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• Marque, S
• Roubaud, V
• Siri, D
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• Marque, S
• Roubaud, V
• Siri, D
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• Marque, S
• Roubaud, V
• Siri, D

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Polymers 2010, 2(3), 353-363; doi:10.3390/polym2030353

Article

Diasteromeric Effect on the Homolysis of the C–ON Bond in Alkoxyamines: A DFT Investigation of 1,3-Diphenylbutyl-TEMPO

Alexandra Blachon, Sylvain R. A. Marque* , Valérie Roubaud and Didier Siri

UMR 6264 LCP CNRS-Aix-Marseille Université, case 521, Avenue Escadrille Normandie-Niemen, 13397 Marseille Cedex 20 France

* Author to whom correspondence should be addressed.

Received: 2 September 2010; in revised form: 16 September 2010 / Accepted: 16 September 2010 / Published: 27 September 2010

(This article belongs to the Special Issue Advanced Polymer Architectures)

Download PDF Full-Text [575 KB, uploaded 27 September 2010 15:49 CEST]

Abstract: The rate constants k_d of the homolysis of the C–ON bond in styryl dyads TEMPO-based alkoxyamines have recently been published (Li et al. Macromolecules 2006, 39, 9201). The diastereoisomers exhibited different values which were higher than for the unimer TEMPO-styryl alkoxyamine 1. At a first glance, the localization of the steric strain was not obvious. To decipher this problem, diastereoisomer models 2 (RR/SS) and 3 (RS/SR), as well as the released alkyl radicals, were calculated at the \B3LYP/6-31G(d) level. It was revealed that the increase in k_d from 1 to 3 was due to the compression (buttressing effect) of the reactive center by the second styryl moiety. The difference in k_d for the diastereoisomer was clearly an activation entropy effect DS^≠ because the alkyl fragment of the RS/SR diastereoismer exhibited the same conformation as the released radical whereas the conformation for the RR/SS diastereoisomer was quite different and thus required the rotation of several bonds to reach the correct TS, which cost DS^≠, and thus lowers k_d.

Keywords: DFT calculations; nitroxide mediated polymerization; remote steric effect

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