Synthesis of Poly(δ-caprolactone) via Bis(phenolate) Rare-Earth Metal Complexes Mediated Ring-Opening Polymerization and Its Chemical Recycling

Abstract

New amine-amino-bis(phenolate) ligands (H 2 L tBu and H 2 L Cl ) with a cyclic tertiary amine (pyrrolidine) as a side arm and t Bu or Cl group on the phenolate ring have been prepared. The alkane elimination reaction between these free ligands and rareearth tris(alkyl)s Ln(CH 2 SiMe 3 ) 3 (THF) 2 afforded the corresponding silylalkyl complexes L tBu LnCH 2 SiMe 3 (THF) (Ln = Y (1), Lu (2)) and L Cl YCH 2 SiMe 3 (THF) (3), where the solid-state structure of complex 1 was unambiguously confirmed by X-ray diffraction (XRD) analysis. These rare-earth metal complexes have been utilized as catalysts for the ring-opening polymerization (ROP) of biobased δ-caprolactone (δCL), either in the absence or presence of alcohols, to give poly(δ-caprolactone) (PδCL) with controlled molecular weight and narrow distribution (Đ < 1.2). The polymerization kinetics of δCL in toluene with yttrium complexes 1 and 3 were investigated. Oligomers prepared with complex 3 alone and the 3/PhCHMeOH binary catalyst system were well characterized with 1 H NMR spectroscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS). Moreover, chemical recycling of the resultant PδCL was achieved with high yield in a solution at ambient temperature (>92%) or in bulk at 130 °C (>82%) by using commercial KO t Bu as a promotor.