Alkoxy-Functionalized Schiff-Base Ligation at Aluminum and Zinc: Synthesis, Structures and ROP Capability

• Main Authors: Xin Zhang, Kai Chen, Melissa Chicoma, Kimberly Goins, Timothy J. Prior, Terence A. Nile, Carl Redshaw
• Format: Article
• Language: English
• Published: MDPI AG 2021-09-01
• Series: Catalysts
• Subjects: Schiff base; organoaluminum; zinc; catalyst; polycaprolactone; copolymer
• Online Access: https://www.mdpi.com/2073-4344/11/9/1090

The Schiff-base compounds 2,4-di-<i>tert</i>-butyl-6-(((3,4,5-trimethoxyphenyl)imino)methyl)phenol (L¹H), 2,4-di-<i>tert</i>-butyl-6-(((2,4,6-trimethoxyphenyl)imino)methyl)phenol (L²H), 2,4-di-<i>tert</i>-butyl-6-(((2,4-trimethoxyphenyl)imino)methyl)phenol) (L³H) derived from anilines bearing methoxy substituents have been employed in the preparation of alkylaluminum and zinc complexes. Molecular structure determinations reveal mono-chelate aluminum complexes of the type [Al(Lⁿ)(Me)₂] (L¹, <b>1</b>; L², <b>2</b>; L³, <b>3</b>), and bis(chelate) complexes for zinc, namely [Zn(Lⁿ)₂] (L¹, <b>5</b>; L², <b>6</b>; L³, <b>7</b>). All complexes have significant activity at 50 °C and higher activity at 100 °C for the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) with good control over the molar mass distribution (<i>M</i>_w<i>/M</i>_n < 2) and molecular weight. Complex <b>1</b> was found to be the most active catalyst, achieving 99% conversion within 18 h at 50 °C and giving polycaprolactone with high molecular weight; results are compared against aniline-derived (i.e., non-methoxy containing) complexes (<b>4</b> and <b>8</b>). Aluminum or zinc complexes derived from L¹ exhibit higher activity as compared with complexes derived from L² and L³. Complex <b>1</b> was also tested as an initiator for the copolymerization of ε-CL and glycolide (GL). The CL-GL copolymers have various microstructures depending on the feed ratio. The crosslinker 4,4′-bioxepane-7,7′-dione was used in the polymerization with ε-CL using <b>1</b>, and well-defined cross-linked PCL was afforded of high molecular weight.