Synthesis and characterization of alkali-metal aryloxo compounds and their catalytic activity for l-lactide polymerization

Three new alkali-metal compounds stabilized by aryloxo groups were synthesized and fully characterized. The reactions of carbon-bridged bis(phenol)s MBMPH2 (MBMPH2 = 2,2′-methylene-bis(6-tert-butyl-4-methylphenol)) with sodium and potassium metals in tetrahydrofuran (THF) gave the desired alkali-metal complexes [MBMPNa2(THF)3]2 (1) and [(MBMPK2)2(THF)5]2 (2), respectively, in high isolated yields. A similar reaction of aminophenol [HNOH] ([HNOH] = N-p-methyl-phenyl(2-hydroxy-3,5-di-tert-butyl)benzylamine) with sodium gave the monosodium salt [HNONa(THF)]2 (3). Compounds 1–3 were well characterized, including X-ray structure determination. Compound 1 has a dimeric structure, and each sodium atom is four-coordinated with four oxygen atoms to form a distorted tetrahedron. Compound 2 has a centrosymmetric tetrameric structure. The coordination environments around the four potassium atoms are different. K1 is four-coordinated, K2 is three-coordinated, K3 is five-coordinated, whereas the coordination sphere of K4 is completed by one aryloxo oxygen atom and two oxygen atoms from two THF molecules and six carbon atoms from one arene ring of the bis(phenolate) ligand. Compound 3 has a dimeric structure, and each of the sodium atoms is four-coordinated to form a distorted tetrahedron. It was found that compounds 1–3 can efficiently initiate the ring opening polymerization of l-lactide in the absence of alcohol, yielding polymers with high molecular weights for a wide range of monomer-to-initiator ratios.

Graphical abstractThree alkali metal aryloxo compounds were synthesized and their structures determined by X-ray structure analysis. It was found that these alkali metal compounds are efficient initiators for the ring-opening polymerization of l-lactide in the absence of alcohol.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Three kinds of alkali metal aryloxo compounds. ► X-ray structure analysis. ► The ring opening polymerization of l-lactide.