2 research outputs found
Isospecific, Chain Shuttling Polymerization of Propylene Oxide Using a Bimetallic Chromium Catalyst: A New Route to Semicrystalline Polyols
Hydroxy-telechelic polyÂ(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
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