Photoinduced cationic polymerization

Scheme 26 Photoinduced cationic polymerization of oxiranes using 1-pyrenyltriarylbismutho-nium salts [97]...

Reaction steps in a photoinduced cationic polymerization are as follows ... 

The preparation of block copolymers by combination of thermally radical and photoinduced cationic polymerization processes has also been reported [151], Indeed, styrene/cyclohexene oxide (CHO) copolymers have been synthesized by using a bifunctional azobenzoin initiator such as ABME, previously described, through a two-step procedure. In the first step, thermal Iree radical polymerization of styrene in the presence of the above azobenzoin initiator gives poly(styrene) prepolymers with benzoin photoactive end groups, as reported in Scheme 38. These prepolymers, upon photolysis and subsequent oxidation to the corresponding carbocations in the presence of l-ethoxy-2-methylpyridinium hexafluoro phosphate (EMP+PFg ), finally give block copolymers by cationic polymerization of cyclohexene oxide (Scheme 45). 

General scheme for photoinduced cationic polymerization is depicted in Scheme 11.1. A photosensitive compound, namely, photoinitiator (PI), absorbs incident light and undergoes decomposition leading to production of initiating species. Active species, namely, a radical cation (R+") in turn, react with cationic polymerizable monomers (M), and yield polymer (Scheme 11.1). 

Phenacyl Sulfonium Salts Being thermally rather stable and highly photoresponsive, phenacyl sulfonium salts are significantly attractive for photoinduced cationic polymerization [41,42]. Despite being easily obtained, they suffer from their poor... 

Phenacylammonium Salts Phenacyl anilinium salts have light absorption 300-350 nm that make them preferable for photoinduced cationic polymerization. Upon irradiation, these salts undergo either a heterolytic cleavage or a homolytic cleavage followed by an electron transfer to yield a cation intermediate that in turn can initiate polymerization of appropriate monomers [16]. Scheme 11.14 shows the mechanism in detail. As can clearly be seen, the photolysis of phenacylammonium salts are irreversible and different than their sulfonium analogs [43]. 

Electron-rich polyaromatic compounds such as anthracene, pyrene, and pery-lene [107] are suitable as photosensitizers as they give redox reactions with DPI salts through exciplex to finally yield the initiating species for photoinduced cationic polymerizations. Scheme 11.28 demonstrates the mechanism of a polymerization followed via exciplex formation through the excited sensitizer with the ground-state onium salt. 

Compounds containing hydroxyl group accelerate photoinduced cationic polymerization of epoxides as explained previously vide ante) [109,110,122]. Accordingly, 1-pyrenemethanol was designed as an accelerator as well as photosensitizer [119]. 

J.V. Crivello and S.Q. Kong, Long-wavelength-absorbing dialkylphenacylsulfonium salt photoinitiators synthesis and photoinduced cationic polymerization. J. Polym. Sci. A Polym. Chem. 2000, 38(9), 1433-1442. 

Cationic Systems Photoinduced cationic polymerization can be initiated directly or indirectly upon exposure to U V light in the presence of onium or pyridinium salts with suitable nucleophilic counterions [33], These initiating systems have been employed in synthesizing block and graft copolymers [34], For example, side-chain benzoin-containing polymers were used as promoters to yield poly(benzoin acrylate-g-cyclohexene oxide) copolymers [35] by using iV-ethoxy-2-methyl-pyridinium... 

SCHEME 13.15 Synthesis of graft copolymer by indirect photoinduced cationic polymerization... 

Photoinduced cationic polymerization using [CpFe(C6H6)]asthe photoinitiator. 

Currently, the most commonly used photoinitiators for photoinduced cationic polymerizations are onium salts, particularly sulfonium salts, with the general struetures shown below. In these illustrations MtX -represents a nucleophilie eounter ion ... 

A series of octafunctional epoxy and 1-propenyl ether monomers bearing the Tg silsesquioxane core have been prepared and characterized. Despite the high functionality and reactivity of these monomers, the efficiency with which the monomers undergo crosslinking is considerably less than expected. Instead, due to steric inhibition effects and the proximity of the functional groups to one another, photoinduced cationic polymerization proceeds mainly by an intramolecular process. 

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