Ring-opening polymerization of ε-caprolactone by Zr(IV) tris(β-diketonates): Electronic character of complexes in initiation and propagation steps

Abstract

A series of zirconium β-diketonates, i.e., tris(acetylacetonato)zirconium(IV) chloride (1), tris(benzoylacetonato)zirconium(IV) chloride (2), tris(dibenzoylmethanato)zirconium(IV) chloride (3), and tris(benzoyltrifluoroacetonato)zirconium(IV) chloride (4) work as efficient catalysts in the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) to generate poly(ε-CL) with repeating unit up to 68. The activity of the complexes was in the order of 2 > 4 > 1 > 3, with more Lewis acidic complexes 2 and 4 gave higher catalytic activity. DFT calculations showed that a trifluoromethyl moiety in the ligand decreases the oxygen nucleophilicity at intermediate complexes, and thus inhibiting the propagation process. Three possible states of the complex before ε-CL coordination in the ROP were proposed as follows: (i) dissociation of one ligand; (ii) dissociation of one coordinated-oxygen of the ligand from zirconium and (iii) direct insertion of sp2-oxygen of ε-CL into Zr (no change of the complex geometry). The thermodynamic analysis via the Gibbs energy calculations at DFT level revealed that a direct insertion of sp2-oxygen of ε-CL into zirconium is the most plausible pathway for the ROP.