Organotin catalyst reaction with Isocyanates

Catalysts. To make the final polyurethane, common catalysts are tertiary amines and organotin compounds, often used in mixtures. Amine catalysts favor the reaction of isocyanate with water, producing urea linkages and CO2, which acts as a blowing agent. On the other hand, organotin catalysts favor the isocyanate/hydroxyl reaction. Additional catalysts and catalyst decomposition products may be present if graft polyols are part of the mixture. The most common of these catalysts are free radical initiators. Phosphorus compounds may be present in the case of carbodiimide-modi-fied MDI. 

Organotin compounds are effective catalysts for the isocyanate-hydroxyl reaction. Tin catalysts have a slight odour, and low amounts are required to achieve a high reaction rate. Examples of organotin catalysts are stannous octoate, stannous oleate, dibutyltin dilaurate and dibutyltin di-2-ethylhexoate. They are often used in conjunction with small concentrations of antioxidants such as tertiary-butyl catechol resorcinol and tartaric acid. 

Complexes of organotin with amidines have been found to be excellent catalysts for the preparation of polyurethane foams, which do not have the disadvantage of any amine odor and, in addition, delayed onset of the isocyanate-hydroxyl reaction An example of a mercapto-delayed organotin catalyst is 2,2,4,4-tetrakis(alkyl)- l,3,2,4-dithia-stannetane.55 Amine salts of amino acids, tertiary amino acids, and tertiary amino acid-nitrile compositions, have been found to be effective as delayed action catalysts for polyurethane synthesis. They are particularly effective when used in combination with an organometallic compound, such as an organotin. ... 

Di-n-butyltin catalysts are being used in the preparation of polyurethane foams. Most polyurethane foams utilize aromatic isocyanates such as toluene diisocyanate (TDI) or diphenylmethane diisocyanate (MDI) as the isocyanate, and a polyester or polyether polyols as the coreactant. Tertiary amine catalysts are used to accelerate the reaction with water and formation of the carbon dioxide blowing agent. To achieve a controlled rate of reaction with the polyol, an organotin catalyst can be used. Polyurethane foams are not only applied in place, but are also cast in a factory as slabstocks. These foam slabs are then cut for use in car seats, mattresses, or home furnishings. DBTDL is an excellent catalyst in high resiliency slabstock foams. DBTDL shows an excellent reaction profile for this application replacement for DBTDL in such an end-use is difficult and requires a substantial reformulation of the foam. 

For environmental reasons, organotin catalysts are being replaced in a nnmber of applications with more benign catalysts. The replacement of an organotin catalyst often reqnires a complete change of formnlation. Most alternate catalyst systems offer a different reaction profile. Table 6.2.10 shows a partial periodic table and the elements that, according to the literature and our own screening studies,i° are active catalysts for the isocyanate reaction. 

Carbamates of tertiary alcohols. The reaction of tertiary alcohols with isocyanates catalyzed by base usually results in dehydration as well as carbamate formation. However, carbamates of tertiary alcohols can be obtained in yields as high as 90% by use of organotin catalysts (stannous octoate, stannous oleate, dibutyltin dilaurate) and an amine. 

Two general classes are very well known. The first is composed of tertiary amines, of which DABCO, tertiary alkylamines, substituted morpholines, piperazines, guanidines, and substituted hydroxy amines are representative types. A wide range of activities is represented in this class, but stronger catalysts are sometimes needed to promote the reaction of secondary hydroxyl groups with isocyanates. Stronger catalysis is supplied by catalysts of the second class, organotin compounds like dibutyltin dilaurate... 

Reactions of isocyanates with alcohols are catalyzed by a variety of compounds, including bases (tertiary amines, alkoxides, and carboxylates), metal salts and chelates, organometallic compounds, acids, and urethanes. The most widely used catalysts in coatings are organotin (IV) compounds, most commonly dibutyltin dilaurate (DBTDL) (dibutylbis[(l-oxododecyl)oxy]stannate) [77-58-7] and tertiary amines, commonly diazabicyclo[2.2.2]octane (DABCO). Combinations of DABCO and DBTDL often act synergistically. 

DBTDL is soluble in a wide range of solvents, comparatively low in cost, colorless, and, in general, highly effective at levels of the order of 0.05 wt%. DBTDL promotes urethane formation without promoting allophanate formation (2) or trimerization (3). While aromatic isocyanates are more reactive than aliphatic isocyanates in uncatalyzed reactions with alcohols, the reactivity of aliphatics can be roughly equal with DBTDL. On the other hand, amine catalysts are more effective with aromatic than aliphatic isocyanates. Carboxylic acids inhibit catalysis by organotin compounds. 

In chiral organotin dibromide- and bistriflate-catalyzed desymmetrization of 2-substituted 1,3-propandiols with phenyl isocyanate the enantiomeric excess of the product was uniquely dependent on the reaction temperature. The chirality of the product was inverted from one enantiomer to anofher upon changing the reaction temperature from 0 to -78 °C (Scheme 12.170) [308]. When, on the ofher hand, this nonracemic organotin dihalide was employed as a catalyst for benzoylation of racemic 1,2-diols, nonenzymatic kinetic resolution was achieved under sophisticated reaction conditions (Scheme 12.171) [309]. 

COMPARISON OF TERTIARY AMINE (DMCHA) WITH ORGANOTIN (DBTDL) CATALYSTS IN THE REACTION OF PHENOL AND BENZYL ALCOHOL WITH PHENYL ISOCYANATE IN DIOXANE AT 25°C... 

Because of the reactive nature of isocyanate groups with the mentioned functional groups, polyisocyanates can be formulated to react under a wide variety of cure conditions. Many substances catalyze these reactions. Common catalysts are organotin compounds, other metallic salts, amines, and acids. This breadth of catalytic species can also introduce problems, because catalysts may be unknowingly introduced from other components of a formulation or even as contaminants of other formulating materials. 

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