Radical polymerization penultimate unit effect

For radicals 1, ktA/ktc shows a marked dependence on the bulk of the substituent (R2). While phenylethyl radicals (2) and cumyl radicals (5) afford predominantly combination, there are indications of a substantial penultimate unit effect. The radicals 6, with an a-neopentyl substituent, give predominantly disproportionation, Termination in AMS polymerization might therefore also give substantial... 

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. 

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. 

It has been known since 1980 that the terminal model for free-radical copolymerization sometimes fails, due to the penultimate unit effect. Direct detection of the penultimate unit effect by ESR has been unsuccessfully attempted many times. In this section, direct detection of the penultimate unit effect using dimeric model radicals generated from dimeric model radical precursors prepared by ATRA is discussed (Fig. 19). The structures of the dimeric model radicals studied are summarized in Fig. 20. For a detailed discussion of the penultimate unit effect, dimeric, monomeric, and polymeric model radicals were examined. The radicals were generated by three methods homolytic cleavage of carbon-bromine bonds of alkyl bromides with hexabutyldistannane, photodecomposition of an azo-initiator, and radical polymerization performed directly in a sample cell in a cavity. 

The penultimate unit effect may play a very important role in ATRR The rate constants of activation of monomeric and dimeric alkyl bromides with a CuBr-bpy (bpy=2,2 -bipyridine) complex as activator were determined. The ATRP relies on the reversible activation of a dormant alkyl halide through halogen abstraction by a transition metal complex to form a radical that participates in the classical free-radical polymerization figure (Fig. 2) prior to deactivation. In this equiUbrium, the alkyl radical (Pm ) is formed in an activated process, with a rate constant kact> by the homolytic cleavage of an alkyl halogen bond (Pm-Z) catalyzed by a transition metal complex in its lower oxidation state (Cu ). The relative values of fcact of the alkyl bromides were determined for CuBr/bpy catalyst systems in acetonitrile at 35°C. These systems followed the order EBriB (30) MBrP (3)>iBBrP (1) for monomeric initia-tors and MMA-MMA-Br (100) MA-MMA-Br (20) > MMA-MA-Br (5) > MA-MA-Br (1) for dimeric initiators. ... 

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. 

In a more recent study, Charleux et studied the theoretical features of the activation-deactivation equilibrium in nitroxide-mediated copolymerization and applied it to the SGl-mediated copolymerization of methyl methacrylate with a low percentage of styrene (typically in the 4-9 mol.% range). They actually demonstrated that the system exhibited all the characteristics of a living/controlled polymerization, which was explained by the following features (1) the overall concentration of propagating radicals was strongly reduced by the copolymerization effect and the irreversible termination reactions undergone by the MMA/SGf system were hence slowed down (2) isolated styrene subunits were incorporated into the chains and the terminal one promoted the reversible deactivation by the SGI nitroxide and (3) the MMA penultimate unit effect enhanced deactivation of the so-formed styryl-SGf... 

Penultimate effects have been observed for many comonomer pairs. Among these are the radical copolymerizations of styrene-fumaronitrile, styrene-diethyl fumarate, ethyl methacrylate-styrene, methyl methacrylate l-vinylpyridine, methyl acrylate-1,3-butadiene, methyl methacrylate-methyl acrylate, styrene-dimethyl itaconate, hexafluoroisobutylene-vinyl acetate, 2,4-dicyano-l-butene-isoprene, and other comonomer pairs [Barb, 1953 Brown and Fujimori, 1987 Buback et al., 2001 Burke et al., 1994a,b, 1995 Cowie et al., 1990 Davis et al., 1990 Fordyce and Ham, 1951 Fukuda et al., 2002 Guyot and Guillot, 1967 Hecht and Ojha, 1969 Hill et al., 1982, 1985 Ma et al., 2001 Motoc et al., 1978 Natansohn et al., 1978 Prementine and Tirrell, 1987 Rounsefell and Pittman, 1979 Van Der Meer et al., 1979 Wu et al., 1990 Yee et al., 2001 Zetterlund et al., 2002]. Although ionic copolymerizations have not been as extensively studied, penultimate effects have been found in some cases. Thus in the anionic polymerization of styrene t-vinylpyri-dine, 4-vinylpyridine adds faster to chains ending in 4-vinylpyridine if the penultimate unit is styrene [Lee et al., 1963]. 

Anionic copolymerizations have been investigated by applying the classical Mayo-Lewis treatment which was originally developed for free-radical chain reaction polymerization [198]. The copolymerization of two monomers (Mj and M2) can be uniquely defined by the following the four elementary kinetic steps in Scheme 7.21, assuming that the reactivity of the chain end (Mj" or ) depends only on the last unit added to the chain end, that is, there are no penultimate effects. 

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