{Phenoxy-imine}aluminum versus -indium Complexes for the Immortal ROP of Lactide: Different Stereocontrol, Different Mechanisms

A series
of dialkylaluminum and -indium {ONR}­MR′2 complexes (M = Al, R′ = Me; M = In, R′ = Me,
CH2SiMe3) stabilized by a phenoxy-imine {ONR}− ligand platform, with variable R-imino
substituents and functionalized by a bulky o-SiPh3 in the phenoxy moiety, has been prepared and structurally
characterized in solution and in the solid state. {ONR}­AlMe2 complexes reacted with alcohols, in particular with alkyl
(S)-H-lactate, to generate the corresponding {ONR}­Al­(OR)2 compounds. On the other hand, the indium
complexes {ONR}­InR′2 proved largely inert
toward alcohols. When they were combined with an alcohol (iPrOH, BnOH), the {ONR}­AlMe2 complexes
promoted the living (immortal) ring-opening polymerization
((i)­ROP) of racemic lactide (rac-LA) with a good control over the molecular weights and various microstructures,
dependent on the R-imino substituent. Complexes having benzyl-type
imino substituents enabled the achievement of significant isotacticity
(Pm up to 0.80), following grossly the
bulkiness of the aryl moiety. The analogous {ONR}­InR′2 proved similarly active for the (i)­ROP of rac-LA in presence of an external alcohol, but the polymerizations
were less controlled and none of the complexes induced stereoselectivity,
except one (3a, Pm = 0.70).
Kinetic studies revealed different rate laws, with an apparent zero-order
dependence on monomer for the aluminum system 1m/iPrOH and a first-order dependence on monomer for the analogous
indium system 3m/iPrOH. On the basis
of the stoichiometric reactivity of model compounds, two different
operative ROP mechanisms are suggested, depending on the nature of
the metal center: Al-based complexes proceed through coordination–insertion,
while In-based complexes are proposed to operate through an activated
monomer mechanism.