Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 22, Issue 22, Pages 7629-7641Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.201600223
Keywords
density functional calculations; lactones; polymers; ring-opening polymerization; yttrium
Categories
Funding
- CNRS
- Region Bretagne
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Yttrium [ amino-alkoxy-bis(phenolate)] amido complexes have been used for the ring-opening polymerization (ROP) of racemic alkyl beta-malolactonates (4-alkoxycarbonyl-2-oxetanones, rac-MLA(R)s) bearing an allyl (All), benzyl (Bz) or methyl (Me) lateral ester function. The nature of the orthosubstituent on the phenolate rings in the metal ancillary dictated the stereocontrol of the ROP, and consequently the syndiotactic enrichment of the resulting polyesters. ROP promoted by catalysts with halogen (Cl, Br)-disubstituted ligands allowed the first reported synthesis of highly syndiotactic PMLA(R)s (Pr >= 0.95); conversely, catalysts bearing bulky alkyl and aryl ortho-substituted ligands proved largely inef-fective. All polymers have been characterized by 1H and C-13{H-1} NMR spectroscopy, MALDI-ToF mass spectrometry and DSC analyses. Statistical and thermal analyses enabled the rationalization of the chain-end control mechanism. Whereas the stereocontrol of the polymerization obeyed a Markov first-order (Mk1) model for the ROP of rac-MLA(Bz) and rac-MLA(All), the ROP of rac-MLA(Me) led to a chain end-control of Markov second-order type (Mk2). DFT computations suggest that the high stereocontrol ability featured by catalysts bearing Cl-and Br-substituted ligands does not likely originate from halogen bonding between the halogen substituent and the growing polyester chain.
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