Kinetics of Cationic Photopolymerization

Cationic polymerization is initiated either by strong Lewis acids such as BFg or PF5 or by Bronsted acids such as H+BF4, H+PF, or H+SbFg. Lewis acids are generated by UV irradiation of aryldiazonium salts, whereas diary-liodonium, triarylsulfonium, and triarylselenium salts produce upon UV 

Reaction steps in a photoinduced cationic polymerization are as follows  

In the initiation step, the monomer, M, is initiated by intermediate protonation followed by the formation of a carbocation, H-M+. Propagation can be terminated by anionic or nucleophilic species, A . If a hydroxy-functional compound (ROH) is present, chain transfer can occur via proton formation. 

There are several differences between the photoinitiated cationic polymerization and the photoinduced free radical polymerization  

The initiating species is a stable compound consumed only by anions and nucleophiles. 

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Kinetics of Cationic Photopolymerization Chemical Systems in UV Processing... 

Cho J-D, Ju H-T, Park Y-S, Hong J-W (2006) Kinetics of cationic photopolymerizations of UV-curable epoxy-based SU8-negative photoresists with and without silica nanoparticles. Macromol Mater Eng 291(9) 1155-1163... 

The systems are designed in order to improve the reaction rate of the mixture and the physical properties of the photopoljmier. The flexibility of the two photoinitiation schemes in one system allows for numerons possibilities in achieving greater control of viscosity, conversion, shrinkage, adhesion, and ultimate strength. The kinetics of hybrid photopolymerization systems are more difficult because two reactive systems (free-radical and cationic) mnst be resolved from one another. Cationic photopolymerization kinetics are more difficnlt to analyze than free-radical kinetics because the pseudo-steady-state assumption is often not valid for the cationic active center concentration, and the natnre and concentration of the cationic active centers is difficult to determine (p. 376 of Ref 33, see also Photopolymerization, Cationic). 

Many different methods have been used to monitor the kinetics of cationic ring-opening photopolymerizations. Among the most commonly used techniques are real-time infrared spearoscopy, differential scanning photocalori-metry, " thin-film calorimetry, and optical pyrometry. ... 

Cho and Hong (2005) used photodifferential scanning calorimetry to investigate the photocuring kinetics of UV-initiated cationic photopolymerization of 1,4-cyclohexane dimethanol divinyl ether (CHVE) monomer with and without a photosensitizer, 2,4-diethylthioxanthone (DETX) in the presence of a diaryliodonium-salt photoinitiator. Two kinetic parameters, the rate constant (k) and the order of the initiation reaction (m), were determined for the CHVE system using an auto-catalytic kinetics model as shown in the following equation ... 

J.D. Cho and J.W. Hong, Curing kinetics of UV-initiated cationic photopolymerization of divinyl ether photosensitized by thioxanthone. J. Appl. Polym. Sci. 2005, 97(3), 1345-1351. 

In this review article we present some typical examples showing how effective RTIR spectroscopy proved to be to evaluate the important kinetic parameters of photoinitiated radical and cationic polymerizations, and to assess the influence of some chemical and physical parameters on both the rate and the extent of the light-induced curing reactions. The photopolymerization of different types of monomer mixtures will also be examined, as RTIR spectroscopy is a unique tool to follow the kinetics of... 

The features and detail of the IPN kinetics were also studied in other works [274-276]. The kinetics of thermally initiated cationic epoxy polymerization and free radical acrylate photopolymerization were investigated in [277]. It was found that the preexistence of one polymer has a significant effect on the polymerization of the second monomer. The reaction kinetics and phase separations were studied for sequential IPNs in [278]. The kinetics of IPN formation was studied for IPNs based on PDMS-cellulose acetate butyrate [279]. All these and other works [280-282] confirm the general regularities of the reaction kinetics and its connection with phase separation in forming systems. 

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