Dialkyltin dicarboxylates

A number of recent studies have been concerned with the effects of commercial heat stabilizers on the photodegradation of PVC. During irradiation at room temperature under air with 253.7-nm light, several dialkyltin dicarboxylates were found to increase the rates of the photooxidation and cross-linking of the polymer (62). However, at 0°C under air in a sunshine weatherometer, photooxidation was shown to be retarded by certain dibutyltin dicarboxylates (63). The latter result was also obtained in experiments involving the use of dibutyltin maleate with irradiation in the (280-400)-nm wavelength region at 38°C under air (50,51,64, 5,66). 

Dialkyltin dicarboxylates, e g. Bu2Sn(02CCu 1123)2, are useful catalysts for polyurethane foam formation and as cross-linking agents for room-temperature vulcanization. Monobutyltins have been used as catalysts in transesterification and esterification reactions, e.g. in the reaction between phthalic anhydride and 2-ethylhexanol. 

Another exception to the tetrahedral geometry for tin complexes are the dialkyltin dicarboxylates, such as di-n-butyltin dilaurate (DBTDL), where the carboxylate can be bidentate and a skew-trapezoidal bipyramidal geometry results (Figure 6.2.2). 

These explanations, however, are unlikely in view of the fact that the amine is certainly a much better ligand than the isocyanate or alcohol and therefore its coordination to the metal ion should be favored over that of the isocyanate. It was shown by Graddon and Ranna (8) that dialkyltin dicarboxylates form a 1 1 complex with tertiary amines quite readily. Using calorimetric techniques, these workers calculated the formation constants in benzene solution for numerous complexes of alkyltin carboxylates with tertiary amines. 

Organotin stabilizers also have a widespread use some examples are dialkyltin dicarboxylate, dialkyltin bis(maleate ester), dialkyltin bis(alkyl-mercaptan), dialkyltin(S,S -mercaptoacid ester) and dialkyltin 0-mercapto-propionate. The last one is supposed to exist in the solid state as a kind of telomer... 

The existence of a ligand exchange reaction between dialkyltin dimercaptides and dicarboxylates (typical poly(vinyl chloride) stabilizers) and dialkyltin dichlorides is demonstrated by 1H and 13C NMR. Dibenzyl- and dibutyltin dimercaptides undergo the reaction instantaneously and quantitatively at 25° C to yield dialkylchlorotin mercaptides. Dialkyltin dicarboxylates and dimethyltin dimercaptides appear to participate in an equilibrium exchange reaction with dialkyltin dichlorides in which the individual components are not identifiable by NMR at 25°C. These results are discussed as they pertain to poly(vinyl chloride) stabilization. 

As catalysts, almost exclusively dialkyltin dicarboxylates are used. Since the catalytically active species is the dialkyltin dihydroxide, the original catalyst compound must be first hydrolysed to become effective. It is therefore imperative that a certain level of moisture, i.e. water in vapor form as a cocatalyst, is present in both the rubber system and the ambient atmosphere. 

Carboxylates of divalent tin are used almost exclusively for the commercial production of flexible foams. Catalysts containing tetravalent tin, e.g. the dialkyltin dicarboxylates, catalyze oxidation at elevated temperatures. Since temperatures exceeding 140° are often reached during the production of polyurethanes, this can lead to severe degradation unless these catalysts are used in conjunction with small amounts of antioxidants such as substituted phenolics. 

One possible mode of stabilizer action is the deactivation of labile sites responsible for initiating dehydrochlorination and it has been suggested that metal carboxylate and dialkyltin dicarboxylate stabilizers function in this way. Using radiotracer techniques it was found that a small amount of stabilizer becomes attached to the polymer, probably by substitution of unstable chlorine atoms with the ligands of the stabilizer to yield esters which are more stable than the original chloride [13], e.g. ... 

A number of organotin polyesters, polyamides, polythiols, polyethers, and polyaminoesters have been synthesized from dialkyltin halides with dicarboxylic acids, diols, diamines, dithiols, urea, thiourea, amino acids, and hydroxyl acids involving polycondensation reactions. These polymers have potential applications as thermostabilizers for PVC. 

The Ligand Exchange Reaction of Some Dialkyltin Dimercaptides and Dicarboxylates with Dialkyltin Dichlorides... 

Dialkyltin Compounds. All of the dialkyltin dimercaptides and dicarboxylates were prepared by reaction of the appropriate dialkyltin oxide (Alfa Inorganics) with the desired mercaptan or carboxylic acid. The reactions were carried out in refluxing benzene or toluene using a Dean-Stark water separator. The yields are quantitative and products require no purification (purity confirmed by H and 13C NMR). The dialkyltin dichlorides were obtained commercially (Alfa Inorganics) and used as received ... 

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