Sterically Induced Ligand Framework Distortion Effects on Catalytic Cyclic Ester Polymerizations

Abstract

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.

Dinuclear Ce(IV) Aryloxides: Highly Active Catalysts for Anhydride/Epoxide Ring-Opening Copolymerization

“…The adamantyl groups could be providing a hydrocarbon pocket that provides additional dispersion forces to facilitate monomer binding/orientation or could be distorting the metal complex geometry and destabilizing the initiating group. A significant distortion from an octahedral geometry is visible in siloxide 1*- p -Ad , and a similar distortion accorded by an adamantyl group was suggested to be responsible for the increased lactone polymerization rate in an Al III catalyst elsewhere . These two factors (substituent volume and dispersion forces) and the fact that the bulky adamantyl group increases the rate highlight the sometimes complex, counterintuitive, and ambiguous nature of steric effects upon catalytic systems.…”

“…A significant distortion from an octahedral geometry is visible in siloxide 1*-p-Ad, and a similar distortion accorded by an adamantyl group was suggested to be responsible for the increased lactone polymerization rate in an Al III catalyst elsewhere. 66 These two factors (substituent volume and dispersion forces) and taking note that the bulky adamantyl group increases the rate, highlights the sometimes complex, counter-intuitive and ambiguous nature of steric effects upon catalytic systems.…”

Effective Regulation of Polycaprolactone Molecular Weight and Oligomers Content Using Tetraphenyltin Catalyst

“…4, which follows the "coordination-insertion" pathway that was reported in several previous studies for the ringopening transesterification polymerization (ROTEP) of cyclic ester catalyzed by other singlesite metal alkoxide complexes. [37][38][39][40][41][42] The best catalyst used in this study is tetraphenyltin. The calculations show that the direct insertion of the phenyl group of tetraphenyltin into the coordinated ester monomer has to cross an extremely high activation barrier (45.4 kcal/mol, supporting information (SI)).…”

Mechanistic Insight into the Ring-Opening Polymerization of ε-Caprolactone and L-Lactide Using Ketiminate-Ligated Aluminum Catalysts

“…The ring-opening polymerization catalyzed by an organometallic complex usually proceeds through a coordination-insertion mechanism [31,32]. Recent works have revealed that coordination number [33,34,35], the size effect of the chelate rings [36], steric and electronic effects of the auxiliary ligands surrounding the aluminum complexes [22,25,34,37,38,39,40,41,42,43,44] as well as the co-initiators would alter the reactivity and the properties of the products. Besides the reaction conditions would be very dependent on the ring size and the bulkiness of the monomers [45,46].…”

“…To get more insight into the mechanism, we decided to employ density functional theory (DFT) calculation to compare the two ring-opening polymerization systems catalyzed by the Al complex bearing a ketiminate ligand. Although only few cases of tetradentate Al catalysts have been evaluated by DFT calculations [50,51,52], it has been proven to be a useful tool to model the reaction intermediates and comprehend the reaction pathways of the tris- and penta-coordinate Al complexes [29,42,44,52,53,54,55,56,57,58,59,60,61,62]. Our aims in this theoretical study are: (a) to realize why polymerization of ε -caprolactone and L -lactide require different reaction temperatures; (b) to understand how the steric and electronic effects on the pendant group of the Al complexes would alter the catalytic abilities.…”