Photo-initiated cationic polymerization

Photo-initiated cationic polymerization combines two distinct chemical reactions. In the first, the cation for the initiator of polymerization is formed in the second, the cation initiates polymerization of the cationic monomer. The first reaction is subject to the laws of photophysics and photochemistry, whilst the second is governed by the laws of thermochemistry. 

TGA analyses were performed for polymer samples having different degrees of cross-linking. The decomposition of the linear oligomer starts at about 200 °C. Once cured and baked, the formed siloxane network is more thermally stable, and the decomposition begins at temperatures higher by 100-150 °C. The results are similar to those reported for analogous Tsi-modified siloxanes cross-linked by means of photo-initiated cationic polymerization of epoxides [8]. 

J. V. Crivello and U. Varlemann, Structure and Reactivity Relationships in the Photo-initiated Cationic Polymerization of 3,4-Epoxycyclohexylmethyl-3, 4 - Epoxycyclohexane Carboxylate. In Photopolymerization, Vol. 673, A. B. Scranton and C. N. Bowman, Eds., American Chemical Society, Washington D.C., 1997, p. 82. 

Crivello and J.H. Lam, Photo initiated cationic polymerization with triarylsulfonium salts, J. Polym. Set Polym. Chem. Ed. 17, 977 (1979). 

The topic of photo-initiated cationic polymerization has been reviewed in-depth recently by Crivello, who has also co-authored a series of reports on the development of diarylhalonium salts for use as photo-initiators. These salts are claimed to decompose in the presence of ultraviolet light and react with solvent molecules to yield Bronsted acids with stable counterions, as shown in reactions (4)—(6), where S—H is a solvent molecule. These acids are the initiating species... 

An Extremely Acidic Medium is Formed During Photo-Initiated Cationic Polymerization in Photolithography... 

In Chapter 17 we will describe the photolithography process, in which 0.25 xm or smaller features in silicon chips can be created. The process involves a photo-initiated cationic polymerization, and /or cross-linking reaction. In some cases, photo-initiated deprotection of phenol groups is used to change the polarity of certain regions of the chip. In all of these applications, the transformations are initiated by very strong, photo-generated acids. 

Using a pH Indicator to Sense Species Other Than the Hydronium Ion 264 Realistic Titrations in Water 265 An Extremely Acidic Medium is Formed During Photo-Initiated Cationic Polymerization in Photolithography 269... 

A recent development is photo-initiated cationic polymerizations. This is very useful for protective coatings, paints, etc. and utilizes the fact that certain quaternary and organic diazonium salts of strong acids can be photo-decomposed with formation of a strong acid, HX. Photo-initiation is also used for curing dental resins (but not using the same initiators as above). 

Decker C, Nguyen thi viet T, Thi HP. Photo-initiated cationic polymerization of epoxides. Polym Intern. 2001 50 986-997. 

The usefulness for this purpose of triarylsulphonium hexafluoroantimonate comes from its considerable thermal stability, stability in the presence of highly reactive monomer, and highly efficient photolysis to yield reactive cations capable of initiating cationic polymerization. These properties arise from the unique chemical composition of the photo-initiator, the effectiveness of which can be shown to be a result of the presence of a very weak cation and a similarly weak anion. 

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. 

More recently, iodonium salts have been widely used as photoinitiators in the polymerization studies of various monomeric precursors, such as copolymerization of butyl vinyl ether and methyl methacrylate by combination of radical and radical promoted cationic mechanisms [22], thermal and photopolymerization of divinyl ethers [23], photopolymerization of vinyl ether networks using an iodonium initiator [24,25], dual photo- and thermally-initiated cationic polymerization of epoxy monomers [26], preparation and properties of elastomers based on a cycloaliphatic diepoxide and poly(tetrahydrofuran) [27], photoinduced crosslinking of divinyl ethers [28], cationic photopolymerization of l,2-epoxy-6-(9-carbazolyl)-4-oxahexane [29], preparation of interpenetrating polymer network hydrogels based on 2-hydroxyethyl methacrylate and N-vinyl-2-pyrrolidone [30], photopolymerization of unsaturated cyclic ethers [31] and many other works. 

Different initiation techniques have been investigated in polymerizations induced by iodonium salts, such as visible laser irradiation [32], dual photo- and thermally initiated cationic polymerization [23, 26] and a two-photon photopolymerization initiation system [33,34]. For example, dual photo- and thermal-initiation systems based on selective inhibition of the photoinifiated cationic ring-opening polymerization of epoxides by dialkyl sulfides have been developed [26]. Such a dual system, iodonium salt/dialkyl sulfide, in the... 

These addition polymerizations occur by means of a chain reaction mechanism involving the propagation of an active center by interaction with the monomer. The active center consists of reactive species, typically, a radical, a cation, or an anion. In the simplest case, a photo initiated addition polymerization can be represented as shown in eqns [16]-[18] of Scheme 8. 

For SCVP of styrenic inimers, the mechanism includes cationic (14 [18], 19 [29]), atom transfer radical (15 [22, 27]), nitroxide-mediated radical (16 [21]), anionic (20 [19]), photo-initiated radical (17 [2], 18 [52-55]), and ruthenium-catalyzed coordinative (21 [56]) polymerization systems. Another example in-... 

Photo-initiators for cationic polymerization may be classified in four groups the diazonium salts, the diaryliodonium salts, the triarylsulphonium salts, and the mixed ligand arene cyclopentadienyl Fe11 salts. 

Once the initiating cation has been formed the polymerization is controlled by temperature (technical difficulties may arise if the control is not adequate). A reaction scheme for cationic polymerization of an epoxide using triarylsulphonium hexafluoroantimonate as photo-initiator is shown in Figure 105. Termination of the polymerization often is adventitious, particularly with anions and other bases (theoretically the reaction can continue until the supply of monomer is exhausted). 

Figure 105 A reaction scheme for cationic polymerization of an epoxide using triarylsulphonium hexafluoroantimonate as photo-initiator...

A purely cationic curing can be induced by Lewis acids such as AICI3, BF3, ZnCl2, TiCU, or FeBr3. The cationic polymerization of the epoxy resins has received little attention and is mentioned only in connection with photo- or radiation chemistry initiation. 

In the case of photo-initiated polymerizations bimodal distributions can occur as a result of concurrent free-radical and cationic growth, these intermediates not being in chemical equilibrium. This arises in the bulk polymerizations of styrene photo-initiated in the presence of tetracyanobenzene and also in the radiation-induced polymerization of alkyl-substituted styrenes.In the former system the higher molecular weight fraction is attributed to a cationic... 

Kinetics and Mechanism. Practical applications of onium salt photo-initiated polymerization generally require use of spectral sensitizers. Under conditions of electron transfer mediated sensitization, the initiating cation, R+, thus derives from the sensitizer, not the onium salt (see above), that is, a Lewis, rather than a Bronsted, acid functions as the polymerization catalyst. As a result overall polymerization quantum yields are very dependent on sensitizer [93,166]. 

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