Photoinitiated cationic polymerization Bronsted acid Initiation

As pointed out in Section 4.2.2, cationic polymerization processes are initiated by photoinitiators, which are essentially precursors generating Lewis and Bronsted acids. The mechanism of the process is ionic, and this chemistry does not function with the type of double bonds and unsaturation found in fhe monomers and oligomers reacting via free radical mechanism. 

The proposed mechanism was identical with that in acid-catalyzed reactions except for the initiation step. Photolysis of the iodonium salt yields cations and cation radicals that react with traces of water or the monomer to form HX [23]. The Bronsted acid HX then functions similarly to other Bronsted acids in the polymerization reactions. 1,3-Diisopropenylbenzene has also been polymerized in a photoinitiated cationic reaction using 70 as the initiator [Eq. (14)] [9]. 

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 ... 

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. 

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