C O M M U N I C A T I O N S
Scheme 2
References
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Cs-Ligated catalyst structure A (Scheme 2) is chiral at metal and
exhibits two enantiotopic lateral coordination sites. Isotactic PMMA
is formed by a site-control mechanism when the reaction follows
a AfBfA cycle in which stereoselective C-C bond formation
occurs predominantly through the same preserved structure A, and
the lateral coordination site for MMA is also the same due to
monomer backside attack (relative to the coordinated ester leaving
group) following each bond formation step.10 On the other hand,
syndiotactic PMMA is formed by a site-control mechanism if both
lateral sites are alternately utilized for stereoselective monomer
enchainment via a AfBfA* cycle (* denotes an enantiomer)
requiring frontside attack of the incoming monomer at the resting
cyclic intermediate B following each C-C bond formation step.
For both site-control cases, the rate of MMA-assisted site epimer-
ization must be slow relative to Michael addition. No site
epimerization was observed for 3 and 4 in CD2Cl2 or in the presence
of 10 or 20 equiv of THF. Consistently, addition of 10 or 20 equiv
of THF decreased the syndiotacticity of the PMMA produced by 3
or 4 in CH2Cl2 by only ∼1.0%, suggesting little to no site
epimerization during polymerization by displacement of the coor-
dinated chain end from the metal prior to monomer coordination.
Hence, the observed differences in reactivity and stereoselectivity
between the Me2C< and Ph2C< bridged Zr systems may be
explained by their different Thorpe-Ingold effect25 in terms of
relative rates of ring closing (Michael addition step) and ring
opening as compared to the rate of MMA-assisted site epimeriza-
tion. In the event of fast site epimerization relative to propagation,
syndiotactic PMMA would not be formed by a site-control
mechanism using Cs-ligated catalysts.
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In summary, we have developed a highly active catalyst system
based on the Cs-ligated ansa-zirconocene bis- and mono(ester
enolate) complexes 1 and 2, leading to the production of highly
syndiotactic poly(methacrylate)s at industrially convenient temper-
atures, thus accomplishing a long-standing scientific goal of the
field. However, a fundamental understanding of the unique features
that the current system possesses and further development of
possibly even better systems based on this understanding require
much further work, which will be a focus of our ongoing studies.
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(21) See Supporting Information for experimental details and further results.
(22) Zhang, Y.; Chen, E. Y.-X. Macromolecules 2008, 41, 36-42.
(23) Hydride abstraction from an enolate methyl group of Cp2ZrMe[OC(O-
tBu)dCMe2] by the trityl cation led to the formation of a zirconium-
carboxylate dication after subsequent elimination of methane and
isobutene: Lian, B.; Toupet, L.; Carpentier, J.-F. Chem.sEur. J. 2004,
10, 4301-4307.
Acknowledgment. This work was supported by the National
Science Foundation (NSF-0718061). We thank Boulder Scientific
Co. for the research gifts of Ph2C(Cp)(Flu)ZrCl2, Ph3CB(C6F5)4,
and B(C6F5)3, Prof. Luigi Cavallo for discussions, as well as the
reviewers for the valuable comments.
(24) The X-ray crystal structure of the similar complex bearing a C2-symmetric
ligand has been reported: Ning, Y.; Chen, E. Y.-X. Macromolecules 2006,
39, 7204-7215.
(25) (a) Jung, M. E.; Piizzi, G. Chem. ReV. 2005, 105, 1735-1766. (b) Beesley,
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Supporting Information Available: Experimental details. This
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