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.
Aluminum alkoxide complexes supported by salen ligands [salen = N, N'-bis(salicylaldimine)-2-methylpropane-1,2-diamine or N, N'-bis(salicylaldimine)-2,2-dimethylpropane-1,3-diamine] with o-adamantyl substituents have been synthesized and investigated for the polymerization of ε-caprolactone. Geometric analysis of the catalysts used for the reaction reveals the metal coordination geometries to be intermediate between square-pyramidal and trigonal-bipyramidal. A detailed kinetic study accompanied by density functional theory modeling of key mechanistic steps of the reaction suggest that, in addition to the length of the backbone linker, the o-aryl substituents have a significant impact on the catalyst's reactivity. Bulky ortho substituents favorably distort the precatalyst geometry and thereby foster the achievement of the rate-limiting transition-state geometry at low energetic cost, thus accelerating the reaction.
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