Polymerization of rac-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) ) 2,2dimethyl-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.

Ring-opening polymerization of lactides by (pyrazol-1-ylmethyl)pyridine Zn(II) and Cu(II) complexes: Kinetics, mechanism and tacticity studies

“…The monomers, D,L-LA and L-LA were purchased from Sigma-Aldrich and used as received. NMR spectra were recorded on a Bruker Ultrashield 400 instrument at room temperature in CDCl 3 ( 1 H at 400 and 13 C at 100 MHz). Mass spectrometry was recorded on a Waters Micromass LCT Premier Mass Spectrometer TOF with nano ACQUITY UPLC System.…”

“…[10][11][12] Furthermore, it is possible to carry out detailed kinetic and mechanistic studies of the polymerization reactions and gain fundamental knowledge on the effect of structure on catalyst activity and polymer microstructure. 13 Although tin(II) octanoate 14 as well as several metal alkoxides 15 are efficient catalysts in the ROP of lactides, design of well-defined catalysts that could produce PLAs with desirable molecular weight and polymer microstructure still remains a major challenge both in academia and in industry. One major drawback of most of these metal-based catalysts is their relative toxicity, which limits the application of the resultant PLAs in medical fields.…”

Review – recent development of ring-opening polymerization of cyclic esters using aluminum complexes

“…Due to bulky substituents on the Schiff base ligand influence the enantiomeric form, these complexes exhibited the ability to control the tacticity of the growing polymer chains and high isotactic (P m = 90%) polymer was obtained using complexes 30 as initiator ( Figure 12). [73] Jones synthesized a novel family of salalen aluminum complexes 34-40, these catalysts showed active for Jacobsen ligand complexes 9 (R,R) and complexes 10(rac) Figure 6. Preparation of complexes 9 (R,R) and -complexes 10(rac) from the Jacobsen ligand.…”

Dimethyl-Aluminium Complexes Bearing Naphthyl-Substituted Pyridine-Alkylamides as Pro-Initiators for the Efficient ROP of ε-Caprolactone

“…As the catalytic properties of the aluminum species employed are greatly influenced by the ancillary ligand (both electronically and sterically), a wide variety of multidentate ligand architectures have been developed and investigated in this field. While early reports focused on dianionic tetradentate ligands [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20], more recent studies have highlighted the considerable potential of using tridentate [21][22][23][24][25][26][27][28][29] and bidentate [20,[30][31][32][33][34][35][36][37][38][39][40][41][42] ligand sets as monoanionic ancillaries for potent and well-behaved AlR2-based (R = alkyl) pro-initiators.…”