Epoxy photoinitiated cationic polymerization

Crivello, J.V. and Lee, J.L., The synthesis, characterization and photoinitiated cationic polymerization of silicon-containing epoxy resins. J. Polym. Sci. Polym. Chem. Ed., 28, 479-503 (1990). 

Structure and Reactivity Relationships in the Photoinitiated Cationic Polymerization of 3,4-Epoxy cyclohexylmethyl-3, 4 -epoxy cyclohexane... 

To evaluate the reactivity of model compounds III-VIII in photoinitiated cationic polymerization, we have employed real-time infrared spectroscopy (RTIR). Thin film samples of the model compounds containing 0.5 mol% of (4-n-octyloxyphenyl)phenyliodonium SbF - as a photoinitiator were irradiated in a FTIR spectrometer at a UV intensity of 20 mW/cm2. During irradiation, the decrease in the absorbance of the epoxy ether band at 860 cm-1 was monitored. 

The results of these photochemical studies form guidelines for the choice of sensitizers, onium salts and other additives potentially useful in the cationic curing of coatings. The sensitized photochemistry of diphenyliodonium hexafluoroarsenate and triphenylsulfonium hexaflurorarsenate was investigated at 366 nm. Product quantum yields are compared to relative rates of photoinitiated cationic polymerization of an epoxy resin. 

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]. 

V. James and S.L. Crivello, Photoinitiated cationic polymerization of epoxy alcohol monomers. J. Polym. Sci. A Polym. Chem. 2000, 38(3), 389-401. 

M. Sangermano, S.N. Falling, and J.V. Crivello, Photoinitiated cationic polymerization of epoxy monomers in the presence of poly(3,4-epoxy-1-butene). J. Macromol. Sci. Pure Appl. Chem. 2002, A39(ll), 1279-1294. 

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. 

Unlike free-radical propagation, photoinitiated cationic polymerizations of epoxides are unaffected by oxygen and thus require no blanketing by an inert atmosphere. However, water and basic materials present in UV-curable epoxy formulations can inhibit cationic cures and should be excluded. 

Fig.2 Influence of the epoxy content on the photoinitiated cationic polymerization of epoxidized liquid natural rubber (ELNR). ( ) Photolysis of the photoinitiator ( A) [TAS] = 3 wt %.

The papers presented in the following chapters represent advances in pressure sensitive adhesives (ultraviolet light activated acrylate monomer - low Tg polyether formulations) photoinitiated cationic polymerization (light activated aryliodonium and arylsulfonium salts of lewis acids in epoxy resin formulations) polymer and formulation design criteria for radiation curable adhesives radiation curable composites (dynamic thermal analysis characterization of electron beam cured... 

Photoinitiated cationic polymerizations have numerous applications involving photoimaging. Epoxy-based photoresists with high resolution derived from commercially available resins have been developed (22) and the use of these same materials in plastic flexographic printing plates has been shown in our laboratory. 

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. 

The compound has functional groups that support dimerization type crosslinking and cationic polymerization upon UV exposure (A, = 300—360 nm).. Photodimerization of the chalcone-epoxy compound was confirmed by UV-visible and IR absorbance changes of the C=C double bond of the chalcone unit. Additions of small amounts of onium salts will also photoinitiate cationic polymerization of the epoxy groups present in the above chalcone-epoxy compound by exposine to UV. This ultra-violet light cured chalcone-epoxy compound was reported to possess excellent thermal stability and compares well with conventional UV-cured Bisphenol A type epoxy resins. (see Chapter 3)... 

Carroy, A.C., Chomienne, F. and Nebout, J.F., Recent progress in the photoinitiated cationic polymerization of cycloaliphatic epoxy systems, Proc. RadTech Europe, 1997, 303-313. 

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

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. 

In addition to the control which can be exercised over photoinitiated cationic polymerizations by manipulation of the structures of the diaryliodonium and triarylsulfonium salts, there are a number of additional factors which also influence these polymerizations. First, the emission spectrum of the irradiation source must be matched as closely as possible to the absorption characteristics of the specific photoinitiator. Fortunately, today there are available commercial light sources which provide intense bands in specific areas of any portion of the ultraviolet spectrum. Since the rate of photolysis of the photoinitiators varies as the first power of the light intensity, a simple doubling of the light intensity doubles the rate of photolysis of the photoinitiator. In practice it has also been observed that the rate of the polymerization of epoxy containing monomers is also doubled by a two fold increase in the light intensity... 

All of these species (XIV, XV) have been for the most part applied towards function in the olefin polymerization arena use of these novel anions for the stabilization of other electrophilic species remains to be explored. Recently, the imidazolide anion XVI, as well as the perfluorinated tetraaryl borate derived from the diborane IX of Chart 2, have been used to stabilize iodonium cations.222 These cations are used as photoinitiators for cationic polymerization of epoxy resins in photolithography applications. While use of the [B(C6F5)4] led to a breakthrough in this area of research,223 higher activities are observed for more WCAs. 

The generation of a protonic acid (HX) is responsible for the initiation of cationic polymerization. Various monomers are polymerized by sulfonium photoinitiators. Especially this system has been shown to be potentially useful for UV curable coatings of metal and plastics with epoxy resins. 

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