Stereoselective Polymerization of rac-Lactide Using a Monoethylaluminum Schiff Base Complex

Abstract

A monoethylaluminum Schiff base complex (2) with formula LAlEt (L = N,N'-(2,2-dimethylpropylene)bis(3,5-di-tert-butylsalicylideneimine) was synthesized and employed for the stereoselective ring-opening polymerization of rac-lactide (rac-LA). The complex 2 was characterized by nuclear magnetic resonance, crystal structure, and elemental analysis. It contains a five-coordinate aluminum atom with distorted trigonal bipyramidal geometry in the solid state. In the presence of 2-propanol, 2 showed high stereoselectivity for the polymerization of rac-LA. The polymerization yielded crystalline poly(rac-LA) with a high melting temperature (193-201 degrees C). NMR, differential scanning calorimetry, and wide-angle X-ray diffraction indicated that the poly(rac-LA) was highly isotactic, and a stereocomplex was formed between poly-l- and poly-d-lactide block sequences. By the analysis of electrospray-ionization mass spectrometry and (1)H NMR, the polymer was demonstrated to be endcapped in both terminals with an isopropyl ester and a hydroxy group, respectively. The polymerization was of first order in rac-LA concentration. The relationship between the rac-LA conversion and molecular weights of the polymer was linear so that the polymerization could be well controlled.

Five‐coordinated active species in the stereoselective polymerization of rac‐lactide using N,N′‐(2,2‐dimethyl‐1,3‐propylene) bis(3,5‐di‐tert‐butyl‐salicylideneimine) aluminum complexes

“…The 1 H NMR spectrum of LAlOCH(CH 3 )COOCH 3 displays dif- H NMR spectra. 20,21 Because the 1 H NMR spectroscopic data for LAlOCH(CH 3 )COOCH 3 are in good agreement with the single crystal structure, we tentatively propose that the five coordinated species in the solid state is retained in solution.…”

“…Similar to alkyl and alkoxide complexes LAlEt and LAlO i Pr, LAlOCH(CH 3 )COOCH 3 is also monomeric and contains a five-coordinated central Al atom in a distorted trigonal bipyramidal (TBP) geometry. The s value of LAlOCH(CH 3 )COOCH 3 (0.86) is significantly higher than those of complexes LAlEt (0.76) and LAlO i Pr (0.78), 20,21 which indicates that the Al center adopts a more TBP geometry than those of LAlEt and LAlO i Pr. 24 The atoms N2 and O1 occupy the axial sites and form an angle of 172.52(14)8, the N1, O2, and O3 atoms occupy the equatorial plane that form angles 121.65(14)8 (O2À ÀAlÀ ÀO3), 121.15(13)8 (N1À ÀAlÀ ÀO2), and 116.31(14)8 (O3À ÀAlÀ ÀN1).…”

“…[5][6][7][8][9][10][11][12][13][14][15][16][17][18] Some of the most significant advances in the field have been seen using Salen-group Al alkyls or alkoxides as catalysts, showing high molecular weight controllability and stereoselectivity for the ROP of racemic lactide (rac-LA). [10][11][12][13][14][15][16][17][18][19][20][21][22] The structures of active species play a key role for a stereoselective polymerization and may give vital information for the understanding of the origin of stereocontrol in this family of lactide polymerization initiating systems. In reference to the structure of active species from Al Salen catalysts (Scheme 1), Baker and coworkers implied that a five-coordinated species was involved when (SalBinap)AlO i Pr was used as a catalyst; however, no experimental evidence was provided.…”

“…20,21 Herein, we report our results on the synthesis, solid and solution structures of the active species in the polymerization of rac-LA, using LAlEt or LAlO i Pr. It was noteworthy that the real initiating species was LAlO i Pr in the case of LAlEt and 2-propanol initiating system.…”

“…20,21 Toluene was distilled from Na-benzophenone. Ethyl acetate was distilled from CaH 2 under the protection of N 2 .…”