Stereoselective Alkali-Metal Catalysts for Highly Isotactic Poly(rac-lactide) Synthesis

Abstract

A high degree of chain end control in the isoselective ring-opening polymerization (ROP) of rac-lactide is a challenging research goal. In this work, eight highly active sodium and potassium phenolates as highly isoselective catalysts for the ROP of rac-lactide are reported. The best isoselectivity value of Pm = 0.94 is achieved. The isoselective mechanism is chain-end control through the analysis of the stereoerrors in the microstructure of a final polymer; thus, isotactic multiblock structure polymers are obtained, and the highest melt point can reach 192.5 °C. The donating group in phenolate can clearly accelerate the ROP reaction, potassium complexes are more active than the analogous sodium complexes, and the big spacial hindrance of the ligand can decrease the activity. The high isoselectivities of these complexes mostly result from their sandwich structure constructed by the plane of the crown and the plane of the anthryl group.

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...…”

Ligands and complexes based on piperidine and their exploitation of the ring opening polymerisation of rac-lactide

“…[23][24][25][26][27][28][29][30][31][32] Group I metals have shown to be highly active for the ROP and in some instances, Na(I) and K(I) complexes have demonstrated strong isotactic tendencies. [33][34][35][36][37][38] Lanthanide based complexes have also received much attention, demonstrating high activities with both heterotactic and isotactic PLA being reported. [39][40][41][42][43][44][45][46][47][48] Relevant to this study is the application of group II, Zn(II) and group IV metal complexes for the ROP of rac-LA.…”

Tetradentate iminophenolate copper complexes in rac-lactide polymerization

“…show the same general polymerization behaviour as 1-5 ( Fig. 9, Tables 2 and S1, entries [42][43][44]. Coordination/dissociation of the 3 rd amino group is thus not responsible for the existence of two active species.…”

“…In a series of excellent publications, Wu and coworkers employed this concept in lactide polymerizations with phenolate complexes of group 1 metals, mostly sodium and potassium. 36, [39][40][41][42][43][44] They could show that phenolate is not incorporated in the polymer chain and acts (mostly) as spectator ligand. The bulky environment (crown-ether coordination around the metal center, highly substituted phenolates) constricts the environment sufficiently to allow stereocontrol, remarkably toward isotactic PLA.…”