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Highly Stereocontrolled Ring‐Opening Polymerization of Racemic Alkyl β‐Malolactonates Mediated by Yttrium [Amino‐alkoxy‐bis(phenolate)] Complexes
Author(s) -
Jaffredo Cédric G.,
Chapurina Yulia,
Kirillov Evgueni,
Carpentier JeanFrançois,
Guillaume Sophie M.
Publication year - 2016
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201600223
Subject(s) - substituent , chemistry , alkoxy group , alkyl , polymerization , ring opening polymerization , polymer chemistry , medicinal chemistry , stereochemistry , polymer , organic chemistry
Yttrium [amino‐alkoxy‐bis(phenolate)]amido complexes have been used for the ring‐opening polymerization (ROP) of racemic alkyl β‐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 ortho ‐substituent 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 ( P r ≥ 0.95); conversely, catalysts bearing bulky alkyl and aryl ortho ‐substituted ligands proved largely ineffective. All polymers have been characterized by 1 H and 13 C{ 1 H} 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.