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Ethylene insertion and β-hydrogen elimination for permethylscandocene alkyl complexes. A study of the chain propagation and termination steps in Ziegler-Natta polymerization of ethylene

Burger, Barbara J. and Thompson, Mark E. and Cotter, W. Donald and Bercaw, John E. (1990) Ethylene insertion and β-hydrogen elimination for permethylscandocene alkyl complexes. A study of the chain propagation and termination steps in Ziegler-Natta polymerization of ethylene. Journal of the American Chemical Society, 112 (4). pp. 1566-1577. ISSN 0002-7863. https://resolver.caltech.edu/CaltechAUTHORS:20180430-145259868

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Abstract

The rates of ethylene insertion into the Sc-C bond for Cp*_2ScR (Cp* = (η^5-C_5Me_5), R = CH_3, CH_2CH_3, CH_2CH_2CH_3) have been measured at -80 °C by ^(13)C NMR; the second order rate constants (M^(-1) S^(-1)) are as follows: R = CH_3, 8.1 (2) X 10^(-4); R = CH_2CH_3,4.4 (2) X 10^(-4); R = CH_2CH_2CH_3, 6.1 (2) X 10^(-3). The slow rate for the ScCH_2CH_3 complex is attributed to a ground-state stabilization by a β-C-H "agostic” interaction. The distributions of molecular weights for ethylene oligomers (CH_3(CH_2)_nCH_3, n = 11-47) produced from known amounts of ethylene and Cp*_2ScCH_2CH_2CH_3 at -80 °C satisfactorily fit a Poisson distribution, indicative of a “living” Ziegler-Natta polymerization system. From the measured, slower initiation rates of insertion for Cp*ScCH_3 and Cp*_2ScCH_2CH_3 and propagation rates equal to that for Cp_2ScCH_2CH_2CH_3, the molecular weight distributions of ethylene oligomers are also accurately predicted. Cp*ScCH_3 undergoes a single insertion with 2-butyne with a moderate enthalpy of activation and a large, negative entropy of activation. The second-order rate constants for the insertion of 3-phenyl-2-propyne, 2-pentyne, and 4-methyl-2-pentyne have been measured. The rates for &hydrogen elimination for members of the series of permethylscandocene alkyl complexes Cp*_2ScCH_2CH_2R (R = H, CH_3, CH_2CH_3, C_6H_5, C_6H_4-p-CH_3, C_6H_4-p-CF_3, C_6H_4-p-NMe_2) have been obtained by rapidly trapping Cp*ScH with 2-butyne. A transition state for the β-hydrogen elimination is indicated with partial charge on the &carbon. Hydrogen is thus transferred to the scandium center as hydride in the β-H elimination process.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://dx.doi.org/10.1021/ja00160a041DOIArticle
Alternate Title:Ethylene insertion and .beta.-hydrogen elimination for permethylscandocene alkyl complexes. A study of the chain propagation and termination steps in Ziegler-Natta polymerization of ethylene
Additional Information:© 1990 American Chemical Society. Received May 5, 1989. This work was supported by the USDOE Office of Basic Energy Research, Office of Basic Energy Sciences (Grant No. DE-FG03-85ER13431) and by the Shell Companies Foundation, which are gratefully acknowledged. The use of the Southern California Regional NMR Facility, supported by the National Science Foundation Grant No. CHE-84-40137 is also gratefully acknowledged. W.D.C. also thanks the National Science Foundation for an NSF Predoctoral Fellowship (1987-1990). We thank Dr. Jay A. Labinger for helpful discussions.
Funders:
Funding AgencyGrant Number
Department of Energy (DOE)DE-FG03-85ER13431
Shell Companies FoundationUNSPECIFIED
NSFCHE-84-40137
NSF Predoctoral FellowshipUNSPECIFIED
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Caltech Division of Chemistry and Chemical Engineering7948
Issue or Number:4
Record Number:CaltechAUTHORS:20180430-145259868
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20180430-145259868
Official Citation:Ethylene insertion and .beta.-hydrogen elimination for permethylscandocene alkyl complexes. A study of the chain propagation and termination steps in Ziegler-Natta polymerization of ethylene Barbara J. Burger, Mark E. Thompson, W. Donald Cotter, and John E. Bercaw Journal of the American Chemical Society 1990 112 (4), 1566-1577 DOI: 10.1021/ja00160a041
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:86131
Collection:CaltechAUTHORS
Deposited By: George Porter
Deposited On:30 Apr 2018 23:10
Last Modified:03 Oct 2019 19:39

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