RESUMEN
Herein we report an efficient strategy for preparing isotactic polyesters and chiral epoxides via enantioselective resolution copolymerization of racemic terminal epoxides with anhydrides, mediated by enantiopure bimetallic complexes in conjunction with a nucleophilic cocatalyst. The chirality of both the axial linker and the diamine backbones of the ligand are responsible for the chiral induction of this kinetic resolution copolymerization process. The catalyst systems exhibit exceptional levels of enantioselectivity with a kinetic resolution coefficient exceeding 300 for various racemic epoxides, affording highly isotactic copolymers (selectivity factors of more than 300) with a completely alternating structure and low polydispersity index. Most of the produced isotactic polyesters are typical semicrystalline materials with melting temperatures in the range from 77 to 160 °C.
RESUMEN
We report a comprehensive understanding of the stereoselective interaction between two opposite enantiomeric polyesters prepared from the regioselective copolymerization of chiral terminal epoxides and cyclic anhydrides. For many of the resultant polyesters, the interactions between polymer chains of opposite chirality are stronger than those of polymer chains with the same chirality, resulting in the formation of a stereocomplex with an enhanced melting point (Tm) and crystallinity. The backbone, tacticity, steric hindrance of the pendant group, and molecular weight of the polyesters have significant effects on stereocomplex formation. Bulky substituent groups favor stereocomplexation, resulting in a greater rise in Tm in comparison to the component enantiomeric polymers. The stereocomplex assembly of discrete (R)- and (S)-poly(phenyl glycidyl ether-alt-phthalic anhydride)s oligomers revealed that the minimum degree of polymerization required for stereocomplex formation is five. Raman spectroscopy and solid-state NMR studies indicate that stereocomplex formation significantly restricts the local mobilities of CâO and C-H groups along the backbone of chains. The reduced mobility results in the enhanced spin-lattice relaxation time and both 1H and 13C downfield shifts due to the strong intermolecular interactions between R- and S-chains.
RESUMEN
An aliphatic polyester has been prepared from ethylene oxide and maleic anhydride that undergoes reversible transformation between amorphous (Tg =18 °C) and crystalline (Tm =124 °C) states through cis-trans isomerization of the C=C bonds in the polymer backbone without any change in either the molecular weight or dispersity of the polymer. A similar transformation was also observed in chiral unsaturated polyesters formed from enantiopure terminal epoxides, such as epichlorohydrin, phenyl glycidyl ether, and (2,3-epoxypropyl)benzene. These unsaturated polyesters with 100 % E-configuration in the crystalline state were prepared by quantitative isomerization of their Z-configuration analogues in the presence of a catalytic amount of diethylamine, while in the presence of benzophenone, irradiation with 365â nm UV light resulted in the transformation of about 30 % trans-alkene to cis-maleate form, thereby affording amorphous polyesters.
RESUMEN
An iron(III) complex of tetradentate N,N'-disubstituted bis(aminophenoxide) (designated as salan, a saturated version of the corresponding salen ligand) with a sterically hindered organic base anchored on the ligand framework, can selectively mediate the conversion of carbonyl sulfide to sulfur-containing polymers by the copolymerization with epoxides. This single-site catalyst exhibits broad substrate scope, and the resultant copolymers have completely alternating structures. In addition, this catalyst is efficient in producing diblock copolymers, suggesting a living polymerization nature.