Initiation by onium salts

Reiser, Photoactive Polymers The Science and Technology of Resists, p. 148, John Wiley Sons, Hoboken, NJ (1989). 

Crivello, in UV Curing Science and Technology, S.P. Pappas, Ed., pp. 24 75, Technology Marketing Corp., Norwalk, CT (1980). 

Organic Chemistry of Synthetic High Polymers, p. 247, Interscience, New York 

The propagation step is fast, aided by the coulombic interaction between the carbo-cation and the negatively polarized ether oxygen, and the strain in the three-membered ring. Termination may occur by the reaction of the carbocation with an adventitious nucleophile (a base, for example) or, very occasionally, with the anion of the initiator.  

Reiser reports that in order not to terminate the reaction and hence inhibit propagation, the counter anion must have very low nucleophilicity, since strong nucleophiles or bases will terminate the reaction immediately. Nevertheless, the polymerization can tolerate a small amount of water (1-2%), which is important for the practical usefulness of the system. Oxygen, which acts as a biradical, shows no effect on cationic polymerization—quite an important practical advantage. Characteristically, the epoxy polymers that are the result of the curing process tend to have excellent mechanical properties, including thermal and dimensional stability, nontoxicity, and chemical inertness. 

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Ledwith and coworkers have studied the cationic polymerization of THF initiated by onium salts in combination with free radical proginators.(20-23) When 2-2-dimethoxy-2-phenylacetophenone (IRGACURE 651) was irradiated in THF, the experiment containing an aryliodonium salt had a seven fold greater polymerization rate than a comparable experiment containing an arylsulfonium salt (21). 

Photoinitiated cationic ring-opening polymerization of epoxy monomers initiated by onium salts are obviously accelerated by addition of benzyl alcohols [108], The best way to understand how benzyl alcohols are effective in the polymerization is determination of acceleration factor (AF). AF is the ratio of the rate of ring-opening polymerization epoxy in the presence of the benzyl alcohol (Rpa) to the rate of that in the absence of the alcohol (Rp). Notably, [M0]a is the initial concentration of the monomer in the presence of the alcohol, and [Mo]a is in the absence [108],... 

The replacement in the above systems of the alkali metal initiators by onium salts, typically tetraalkylammonium or tetraalkylphosphonium halides, drastically improves the synthesis of polyethers. For instance, poly(POx)s of controlled molar masses, up to 150000gmor, ° are obtained in very short reaction times using tetrabutylammonium chloride and bromide as initiating species in the presence of a slight excess of... 

Cationic photopoiymerization is typically initiated by onium salts, for example, diaryliodonium salts Ar2C MtX or triarylsulfonium salts AtsS, MtK " (MtK = PF4", BFs", SbFs , etc.). The irradiation of photoinitiator generates a number of reactive species that subsequently react with solvent or monomer to give protonic acid HMtK . 

An interesting example of oxygen activation by onium salts of nontransition elements has been claimed. Sulfonium salts were found to be especially effective,211 and it was proposed212-214b that dioxygen complexes of sulfonium compounds can initiate autoxidations via hydrogen transfer with hydrocarbons, e.g.,... 

Free Radical Promoted. Many photolytically formed radicals can be oxidized by onium salts. The cations thus generated are used as initiating species for cationic polymerizations [82-84]. 

Kinetics and mechanism of chemically activated peroxides decomposition in solution especially activated by onium salts are very sensitive to the medimn properties and initial reactants concentration. Thus keeping to the equal conditions for decomposition and initiation reactions proceeding is of grate importance for radical forming estimation at peroxide decomposition investigation in solution. 

Although in photocurable formulations, difunctional oxirane derivatives are employed for mechanistic studies, monofunctional oxiranes are used including cyclohexene oxide, styrene oxide, or phenyl glycidyl ether. These studies indicate that the cationic polymerizations proceeding as a result of photoinitiation by onium salts have typical characteristics of polymerizations initiated by strong protonic adds. Thus, initiation involves protonation of oxirane ring while propagation proceeds on tertiary oxonium ions as active species, that is, by the ACE mechanism. 

Fig. 20. Proposed photochemical mechanisms for the generation of acid from sulfonium salt photolysis. Shown ate examples illustrating photon absorption by the onium salt (direct irradiation) as well as electron transfer sensitization, initiated by irradiation of an aromatic hydrocarbon.

However, in the presence of a suitable Lewis base the polymerization becomes living, due to the nucleophihc stabilization of the growing cation generated by the added base. (3) Initiator, strong Lewis acid and onium salt as additive The previous method cannot be easily applied in polar media. In this case the living cationic polymerization is promoted by the addition of salts with nucleophihc anions, such as ammonium and phosphonium derivatives. 

Here A, lA, and3A represent anthracene in the ground state, the first excited singlet state and first excited triplet state, respectively. In addition, I represents the onium salt initiation, while Rs and Rt correspond to the reactive centers formed by reaction of the onium salt with the excited singlet and triplet state anthracene, respectively. 

Onium salts such as group 5 ammonium, phosphonium, arsonium, group 6 sulfonium, group 7 chloronium, bromonium and iodonium undergo PET bond cleavage reactions that is accompanied by the production of a Bronsted acid. The photochemically produced acid has been used effectively to initiate cationic... 

The quantum yields of onium salt decomposition (cj)in) give important hints of the effectivity of such systems for polymerization initiation. For some combinations the 4 In are listed in Table 6. Various systems exist, in which almost all photo-chemically formed Riu were consumed by the R X+ see the small differences in <(> and in. Data of the action of these photoinitiator systems in radical polymerization of MMA are given in Table 6. In order to discuss the dependence of the 4>in-values it must be kept in mind that acyl radicals, produced simultaneously with R4u (see Scheme 12), also initiate the polymerization chain. But, as can be seen by the ratio of polymerization rates in presence (Rp) and in absence of onium salts (R ), the decomposition of R X+ by Riu increases the 4>p-values. 

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