Metal Ion Initiated Charge-Transfer Photopolymerization of Tetrahydrofuran. A Mechanistic Investigation

Irradiation of dilute solutions of salts of Ag(I), Cu(II), Cu(I), and Tl(I) having suitable noncoordinating anions in tetrahydrofuran results in the formation of polytetrahydrofuran in yields comparable to those for acid-catalyzed (dark) polymerization. The yield of polymer is dependent upon the identity and concentration of metal ion, the anion, the duration of irradiation, and the dark time following irradiation. The observed polymer-yield dependencies upon metal ion and anion at low conversions are Cu(II) > Ag(I) > Cu(I) ≫ T1(I) and PF₆⁻> SbF₆⁻≫ OTf⁻> BF₄⁻. Both the counterion dependence and increase in polymer yield with dark time following photoinitiation are indicative of a cationic ring-opening polymerization process. Initiation of polymerization is proposed to occur via ligand-to-metal charge-transfer excitation of the metal ion-ether complex to generate oxidized ether and reduced metal, which precipitates from solution. The ether cation radical reacts further to form reactive intermediates known to initiate the cationic ring-opening polymerization of tetrahydrofuran. Evidence in support of the proposed mechanism of initiation and polymerization is presented. The initiation of thermal and photochemical ring-opening polymerization of several other ethers by metal salts is also described.