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

Polymers

Alexandra Blachon²  Sylvain R. A. Marque¹  Valérie Roubaud² 

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

关键词: DFT calculations;    nitroxide mediated polymerization;    remote steric effect;   

DOI  :  10.3390/polym2030353

来源: mdpi

【 摘 要 】

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 ΔS^≠ 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 ΔS^≠, and thus lowers k_d.

【 授权许可】

CC BY   
© 2010 by the authors; licensee MDPI, Basel, Switzerland.