Dialkyl-4-hydroxyphenylsulfonium salts

Photoinitiation of radical polymerization has long been known. Recently, a group of photoinitiators for cationic polymerization hase been discovered and developed by Crivello et al.1J. They include diaryliodonium (7),2) triarylsulfonium (2), 3 5) dialkylphenacylsulfonium (J), 6) and dialkyl-4-hydroxyphenylsulfonium salts (4) 7). 

Many different photoinitiators based on onium -type compounds with anions of low nucleophilicity also have been described in the literature as effective catalysts for the polymerization of epoxides Thus, diaryliodonium salts diaryliodosyl salts triarylsulfonium salts and related compoundstri-phenylsulfoxonium saltsdialkylphenacylsulfonium salts and dialkyl-4-hydroxyphenylsulfonium salts seem to be most suitable as photoinitiators for epoxy curing. Some of the principles of the reaction mechanism involving these initiators are discussed in detail in the following Sections. Various other onium photoinitiators such as diarylchloronium and diarylbromonium salts , thiopyrylium salts 3), triarylselenonium salts and onium salts of group Va elements >... 

The syntheses of diaryliodonium, triarylsulfonium, dialkylphen-acylsulfonium, and dialkyl-4-hydroxyphenylsulfonium salts was thoroughly reviewed by Crivello [25] in 1984. Prot and Karpinski [26] reviewed the... 

Two additional classes of sulfonium salt initiators, also introduced by Crivello and Lam, are the dialkylphenacylsulfonium salts (II) (12,13) and the dialkyl-4-hydroxyphenylsulfonium salts (III) (13,14). The initiating species generated upon... 

The major portion of the article will be devoted to a discussion of the synthesis and mechanistic aspects of photoinitiation by individual photoinitiator systems. Cationic polymerizations induced by these photoinitiators are dark, i.e., non-photo-chemical, processes which are governed by the same parameters which must be taken into account in polymerizations which occur in the presence of conventional initiators such as Lewis and Bronsted acids. Accordingly, cationic polymerizations induced by halogen and sulfur based onium salt photoinitiators will be discussed only from the context in which they are influenced by factors which have their origin in the photoinitiator. Since the photochemistry of diaryliodonium and triarylsulfonium salts is similar, these two types of photoinitiators will be discussed together. The photolysis of dialkylphenacylsulfonium and dialkyl-4-hydroxyphenylsulfonium salts proceeds by a different mechanism, and they will be discussed separately. 

Dialkylphenacylsulfonium and dialkyl-4-hydroxyphenylsulfonium salts are in general colorless crystalline compounds of well defined character which are insoluble in aqueous media but soluble in many common organic solvents from which they may be purified by crystallization,... 

Dialkyl-4-hydroxyphenylsulfonium salts are also readily accessible in excellent yields by the two synthetic routes shown below... 

Table 7. Structure and Properties of Dialkyl-4-Hydroxyphenylsulfonium Salts...

The photochemistry of dialkyl-4-hydroxyphenylsulfonium salts has received little attention in the literature. Recently, on the basis primarily of NMR data and analysis of the photolysis products, Crivello and Lam have proposed the following mechanism for the photolysis of these compounds which is related to that of the dialkylphenacylsulfonium salts. 

Irradiation of dialkylphenacylsulfonium and dialkyl-4-hydroxyphenylsulfonium salts at wavelengths from 190 to 360 nm in the presence of suitable monomers results in facile cationic polymerization. Shown in the following scheme is the mechanism which has been proposed for photoinitiation using these onium salts The first step involves the reversible photogeneration of the respective ylides and the solvated acid In the following nonphotochemical or dark step, initiation takes place by protonation of a monomer molecule by HX,s, to generate a carbenium or onium species. In the case where the solvent is the monomer, photolysis and protona-... 

The photolysis of dialkyl-4-hydroxyphenylsulfonium salts is not photosensitized by aromatic hydrocarbons. Instead, aromatic ketones are excellent photosensitizers for these photoinitiators Scheme 12 gives the mechanism which has been proposed for the photosensitization of dialkyl-4-hydroxyphenylsulfonium salts. 

Fig. 5. Mechanism of ylid and Bronsted acid formation by photolysis of dialkylphenacyl sulfonium salt and dialkyl- -hydroxyphenylsulfonium salt (X" = PF, BF, SbFg etc.)...

Considerable variation in both the aryl and alkyl groups in the above structures can be tolerated while maintaining the photoactivity of these salts. Through the introduction of substituents and structural modifications, it is possible to manipulate both the spectral absorption characteristics and the photosensitivity of both types of photoinitiators. In addition to those sulfonium salts represented by structures III and IV, several related compounds such as alkylarylphenacylsulfonium salts and dialkyl-2-hydroxyphenylsulfonium salts have also been observed to be excellent cationic photoinitiators... 

Dialkylphenacylsulfonium salts (111) and dialkyl-4-hydroxyphenylsulfonium (IV) salts have recently been described as useful photoinitiators for cationic polymerization 1 2-123)... 

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