Highly Iso-Selective and Active Catalysts of Sodium and Potassium Monophenoxides Capped by a Crown Ether for the Ring-Opening Polymerization of rac-Lactide

Abstract

Sodium and potassium complexes supported by a bulky monophenoxy with one xanthenyl group at the orthoposition and 18-crown-6 or 15-crown-5 as an auxiliary ligand were synthesized and characterized. These complexes are highly iso-selective and active catalysts for the controlled ringopening polymerization of rac-lactide. The best isotacticity (P m ) achieved was 0.86, which is the highest iso-selectivity reported to date for an alkali-metal complex. In addition, the corresponding polymer exhibited a high T m of 182°C. Furthermore, the polymerization looks like an anti-Arrhenius reaction, which is slower at high temperatures than at low temperatures. ■ INTRODUCTIONPolylactide (PLA) derived from a renewable resource has gained considerable attention for its nontoxicity, biodegradability and biocompatibility and has been used in a wide range of applications in the medical, food, packaging, and agricultural fields as a replacement for oil-based materials. 1 The ringopening polymerization (ROP) of lactides is an efficient method for synthesizing polylactide because of its advantages of a well-controlled molecular weight and low polydispersity (PDI). 2 The physical and chemical properties of polylactide are highly dependent on its stereochemistry, 3 hence, the stereocontrolled ROP of rac-lactide is currently a valuable and challenging research goal that can afford PLA with various stereo microstructures. Toward this end, numerous initiators, including metal complexes of aluminum, 4 zinc, 5 lanthanides, 4d,6 indium, 7 and magnesium, 5d,8 have been reported to exhibit good selectivities. 2,9 Compared to diverse heterotactic-selective systems, highly iso-selective initiators have rarely been reported. Among these initiators, only aluminum, indium, lanthanides, and zinc complexes have been extensively investigated because of their outstanding isotactic selectivities. No highly isoselective sodium or potassium metal complexes have been reported in this area until now. Sodium and potassium are innocuous, abundant elements in the human body and are suitable for the catalytic synthesis of polylactides for use in medical-related fields, 10 thus exploring sodium or potassium complexes as catalysts for ring-opening polymerization of lactide is very valuable.Recent studies have shown that bulky ligand-supported sodium and potassium complexes can catalyze the ROP of lactide in a controlled manner with high activities. 11 Usually, the selectivities of alkali-metal complexes are lower and rarely reported, for example, some simple sodium aryloxides reported by Davidson group can iso-selectively catalyze the ROP of raclactide giving a modest isotactic-enriched PLA (P m = 0.63), 12 Kasperczyk reported lithium tert-butoxide can initiate the ROP of rac-lactide affording a heterotactic-enriched PLA (P r = 0.76). 13 The lower selectivity may result from the weak interaction between the incoming lactide and the active end of the polylactide or ligand in alkali-metal complex catalytic system, despite the electronic modulation of the ligand. To...

Isoselective Ring‐Opening Polymerization of rac‐Lactide Catalyzed by Sodium/potassium Tetradentate Aminobisphenolate Ion‐paired Complexes

“…To date, many good metal complexes have been used for the stereoselective ROP of rac-lactide, such as aluminium, [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] indium, [23][24][25][26][27] zinc, [28][29][30][31][32][33] and other metal complexes. [34][35][36][37] In the past few years, we firstly reported some highly isoselective sodium/potassium complexes for the ROP of rac-lactide, [38][39][40][41][42] which is interestingb ecause sodium/p otassium metal ions are nontoxic and suitable for the synthesis of medical-related polylactide.N ormally the low Lewis acidity of sodium/potassium ion will cause the interactions betweens ubstrates be weak and the stereoselectivities of organic or polymerization reactions consequently are not very high;t he charm of coordinationc hemistry is that auxiliary ligands can construct ac onfined environmenta rounda ctives odium/p otassium ion centre and can synergistically improve the interactions betweenl igand, monomer,a nd polymer chains, then the stereoselectivities of sodium/potassium complexes can increase.I np revioush ighly isoselective sodium/potassium complex systems, the auxiliary ligands mainly focusedo nc rown ethers and one chargedb ulky phenol, andt he good isoselectivity even can reachtoP m = 0.94 at À70 8C. The crown ether ligands seem to be indispensable in previouss odium/potassium systemsf or high isoselectivities because of the confined space between crown ether and bulky substituted groups of phenol ligand.H owever,a...…”

Ring-opening polymerization of rac-lactide catalyzed by crown ether complexes of sodium and potassium iminophenoxides

“…The K1-C7 distance of 2.623(3)Å is within a normal range compared with the reported crown ether potassium phenoxide complexes. [94][95][96][97][98][99][100] The O(7)-C(15) distance of 1.289(5)Å is comparable with those in crown ether potassium complex bearing 2-(anthracen-9-yl) phenolate ligands.…”

“…93 Recently Wu and co-workers reported that crown ether complexes of sodium and potassium mono-phenoxides exhibited high catalytic activity and high iso-selectivity for the ROP of rac-lactide. [94][95][96][97][98][99][100][101] However, the phenoxide ligands are required to have very bulky ortho-substituents on the aromatic rings to achieve good selectivity. [94][95][96][97][98][99][100][101] It is interesting to design simpler ligands to get high catalytic selectivity of complexes.…”

“…A plausible reason could be the presence of transesterication side reaction in the process of polymerization. [94][95][96][97][98][99][100][101] The relatively wide molecular weight distributions (1.18 to 1.39) and MALDI-TOF MS analysis ( The sodium and potassium complexes 1 and 4 were selected for a more detailed study on their catalysis. At rst, higher monomer loadings were tested using 1/BnOH and 4/BnOH, respectively (Table 1, entries 8-11 and 14-17).…”

Zinc complexes supported by (benzimidazolyl)pyridine alcohol ligands as highly efficient initiators for ring-opening polymerization of ε-caprolactone

“…The solution became very sticky quickly as the increase of [CL] 0 /[Zn] 0 ratio so that the magnetic stir bar was stuck. (6) 116.49 (5) The polymerization rate was so fast that the influence of substituent R at the benzimidazolyl-N atom on the activity was difficult to be detected; however, the different R produced PCLs with different number-average molecular weights (M n ) and PDIs even if the same reaction parameters were used. The catalytic systems kept highly active in a broad temperature range over 0e90 C; whereas, the higher reaction temperature resulted in lower M n and broader PDI of PCLs (entries 2, 5 and 6 in Table 2).…”