Epoxide polymerization reaction mechanism

These results indicate that vacuum curing occurs through a radical reaction mechanism and is terminated by reaction of the ring-opened epoxy group with the azide group (not nitrene) under exposure. There is a possibility that polymerization initiated by an exposure-induced radical cation may occur. Furthermore, it is thought that reaction products from both the azide and epoxide serve as dissolution inhibitors, because the sensitivity of EAP is almost the same as that of EP, as shown in Figures 1 and 2. 

Many different photoinitiators based on onium -type compounds with anions of low nucleophilicity also have been described in the literature as effective catalysts for the polymerization of epoxides Thus, diaryliodonium salts diaryliodosyl salts triarylsulfonium salts and related compoundstri-phenylsulfoxonium saltsdialkylphenacylsulfonium salts and dialkyl-4-hydroxyphenylsulfonium salts seem to be most suitable as photoinitiators for epoxy curing. Some of the principles of the reaction mechanism involving these initiators are discussed in detail in the following Sections. Various other onium photoinitiators such as diarylchloronium and diarylbromonium salts , thiopyrylium salts 3), triarylselenonium salts and onium salts of group Va elements >... 

Although ethylene and substituted ethylenes are the monomers most commonly used for chain-growth polymerization reactions, other compounds can polymerize as well. For example, epoxides undergo chain-growth polymerization reactions. If the initiator is a nucleophile such as HO or RO , polymerization occurs by an anionic mechanism. 

If the initiator is a Lewis acid or a proton-donating acid, epoxides are polymerized by a cationic mechanism Polymerization reactions that involve ring-opening reactions, such as the polymerization of propylene oxide, are called ring-opening polymerizations. 

Epoxide monomers readily undergo cationic polymerization whereby chain propagation is based on the attack of a carbocation on the negatively polarized oxygen of the oxirane (XL). Chain initiation is either by another cation or by a strong electrophile, such as a Lewis acid (BF3) or a protonic Brpnsted acid. The process is illustrated in the reaction mechanism (Scheme 6.40). ... 

Polymerizations of epoxides by a coordinated anionic mechanism result in high molecular weight products. The details of the reaction mechanism have not been fully resolved yet, but it is conunonly believed to involve coordination of the monomers to electrophilic centers of the catalyst. This is followed by an activation for an attack by the anion. The mechanism can be illustrated by the following reactions ... 

Kutal et reviewed the chemistry of several iron (II) metallocenes that are effective photoinitiators for ionic polymerization reactions. Photoexcitation of ferrocene and 1,1-dibenzoyl -ferrocenes in solutions of ethyl-a-cyanoacrylate produces anionic species that initiate the polymerization of electrophilic monomers. Irradiation of CsHs-Fe (t] - arene) in epoxide containing media generates several cationic species capable of initiating ringopening polymerizations. It was concluded that iron(II) metallocenes exhibit a diversity of photoinitiation mechanisms. 

The cure mechanisms of structural adhesives depends on the resin chemistry and can be based on either Chain polymerization or Step polymerization reactions. The condensation cure mechanism is the most important for phenolic adhesives. Epoxide adhesives and Polyurethane adhesives, which require stoichiometric or balanced combination ratios as well as thorough mixing for maximum performance. Off-ratio mixing or incomplete mixing can alter significantly the final adhesive structure and effect the resulting properties. 

Detailed studies on the mechanisms of initiation and propagation of epoxide polymerization have been conducted using well-defined and stable initiators. Ethylene oxide (EO) was generally chosen as the reference monomer, owing to the living character of its polymerization, whereas the anionic polymerization of most other epoxides, including propylene oxide (POx), is subject to side reactions. 

In contrast to conventional anionic polymerizations, in which epoxide insertion involves an inversion of the configuration at the carbon atom of the epoxide ring where cleaved, some porphyrin systems were shown to operate by a nondisso-ciarive reaction mechanism, with retention of configuration at the carbon atom of the epoxide ring where cleaved (Scheme 28). 

The radiation-induced cationic polymerization in the presence of onium salts has attained practical importance for the EB curing of systems containing epoxides or vinyl ethers [22,23]. The chemical structures of typical compounds were presented in Table 3.23. THE does not play a role in this context, because of its very low propagation rate constant (kp 4 x 10 lmol s ). A reaction mechanism for the polymerization of vinyl ethers in the presence of an iodonium salt, as proposed by Crivello [22], is shown in Scheme 5.8. 

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