PHOTOIONIZATION OF ALPHA-ALKOXYBENZYL RADICALS TO YIELD ALPHA-ALKOXYBENZYL CATIONS - PHOTOCHEMISTRY OF OMEGA,OMEGA-DIMETHOXY-OMEGA-PHENYLACETOPHENONE IN POLAR-SOLVENTS AT HIGH LIGHT INTENSITIES

On photolysis of the photopolymerization initiator PhC(O)C(OMe)(2)Ph i n aqueous or alcoholic solution with 248 nm laser pulses or with 248 m n followed by 308 nm pulses (two-laser-two-colour technique), the shor t-lived species PhC'(OMe)(2), PhC+(OMe)(2) and e(solv)(-) were detecte d by their optical absorptions. The carbocation PhC+(OMe)(2) and e(sol v)(-) are the products of the monophotonic ionization of the radical P hC'(OMe)(2) [phi(308 nm)= 0.2 in aqueous solution] generated from the parent, PhC(O)C(OMe)(2)Ph, by alpha-cleavage [phi(248 nm) ca. 0.5]. Ph C+(OMe)(2) reacts with water to produce the hemi-orthoester PhC(OMe)(2 )OH and H+ which was identified by time-resolved conductance. PhC(OMe) (2)OH decomposes to yield PhC(O)OMe and MeOH (identified by GC). The a lpha-alkoxybenzyl radicals (PhCHOR)-O-., produced by 248 nm photoinduc ed alpha-cleavage of the benzoin ethers PhC(O)CH(OR)Ph (R = Me, Et, Pr -i, Bu-t), also undergo ionization upon 248 or 308 nm photolysis. The rate constants for reaction of (PhCHOR)-O-. with the oxidants Fe(CN)(6 )(3-), Ir(Cl)(6)(2-), O-2 and PhI2+ are on average 5.9 x 10(9), 4.2 x 10(9), 2.8 x 10(9) and 1.7 x 10(8) dm(3) mol(-1) s(-1), respectively. From the facile photoionization of the alpha-alkoxybenzyl radicals it is concluded that this process may also take place under the typical h igh-intensity light conditions used in industrial photocuring, i.e. th at the latter may involve cationic polymerization.