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Yttrium– and Aluminum–Bis(phenolate)pyridine Complexes: Catalysts and Model Compounds of the Intermediates for the Stereoselective Ring-Opening Polymerization of Racemic Lactide and β‑Butyrolactone
journal contribution
posted on 2014-01-13, 00:00 authored by Joice
S. Klitzke, Thierry Roisnel, Evgeny Kirillov, Osvaldo de L. Casagrande, Jean-François CarpentierYttrium
and aluminum complexes of an original pyridine-bis(phenolate)
ligand have been prepared. Reactions of {ONOMe,Cumyl}Y[N(SiHMe2)2](THF)(Et2O) (1) with 1 equiv of methyl (R)-3-hydroxybutyrate and
methyl (S,S)-lactyllactate afforded respectively
{ONOMe,Cumyl}Y((R)-OCH(CH3)CH2COOMe) (2) and {ONOMe,Cumyl}Y((S,S)-OCH(CH3)CO2CH(CH3)CO2Me) (3), which are rare
models of the active species in the ring-opening polymerization (ROP)
of β-butyrolactone (BBL) and lactide (LA), respectively. 13C NMR data for 2 and 3 indicate
that, in solution, the carbonyl groups coordinate onto the yttrium
centers, forming mononuclear species with six- and five-membered cycles,
respectively. The aluminum compounds {ONOMe,Cumyl}Al(iPr (S)-lactate) (5), {ONOMe,Cumyl}Al((R)-OCH(CH3)CH2COOCH3) (6), and {ONOMe,Cumyl}Al((rac)-OCH(CF3)CH2CO2C2H5) (7) were prepared
analogously from the parent methyl complex {ONOMe,Cumyl}AlMe (4). NMR data and the solid-state structure of 6 confirm the coordination of the carbonyl group. Yttrium
compounds 1 and 2 are active initiators
for the ROP of racemic LA and BBL. Their performances (activity, control
of the molecular weights, tacticity) are much affected by the nature
of the solvent or by the addition of just a few equivalents of pyridine.
Optimal conditions are quite contrasted for the ROP of rac-LA and rac-BBL, highlighting fundamental differences
between these two monomers. In the best cases, highly heterotactic
PLAs (Pr up to 0.96) and syndiotactic-enriched
PHB (Pr up to 0.86), with good control
over the molecular weights, were obtained.