Phosphoric and phosphoramidic acids as bifunctional catalysts for the ring-opening polymerization of ε-caprolactone: a combined experimental and theoretical study

The ring-opening polymerization of ε-caprolactone catalyzed by the phosphoric acid (PhO)₂P(O)OH (PA) or the phosphoramidic acid (PhO)₂P(O)NH(SO₂CF₃) (PAA) has been investigated experimentally and computationally. The two catalysts promote polymerization under mild conditions, with PAA being slightly more active than PA. The controlled character of the polymerization has been examined thoroughly, and the polymerization conditions were optimized so as to afford well-defined polycaprolactones of molar masses up to 15 000 g mol⁻¹ and narrow molar mass distributions. Density Functional Theory calculations support a bifunctional mechanism for these organo-catalyzed ROPs, with participation of both the acidic proton and basic PO (/SO) moiety of the phosphoric and phosphoramidic acids.