Ring-Opening Homo- and Copolymerization of Cyclic Esters Catalyzed by Iron(III) Triflate

Biomass-based and biodegradable poly(l-lactide) (PLLA) is synthesized by ring-opening polymerization of l-lactide (LLA), for which tin(II) 2-ethylhexanoate is a major catalyst. However, the potential toxicity of tin can be a problem, especially in biomedical applications. In this study, we focused on iron, which is a non-toxic metal and an abundant resource. We investigated the ring-opening homo- and copolymerization of cyclic esters such as LLA and ε-caprolactone (CL) catalyzed by iron(III) triflate, Fe(OTf)3, which is commercially available and known as a Lewis acid. In the polymerization of LLA in toluene at 110 °C, Fe(OTf)3 showed relatively high activity and yielded PLLA with unimodal molecular weight distribution. The addition of 1,8-bis(dimethylamino)naphthalene (proton sponge: PS) to the Fe(OTf)3 catalyst system increased the yield and molecular weight of the resulting polymer. In contrast, the polymerization of CL by Fe(OTf)3 was decelerated by the presence of PS. The Fe(OTf)3 system was found to have an exceptionally high preference for CL over LLA in the copolymerization of LLA and CL, with the reactivity ratio of rLLA = 0.51 and rCL = 6.9. In contrast, the Fe(OTf)3–2PS system exhibited an LLA preference with rLLA = 15 and rCL = 0.22, indicating that the comonomer selectivity changed depending on the presence or absence of PS. While the LLA polymerization rate by the Fe(OTf)3 system showed a second-order dependence on the Fe(OTf)3 concentration, that of the Fe(OTf)3–PS system showed a first-order dependence on the Fe(OTf)3–PS concentration.