Polypropylene glycol
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Polypropylene glycol or polypropylene oxide is the polymer of propylene glycol. Chemically it is a polyether. The term polypropylene glycol or PPG is reserved for low to medium range molar mass polymer when the nature of the end-group which is usually a hydroxyl group, still matters. The term "oxide" is used for high molar mass polymer when endgroups no longer affect polymer properties. In 2003, 60% of the annual production of propylene oxide of 6.6x106 tonnes was converted into the polymer 1.
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[edit] Polymerization
Polypropylene glycol is produced by anionic ring-opening polymerization of propylene oxide. The initiator is an alcohol and the catalyst a base, usually potassium hydroxide. When the initiator is ethylene glycol or water the polymer is linear. With a multifunctional initiator like glycerine, pentaerythritol or sorbitol the polymer branches out.
Conventional polymerization of propylene oxide results in an atactic polymer. The isotactic polymer only exist in the laboratory. It can be produced from optically active propylene glycol but at a high cost. A salen cobalt catalyst has recently been reported to provide isotactic polymerization of the racemic propylene glycol 1.
[edit] Properties
PPG has many properties in common with polyethylene glycol. The polymer is a liquid at room temperature. Solubility in water decreases rapidly with increasing molar mass. Secondary hydroxyl groups in PPG are less reactive than primary hydroxyl groups in polyethylene glycol.
[edit] Uses
PPG is used in many formulations for polyurethanes. It is used as a rheology modifier.
[edit] External links
[edit] References
- 1 A Highly Active, Isospecific Cobalt Catalyst for Propylene Oxide Polymerization Kathryn L. Peretti, Hiroharu Ajiro, Claire T. Cohen, Emil B. Lobkovsky, and Geoffrey W. Coates J. Am. Chem. Soc., 127 (33), 11566 -11567, 2005. Abstract
