Isospecific Polymerization of Racemic Epoxides: a Catalyst System for the Synthesis of Highly Isotactic Polyethers

A highly active bimetallic cobalt catalyst system is reported for the polymerization of racemic terminal epoxides to yield isotactic polyethers.We acknowledge the NSF (CHE-0809778) and the King Abdullah University of Science and Technology (KAUST; Award No. KUS-C1-018-02) for support of this research. The authors are grateful for the help of T. Mourey at Kodak for polymer molecular weight determinatio

Enantioselective Polymerization of Epoxides Using Biaryl-Linked Bimetallic Cobalt Catalysts: A Mechanistic Study

The enantioselective polymerization of propylene oxide (PO) using biaryl-linked bimetallic salen Co catalysts was investigated experimentally and theoretically. Five key aspects of this catalytic system were examined: (1) the structural features of the catalyst, (2) the regio- and stereoselectivity of the chain-growth step, (3) the probable oxidation and electronic state of Co during the polymerization, (4) the role of the cocatalyst, and (5) the mechanism of monomer enchainment. Several important insights were revealed. First, density functional theory (DFT) calculations provided detailed structural information regarding the regio- and stereoselective chain-growth step. Specifically, the absolute stereochemistry of the binaphthol linker determines the enantiomer preference in the polymerization, and the interaction between the salen ligand and the growing polymer chain is a fundamental aspect of enantioselectivity. Second, a new bimetallic catalyst with a conformationally flexible biphenol linker was synthesized and found to enantioselectively polymerize PO, though with lower enantioselectivity than the binaphthol linked catalysts. Third, DFT calculations revealed that the active form of the catalyst has two active exo anionic ligands (chloride or carboxylate) and an endo polymer alkoxide which can ring-open an adjacent cobalt-coordinated epoxide. Fourth, calculations showed that initiation is favored by an endo chloride ligand, while propagation is favored by the presence of two exo carboxylate ligands

Enantioselective Polymerization of Epoxides Using Biaryl-Linked Bimetallic Cobalt Catalysts: A Mechanistic Study

The enantioselective polymerization of propylene oxide (PO) using biaryl-linked bimetallic salen Co catalysts was investigated experimentally and theoretically. Five key aspects of this catalytic system were examined: (1) the structural features of the catalyst, (2) the regio- and stereoselectivity of the chain-growth step, (3) the probable oxidation and electronic state of Co during the polymerization, (4) the role of the cocatalyst, and (5) the mechanism of monomer enchainment. Several important insights were revealed. First, density functional theory (DFT) calculations provided detailed structural information regarding the regio- and stereoselective chain-growth step. Specifically, the absolute stereochemistry of the binaphthol linker determines the enantiomer preference in the polymerization, and the interaction between the salen ligand and the growing polymer chain is a fundamental aspect of enantioselectivity. Second, a new bimetallic catalyst with a conformationally flexible biphenol linker was synthesized and found to enantioselectively polymerize PO, though with lower enantioselectivity than the binaphthol linked catalysts. Third, DFT calculations revealed that the active form of the catalyst has two active <i>exo</i> anionic ligands (chloride or carboxylate) and an <i>endo</i> polymer alkoxide which can ring-open an adjacent cobalt-coordinated epoxide. Fourth, calculations showed that initiation is favored by an <i>endo</i> chloride ligand, while propagation is favored by the presence of two <i>exo</i> carboxylate ligands

Isospecific, Chain Shuttling Polymerization of Propylene Oxide Using a Bimetallic Chromium Catalyst: A New Route to Semicrystalline Polyols

Hydroxy-telechelic poly­(propylene oxide) (PPO) is widely used industrially as a midsegment in polyurethane synthesis. These atactic polymers are produced from racemic propylene oxide using chain shuttling agents and double-metal cyanide catalysts. Unlike atactic PPO, isotactic PPO is semicrystalline with a melting temperature of approximately 67 °C. Currently there is no practical route to hydroxy-telechelic isotactic PPO using racemic propylene oxide as the monomer. In this paper, hydroxy-telechelic isotactic PPO is synthesized from racemic propylene oxide with control of molecular weight using enantioselective and isoselective bimetallic catalysts in conjunction with chain shuttling agents. The discovery of an easily accessible bimetallic chromium catalyst is reported for this transformation. Diol, triol, and polymeric chain shuttling agents are used to give hydroxy-telechelic isotactic PPO of varying functionality and structure. Detailed quantum chemical studies are used to reveal the polymerization mechanism and origin of stereoselectivity

Identifying residual structure in intrinsically disordered systems: a 2D IR spectroscopic study of the GVGXPGVG peptide.

Item does not contain fulltextThe peptide amide-I vibration of a proline turn encodes information on the turn structure. In this study, FTIR, two-dimensional IR spectroscopy and molecular dynamics simulations were employed to characterize the varying turn conformations that exist in the GVGX(L)PGVG family of disordered peptides. This analysis revealed that changing the size of the side chain at the X amino acid site from Gly to Ala to Val substantially alters the conformation of the peptide. To quantify this effect, proline peak shifts and intensity changes were compared to a structure-based spectroscopic model. These simulated spectra were used to assign the population of type-II beta turns, bulged turns, and irregular beta turns for each peptide. Of particular interest was the Val variant commonly found in the protein elastin, which contained a 25% population of irregular beta turns containing two peptide hydrogen bonds to the proline C horizontal lineO