No abstract

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.

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.

Bimetallic chromium catalysts are investigated for the enantioselective polymerization of propylene oxide. The catalyst is composed of two salen chromium species linked by an alkyl chain, the length of which significantly impacts the rate of polymerization. While the use of a chloride initiator on the catalyst resulted in bimodal molecular weight distributions, switching to a trifluoroacetate initiating group and adding a diol chain transfer agent afforded polymers of controllable molecular weight with low, unimodal dispersities.

Sealing of wellbores in geothermal and tight oil/gas reservoirs by filling the annulus with cement is a well-established practice. Failure of the cement as a result of physical and/or chemical stress is a common problem with serious environmental and financial consequences. Numerous alternative cement blends have been proposed for the oil and gas industry. Most of these possess poor mechanical properties, or are not designed to work in high temperature environments. This work reports on a novel polymer-cement composite with remarkable self-healing ability that maintains the required properties of typical wellbore cements and may be stable at most geothermal temperatures. We combine for the first time experimental analysis of physical and chemical properties with density functional theory simulations to evaluate cement performance. The thermal stability and mechanical strength are attributed to the formation of a number of chemical interactions between the polymer and cement matrix including covalent bonds, hydrogen bonding, and van der Waals interactions. Self-healing was demonstrated by sealing fractures with 0.3–0.5 mm apertures, 2 orders of magnitude larger than typical wellbore fractures. This polymer-cement composite represents a major advance in wellbore cementing that could improve the environmental safety and economics of enhanced geothermal energy and tight oil/gas production.