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Winner of the IUPAC Prize
for Young Chemists - 2001

 

Michelle Louise Coote wins one of the five IUPAC Prize for Young Chemists, for her Ph.D. thesis work entitled "The origin of the penultimate unit effect in free-radical copolymerization"

Current address (at the time of application)

IRC in Polymer Science
University of Durham
Durham, DH1 3LE, UK

Starting May 2001
Research School of Chemistry
Australian National University
Canberra ACT 0200, Australia
E-mail: mcoote@rsc.anu.edu.au

Academic degrees

  • Ph.D.: University of New South Wales, Sydney, Australia - specialization: polymer chemistry, February 2000.
  • B.Sc. (Hons. Class 1 and the University Medal). University of New South Wales, Sydney, Australia - specialization: industrial chemistry, December 1995.

Ph.D. Thesis

Title The origin of the penultimate unit effect in free-radical copolymerization
Adviser Professor Thomas P. Davis
Thesis Committee T. Fukuda, Institute for Chemical Research, Kyoto University, Kyoto, Japan; K. Matyjaszewski, Dept. of Chemistry, Carnegie Mellon University, Pittsburgh PA, USA; G. Moad, CSIRO Molecular Science, Clayton South, Victoria, Australia.

Essay

Free radicals are fundamentally important in a wide range of chemical and biological processes, including the degradation of synthetic and biological polymers, stratospheric and atmospheric processes, several important enzymatic processes such as photosynthesis and the biosynthesis of DNA, and even the growth of complex chemicals in interstellar clouds. The objective of my PhD research was to contribute to a deeper understanding of the fundamental influences on radical reactivity and selectivity, with a specific focus on remote substituent effects.

My research was motivated by the specific problem of kinetic modeling in free-radical copolymerization. Free-radical polymerization is one of the most important bulk chemical industries and copolymerization is often utilized to impart the properties of more than one type of monomer to a product. The properties of the polymer depend heavily on such things as its molecular weight distribution, chemical composition, and sequence distribution -and these are governed by the rates of the various reactions that occur during the polymerization process. Modeling copolymerization is very complicated as, not only are there different classes of reaction to consider (such as initiation, propagation, termination, and chain-transfer), but the radicals undergoing these reactions can also have countless different chemical compositions (being composed of various combinations of the different monomer units). To develop useful models it is necessary to make simplifying assumptions about the fundamental influences on radical reactivity, so that the large set of chemically different reactions can be grouped together into a smaller number of sets of kinetically equivalent reactions.

For nearly 50 years it was assumed that remote substituent effects were unimportant in the propagation step of free-radical polymerization, and a simplified model based on this assumption -the terminal model- was adopted for modeling the composition, sequence distribution and overall propagation rate. By treating the reactivity ratios of the monomers as adjustable parameters, this model could be made to fit the composition data for almost all systems tested. However, although widely used, the model was not critically tested until the seminal work of Prof. Fukuda in 1985. Unlike previous workers, Fukuda actually used the parameters obtained from a terminal model fit to composition data, to predict the propagation rate coefficients for the same system, and compared them against independently measured values. When subjected to this critical testing, the model failed comprehensively. This failure has since been demonstrated for a number of other systems, and it is now generally accepted that the terminal model cannot be made to describe simultaneously the composition and propagation rate coefficients of ordinary copolymerization systems.

While it is now clear that the terminal model is incorrect, there has been a reluctance to call into question the large number of terminal model reactivity ratios that have been measured over the previous 50 years, as well as the numerous (terminal-model based) empirical schemes for explaining reactivity in free-radical polymerization that have been proposed. In order to retain this earlier work, Fukuda proposed a compromise model in which remote substituent effects were considered to be unimportant in models for copolymer composition but important in models for propagation kinetics. In chemical terms this amounted to the assumption that the penultimate unit affects radical reactivity but not selectivity. This 'restricted penultimate' model has now been widely adopted, but the physical validity of its assumptions had not, until my work, been tested.

My research was an extensive theoretical and experimental examination of the nature of the penultimate unit effect, with the aim of establishing whether or not such effects exist, and whether they are more important in radical selectivity or reactivity. This would facilitate a critical test of Fukuda's restricted model and would thus establish whether or not the previous 50 years of work based upon the terminal model is valid.

In a series of high-level ab initio molecular orbital calculations of the reaction barrier (and other related quantities) in a series of model propagation reactions, I provided direct evidence for the existence of significant penultimate unit effects. Furthermore, the calculated effects were strongly dependent on the nature of the reacting monomer (ie. there were effects on selectivity), and were likely to be polar in origin. Although my calculations revealed significant penultimate unit effects on radical stability, such effects largely canceled from the reaction barrier owing to the early transition state. I thus showed that penultimate unit effects on selectivity are much more significant than those on reactivity, which in turn shows that Fukuda's restricted model -and thus the terminal model composition equation- is not physically valid.

In order to provide experimental support for this result, I also performed a series of kinetic studies. I initially attempted to discriminate between alternative versions of the penultimate model purely on their capacity to describe simultaneously propagation and composition data for typical co- and ter-polymerization systems. However, despite compiling the most extensive data-sets to date, using the most accurate available experimental methods, I found that, owing to their adjustable parameters, any number of models could be made to fit the data. Nonetheless, by designing model copolymerization systems, such as pairs of sterically similar but electronically different para-substituted styrene systems, I was able to provide direct evidence against the restricted model. In an additional study, I showed that the penultimate unit effect is temperature dependent, and thus has a significant enthalpic component, which (coupled with previous studies of solvent effects) is further evidence for polar penultimate unit effects.

In conclusion, my research provided evidence for the existence of penultimate unit effects, and indicated that these effects will be more important in selectivity than reactivity. These results imply that the failure of the terminal model to describe propagation rate coefficients must be taken as evidence of its failure to describe composition data. This result not only entails that current kinetic modeling of free-radical polymerization needs to revised, but it also suggests that many of our fundamental ideas about radical reactivity in general -in particular, about the importance of remote substituent effects- need to be re-examined .


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