Polymerization of <i>rac</i>-Lactide Using Schiff Base Aluminum Catalysts:  Structure, Activity, and Stereoselectivity

Abstract

A series of aluminum ethyls and isopropoxides based upon N,N,O,O-tetradentate Schiff base ligand framework have been prepared. X-ray diffraction analysis and 1 H NMR confirmed that these Schiff base aluminum ethyls and isopropoxides were all monomeric species with a five-coordinated central aluminum in their solid structures. Compared to the aluminum ethyls which all retain their monomeric structure in the solution, the dinucleating phenomenons of aluminum isopropoxides with less steric hindered substituents in the solution have also been observed. The activities and stereoselectivities of these complexes toward the ring-opening polymerization of rac -lactide have been investigated. Polymerization experiments indicated that (SB- 2d )AlO i Pr [(SB- 2d ) = 2,2-dimethyl-1,3-propylenebis(3,5-di- tert -butylsalicylideneiminato)] exhibited the highest stereoselectivity and (SB- 3b )AlO i Pr [(SB- 3b ) = 2,2-dimethyl-1,3-propylenebis(3,5-dichlorinesalicylideneiminato)] possessed the highest activity among these aluminum isopropoxides. The substituents and the mode of the bridging part between the two nitrogen atoms both exerted significant influences upon the progress of the polymerizations, influencing either the tacticity of isolated polymers or the rate of polymerization. The polymerization kinetics using (SB- 3b )AlO i Pr as a catalyst were studied in details, and the experimental results revealed that the rate of polymerization was first-order in [LA] and 1.81th-order with respect to (SB- 3b )AlO i Pr, which indicated that the propagating species was not uniform in the system without the protection of steric hindered substituents. Furthermore, the polymerization by initiating with (SB- 3b )AlO i Pr could be progressed at low temperatures (0 °C) without the change of stereoselectivity.