Iron Tris(dibenzoylmethane)-Centered Polylactide Stars:  Multiple Roles for the Metal Complex in Lactide Ring-Opening Polymerization

Poly(lactic acid) (PLA)-supported dibenzoylmethane (dbm) and corresponding metal complexes have potential applications as biomaterials and catalysts. Using hydroxyl-functionalized dbm (i.e., dbmOH) as the initiator and Sn(oct)₂ as the catalyst, lactide ring-opening polymerizations were unexpectedly slow (6 h), and PDIs broadened when molecular weights >10 000 were targeted. Because interactions between the dbm ligand and Sn(oct)₂ may be responsible for the diminished catalyst activity and molecular weight control, iron(III) was employed as a protecting group to form Fe(dbmOH)₃. Shorter reaction times (10 min) were noted with this trifunctional metalloinitiator and Sn(oct)₂, and higher molecular weights were achievable. Moreover, it was discovered that Fe(dbmOH)₃ serves not only as an initiator but also as a catalyst and activating group for the polymerization. Even without the tin catalyst, iron-centered polymers with low PDIs (<1.1) were obtained within 10 min (∼70% monomer conversion). The resulting Fe(dbmPLA)₃ stars were demetalated by acid treatment to generate dbmPLA for subsequent coordination to other metals. To explore the scope of iron β-diketonate complexes as catalysts, additional studies were performed with dbmOH and benzyl alcohol initiators using either iron tris(dbm) or iron tris(acac) (acac = acetylacetonate) as the catalyst. Here too, PLA products were obtained, and the iron catalysts were readily separated by treatment with aqueous HCl.