Ring-opening polymerization of ε-caprolactone by lithium piperazinyl-aminephenolate complexes: synthesis, characterization and kinetic studies

Abstract

A series of lithium complexes were prepared from 2(N-piperazinyl-N′-methyl)-2-methylene-4-R′-6-Rphenols ([ONN] RR′ ) and characterized through elemental analysis, 1 H and 13 C{ 1 H} NMR spectroscopy, and X-ray crystallography. Treatment of the ligands with n-butyllithium afforded {Li [ONN] RR′ } 3 [R = Me, R′ = t Bu, (1); R = R′ = t Bu (2); R = R′ = t Am, (3), t Am = C(CH 3 ) 2 CH 2 CH 3 ], with trimetallic structures in the solid-state as shown by single-crystal X-ray diffraction. The reactivity of these complexes in the ring-opening polymerization of ε-caprolactone (ε-CL), as well as the influences of monomer concentration, monomer/Li molar ratio, polymerization temperature and time, was studied. Rates of polymerization were first order with respect to both monomer and lithium concentrations, and activation energies for the reactions were determined. MALDI-TOF MS analysis revealed that transesterification had occurred during the polymerization.

Aluminum Methyl and Chloro Complexes Bearing Monoanionic Aminephenolate Ligands: Synthesis, Characterization, and Use in Polymerizations

“…This probably indicates that polymerization using 1/BnOH is more controlled than that using 2/BnOH. This is somewhat surprising given previous ROP reactions studied using Li and Zn complexes of these ligands, 10,27 where narrower M w /M n polymers were obtained from complexes bearing more sterically demanding ligands. It was also observed that in all cases using 1-3, the measured number average molecular weight of the polymers obtained via GPC were lower than the calculated values based on initial [Al]:[ε-CL] ratios.…”

“…Some complexes of these ligands (L1-L3) have been reported by our group and others, and will allow us to compare reactivity where data are available from these prior studies. 10,27,28,31,32,39 Compounds 1-5 were synthesized through alkane elimination reactions using two equivalents of the appropriate protio ligands with the corresponding alkyl/halide aluminum precursors (AlMe 3 or Et 2 AlCl) in dry toluene as summarized in Scheme 27 Al NMR spectroscopy. The 1 H NMR spectra of 1-3 confirmed that the complexes were monometallic with two coordinated aminephenolate ligands.…”

“…Lithium complexes (I) of the same ligand showed good activity in catalytic ROP of ε-caprolactone. 10 Similarly, Carpentier, Sarazin and co-workers employed zinc complexes of a morpholinyl derived ligand (III) to achieve immortal ROP of cyclic esters. 28 The same group has investigated the reactivity of tin complexes of these ligands in immortal ROP of lactide and trimethylene carbonate.…”

“…Some examples of previously reported piperazinyl and morpholinyl aminephenolate based complexes (R = alkyl or aryl). 10,27,28,39,40 Recently, we reported the synthesis of piperazinyl-aminephenolate zinc complexes (II), which showed good activity in ROP of rac-lactide and ε-caprolactone. 27 These zinc complexes, however, showed poor activity in the cycloaddition of carbon dioxide and epoxides.…”

“…Thus, amine-phenolate and related ligands possessing a mixed set of N-and O-donor atoms have emerged as attractive candidates due to their ability to stabilize a wide range of metal centers and the ease of systematic steric manipulation by variation of the backbone and phenol substituents. 9 A number of main-group and transition metal complexes including lithium, [10][11][12][13][14][15][16] magnesium, [17][18][19][20][21][22][23] larger alkaline earths, 24 rare-earths, 25,26 zinc, [27][28][29][30][31][32][33][34][35][36] aluminum, [37][38][39][40][41][42][43][44] , tin, 45 zirconium 46,47 and titanium 48 complexes stabilized by these and related ligands have been prepared and some are effective initiators for ROP of cyclic esters such as lactide and ε-caprolactone (e.g., I-IV, Figure 1). Among the catalysts/initiators studied, aluminum complexes with N,O-chelate and related ligands have attracted much attention due to their high activity and good polymer molecular weight control.…”