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

Abstract

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 ( approximately 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 beta-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.