Lithium, sodium, potassium and calcium amine-bis(phenolate) complexes in the ring-opening polymerization of rac-lactide†
Compounds of Li, Na, K and Ca of a tetradentate amino-bis(phenolato) ligand were prepared. Bimetallic compounds formulated as M2[L](THF)n (where M = Na, n = 1 (1·THF) or Li, n = 1 (2·THF)) were synthesized via the reaction of H2[L] (where [L] = 2-pyridylmethylamino-N,N-bis(2-methylene-4-methoxy-6-tert-butylphenolato) with sodium hydride or n-butyllithium, respectively, in THF. Monometallic complexes MH[L](THF)n (where M = Na, n = 1 (3·THF), Li, n = 0 (4) and K, n = 0 (5)) were obtained by reaction of H2[L] with MN(SiMe3)2 where M = Na, Li, or K. Calcium complex Ca[L](THF) (6·THF) was synthesized in two ways; reaction of Na2[L] with calcium iodide in THF, and reaction of Ca[N(SiMe3)2]2 with H2[L] in toluene. Compounds 1–6 exhibit activity for rac-lactide polymerization under melt and solution conditions. Moderate control of polymer molecular weights was achieved in toluene, whereas polydisperse polymer was obtained under solvent free conditions. MALDI-TOF MS analysis of the polymer end groups revealed a predominantly cyclic nature for the polylactides.