Photocatalyzed ring-opening polymerization of ε-caprolactone

Photocatalyzed polymerization by semiconductors has recently contributed to the methods of polymer synthesis. In this work, we developed a new type of photocatalyzed polymerization with ring-opening polymerization of lactone. A complex catalyst was designed consisting of nitrogen-doped TiO₂ (N–TiO₂), iodonium salt (Ph₂I⁺PF₆⁻) and H₂O. Poly(ε-caprolactone) (PCL) with a narrow molecular weight distribution was synthesized under visible light irradiation. By using electron spin resonance (ESR), in situ¹H-nuclear magnetic resonance (¹H-NMR) and MALDI-TOF mass spectrometry, the mechanism studies clearly presented the reactions of the three components in the ternary catalyst. This polymerization was ruled by the route in which ε-caprolactone (ε-CL) was activated by H⁺. H⁺ was photogenerated from the oxidation reaction of H₂O. Meanwhile, H⁺ was stabilized by the photogenerated PF₆⁻, forming Brønsted acid H⁺PF₆⁻. The relationship between the monomer conversions and PCL molecular weights was studied.