Photoinitiated cationic polymerization structure

Structure and Reactivity Relationships in the Photoinitiated Cationic Polymerization of 3,4-Epoxy cyclohexylmethyl-3, 4 -epoxy cyclohexane... 

The reactivity of I in photoinitiated cationic polymerization is due to several factors associated with the structure of this monomer. Most importantly, the presence of the ester groups in I which can interact with oxiranium ions generated at either of the two epoxide groups both intra- and intermolecularly produces dioxacarbenium ions of reduced activity in the propagation reaction. Taking this into account, a series of diepoxides were prepared which did not possess ester groups. Some of these monomers show enhanced reactivity as measured by RTIR in photoinitiated cationic polymerization compared to I. 

Photoinitiated cationic polymerization of benzoxazines by onium salts was investigated mechanistically. It was postulated that the first step involves the addition of photocemically generated proton (or carbocation) either to oxygen or to nitrogen atom. Then, the polymerization proceeds via two different routes leading to the formation of different structures (Scheme 11.34) [2,118]. 

A novel well-defined macromonomer of epoxy end-functionalized poly(V -capro-lactone) (PCL) was synthesized and its reactivity in photoinitiated cationic polymerization was examined [28]. PCL macromonomer as the comonomer allowed a rather simple incorporation of PCL side chains into poly(cyclohexene oxide) (PCHO) backbone. This way PCHO-g-PCL copolymer with random sequences of the structure shown in Scheme 13.16 is formed. 

Over the past ten years the development of onium salt and other cationic photoinitiators has moved from the realm of speculative investigation to the point today at which they are being employed in numerous commercial applications. Much work still needs to be done in this fields particularly to improve our understanding of the relationship between the structure and the photosensitivity of these photoinitiators. As the field advances, one can expect still other new classes of onium salt photoinitiators to be developed as well as continued improvements to be made in the efficiency of the present systems. An understanding of the mechanism of photosensitization should lead to discovery of more efficient photosensitizers and a further broadening of their spectral response in photoinitiated cationic polymerization. 

In addition to the control which can be exercised over photoinitiated cationic polymerizations by manipulation of the structures of the diaryliodonium and triarylsulfonium salts, there are a number of additional factors which also influence these polymerizations. First, the emission spectrum of the irradiation source must be matched as closely as possible to the absorption characteristics of the specific photoinitiator. Fortunately, today there are available commercial light sources which provide intense bands in specific areas of any portion of the ultraviolet spectrum. Since the rate of photolysis of the photoinitiators varies as the first power of the light intensity, a simple doubling of the light intensity doubles the rate of photolysis of the photoinitiator. In practice it has also been observed that the rate of the polymerization of epoxy containing monomers is also doubled by a two fold increase in the light intensity... 

As in the case of the diaryliodonium and triarylsulfonium salts, photoinitiated cationic polymerizations employing dialkylphenacylsulfonium and dialkyl-4-hydroxy-phenylsulfonium salts exhibit a marked dependency on the structures of both the cation and anion portions of the photoinitiator. Here also, the most efficient photoinitiators are those bearing anions having the poorest nucleophilic character possible i.e. BF4, PF, AsFg, and SbF. ... 

Cationic polymerizations induced by thermally and photochemically latent N-benzyl and IV-alkoxy pyridinium salts, respectively, are reviewed. IV-Benzyl pyridinium salts with a wide range of substituents of phenyl, benzylic carbon and pyridine moiety act as thermally latent catalysts to initiate the cationic polymerization of various monomers. Their initiation activities were evaluated with the emphasis on the structure-activity relationship. The mechanisms of photoinitiation by direct and indirect sensitization of IV-alkoxy pyridinium salts are presented. The indirect action can be based on electron transfer reactions between pyridinium salt and (a) photochemically generated free radicals, (b) photoexcited sensitizer, and (c) electron rich compounds in the photoexcited charge transfer complexes. IV-Alkoxy pyridinium salts also participate in ascorbate assisted redox reactions to generate reactive species capable of initiating cationic polymerization. The application of pyridinium salts to the synthesis of block copolymers of monomers polymerizable with different mechanisms are described. 

