Synthesis, characterization and catalytic activity of Salen–(sodium)2 and (Salen)2–lanthanum–sodium complexes

Salen–Na2(1) and (Salen)2–La–Na(2) complexes have been prepared and structurally characterized [Salen = N,N′-bis (3,5-di-tert-butylsalicylidene)-1,3-propanediamine]. Experimental results show that the two complexes are efficient catalysts for the ring-opening polymerization (ROP) of l-lactide. According to the data of 1H NMR and electrospray-ionization (+ESI) mass spectrum, it is suggested that the obtained polymer is mainly cyclic polylactide (PLA) for a short PLA chain, but a mixture of cyclic and linear PLA for a long PLA chain. Structure–activity analysis of the two complexes is also done. We investigated why Salen–Na2 complex is more active (Salen)2–La–Na complex as the catalyst in the ROP of l-lactide and why the PLA catalyzed by complex 2 has lower PDIs than that catalyzed by complex 1.