Urethane reaction catalysis, mechanism

Based on the presented data, the probable mechanism of the catalysis of urethane reaction by the DBTDL catalyst is depicted in Figures 6 and 7. 

Various catalysts are used to prepare polyurethane at a relatively low temperature and with a much faster rate of polymerisation than would be the case with an uncatalysed reaction. Catalysts may be classified into two broad categories namely, amine (basic) compounds and organometalhc complex compounds. Tertiary amine is stiU one of the most frequently used urethane catalysts. Commonly used amine catalysts are triethylenedi-amine (TEDA), l,4-diazabicyclo[2.2.2]octane (DABCO), triethylamine (TEA), dimethylethanolamine (DMEA) and dimethylcyclohexylamine (DMCHA). The catalysis mechanism of tertiary amine catalysed urethane reaction involves complexation of the amine with isocyanate groups, followed by reaction of the complex with alcohol to produce polyurethane. A list of catalysts used in polyurethane preparation is given in Table 6.4. 

Organometalhc compounds based on lead, tin, bismuth and zinc are also used to catalyse a urethane reaction. Bismuth and zinc carboxylates are used because of the toxicity and disposal problems of lead and tin. Nowadays, alkyl tin carboxylates, oxides and mercaptide oxides such as dibutyltin dUaurate (DBTDL),dioctyltin mercaptide, stannous octoate and dibutyltin oxide are used successfully in all types of polyurethane applications (Table 6.4), among which DBTDL was found to be the most widely used catalyst. The catalytic effect of organometaUic compounds is due to their capacity to form a complex with the isocyanates and polyols. The catalysis mechanism involves interaction of the metal cation with isocyanate and hydroxyl groups, followed by rearrangement of the resulting complex to yield the final urethane product. 

I. S. Bechara, The Mechanism of Tin-Amine Synergism in the Catalysis of Isocyanate Reaction with Alcohols, in Urethane Chemistry and Applications, ACS Symposium Series 172, K. N. Edwards, (Ed.), American Chemical Society, Washington, DC, 1981. 

The catalysts of reactions between 4,4 -diphenylmethane diisocyanate (MDI) and alcohols in N,N-dimethylformamide (DMF) by dibutylin dilaurate has been investigated. The reaction rate of the catalyzed urethane formation in DMF is proportional to the square root of dibutylin dilaurate concentration. This result differs from that of similar studies on apolar solvents. The catalysis in DMF can be explained very well by a mechanism in which a small amount of the dibutylin dilaurate dissociates into a catalytic active species. 

T n continuation of a study of the uncatalyzed reactions between MDI (4,4 -diphenylmethane diisocyanate) and alcohols in DMF (N,N-dimethylformamide) (J), the effect of dibutyltin dilaurate on the same reactions has been studied. The results were compared with those found in studies on the mechanism of catalysis of urethane formation in apolar solvents (2-6). 

For the catalysis of isocyanate-alcohol reactions in apolar solvents, several mechanisms have been proposed. However, the results of the kinetic measurements in DMF could not be explained with these mechanisms. So we concluded that, in the polar solvent DMF, the mechanism of the catalyzed urethane formation differs from the published mechanisms in apolar solvents. The behavior in DMF can be explained from a mechanism in which dibutyltin dilaurate dissociates into a catalytic active species. 

If the mechanism of the catalysis of isocyanate-alcohol reaction by tin carboxylates does indeed proceed via the alkoxide as proposed by Bloodworth and Davies (11), then the synergism of the amine to tin can readily be explained from the above equilibrium. The amine will assist in the alcoholysis step and speed up the decomposition of the tin-carbamate complex by the alcohol to the urethane and tin alkoxide. 

---