Synthesis of Stereoregular Polymers through Ring-Opening
Metathesis Polymerization
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Abstract
ConspectusSome of the most readily available and inexpensive
monomers for
ring-opening metathesis polymerization (ROMP) are norbornenes or substituted
norbornadienes. Polymers made from them have tacticities (the stereochemical
relationship between monomer units in the polymer chain) that remain
after the CC bonds in the polymer backbone are hydrogenated.
Formation of polymers with exclusively a single structure (one tacticity)
was rare until approximately 20 years ago, when well-defined ROMP
catalysts based on molybdenum imido alkylidene complexes that contain
a chiral biphenolate or binaphtholate ligand were shown to yield <i>cis</i>,<i>isotactic</i>-poly(2,3-dicarbomethoxynorbornadiene)
and related polymers through addition of the monomer to the same side
of the MC bond in each step. Over the past few years, molybdenum
and tungsten monoaryloxide pyrrolide (MAP) imido alkylidene initiators
have been found to produce <i>cis</i>,<i>syndiotactic</i> polynorbornenes and substituted norbornadienes through addition
of the monomer to one side of the MC bond in one step followed
by addition to the other side of the MC bond in the next step.
This “stereogenic metal control” is possible as a consequence
of the fact that the configuration of the stereogenic metal center
switches with each step in the polymerization. Stereogenic metal control
also allows syndiotactic polymers to be prepared from racemic monomers
in which enantiomers of the monomer are incorporated alternately into
the main chain. Because pure <i>trans</i> polymers have
not yet been prepared through some predictable mechanism of stereochemical
control, it seems unlikely that all four basic polymer structures
from a single given monomer can be prepared simply by choosing the
right initiator. However, because tactic, and relatively oxygen-stable,
hydrogenated polymers are often a desirable goal, the ability to form
pure <i>cis</i>,<i>isotactic</i> polymers (through
enantiomorphic site control) and <i>cis</i>,<i>syndiotactic</i> polymers (through stereogenic metal control) is sufficient for preparing
hydrogenated polymers with a single structure. It is hoped that the
principles of forming polymers that have a single structure through
ring-opening metathesis polymerization will be general for a relatively
large number of monomers and that some important problems in ROMP
polymer chemistry can benefit from knowledge of polymer structure
at a molecular level. With an increase in knowledge concerning the
mechanistic details of polymerization by well-defined initiators,
more elaborate ROMP polymers and copolymers with stereoregular structures
may be possible