Electron rich salen-AlCl catalysts as efficient initiators for the ring-opening polymerisation of rac-lactide

Abstract

The ring-opening polymerisation of lactide is a useful means to prepare biodegradable materials with well controlled polymer architectures and bespoke material properties. While homogeneous ligand-Al-OR complexes have shown great success in this field, initiation from ligand-AlCl complexes has lagged behind. Here, we report four new salen-AlCl complexes featuring NEt2-substituents, which display high catalyst activities towards rac-lactide ring-opening polymerisation (kobs < 1.9 x 10 -3 s -1 ) via in situ activation with a single equivalent of propylene oxide. Incorporating Lewis basic NEt2 groups into the ligand scaffold not only improves the initiation efficiency but also avoids the need for a Lewis basic co-catalyst and excess epoxide. Notably, studies of our amino-substituted catalysts reveal that the formation of a hexacoordinate aluminate species may hinder rather than enhance catalytic activity.

Lithium Half‐Salen Complexes: Synthesis, Structural Characterization and Studies as Catalysts for rac‐Lactide Ring‐Opening Polymerization

“…Different classes of catalysts have shown success, yet the industrial use of organocatalysts is currently limited by potential toxicity and economic concerns, [6,7] while enzymatic catalysts often give low yields and require long polymerization times with high catalyst loadings [8,9] . Organometallic catalysts are the most widely used, and efficient systems have been developed including those based on Al, [10–13] Zn, [14–16] or lanthanides [17–19] …”

“…Different classes of catalysts have shown success, yet the industrial use of organocatalysts is currently limited by potential toxicity and economic concerns, [6,7] while enzymatic catalysts often give low yields and require long polymerization times with high catalyst loadings. [8,9] Organometallic catalysts are the most widely used, and efficient systems have been developed including those based on Al, [10][11][12][13] Zn, [14][15][16] or lanthanides. [17][18][19] Most organometallic catalysts are proposed to operate through a coordination-insertion mechanism (Scheme 1, left), where LA coordination to a Lewis acidic metal center activates the monomer towards nucleophilic attack from a metal-alkoxide group (sometimes formed through in situ alcoholysis of a metal-alkyl precursor), resulting in LA ring-opening and enchainment.…”

Towards a selective synthetic route for cobalt amino acid complexes and their application in ring opening polymerization of rac-lactide

“…Based on previous literature reports, it is also plausible that two competing mechanisms are active, promoted from two different initiating groups. [73][74][75] Extended reaction times without incorporation of BnOH resulted in observed broadened polydispersities (Đ ¼ 1.13-1.40, Table 2, entries 4-6), suggesting that chain transfer and intermolecular transesterication reactions may predominate at high conversion >60%, which could be ascribable to the presence of distinct amounts of water in commercial grade rac-lactide and the crystal water in the lattice of the complexes accelerating trans-esterication reaction. Due to the absence of initiator groups or vacant active site in the metal center a mechanistic hypothesis can be proposed to rationalize the formation of PLA as shown in To gather further information on the polymer microstructure, tacticity of the polymers was assessed by relative integration of methine resonances in the homonuclear decoupled 1 H { 1 H} NMR spectrum (Fig.…”

Synthesis and characterization of Ti(IV), Zr(IV) and Al(III) salen-based complexes

“…Moreover, the incorporation of chiral centers within the salen backbone reveals an important role in enantioselective catalysis [9][10][11][12]. Most of the catalytic studies have been focused on middle and late d-block transition metal-salen complexes as catalysts for olefin epoxidation [13][14], cyclopropanation [15][16][17], aziridination [18][19][20], sulfoxidation [21][22][23], ring-opening polymerization of cyclic esters [24][25][26][27][28][29] and copolymerization of CO2 and epoxides [30][31][32]. Early transition metal complexes supported by salen ligands are relatively less reported [33][34][35][36][37].…”

“…Nevertheless, vanadium derivatives have been widely explored for diverse applications [38][39][40][41]. Despite a large variety of mononuclear aluminum complexes supported by salen-type ligands have been reported mainly as catalysts for the ringopening polymerization of cyclic esters [26][27][28], bimetallic derivatives have not been well studied [42][43][44]. In this work, we present the synthesis and structural characterization of new Ti(IV), Zr(IV) and Al(III) complexes based on meso-(R,S)diphenylethylene-salen ligand.…”