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An Ab Initio Investigation of the Chain-Length Dependence of the Addition–Fragmentation Equilibria in RAFT Polymerization

Ching Yeh Lin A and Michelle L. Coote A B
+ Author Affiliations
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A Australian Research Council Centre of Excellence for Free-Radical Chemistry and Biotechnology, Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia.

B Corresponding author. Email: mcoote@rsc.anu.edu.au




Professor Michelle Coote is a graduate of the University of New South Wales, where she completed a B.Sc. (Hons) in industrial chemistry (1995), followed by a Ph.D. in polymer chemistry (2000). Following postdoctoral work at the University of Durham, UK, she joined the Research School of Chemistry, Australian National University in 2001, initially as a postdoctoral fellow with Professor Leo Radom. She established her own research group in 2004 and has recently taken up an ARC Future Fellowship. She has published extensively in the fields of polymer chemistry, radical chemistry, and computational quantum chemistry, and is a member of the ARC Centre of Excellence for Free-Radical Chemistry and Biotechnology. She has received many awards including the 2001 IUPAC prize for young scientists, the RACI Cornforth medal (2000), Rennie medal (2006) and David Sangster Polymer Science and Technology Achievement Award (2010), and the Le Fevre Memorial Prize of the Australian Academy of Science (2010).

Australian Journal of Chemistry 64(6) 747-756 https://doi.org/10.1071/CH11069
Submitted: 11 February 2011  Accepted: 8 March 2011   Published: 27 June 2011

Abstract

Ab initio molecular orbital theory has been used to study and explain the effects of chain length on the addition–fragmentation equilibrium constant in reversible addition–fragmentation chain transfer (RAFT) polymerization. New data is presented for azobisisobutyronitrile-initiated t-butyl dithiobenzoate-mediated polymerization of methyl methacrylate, and 2-(((ethylthio)carbonothioyl)thio)propanoic acid-mediated polymerization of acrylamide, and compared with published results for a dithiobenzoate-mediated polymerization of styrene and a trithiocarbonate-mediated polymerization of methyl acrylate. The effects of primary and penultimate substituents on the addition–fragmentation equilibrium constants in RAFT polymerization can be very large (up to eight orders and four orders of magnitude respectively) and should be taken into account in kinetic models. Antepenultimate unit effects are relatively small, implying that, for most systems, chain length effects have largely converged by the dimer stage. However, for sterically bulky monomers capable of undergoing anchimeric interactions such as hydrogen bonding, the onset and convergence of these substituent effects is delayed to slightly longer chain lengths. The magnitude and direction of chain-length effects in the addition–fragmentation equilibrium constants varies considerably with the nature of the RAFT agent, the initiating species, the propagating radical, and the solvent. The observed substituent effects arise primarily in the differing stabilities of the attacking radicals, but are further modified by homoanomeric effects and, where possible, hydrogen-bonding interactions.


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