Living ring-opening homo- and copolymerisation of ε-caprolactone andl-lactide by cyclic β-ketiminato aluminium complexes

Abstract

A series of novel aluminium complexes containing cyclic β-ketiminato ligands of type Me2Al{O-[(ArN=CHC4H4(C6H4))]} (3a, Ar = 2,6-(i)Pr2C6H3; 3b, Ar = C6H5; 3c, Ar = C6F5) have been prepared in high yields. These complexes were identified by (1)H, (13)C NMR spectroscopy and elemental analysis. X-ray structural analyses for 3a-c revealed that these complexes have a distorted tetrahedral geometry around Al, and both bond distances and bond angles were considerably influenced by the ligand structure. These complexes were tested as catalyst precursors for ring-opening polymerisation of ε-caprolactone (ε-CL) and L-lactide (L-LA) in the presence of 2-propanol as an initiator. Complex 3a could polymerize ε-CL in a controlled manner with high efficiency. Based on the living characteristics, the preparation of well-defined block copolymers PCL-b-PLLA via sequential addition of monomers was performed by 3a. Note that complex 3c exhibited rather high catalytic activity for the ROP of L-LA with narrow molecular weight distribution. The monomer conversion reached completion only in 4 h when the L-LA/Al molar ratio was 100 at 80 °C. PLLA-b-PCL copolymers were thus easily produced by 3c.

Ring-opening polymerization of rac-lactide using anilinotropone-based aluminum complexes-sidearm effect on the catalysis

“…The 1 H NMR patterns are consistent with a mirror symmetry passing through the tropone seven member ring, leading to equivalent AlMe 2 groups. [15][16][17][18][19][20]. For example, in complex A (Scheme 1), the distances are 1.759 Å and 1.975 Å, respectively [16].…”

“…It is well known that the catalytic performances of metal complexes are greatly affected by the steric and electronic properties of the coordinating ligands. There have been numerous reports on aluminum complexes bearing [N,O] type ligands (such as salicylaldiminato aluminum complex, Scheme 1a, A) [15][16][17][18][19][20], or the structurally similar tetradentate [O,N,N,O] ligands such as salen (Scheme 1a, B) [21], salan [22], and salalen ligands [23]. Almost all of these ligands feature a six-member aluminum chelate ring.…”

“…The observed rate constant of complex 3 is 1.3 h -1 , which is among the most active aluminum catalysts for the ROP of rac-LA. Previously, Li et al reported that a β-ketiminato aluminum complex exhibited the highest efficiency for the ROP of L-LA among the low coordinated Schiff-base aluminum complexes, which can achieve near completion in 4h at 80 o C and a [lactide]/[catalyst]/[ i PrOH] ratio of 100/1/1[15].…”

Organocatalyzed ring-opening copolymerization of α-bromo-γ-butyrolactone with ε-caprolactone for the synthesis of functional aliphatic polyesters – pre-polymers for graft copolymerization

“…A commonly used technique to prepare aliphatic polyesters is controlled ring-opening polymerization (CROP) which can construct well-defined polymers. However, up to now, many CROPs of εCL are catalyzed by metal-containing catalysts, such as stannous octoate [Sn(Oct) 2 ] [8,9], zinc-containing catalyst [10], iron-containing catalyst [11], magnesium-containing catalyst [12] and aluminum-containing catalyst [13], to name a few. It is attractive that the recent development of metal-free organocatalysts for CROP brings a new horizon of preparing aliphatic polyesters.…”

Synthesis of lactide/ɛ‐caprolactone quasi‐random copolymer by using rationally designed mononuclear aluminum complexes with modified β‐ketiminato ligand

“…Our group has dedicated to developing novel aluminum catalysts for LA/ ɛ ‐CL copolymerization, and many kinds of them, ranging from mononuclear to binuclear catalysts, showed great success in synthesizing block and gradient copolyesters. For example, a mononuclear aluminum complexes containing N , O ‐bidentate β‐ketiminato ligands (M, Scheme ) could catalyze copolymerization of CL and LA in a controlled manner . However, its less crowded space did not suppress the coordination of LA to Al center enough, only yielding gradient structure in the copolymerization of ɛ ‐CL/LA.…”