Cationic chain polymerization photoinitiation

The radiolysis of olefinic monomers results in the formation of cations, anions, and free radicals as described above. It is then possible for these species to initiate chain polymerizations. Whether a polymerization is initiated by the radicals, cations, or anions depends on the monomer and reaction conditions. Most radiation polymerizations are radical polymerizations, especially at higher temperatures where ionic species are not stable and dissociate to yield radicals. Radiolytic initiation can also be achieved using initiators, like those used in thermally initiated and photoinitiated polymerizations, which undergo decomposition on irradiation. 

Figure 8, Chain reaction photoinitiation. The efficiency of photoinitiation can be increased by chain reactions. Hydrogen abstraction by triplet excited benzophenone forms a THF free radical. Subsequent oxidation by the aryliodonium salt produces the THF cation capable of initiating polymerization, and a phenyl radical. Hydrogen abstraction by the phenyl radical produces the THF free radical completing the...

Since the photolysis of the salt is reversible, the formation of the proton is scavenged by the ylide to form the starting photoinitiator when irradiation ceases [43]. During irradiation, polymerization takes place via protonation of a monomer followed by sequential monomer addition. Notably, a termination via addition of a ylide to the growing cationic chain end is feasible. The overall polymerization process is summarized in Scheme 11.13. 

Organometallic molecules have been used as photoinitiators in cationic, anionic, and radical (chain) polymerization reactions. Examples of each type abound, but only selected examples will be discussed here because the fundamental principle is the same in each case irradiation is used to generate an intermediate species (an ion or a radical species) that can initiate a polymerization reaction. 

Cationic polymerizations proceed as chain reactions involving initiation and propagation. In most cases, there is no termination by neutralization, and the growing chains are terminated by nucleophilic impurities. Cationic chain reactions are photoinitiated by special initiators (Table 3.10) which date back to the pioneering studies of Crivello on sulfonium and iodonium salts containing non-nucleophilic anions such as and AsFs ... 

Polymers with indane units in the main chain have also been obtained by the photoinitiated cationic polymerization of bis(4-isopropenylphen-oxy)alkanes [Eq. (13)] [23,24]. 

The main advantages of cationic photoinitiators is that they have high reaction rates and require a low energy. They can operate at a low temperature, they are not inhibited by oxygen, they do not promote the polymerization of epoxy groups in the dark, and they are often stable at elevated temperatures. Some disadvantages exist that is, inhibition by bases, chain-transfer reaction by water, and the presence of acids in cured products. 

Photoinitiated polymerization uses the energy of light for the rapid conversion of monomeric liquids to solid polymeric products. The term photopolymerization implies that the initiation step of a radical, cationic, or anionic chain reaction producing a macromolecule requires the absorption of a photon. Since the absorption of one photon may start the reaction of up to 10 monomeric units, photoinitiated polymerization is, in practice, one of the most powerful chemical amplification techniques. 

M. Sangermano, M.A. Tasdelen, and Y. Yagci, Photoinitiated curing of mono- and hifunctional Epoxides hy combination of active chain end and activated monomer cationic polymerization methods. J. Polym. Sci. A Polym. Chem. 2007, 45(21), 4914-4920. 

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. 

J. V. Crivello In photoinitiated cationic polymerizations one does observe considerable dark polymerization after irradiation has ceased. This polymerization will basically be limited by diffusion rates of the monomer and the growing chain ends. When multifunctional monomers are used, network polymers are formed. In these systems since the growing chain end is attached to the matrix, it cannot diffuse and the rate of... 

The mechanism shown above involves a three-step radieal chain process where aryl radicals are converted to oxidizable radicals (R ) that interact with the onium salt. Subsequently, the cations generated by this process initiate polymerizations, while the reduced onium salts that undergo irreversible decomposition to regenerate aryl radicals. The overall consequence is an acceleration of the initiation process due to an increase in the number of initiating cationic species. In addition, Sangermano and Crivello reported that certain onium salt photoinitiators can be reduced by free radicals produced... 

Epoxide adhesives comprise epoxy resin, many of which are prepared from phenols and epichlorohydrin, for example, the diglycidyl ether of bis-phenol A or bis-phenol F usually, these resins are a mixtnre of molecular weights blended to fit the applications. The most-common cnratives for epoxy resins are polyanfines (used in stoichiometric amounts), usually a chain-extended primary aliphatic amine, for example, diethylene triamine or triethylene tetraamine or chain-extended equivalents, which react rapidly with the epoxy resin at room temperature. Aromatic amines react slowly at room temperature but rapidly at higher temperatures. Most epoxide adhesives also contain catalysts, typically, tertiary amines. Dicyanimide is the most-common curative for one-component high-temperature-cured epoxide adhesives. Mercaptans or anhydrides are used as curatives for epoxide adhesives for specialist applications, for example, for high-speed room-temperature cures or for electronic applications. A smaller number of epoxide adhesive are cured by cationic polymerization catalysed by Lewis acids photogenerated at the point of application. Lewis acid photoinitiators include diaryliodonium and triarly sulphonium salts. See Radiation-cured adhesives. 

The last class of dielectric coatings to be mentioned here is formed by the UV cationic epoxy systems. In most cases photoinitiators as invented by Crivello are used to initiate the chain crosslinking polymerization of di-epoxides or mixtures thereof with monoepoxides. 

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