Photosensitized cationic polymerization of cyclohexene oxide: A mechanistic study concerning the use of pyridinium-type salts

The photoinitiation of the polymerization of bulk cyclohexene oxide (CHO) containing N-ethoxy-2-methyl pyridinium hexafluorophosphate (EMP⁺PF^- ₆) and either anthracene or thioxanthone (TX) at λ_inc > 340 nm was studied. Regarding the action of anthracene it is notable that upon u.v. irradiation of a CHO solution of poly(tetrahydrofuran) bearing terminal anthryl groups a block copolymer, poly(tetrahydrofuran-block-cyclohexane oxide), is formed. The optical absorption spectrum of the block copolymer does not possess bands characteristic for anthracene. Therefore, the following mechanism is postulated; electron transfer from singlet excited anthracene molecules to EMP⁺ ions results in the formation of anthracene radical cations that react with ethoxyl radicals stemming from the decomposition of EMP• radicals. 9-Ethoxy-9,10-dihydroanthryl ions generated in this way react with CHO thus initiating its polymerization. Regarding the action of TX it seems that the polymerization of CHO is essentially initiated by protons. The generation of protons has been evidenced. In propylene carbonate solution protons are generated with Φ(H⁺) = 0.27 whereas Φ(-TX) = 0.028. The postulated mechanism is based on the reaction of triplets, ³TX*, with both CHO (k_RH = 3 × 10⁴ 1mol^-1 s^-1) and EMP⁺ ions (k_ET = 4 × 10⁷ 1mol^-1 s^-1). At low concentration of EMP⁺PF^- ₆ (6.8 × 10^-4 mol^-1 1^-1) ³TX* molecules react almost exclusively (93%) with CHO and it appears that ketyl radicals thus formed react with EMP⁺ ions, a process eventually resulting in the formation of protons and the regeneration of TX. At relatively high concentration of EMP⁺PF^- ₆ (6.8 × 10^-3 mol 1^-1), thioxanthone triplets are largely (47%) deactivated by electron transfer to EMP⁺ ions. The importance of this reaction with respect to its contribution to the initiation of the polymerization of CHO has not yet been revealed.