Mechanism of the Polymerization of rac-Lactide by Fast Zinc Alkoxide Catalysts

Abstract

The ring-opening transesterification polymerization (ROTEP) of rac-lactide (rac-LA) using LZn catalysts (L = ligand having phenolate, amine, and pyridine donors with variable para substituents X on the bound phenolate donor; X = NO, Br, t-Bu, OMe) was evaluated through kinetics experiments and density functional theory, with the aim of determining how electronic modulation of the ligand framework influences polymerization rate, selectivity, and control. After determination that zinc-ethyl precatalysts required 24 h of reaction with benzyl alcohol to convert to active alkoxide complexes, the subsequently formed species proved to be active and fairly selective, polymerizing up to 300 equiv of rac-LA in 6-10 min while yielding isotactic (P = 0.72-0.78) polylactide (PLA) with low dispersities: Đ = 1.06-1.17. In contrast to previous work with aluminum catalysts for which electronic effects of ligand substituents were significant (Hammett ρ = +1.2-1.4), the LZn systems exhibited much less of an effect (ρ = +0.3). Density functional calculations revealed details of the initiation and propagation steps, enabling insights into the high isotacticity and the insensitivity of the rate on the identity of X.

Preparation of multiblock copolymers via step-wise addition of l-lactide and trimethylene carbonate

“…To gain a better understanding of the mechanism, we turned to density functional theory. 66 , 67 All calculations were carried out with the Gaussian 09 program package 68 on the Extreme Science and Engineering Discovery Environment (XSEDE). 69 The methyl groups on the pyrazole substituents were replaced by hydrogen atoms and the phenyl groups on PPh 2 were replaced by methyl groups to simplify the calculations (for more details about calculations, see the ESI † ).…”

Mono- and dimeric zinc(ii) complexes for PLA production and degradation into methyl lactate – a chemical recycling method

“…In summary, all Zn(II)-initiators exhibited high catalyst activity in the polymerisation of rac-LA, consistent with previous examples in the literature. 41,42,45,46,[50][51][52][53][54][55][56][57][58][59][60][61][62][63] Polymerisation kinetics Generally, Zn 2 (1-3) 2 Et 2 outperformed their monomeric counterparts under both melt and solution conditions. A mono-vs. dimeric system kinetic study was envisaged but not possible due to the extremely high activity of the latter.…”

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

“…[1][2][3][4][5][6] Because the tacticityc an remarkably affect the physicalp roperties of polylactide, the stereoselective ring-opening polymerization (ROP) of rac-lactidei ss till ac urrent research hotspot. 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 ...…”

“…Because the tacticity can remarkably affect the physical properties of polylactide, the stereoselective ring‐opening polymerization (ROP) of rac ‐lactide is still a current research hotspot. To date, many good metal complexes have been used for the stereoselective ROP of rac ‐lactide, such as aluminium, indium, zinc, and other metal complexes . In the past few years, we firstly reported some highly isoselective sodium/potassium complexes for the ROP of rac ‐lactide, which is interesting because sodium/ potassium metal ions are nontoxic and suitable for the synthesis of medical‐related polylactide.…”