Mechanistic Studies of ε-Caprolactone Polymerization by (salen)AlOR Complexes and a Predictive Model for Cyclic Ester Polymerizations

Abstract

Aluminum alkoxide complexes (2) of salen ligands with a three-carbon linker and para substituents having variable electron-withdrawing capabilities (X = NO2, Br, OMe) were prepared, and the kinetics of their ring-opening polymerization (ROP) of ε-caprolactone (CL) were investigated as a function of temperature, with the aim of drawing comparisons to similar systems with two-carbon linkers investigated previously (1). While 1 and 2 exhibit saturation kinetics and similar dependences of their ROP rates on substituents X (invariant Keq, similar Hammett ρ = +1.4(1) and 1.2(1) for k2, respectively), ROP by 2 was significantly faster than for 1. Theoretical calculations confirm that, while the reactant structures differ, the transition state geometries are quite similar, and by analyzing the energetics of the involved distortions accompanying the structural changes, a significant contribution to the basis for the rate differences was identified. Using this knowledge, a simplified computational method for evaluating ligand structural influences on cyclic ester ROP rates is proposed that may have utility for future catalyst design.

Zn(II) and Cu(II) unsymmetrical formamidine complexes as effective initiators for ring‐opening polymerization of cyclic esters

“…However, the data for Cu(II) complexes deviated from fitted first order kinetics plot over the entire reaction time pointing to a pseudo first order kinetics behaviour. This trend can also be rationalized by the higher activation energy required to generate the active species presumably due to dissimilar transition state structure geometry around the metal centres as suggested by Marlier et al . The apparent rate constant ( k app ) for each complex was obtained from the slope of ln([M] 0 /[M] t ) vs t , and are summarized in Table .…”

Preparation of single-site tin(IV) compounds and their use in the polymerization of ε-caprolactone

“…These data suggest that first ε-caprolactone attacks to tin(Sn) center and then the nucleophile Cl − ion attacks C=O carbon atom in the ε-caprolactone unit as seen in Scheme 2 . [ 27 ] Then, lactone exocyclic oxygen coordinates to the tin atom. Propagation step involves successive ring-opening by an anionic coordination–insertion mechanism.…”

Exploiting Noncovalent Interactions for Room-Temperature Heteroselective rac-Lactide Polymerization Using Aluminum Catalysts

“…initiation (first monomer insertion) and propagation (second monomer insertion). 15,26,29,[41][42][43][44][45] Initiation mechanism and non-covalent interactions (NCI). During the initiation step, it was found that the L-lactide can displace coordinated molecules of THF (∆∆G = -1.5 kcal/mol, Figure S40-41) 46 and docks on the top of the catalyst due to favourable NCIs with the ligand and the metal centre, including a hydrogen bond between one oxygen atom of the lactide carbonyl and one NH bond of the ligand as highlighted in the NCI surfaces in Figure 6 (bright blue dot).…”