Active Ga-catalysts for the ring opening homo- and copolymerization of cyclic esters, and copolymerization of epoxide and anhydrides

Abstract

Ring opening homo- and copolymerization of lactide (LA) and ε-caprolactone (ε-CL) in solution with several Ga catalysts yielded isotactic-enriched PLAs and well defined diblock copolymers by coordination insertion mechanism (CIM).

Synthesis and Catalytic Activity of Gallium Schiff‐base Complexes in the Ring‐Opening Homo‐ and Copolymerization of Cyclic Esters

“…Each catalyst polymerizes ϵ ‐CL faster (turn over frequencies (TOF) 1090 h −1 1 , 570 h −1 2 , 780 h −1 3 ) than rac ‐LA (TOF 780 h −1 1 , 450 h −1 2 , 570 h −1 3 ) and the catalytic activity in both homopolymerization reactions follow the order 1 > 3 > 2 . The results show that the presence of electron donating t ‐Bu groups (+ I effect) at the ortho and para positions of the imino(phenolate) substituent increases the catalytic activity, whereas electron withdrawing Cl substituents (− I effect) diminish the catalytic activity, confirming the trends recently reported for analogous Me‐substituted complexes L 1–3 2 Ga 4 (Me) 8 [21] and others [22,25,26] . More importantly, the t ‐Bu‐substituted complexes L 1–3 2 Ga 4 ( t ‐Bu) 8 1 – 3 are catalytically more active than the corresponding Me‐substituted derivatives L 1–3 2 Ga 4 Me 8 , [21] most likely due to the higher basicity of t ‐butyl compared to the methyl substituent, which enforces the protonation reaction with BnOH and faster formation of the active (BnO‐substituted) catalyst.…”

“…The gallium atoms each adopt distorted tetrahedral coordination spheres. The central bond lengths and angles within the ligands and the Ga 2 O 2 ring match well with those of L 1–3 2 Ga 4 Me 8 [21] . The diffusion coefficients of complexes 1 (4.53 ⋅ 10–10 m 2 s −1 ), 2 (4.37 ⋅ 10–10 m 2 s −1 ), and 3 (4.14 ⋅ 10–10 m 2 s −1 ) were determined by diffusion‐ordered NMR spectroscopy (DOSY, Figures S10–S12) in non‐coordinating solvents (CDCl 3 , C 6 D 6 ), and the hydrodynamic radii ( R H ) in solution calculated using the Stokes‐Einstein equation (8.89 Å 1 , 9.22 Å 2 , 9.73 Å 3 ) [24] .…”

“…Complex 1 (Figure 1) crystallizes in the monoclinic space group C 2/ c with four molecules in the unit cell, complex 2 (Figure 2) in the orthorhombic space group Pbca with eight molecules in the unit cell and complex 3 (Figure 3 ) in the triclinic space group P ‐1 with two molecules in the unit cell (for crystallographic details see Table S1). 1 – 3 form tetranuclear complexes in the solid state and each contain a central four‐membered Ga 2 O 2 ring as was previously observed for the corresponding Me‐substituted complexes L 1–3 2 Ga 4 Me 8 [21] . The Schiff‐base ligands L 1–3 furthermore coordinate to a second gallium atom in a chelating N,O binding mode.…”

“…Unfortunately, the number of catalytically active gallium complexes is still low. We recently reported on tetranuclear gallium imino(phenolate) complexes L 1–3 2 Ga 4 Me 8 (Scheme 1, type A ) which showed good activities and selectivities in the ROP and ROCOP of cyclic esters and the copolymerization of epoxide and anhydrides [21] . The corresponding tetranuclear Mg(II) and Zn(II) complexes (Scheme 1, type B ) also showed high catalytic activities in the ROP of rac ‐LA and ϵ ‐CL even under solvent free conditions with low loading of catalyst.…”

Recent Developments in Ring-Opening Copolymerization of Epoxides With CO2 and Cyclic Anhydrides for Biomedical Applications

“…Among them, thiol-ene click chemistry is the most widely used method (Geschwind et al, 2013;Ntoukam et al, 2020). Besides, the formation of block copolymers or graft copolymers, bearing distinctive block properties (Ghosh et al, 2020;Gregory et al, 2020), could also be a facile approach toward functional materials, such as self-assembled nanomaterials. This section provides an overview of post-polymerization modification for functionalized polycarbonates and polyesters obtained by ROCOP in recent years.…”