Photo-initiated cationic polymerization epoxides

TGA analyses were performed for polymer samples having different degrees of cross-linking. The decomposition of the linear oligomer starts at about 200 °C. Once cured and baked, the formed siloxane network is more thermally stable, and the decomposition begins at temperatures higher by 100-150 °C. The results are similar to those reported for analogous Tsi-modified siloxanes cross-linked by means of photo-initiated cationic polymerization of epoxides [8]. 

Decker C, Nguyen thi viet T, Thi HP. Photo-initiated cationic polymerization of epoxides. Polym Intern. 2001 50 986-997. 

Different initiation techniques have been investigated in polymerizations induced by iodonium salts, such as visible laser irradiation [32], dual photo- and thermally initiated cationic polymerization [23, 26] and a two-photon photopolymerization initiation system [33,34]. For example, dual photo- and thermal-initiation systems based on selective inhibition of the photoinifiated cationic ring-opening polymerization of epoxides by dialkyl sulfides have been developed [26]. Such a dual system, iodonium salt/dialkyl sulfide, in the... 

Once the initiating cation has been formed the polymerization is controlled by temperature (technical difficulties may arise if the control is not adequate). A reaction scheme for cationic polymerization of an epoxide using triarylsulphonium hexafluoroantimonate as photo-initiator is shown in Figure 105. Termination of the polymerization often is adventitious, particularly with anions and other bases (theoretically the reaction can continue until the supply of monomer is exhausted). 

Figure 105 A reaction scheme for cationic polymerization of an epoxide using triarylsulphonium hexafluoroantimonate as photo-initiator...

Interestingly, arene iron salts 62 have also proven to be effective catalysts for the free radical polymerization of epoxides, acrylates, etc. [3,36]. Similar to cationic polymerization, radical processes are also photo chemically initiated reactions but are initiated in the presence of radical precursor additives (halogenat-ed solvents, bis(p-iV,i -dimethylaminobenzylide)cyclopentanone, etc.). 

Preparation of a novel cationic photoinitiator [Cp-Fe-diphenylether] PFe was reported. It was claimed that polymerizations of epoxy resins and eyeloaliphatic epoxides can be photo initiated by this compound effectively. Addition of benzoyl peroxide to this photoinitiator promotes the... 

A reaction scheme similar to that outlined above has been proposed to account for the photo-initiation of cationic polymerizations by triarylsulphonium salts, and a systematic comparison of the [x>Iymerization behaviour of various epoxides in the presence of sulphonium and iodonium salts has been conducted. More recently the concept of sulphonium salt photo-initiators has been extended to include thiopyrylium salts. ... 

The bulky anion then stabilizes the intermediate adduct from protonation of the epoxy group and then facilitates insertion of epoxide at the cationic propagation site. Rapid polymerization can then occur. Cationic photopolymerization of epoxides often involves the photo-generation of acid from an initiator such as diaryliodonium or triaryl sulfonium salts (Crivello, 1999). The anions are important in controlling the addition at the cationic site and are typically BF4 and PFg. The reactivity of the system depends also on the structure of the epoxide. 

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