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.
Bibliographical noteFunding Information:
Funding for this project was provided by the Center for Sustainable Polymers, a National Science Foundation supported Center for Chemical Innovation (CHE-1413862). The X-ray diffraction experiments were performed using a crystal diffractometer acquired through NSF-MRI Award CHE- 1229400. The NMR experiments were performed on Bruker Avance III 500 MHz spectrometers acquired through NIH Award S10OD011952. We thank Dr. Letitia Yao for her help with NMR kinetics experiments.
© 2016 American Chemical Society.
- ligand effects
- ring-opening polymerization
- saturation kinetics