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Photoinduced Cationic Polymerization

Photoinitiated cationic polymerizations are widely used in different applications like photocurable coatings, inks, and adhesives.[1] This type of reactions is usually initiated by photoinitiators that are based on iodonium and sulfonium salts that absorb at λ < 300 nm and in the range of 380 nm) to 400 nm).[2] Contrary to the free-radical photopolymerization, which experiences rapid termination of the polymerization process when the light source is removed (due to radical-radical termination reactions), the cationic polymerization processes proceed long after the irradiation has ceased. Typically, the reaction stops when almost all monomer has been consumed.

Due to a better solubility in non polar monomers, diaryliodonium salts are usually preferred instead of the corresponding triarylsulfonium salts. For the cationic polymerization the four

most commonly used cations are in the order of popularity:[1]

cationeq1

 The mechanism of the UV-induced photolysis of diaryliodonium salts (Ar2I+MtXn) is very complex, but it is possible to show it in an abbreviated form:

           cationeq2                                                                            

Photoexcitation of the diaryliodoniumsalt is the first step, followed by both heterolytic and homolytic cleavages of the carboniodine bond of the resulting excited singlet state. This leads to simultaneously formed free-radical, cationic and cation-radical fragments. The aryl cations and aryliodine radicals generated during this process are highly reactive and react with monomers or impurities to give the protonic acids, the so called “super acid” HMtXn. This “super acid” is able to react with the monomer (M) and induce the cationic polymerization:[3]

                                          cationeq3                                                 

The subsequent propagation steps are identical to common cationic polymerizations.

                              cationeq4                                                               

Typically, there is no chain termination reaction, the reaction stops with the total consumption of the monomer. Common monomers (M) for this reaction are shown in figure 1.

 photocationicmonomers

Figure 1: Common monomers for cationic polymerization.[1

 

  1. ORTYL, J.; POPIELARZ, R.: New photoinitiators for cationic polymerization, Polimery, 2012, 57 (7-8), 510–517.
  2. LALEVÉE, J.; BLANCHARD, N.; TEHFE, M.-A.; CHANY, A.-C.; FOUASSIER, J.-P.: New Boryl Radicals Derived from N-Heteroaryl Boranes: Generation and Reactivity, Chemistry–A European Journal, 2010, 16 (43), 12920–12927.
  3. CRIVELLO, J. V.: UV and electron beam-induced cationic polymerization, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1999, 151 (1), 8–21.

 

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