Chain transfer penultimate unit effects

Penultimate unit effects are also important in both substitution40"41 and in addition-fragmentation chain transfer.42"44 Some examples are provided in Sections 6.2, 6.2.2.4, 6.2.3.4 and 9.5. 

Living polymerizations continue to attract the attention of theorists studying transfer. Chinese authors have analyzed the penultimate unit effect on transfer to a monomer mathematically [53]. According to their conclusions, the penultimate effect is important when the activities of the growth centres on the polymer chain and on the monomer (after transfer) are widely different, otherwise it can be neglected. 

ESR spectroscopy was successfully applied to quantify radical concentration in the polymerizations [4, 8-11], However, the direct detection method of ESR did not reveal information on many additional points that ate very significant in radical polymerization chemistry so far. For example, the length of propagating chain is not known, direct observation of the penultimate unit effect is almost impossible, and detailed mechanisms of radical reactions remain extremely difficult to examine. These problems have not yet been fully resolved but the development of controlled radical polymerization techniques, especially atom transfer radical polymerization (ATRP), enables us to resolve some of these problems. 

Bamford and Basahel have investigated the importance of penultimate unit effects on the reactivity of /7-butanethiol in a number of copolymerizations (S-MMA, S-MA) using the technique of "moderated copolymerization". Their data indicate that penultimate unit effects are unimportant in these systems. More recently, de la Fuente and Madruga" " have come to similar conclusions for the reactivity of dodecanethiol in BA-MMA copolymerization. This contrasts with findings for transfer to carbon tetrabromide (Section 6.2.2.4). It has also been found, again in contrast with halocarbons, that C,r for various primary and secondary thiols is essentially independent of chain length for chain lengths > 2 (Table 6.1). 

This chapter describes the application of electron spin resonance (ESR) spectroscopy and controlled radical polymerization techniques to basic research on the chemistry of radical polymerizations. This combination can provide information on the chain length of propagating radicals, chain-transfer reactions to polymers, and penultimate unit effects in copolymerization, topics that have been difficult or impossible to study by direct detection of radicals. 

In this chapter, three examples of the application of ESR to conventional radical polymerizations based on controlled/living radical polymerizations wUl be demonstrated. The first example is estimation of the effect of chain length on propagating radicals. The second example is the detection of chain-transfer reactions on the propagating radicals in polymerization of tert-butyl acrylate (tBA). The third example is investigation of penultimate unit effects using ESR analysis of dimeric model radicals of (meth)acrylates prepared by ATRA. 

Development of controlled radical polymerization techniques has stimulated basic research on radical chemistry in conventional radical polymerizations. Information on the effect of chain lengths on propagating radicals, chain-transfer reactions to polymers, and penultimate unit effects has been obtained from ESR observation of model radicals generated from radical precursors prepared by ATRP. Previously, it has been extremely difficult, even impossible, to obtain such information from ESR spectra during conventional radical polymerizations. The ESR study of radical polymerizations has made remarkable progress as a result of the combination of study of radicals formed as a result of various kinds of controlled radical polymerization techniques. 

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