Dlaryllodonlum salts

In recent years, research at this laboratory has centered about the development of new Initiators for cationic polymerization. Among the most Interesting and useful of these Initiators are dlaryllodonlum salts whose structure Is shown below. 

These compounds are stable, colorless, crystalline, Ionic salts which are readily soluble In organic solvents but nearly Insoluble In water. Especially useful Is their excellent solubility In a wide variety of catlonlcally polymerizable monomers. Unlike carbenlum salts such as trltyl and tropyllum salts and trlalkyloxonlum salts which spontaneously Initiate cationic polymerization on contact with susceptible monomers, solutions of dlaryllodonlum salts In these same monomers are stable and show no tendency to polymerize even when heated to temperatures up to 150°C. 

The observation that only a very small portion of the polymer chains which were produced using dlaryllodonlum salts contain end groups which are derived from initiator fragments suggests that the process shown in Equation 3 in which Br nsted acids are formed is dominant. 

The key feature of this mechanism Is that the excited photosensltl-zer, P. Is oxidized by the dlaryllodonlum salt which Is correspondingly reduced. This mechanism Is substantiated by first, the direct experimental observation of photosensitizer cation-radical species by UV and ESR spectroscopy(6) and second, by a direct correlation between the activity of a photosensitizer and the reduction potential of Its excited state relative to the dlaryllodonl salt(3, ). It Is Interesting to note that the cation-radical, PT, derived from the photosensitizer rather than from the photolnltlator Is responsible for Initiating polymerization In this Instance. 

Initiators Activated by Elevated Temperatures. Although, as mentioned earlier, dlaryllodonlum salts possess considerable thermal stability and, therefore, cannot be used directly In thermally activated polymerizations, we sought to find some way In which the thermal latency of these Initiators could be broken. This appeared to be possible on the basis of a recent general reaction discovered In our laboratory(2., ). Dlaryllodonlum salts undergo facile reac-... 

A number of experiments designed to elucidate the nature of the catalysis by copper were carried out. It was observed that when Cu(II) compounds were combined with dlaryllodonlum salts In completely unreactlve solvents such as chlorobenzene, there was no reaction even at elevated temperatures. In contrast, dlaryllodonlum salts reacted rapidly and quantitatively even at 25 C In various solvents In the presence of catalytic amounts of a Cu(I) compound. Analysis of the products of this latter reaction shown In Equation 9... 

In light of the above observations, the mechanism shown In Equations 10-13 has been proposed for the thermal Initiation by dlaryllodonlum salts In the presence of Cu(II) catalysts( ). 

The reaction of the Cu(I) species with the dlaryllodonlum salt results in the formation of a proposed organometallic intermediate, [ArCu(III)LX], whose structure has not been fully elucidated due to its lability. This intermediate undergoes as Its primary reaction, an electrophilic attack on the monomer, M, to initiate cationic polymerization. The above mechanism predicts and it has been confirmed that only trace amounts of reducing agents are required to convert a catalytic quantity of Cu(II) compound to Its lower valence state. Once the Cu(I) Is formed. It Is continually recycled between Equations 11 and 12 until all the dlaryllodonlum salt has been consumed. Another consequence of this mechanism Is that polymers prepared using these catalysts should possess aromatic end groups which... 

Free Radical Initiators as Reducing Agents for Dlaryllodonlua Salts. A final method by which dlaryllodonlum salts can be used as thermal Initiators of cationic polymerization has recently been reported by Ledwlth and his coworkers(14,15) and Is shown In Equations 17-19. 

Free radicals produced by the thermolysis of typical radical Initiators as AIBN, benzoplnacole and phenylazotrlphenylmethane reduce the dlaryllodonlum salt generating aryl radicals and solvent derived radicals which In a chain reaction Induce the decomposition of more dlaryllodonlum salt. Through the selection of particular radical Initiators with specific decomposition rates, It Is possible to adjust the Initiation temperature of the cationic polymerization with considerable latitude. 

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