Zwitterions, sulfonium cyclic

The volatility of difunctional isocyanates (such as tolylene diisocyanates, hexamethylene diisocyanate, etc.) creates many environmental problems in the urethane industry. These difficulties can be overcome by preparation of NCO-terminated oligomers with low vapor pressure. One approach is the preparation of NCO-ter-minated oligomers by partial cyclotrimerization of difunctional isocyanates. Usually this is achieved by a multi-step process which includes also deactivation of the catalyst at a certain conversion. During our work on cyclotrimerization of isocyanates we found that cyclic sulfonium zwitterions are very active cyclotrimerization catalysts (2). Recently we found that cyclic sulfonium zwitterions under certain reaction conditions act as anionic initiators. This behavior of cyclic sulfonium zwitterions permits preparation of isocyanate oligomers containing isocyanurate rings by a one-step procedure, eliminating the deactivation step. 

In this communication this unusual behavior of cyclic sulfonium zwitterions will be discussed. 

The kinetics of catalysis of cyclotrimerization was studied on the model system phenyl isocyanate/ace-tonitrile (solvent). Acetonitrile (AN, 99.64%, from Vinstron Corp.) was purified by refluxing with phosphorus pentoxide (5 g/1), then with calcium hydride (2 g/1) followed by distillation under nitrogen. Phenyl isocyanate was obtained from the Upjohn Company with a purity of 99.5%, and was purified by distillation. Tolylene diisocyanates (2,4 and 80/20 2,4/2,6 isomers) were obtained from the Mobay Chemical Co., and were purified by distillation. Cyclic sulfonium zwitterions (SZ) were obtained from the Dow Chemical Co. 

It was found that the cyclic sulfonium zwitterions of the general formula IV are very active catalysts for cyclotrimerization of isocyanates (2). 

However, cyclic sulfonium zwitterions without two molecules of water are unstable and polymerize due to the nucleophilic attack of the anion (0 ) on the tetra-hydrothiophenium ring followed by ring opening with consecutive polymerization. Products of polymerization are low molecular weight linear polymers and cyclic dimers and trimers (6,7) ... 

The effect of relative permitivity D on the cyclo-trimerization of phenyl isocyanate was studied in the solvent system acetonitrile (AN) - ethylacetate (EA) using cyclic sulfonium zwitterion VI as catalyst. It was found that the rate constant increased with the increase of relative permitivity D of the solvent system, the experimental data correlate well with the Kirkwood equation ... 

In diluted aprotic solutions of isocyanates, the reaction rate between isocyanates and stabilizing water of cyclic sulfonium zwitterions is negligible but becomes important at high concentrations of isocyanates or high reaction temperatures. 

EFFECT OF SUBSTITUTION ON CYCLOTRIMERIZATION ACTIVITY OF CYCLIC SULFONIUM ZWITTERIONS... 

The deactivation reaction transfers an active catalyst into the inert (non-reactive) polymer. This phenomenon, when cyclic sulfonium zwitterions act as anionic initiators, can be utilized for the control of the cyclotrimerization of difunctional isocyanates. Therefore the degree of oligomerization of difunctional isocyanates can be controlled by the concentration of the initiator, rate of addition of the initiator, as well as by the temperature of the reaction system. 

Figure 1. Effect of relative permitivity D of the solvent system on cyclotrimeriza-tion of isocyanates Catalyst cyclic sulfonium zwitterion VI 25°C solvent system, acetonitrile (D = 37.5) and ethyl acetate (D = 6.02).

Charge cancellation polymerizations must be considered as special cases. Some degree of charge separation already exists in acetonitrile oxide but a base is required to initiate its polymerization. Although linked by a conjugated system ionisation is essentially complete in cyclic sulfonium zwitterion monomers. No initiator is required and monomeric units add at both ends of the p>olymer chain. The initial step in the general scheme above is best considered as part of monomer synthesis. 

Evidence to support hypothesis (ii) comes from Schmidt s observation that when salt is added, fewer macrocycles are formed during cyclic sulfonium zwitterion polymerizations. 

In the copolymerization with PPL, only route a has been considered by Saegusa, and transfer was not taken into account. Macrocyclization by end-to-end closure is another possibility, however, these macrocycles cannot easily be isolated and studied. The presence of macrocycles may be corroborated by comparison of DPn determined from end-groups with those measured by other methods e.g. GPC, osmometry, etc., i.e. when cyclic and linear molecules are counted separately. Such macrocycles were indeed observed by Schmidt in the polymerization of aryl sulfonium zwitterion 86) ... 

D. L. Schmidt. H. B. Smith, M. Yoshimine, and M. J. Hatch, Preparation and properties of polymers from aryl cyclic sulfonium zwitterions, J. Polym. Sci. (Chem.) 10, 2951 (1972). 

In both methods 1 and 2, the hydrochloride salts are converted to the zwltterlon with anion exchange resin (0H form). The tetrahydrothlophene substitution goes essentially all para to the phenolic hydroxy. If the para position is blocked, the tetrahydrothlophene substitutes In the ortho position no meta substitution has been observed. Phenolic compounds that have electron-withdrawing groups (such as Cl) will not react under conditions of either method 1 or 2. Chlorine-containing derivatives may be prepared by chlorination of the appropriate zwitterion hydrochloride (9 ). Six mend>ered cyclic sulfonium zwitterions can be prepared in only very low yields by methods 1 and 2. These compounds may be obtained by the reaction of 1,5-dibromopentanes with 4-(methylthio)phenol in refluxing chlorobenzene to yield the sulfonium salt and methyl bromide (method 3) 9). Ifethod 3 is also applicable to the preparation of meta substituted derivatives (13). 

SCHMIDT Polymerization of Aryl Cyclic Sulfonium Zwitterions 323... 

The polymerization of LXIII, a stable isolable zwitterion, on heating or photolysis involves nucleophilic attack by phenoxide anion on the cyclic sulfonium ring [Hoyle et al., 2001 Odian et al., 1990]. The reaction proceeds as a step polymerization. 

The l,2-oxathietane-4-ones described in the older literature are really acyclic, zwitterionic sulfonium carboxylates (e.g., 490). They were written incorrectly as cyclic sulfuranes (e.g., 491). 

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