammable

Safety Profile

Poison by intraperitoneal and intravenous routes.When heated to decomposition it emits acrid smoke and irritating fumes.

Definition

ChEBI: A macromolecule composed of repeating propyleneoxy units.

General Description

Colorless liquid that is odorless or has a mild sweet odor. May float or sink in water.