Bulk solvent-free melt ring-opening polymerization (ROP) of L-lactide catalyzed by Ni(II) and Ni(II)–Ln(III) complexes based on the acyclic Salen-type Schiff-base ligand

A monometallic (Ni2+, 1) and a series of heterometallic (Ni2+–Ln3+, Ln = Ce (2); Ln = Nd (3); Ln = Sm (4); Ln = Eu (5); Ln = Tb (6); Ln = Ho (7); Ln = Tm (8)) complexes based on the acyclic Salen-type Schiff-base ligands H2L (H2L = N,N′-bis(3-methoxysalicylidene)propane-1,2-diamine) were synthesized and characterized by FT-IR, ESI-MS and X-ray crystallography. The catalysis results showed that the two kinds of complexes with different active species, could efficiently catalyze the bulk solvent-free melt ring-opening polymerization (ROP) of l-lactide with moderate molecular weights and narrow molecular weight distributions. Especially for the series of bimetallic complexes 2–8, the involvement of rare ions slightly decreased the catalytic activities on the ROP of l-lactide, while was in favor of the increase of polymeric molecular weights (Mw or Mn) and the polymerization controllability, and the type of rare ions was important and influential factor contributing to the catalytic behaviors.