Diaryliodonium salts thermal initiators

J.V. Crivello and J.L. Lee, The synthesis and characterization of polymer-hound diaryliodonium salts and their use in photo and thermally initiated cationic polymerization. Polym. Bull. 1986, 16(4), 243-248. 

Crivello reported that only diaryliodonium salts were used as catiOTiic photoinitiators in this study. Photo activation was carried out by UV irradiation prior to thermal initiation. Initiation was carried out using an electrically heated wire immersed in the monomer. The velocity of the resulting propagating front is quite high with the temperature of the front reaching 110°C. 

In the presence of triarylsulfonium and diaryliodonium salts, polymerization continues even if UV irradiation is terminated. This phenomenon is called dark cure and is due to the living nature of the superacid generated cation. The cure regime can be thought of as UV-initiated but thermally cured. Thermally initiated cationic catalysts are also available (129). 

The most recent work of the General Electric group has addressed the development of diaryliodonium salts as thermal (15) or redox (16) initiators of cationic polymerization. These systems contain Cu (added as such or generated by reduction of added Cu ), which serves to reduce the iodonium salt. Reduction of the iodo-nium salt produces the cationating agent (either H+ or Ar" ) which initiates chain growth. The authors suggest eq. 2 as the mechanism of initiation (15). 

The presence of phenoxy endgroups in polyethers prepared by thermal initiation of epoxides with diaryliodonium salts was verified by ultraviolet spectroscopy (15), but the authors do not comment on the fraction of such endgroups (i.e., on the relative importance of Eqns. 2c and 2e). 

The initial development of diaryliodonium salt cationic photoinitiators was quickly followed by the nearly parallel discovery of triarylsulfonium salts as a second general class of highly efficient and thermally stable cationic photoinitiators. Along with triarylsulfonium salts, 77, their S-aryl sulfiir-heterocyclic analogs display good photosensitivity and function well in photoinitiated cationic polymerizations. 

Ledwith has described several successful thermally initiated polymerizations in which free radical initiators such as AIBN and benzpinacol are used in combination with diaryliodonium and triarylsulfonium salts A typical mechanism proposed... 

Two other patents have also appeared describing the use of alternate free radical sources as reducing agents for diaryliodonium and triarylsulfonium salts Crivello and his coworkers have discovered several other types of reactions which can be used to thermally induce cationic polymerization using onium salt photoinitiators. Diaryliodonium salts are thermally decomposed in the presence of catalytic amounts of copper compounds An organocopper compound is proposed as an intermediate which undergoes electrophilic arylation of cationally polymerizable monomers as shown in Scheme 14 to initiate polymerization. 

The evolution of nitrogen on photolysis of the aryIdiazonium salts appears to have limited the use of these systems to thin film applications such as container coatings and photoresists (23). Other efficient photoinitiators that do not produce highly volatile products have been disclosed (24-27). These systems are based on the photolysis of diaryliodonium and triarylsulfonium salts. Structures I and II, respectively. These salts are highly thermally stable salts that upon irradiation liberate strong Bronsted acids of the HX type (Reactions 43 and 44) that subsequently initiate cationic polymerization of the oxirane rings ... 

Subsequent studies on polymerization of the bisaliphatic epoxy compound 3,4-epoxycyclohexylmethyl, 3,4-epoxy-cydohexylcarboxylate (see Chart 1.1) revealed that the compound does not polymerize thermally in the absence of an initiator. However, in the presence of diaryliodonium or triphenylsulfonium salts, an ionic polymerization was initiated and the onset of the polymerization shifted to higher temperatures upon microwave heating (see Table 1.5). This microwave effect could be explained in terms of the thermodynamic properties being different under the influence of a microwave field [11]. 

---