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Mechanism of Isotactic Polypropylene Formation with C_1-Symmetric Metallocene Catalysts

Miller, Stephen A. and Bercaw, John E. (2006) Mechanism of Isotactic Polypropylene Formation with C_1-Symmetric Metallocene Catalysts. Organometallics, 25 (15). pp. 3576-3592. ISSN 0276-7333. doi:10.1021/om050841k.

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The mechanism for isotactic polypropylene formation by the C_1-symmetric catalyst system [Me_2C(3-tert-butyl-C_5H_3)(C_(13)H_8)]ZrCl_2/MAO (MAO = methylaluminoxane, C13H8 = fluorenylidene) has been examined. Evidence supports an alternating mechanism, where both sites of the metallocene wedge are utilized for monomer insertion, rather than the previously proposed site epimerization (inversion at Zr) following each monomer insertion. As the polymerization temperature increases (0 to 60 °C) with lower concentrations of propylene, the site epimerization mechanism begins to compete, as evidenced by an increase in isotacticity. The alternating mechanism also accounts for polypropylene microstructures obtained with Me_2C(3-R-C_5H_3)(Oct)ZrCl_2/MAO, where Oct = octamethyloctahydrodibenzofluorenylidene and R = methyl, cyclohexyl, diphenylmethyl, and with Me_2C(3-tert-butyl-4-Me-C_5H_2)(Oct)ZrCl_2/MAO. For an Oct-containing catalyst system with R = 2-methyl-2-adamantyl, unprecedentedly high (for a fluorenyl-based metallocene catalyst) isotacticity ([mmmm] > 99%) is obtained; the polymer prepared at 0 °C has T_m = 167 °C and M_w = 370 000.

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Miller, Stephen A.0000-0002-3181-1426
Additional Information:© 2006 American Chemical Society. Received September 29, 2005. Publication Date (Web): June 16, 2006. This work has been supported by USDOE Office of Basic Energy Sciences (Grant No. DE-FG03-85ER13431), Exxon Chemicals America, and the National Science Foundation (Grant No. CHE-0131180). A DOD National Defense Science and Engineering Fellowship awarded to S.M. is also acknowledged. The authors thank Dr. Michael Day and Mr. Lawrence Henling for obtaining the X-ray crystal structure, as well as Dr. Terry Burkhardt at ExxonMobil and Dr. Andreas Ernst at BP-Amoco for assistance in obtaining molecular weight data.
Funding AgencyGrant Number
Department of Energy (DOE)DE-FG03-85ER13431
Exxon Chemicals AmericaUNSPECIFIED
National Defense Science and Engineering Graduate (NDSEG) FellowshipUNSPECIFIED
Issue or Number:15
Record Number:CaltechAUTHORS:20170607-133944016
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Official Citation:Mechanism of Isotactic Polypropylene Formation with C1-Symmetric Metallocene Catalysts Stephen A. Miller and John E. Bercaw Organometallics 2006 25 (15), 3576-3592 DOI: 10.1021/om050841k
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:78007
Deposited By: Tony Diaz
Deposited On:07 Jun 2017 22:36
Last Modified:15 Nov 2021 17:35

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