Although both positive and negative working photoresists based on photoinduced condensation and free-radical chemistry are well-known, cationic polymerization chemistry has received little attention for the fabrication of photoresists. The recent development of several new classes of practical photoinitiators for cationic polymerization has now made it possible to utilize this chemistry in a number of ways to produce highly sensitive photoresists (1-6). The facile synthesis of onium salts I-III together with their ready structural modification to manipulate... 

Photoinitiators for Cationic Polymerization. Recently a class of photoinitiators for cationic polymerization was discovered by Crivello et al. (55). This class includes diaryliodonium (Structure I) (56. 57). triaryIsulfonium (Structure II) (58-62). dialkyIphenacylsulfonium (Structure III) (63). dialkyl-4-hydroxy-phenylsulfonium salts (Structure IV) (64). and triaryIselenonium... 

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 ... 

In the same groundbreaking paper [21], CriveUo and coworkers also demonstrated that the anion plays no role in determining the photosensitivity of the iodonium salt and the photolysis rates of diaryliodonium salts having the same cations but different non-nucleophilic counterions (BF4, PFs, AsFs", or SbFe ) are identical. Likewise, the cation structure has little effect on the photodecomposition of diaryliodonium salts. The utility of iodonium salts as photoinitiators has been demonstrated in several cationic polymerizations using olefins, epoxides, cycUe ethers, lactones and cyclic sulfides as the monomers [21],... 

Crivello s pioneering work on onium salt-type photoinitiators (sulfonium and io-donium salts) gave great impetus to investigations of cationic polymerizations [5, 6]. A common feature of mechanisms proposed in relation to onium salt-type initiators of the general structure is the generation of... 

Over the past several years, there have been developed several new classes of onium salt photoinitiators capable of initiating cationic polymerization. The most significant of these are aryldiazonium salts, diaryliodonium salts, triarylsulfonium salts, and dialkylphenacyl-sulfonium salts. The mechanisms involved in the photolysis of these compounds have been elucidated and will be discussed. In general, on irradiation acidic species are generated which interact with the monomer to initiate polymerization. Using photosensitive onium salts, it is possible to carryout the polymerization of virtually all known cationically polymerizable monomers. A discussion of the various structurally related and experimental parameters will be presented and illustrated with several monomer systems. Lastly, some new developments which make possible the combined radical and cationic polymerization to generate interpenetrating networks will be described. 

As in the case of other sulfonium salts, the above simple halide-containing salts are not directly suitable for use as photoinitiators because of the tendency of these anions to act as terminating agents n in cationic polymerizations. Direct metathesis of the sulfonium halides with the appropriate acid or alkali metal salt of such weakly nucleophilic anions as PF, and SbFg, and AsFg" suffices to convert them to the active photoinitiators. Tables 6 and 7 give the structures and UV absorption characteristics of some representative dialkylphenacylsulfonium and dialkyl-4-hydroxyphenyl-... 

Previously, the same author [52] reported that compounds containing the tricoordinated sulfur cation, such as the triphenylsulfonium salt, worked as effective initiators in the free radical polymerization of MMA and styrene [52]. Because of the structural similarity of sulfonium salt and ylide, diphenyloxosulfonium bis-(me-thoxycarbonyl) methylide (POSY) (Scheme 28), which contains a tetracoordinated sulfur cation, was used as a photoinitiator by Kondo et al. [63] for the polymerization of MMA and styrene. The photopolymerization was carried out with a high-pressure mercury lamp the orders of reaction with respect to [POSY] and [MMA] were 0.5 and 1.0, respectively, as expected for radical polymerization. 

Photoinitiated epoxy crosslinking is generally based (with one exception on cationic ring-opening polymerization of the oxirane group, yielding polyether structures. 

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