Journal
JOURNAL OF POLYMER SCIENCE PART A-POLYMER CHEMISTRY
Volume 46, Issue 16, Pages 5663-5697Publisher
WILEY-BLACKWELL
DOI: 10.1002/pola.22888
Keywords
bond dissociation energy; living radical polymerization; molecular modeling; SET-LRP; single electron-transfer
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Funding
- NSF
- [NSF-DMR-0548559]
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The heterolytic dissociation process associated with the activation of Single Electron-Transfer Living Radical Polymerization is examined through the use of energy profile modeling. Monomer and initiator structure is correlated with the approximate activation barriers, energies of electrostatic ion-radical pair formation, and stability of ion-radical pair generated from the counteranion halide leaving group and the radical atom with partial positive charge density induced by its electron-withdrawing substituent. Energy profiles permit access not just to one, but to all local minima, in the dissociation pathway and the identification of a global minimum. The location and energy of this global minimum allows for the placement of various initiators and dormant propagating macroradicals on the spectrum between stepwise and concerted dissociative electron-transfer. The barrier for the activation step for alkylhalides derived from acrylates, vinyl halides, and styrenes, as well as from initiators healing electron-withdrawing groups is decreased in comparison to relatively more electron-rich alkyl halides. This rate enhancement is explained through the sticky dissociative model wherein electron-transfer is accelerated by the formation of strong ion-radical pairs between radicals with partial positive charge density and their counteranion leaving group. Greater electron-with-drawing capacity of the alkyl halide substituent increases the stability of the ion-radical pair, reduces its equilibrium bond length, and accelerates electron-transfer. (C) 2008 Wiley Periodicals, Inc.
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