page_title: Polyurethane
section_title: Chemistry
hierarchical_section_title: Polyurethane / Chemistry
caption_attribution_description: English: Reaction mechanism for tertiary amine-catalyzed polyurethane polymerization
context_page_description: Polyurethane is a polymer composed of organic units joined by carbamate links. While most polyurethanes are thermosetting polymers that do not melt when heated, thermoplastic polyurethanes are also available.
  Polyurethane polymers are traditionally and most commonly formed by reacting a di- or triisocyanate with a polyol. Since polyurethanes contain two types of monomers, which polymerise one after the other, they are classed as alternating copolymers. Both the isocyanates and polyols used to make polyurethanes contain, on average, two or more functional groups per molecule.
  Polyurethanes are used in the manufacture of high-resilience foam seating, rigid foam insulation panels, microcellular foam seals and gaskets, spray foam, durable elastomeric wheels and tires, automotive suspension bushings, electrical potting compounds, high-performance adhesives, surface coatings and sealants, synthetic fibers, carpet underlay, hard-plastic parts, condoms, and hoses.
context_section_description: Polyurethanes are in the class of compounds called reaction polymers, which include epoxies, unsaturated polyesters, and phenolics.  Polyurethanes are produced by reacting an isocyanate containing two or more isocyanate groups per molecule (R−(N=C=O)ₙ) with a polyol containing on average two or more hydroxyl groups per molecule (R′−(OH)ₙ) in the presence of a catalyst or by activation with ultraviolet light.
  The properties of a polyurethane are greatly influenced by the types of isocyanates and polyols used to make it. Long, flexible segments, contributed by the polyol, give soft, elastic polymer. High amounts of crosslinking give tough or rigid polymers. Long chains and low crosslinking give a polymer that is very stretchy, short chains with many crosslinks produce a hard polymer while long chains and intermediate crosslinking give a polymer useful for making foam. The crosslinking present in polyurethanes means that the polymer consists of a three-dimensional network and molecular weight is very high. In some respects a piece of polyurethane can be regarded as one giant molecule. One consequence of this is that typical polyurethanes do not soften or melt when they are heated; they are thermosetting polymers. The choices available for the isocyanates and polyols, in addition to other additives and processing conditions allow polyurethanes to have the very wide range of properties that make them such widely used polymers.
  Isocyanates are very reactive materials. This makes them useful in making polymers but also requires special care in handling and use.  The aromatic isocyanates, diphenylmethane diisocyanate (MDI) or toluene diisocyanate (TDI) are more reactive than aliphatic isocyanates, such as hexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI). Most of the isocyanates are difunctional, that is they have exactly two isocyanate groups per molecule. An important exception to this is polymeric diphenylmethane diisocyanate, which is a mixture of molecules with two, three, and four or more isocyanate groups. In cases like this the material has an average functionality greater than two, commonly 2.7.
  Polyols are polymers in their own right and have on average two or more hydroxyl groups per molecule. Polyether polyols are mostly made by co-polymerizing ethylene oxide and propylene oxide with a suitable polyol precursor. Polyester polyols are made similarly to polyether polymers. Polyols used to make polyurethanes are mixtures of similar molecules with distinct molecular weights, which is why the "average functionality" is often mentioned. Despite being complex mixtures, industrial grade polyols are sufficiently well controlled to produce polyurethanes with consistent properties. Polyol chain length and functionality contribute much to the polyurethane properties. Polyols used to make rigid polyurethanes have molecular weights in the hundreds, while those used to make flexible polyurethanes have molecular weights in the thousands.
  The polymerization reaction makes a polymer containing the urethane linkage, −RNHCOOR′− and is catalyzed by tertiary amines, such as 1,4-diazabicyclo[2.2.2]octane (also called DABCO), and metallic compounds, such as dibutyltin dilaurate or bismuth octanoate. Alternatively, it can be promoted by ultraviolet light. This is often referred to as the gellation reaction or simply gelling.
  If water is present in the reaction mixture (it is often added intentionally to make foams), the isocyanate reacts with water to form a urea linkage and carbon dioxide gas and the resulting polymer contains both urethane and urea linkages. This reaction is referred to as the blowing reaction and is catalyzed by tertiary amines like bis-(2-dimethylaminoethyl)ether.
  A third reaction, particularly important in making insulating rigid foams is the isocyanate trimerization reaction, which is catalyzed by potassium octoate, for example.
  One of the most desirable attributes of polyurethanes is their ability to be turned into foam. Making a foam requires the formation of a gas at the same time as the urethane po
caption: PU reaction mechanism catalyzed by a tertiary amine
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