Non-symmetric β-Diketiminate Magnesium Complexes as Initiators for Ring-Opening Polymerization/Copolymerization of Lactide, ε-Caprolactone

Magnesium silylamido complexes 1 similar to 7 bearing non-symmetric beta-diketiminate ligands were synthesized via the reactions of corresponding proligands HL1 similar to 7 with one equivalent of Mg[N(SiMe3)(2)](2) at 80 degrees C in toluene. All complexes were characterized by H-1 NMR, C-13 NMR and elemental analysis. The monomeric nature of complexes 1 and 5 in the solid state was further confirmed by X-ray diffraction studies. In both complexes, the metal center is tri-coordinated by the beta-diketiminate ligand and one silylamido group. The very close bond lengths of two Mg-N bonds as well as close C-N distances of the chelate ring indicate significant delocalization. The apparent deviation of the magnesium center from the ligand backbone plane suggests a certain eta(n)-coordination nature of the ligand to the metal center. These magnesium silylamido complexes showed good catalytic activities for the ring-opening polymerization of rac-lactide under ambient conditions, and could polymerize 300 equivalents of rac-lactide to high molecular weight polymers within short time in THF. The solvent effect played a critical role during the polymerization process. All complexes showed higher catalytic activity in THF than in toluene. Taking complex 2 as an example, a monomer conversion of 96% could be achieved within 10 min in THF, whereas a conversion of 80% could only be achieved within extended polymerization time of 210 min in toluene (Table 1, Entries 6 and 9, [LA](0)/[Mg](0) = 100). Upon the addition of isopropanol, the activities of magnesium silylamido complexes 1 similar to 7 increased significantly. For instance, when the polymerization runs were carried in THF in the presence of isopropanol, the reaction time could be reduced to 10 similar to 20 min even with a high molar ratio of [LA](0)/[Mg](0) = 300. Moreover, the type and location of the substituent(s) on the N-aryl group of the beta-diketiminate ligand exerts a significant influence on the catalytic activity of the corresponding complex toward the polymerization of rac-lactide. In the absence of excess isopropanol, the introduction of sterically demanding substituent to the ortho-position of N-aryl ring leads to a decrease of the polymerization activity; but the influence of electron-withdrawing group is different in different solvents (toluene or THF). These magnesium complexes could produce heterotactic polymers in THF (P-r=0.64 similar to 0.80) and atactic polymers in toluene (P-r=0.45 similar to 0.58). Magnesium complexes 1 similar to 7 also displayed high catalytic activities for the ring-opening polymerization (ROP) of epsilon-caprolactone, among them complexes bearing beta-diketiminate ligand with bulky ortho-substituted N-aryl rings showed higher activities. Generally, the ROPs of epsilon-caprolactone initiated by these magnesium silylamido complexes were not well-controlled, giving moderately distributed polymers (M-w/M-n= 1.37 similar to 1.67). Additionally, diblock copolymers of L-lactide and epsilon-caprolactone were obtained by using 2 as the initiator via both sequential feeding of two monomers (in either order) and the one-pot method. The formation of diblock copolymers were verified by H-1 NMR, C-13 NMR, DSC and GPC analysis.