Stereoselective Ring-Opening Polymerization of Racemic Lactide Using Aluminum-Achiral Ligand Complexes:  Exploration of a Chain-End Control Mechanism

Abstract

Stereoselective polymerization of racemic lactide (rac-LA) was examined using Al-achiral ligand complexes. By introduction of substituents in aromatic rings of Schiff base ligands, a higher selectivity was obtained without any chiral auxiliaries in the catalyst via a chain-end control mechanism. The T(m) values (T(m) 170-192 degrees C) were comparable to or higher than that of homochiral polymer, poly(L-LA) (T(m) 162 degrees C), and a thermally more stable polylactide than poly(L-LA) was prepared from rac-LA.

Study of ligand substituent effects on the rate and stereoselectivity of lactide polymerization using aluminum salen-type initiators

“…High levels of transesterification are particularly pronounced for the smaller salen ligands, such as 1 and 2. However, changing the backbone structure from an ethylene to a propylene linker typically leads to a marked reduction in the degree of transesterification, an effect also noted by Nomura et al (6).…”

“…catalysis ͉ polyesters I n recent years, Al(salen) complexes [where salen is N,NЈ-bis(salicylaldimine)-1,2-ethylenediamine] have been widely investigated for their ability to initiate the stereocontrolled polymerization of lactide (LA) (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17) to give a material, polylactide (PLA), which has a range of biomedical, pharmaceutical, and agricultural applications (18)(19)(20)(21). A convenient synthetic route to PLA is the ring-opening polymerization (ROP) of LA, the cyclic diester of lactic acid, which is derivable from renewable resources such as corn starch, sugars, and dairy produce.…”

“…Further tacticity enhancements have been reported for the dimethyl-substituted derivative XI (7). The mechanism of stereoselective polymerization using these achiral ligand-based complexes is postulated to operate by means of a chain end control process (6), whereas an enantiomorphic-site control mechanism is proposed for the chiral salen-based complexes (10). However, recent studies have highlighted some complexities in ascribing the mechanism to chain-end control or enantiomorphic-site control (16).…”

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

“…4 The stereoselective ring-opening polymerization (ROP) of lactide represents an attractive method for the control of microstructure of PLAs, and hence, the stereoselective ROP of lactide has become a key focus of attention. [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.…”

An Overview of the Synthesis and Synthetic Mechanism of Poly (Lactic acid)

“…Many nontoxic catalysts derived from magnesium, calcium, zinc, alkali metals, and aluminum have been developed for the ROP of lactides to solve pollution problems caused by heavy metal catalysts [33][34][35][36][37][38].…”