Solvent Effects on an Organotin-Catalyzed Alcohol-Isocyanate Reaction

Abstract

The pseudo-first order kinetics of the reaction of cyclopentanol with phenylisocyanate, catalyzed by dibutyltin di(2-ethylhexanoate) (DBTDEH), was studied by means of UV and IR spectrophotometry. Reaction orders with respect to catalyst and to monomeric alcohol [1] were considered in toluene (Tol), di(nbutyl)ether (DBE) and acetonitrile (ACN). The results, which were interpreted with the aid of rate equations previously derived [2-4] on the basis of a reaction scheme involving an alkoxytin(IV) intermediate, indicate that pre-reaction complexes of solvent-dependent composition are formed in the rate determining step of both catalyzed and uncatalyzed mechanisms [5], INTRODUCTIONPrior studies related to the organotin-catalyzed nucleophilic addition of alcohol in excess to isocyanate have shown that this reaction is first order in isocyanate. The so obtained pseudo-first order rate constants do not vary linearly with the concentration of alcohol or of catalyst. Various kinetic behaviours were reported. For Borkent [6] the order is close to 0.5 in catalyst and results from the presence of a cationic active intermediate R 3 Sn + while for Van der Weij [7] the value 0.5 found with respect to both catalyst and alcohol reflects the formation of an ionizable alcohol-catalyst complex. Such interpretations are questionable since these reactions are also easy in apolar solvents. For Entelis et al.[8] a ceiling of the rate, observed at high alcohol and catalyst concentrations, should be due to a partial blocking of reacting species by nonproductive reactant(s)-catalyst homo-and/or heteroassociates. Bloodworth and Davies [9], considering the high reactivity of tin alkoxides towards isocyanate, suggested the tin carboxylate to be partly alcoholyzed to an active alkoxytin catalyst (scheme 1). This type of reaction scheme is also supported by some of our experiments dealing with the decelerating effect exerted by carboxylic acids or thiols [2,3], However, in the former works, the order was generally determined with respect to the overall alcohol content. The molecular meaning of the found values is questionable since associated OH groups exhibit a much smaller reactivity than free ones, so that any interpretation of the kinetic influence of alcohol must logically take into account its solvent-dependent sharing among free and associated forms [1],

Quantification of simultaneous solvent evaporation and chemical curing in thermoset coatings

“…2 confirm the conclusion reached by Draye et al 13 that the catalytic reaction order with respect to alcohol should be half order for the type of solvent used here. A simulation using a reaction order of two was conducted and in this case the simulated average film conversion values were smaller than the experimental values after vitrification, where the concentration of OH groups become low (the catalytic rate constant could not be increased further because then the simulation of the surface conversion did not match).…”

“…In the presence of 50 ppm DBTDL (50°C), they found that the ratio of catalytic to noncatalytic reaction rate was 23. Draye et al 13 investigated the reaction of cyclopentanol with phenylisocyanate catalyzed by dibutyltin di(2-ethylhexanoate) in toluene, di(n-butyl)ether and acetonitrile solvents. Their results indicate that activated complexes of solvent-dependent composition are formed prior to the rate-determining step, similar to what was found for the noncatalyzed reaction.…”