Ethers, vinyl photoinitiated cationic polymerization

S.K. Rajaraman, W.A. Mowers, and J.V. Crivello, Interaction of epoxy and vinyl ethers during photoinitiated cationic polymerization. J. Polym. Sci. A Polym. Chem. 1999, 57(21), 4007 1018. 

Rajaraman SK, Mowers WA, Crivello JV. Interaction of vinyl ethers during photoinitiated cationic polymerization. J Polym Sci Polym Chem Ed. 1999 37 4007-4018. 

Free radical promoted, cationic polymerization also occurs upon irradiation of pyridinium salts in the presence of acylphosphine oxides. But phosphonyl radicals formed are not oxidized even by much stronger oxidants such as iodonium ions as was demonstrated by laser flash photolysis studies [51, 52]. The electron donor radical generating process involves either hydrogen abstraction or the addition of phosphorus centered or benzoyl radicals to vinyl ether monomers [53]. Typical reactions for the photoinitiated cationic polymerization of butyl vinyl ether by using acylphosphine oxide-pyridinium salt combination are shown in Scheme 10. 

SCHEME 11.33 General mechanism for photoinitiated cationic polymerization of alkyl vinyl ethers (a) and epoxides (b). 

Photoinitiated Cationic Polymerization of Unusual Monomers Recently, novel monomers and macromonomers (Chart 11.12), namely, benzoxazines, monothiocarbonates, thiophene, and epoxy and vinyl ether functional polymers were reported to undergo photoinitiated cationic polymerization [2,118], Among them, benzoxazine monomers deserves a special attention as they yield thermosets with several excellent characteristics including heat resistance good flame retardance stable dielectric constants low water absorption and perfect dimensional stability. This type of thermosets is also obtained by thermally activated ring-opening polymerization at elevated temperatures without catalysts [19]. 

The photoinitiated cationic polymerization of liquid epoxidized polyisoprene is an efficient method to generate rapidly crosslinked elastomers. In the presence of a triarylsulfonium salt, the reaction develops readily upon UV exposure, with formation of both inter and intramolecular ether linkages. The formulation reactivity can be substantially enhanced by the addition of a difunctional vinyl ether or acrylate monomer, which acts as a reactive diluent and leads to the formation of an... 

The virtues of photoinitiated cationic polymerization are rapid polymerization without oxygen inhibition, minimal sensitivity to water, and the ability to polymerize vinyl ethers, oxiranes (epoxides), and other heterocyclic monomers (see Table 10.7) that do not polymerize by a free radical mechanism. 

Figure 9 Photoinitiated cationic polymerization of a di vinyl ether of triethyleneglycol and of an aliphatic dicycloepoxide. Importance of the dark polymerization after a 1 s UV exposure (—)...

There are three t)q)es of eompositions that eure by eationie meehanism. One of them uses aryldiazonium salts to initiate the reaetion. The seeond one utilizes onium salts and the third one organometallie eomplexes. The most prominent ones are those that eure with the aid of onium salt photoinitiators. Many cationic curable compositions consist of mixtures of compounds with oxirane rings. They may also be mixtures of vinyl ether. In addition, some compositions contain both, epoxides and vinyl ethers. More recent compositions might also include silicone based monomers with epoxide groups. Thus, Crivello and Lee described a synthesis of a series of silicon-epoxy monomers that undergo rapid and efficient photoinitiated cationic polymerizations. Such compounds can be prepared by direct hydrosilylation of olefmic epoxides. 

Dougherty, J.A. and Crivello, J.V., Photoinitiated cationic polymerization of vinyl ether-epoxy blends, Polym. Mater. Sci. Eng., 1995, 72, 410-412. 

KAH 09] Kahveci M.U., Uygun M., Tasdelen M.A. et al., Photoinitiated cationic polymerization of vinyl ethers using substituted vinyl halides , Macromolecules, vol. 42, pp. 4443-4448, 2009. 

The present work focuses on the synthesis of novel multifunctional epoxy and vinyl ether monomers bearing the Tg silsesquioxane core and a study of their behaviour under photoinitiated cationic polymerization conditions. 

Included among the many types of vinyl monomers that have been subjected to photoinitiated cationic polymerization are styrene," substituted styrenes, a-methylstyrenes, N-vinylcarbazole, alkyl vinyl ethers, prop-l-en-l-yl ethers, ketene acetals, and alkoxyallenes. Most useful in the crosslinking photopolymerizations employed for UV curing applications are multifunctional vinyl ethers and multifunctional prop-l-en-l-yl ethers. A number of multifunctional vinyl ether monomers are available from commercial sources, while multifunctional prop-l-en-l-yl ethers can be readily prepared by catalytic isomerization from their corresponding allyl ether precursors. The photoinitiated cationic... 

Cationic polymerization in hot melts has been applied to epoxidized polymers [38,39]. No hot melts based on vinyl ether or other cation-sensitive functionalized polymers have been described in the literature. With cationic systems, it is important that the other ingredients in the adhesive be of low basicity to avoid scavenging the initiating acid generated by the photoinitiator. 

However, unlike the triaryIsulfonium salts, these compounds undergo reversible photoinduced ylid formation rather than homolytic carbon-sulfur bond cleavage. Because the rate of the thermal back reaction is appreciable at room temperature, only those monomers that are more nucleophilic than the ylid will polymerize. Epoxides, vinyl ethers, and cyclic acetals undergo facile cationic polymerization when irradiated in the presence of dialkylphenacylsulfonium salts as photoinitiators. 

A similar study has been performed on EPI blends in which the vinyl ether was replaced by an acrylate monomer (HDDA) to produce, by different mechanisms, two interpenetrating polymer networks. With the onium salt as sole photoinitiator, the cationic polymerization of the EPI epoxy groups occurred as fast in the formulation containing 20% of HDDA by weight as in the EPI/DVE-3 blend, to reach nearly 100% conversion within 0.6 s (Fig. 11). The polymerization quantum yield was found to be similar to that measured in the EPI/vinyl ether blend Op 650 mol E. By contrast, the acrylate double bonds were found to polymerize at a much slower pace, most probably because of the low reactivity of the free radicals generated by the cationic-type photoinitiator. 

In systems 1-4 radical polymerization takes place. Decomposition of the photoinitiator (e.g., benzoin and its derivatives) leads to formation of free radicals which react with the carbon-carbon double bonds. In system 5 acidic reactive species are formed which react with cycloaliphatic epoxy compounds and vinyl ethers to form a cross-linked network. Sulfonium, and iodonium salts are used to initiate this cationic polymerization, a typical example is PhjS PFg. ... 

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. 

Recently, photoinitiators have been developed which cationically polymerize vinyl ether and epoxy functional coatings (3,21). The last section of this paper will briefly introduce this new technology. 

Free radical polymerization is disturbed by oxygen, as shown in Chapter 1, while cationic polymerization shows no effect from oxygen. Epoxy or vinyl-ether compounds are used for cationic polymerization. Typical cationic polymers are shown in Figure 4.10. Photoinitiators are photoacid-generating compounds, di-(p-toluene)iodonium hexafluoroantimonate and triphenyl-sulfonium hexafluorophosphate, as shown in Figure 4.11. 

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