Lactide Cyclopolymerization Kinetics, X-ray Structure, and Solution Dynamics of (tBu-SalAmEE)Al and a Cautionary Tale Of Polymetalate Formation
The complex (tBu-SalAmEE)Al (tBu-SalAmEEH3 = N,N-bis(3,5-di-tert-butyl-2-hydroxybenzyl)-2-(2-aminoethoxy)ethanol, 1) catalyzes the ring-expansion polymerization of lactide to form cyclic poly(lactide) (cPLA). The X-ray structure of 1 was determined, its polymerization kinetics were examined and its interactions with Lewis bases were observed. The data from these experiments are consistent with a coordination–insertion mechanism whose rate-determining step is catalyst rearrangement by loss of a hemilabile, datively bound, bridging ligand ether. cPLA was examined by thermogravimetric analysis and found more stable than its linear counterpart. In the course of these studies, we unexpectedly observed the formation of polymetalate (AlMe(tBu-SalAmEE)AlMe2)2 (6), which was characterized (X-ray, EA, and 1H and 13C NMR).