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

The Schiff-base compounds 2,4-di-tert-butyl-6-(((3,4,5-trimethoxyphenyl)imino)methyl)phenol (L1H), 2,4-di-tert-butyl-6-(((2,4,6-trimethoxyphenyl)imino)methyl)phenol (L2H), 2,4-di-tert-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, 1; L2, 2; L3, 3), and bis(chelate) complexes for zinc, namely [Zn(Ln)2] (L1, 5; L2, 6; L3, 7). All complexes have significant activity at 50 °C and higher activity at 100 °C for the ring-opening polymerization (ROP) of e-caprolactone (e-CL) with good control over the molar mass distribution (Mw/Mn < 2) and molecular weight. Complex 1 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 (4 and 8). Aluminum or zinc complexes derived from L1 exhibit higher activity as compared with complexes derived from L2 and L3. Complex 1 was also tested as an initiator for the copolymerization of e-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 e-CL using 1, and well-defined cross-linked PCL was afforded of high molecular weight.