Copolymerization penultimate unit effects

Problems arise with any of the abovementioned methods in the measurement of transfer constants for very active transfer agents. Bamford 8 proposed the technique of moderated copolymerization. In these experiments, the monomer of interest is copolymcrizcd with an excess of a moderating monomer that has a much lower (preferably negligible) transfer constant. The method has also been applied to evaluate penultimate unit effects on the transfer constant.28-j0... 

The general features of the penultimate model in what have become known as the explicit and implicit forms are described in Section 7.3.1.2.1. Evidence for remote unit effects coming from small molecule radical chemistry and experiments other than copolymerization is discussed in Section 7.3.1.2.2. In Sections 7.3.1.2.3 and 7.3.1.2.4 specific copolymerizations are discussed. Finally, in Section 7.3.1.2.5, we consider the origin of the penultimate unit effects. A general recommendation is that when trying to decide on the mechanism of a copolymerization, first consider the explicit penultimate model."... 

Rased on the above data, it would seem unusual if reactivity of the propagating species in copolymerization were insensitive to the nature of the last added monomer units. However, while there are ample experimental data to suggest that copolymerizations should be subject to penultimate unit effects that affect the rate and/or copolymer composition, the origin and magnitude of the effect is not always easily predictable. 

Perhaps because of this complexity, few studies on determining kld/ktt, in cross termination in copolymerization have been reported and most of the available data come from model studies, it is also usually assumed, without specific justification, that penultimate unit effects are unimportant in determining which reactions occur and that values of k klt for the homotermination reactions are similar to those in the corresponding homopolymerizations. 

S. Losio, P. Stagnaro, T. Motta, M.C. Sacchi, F. Piemontesi, and M. Gal-imberti, Penultimate-unit effect in ethene/4-methyl-l-pentene copolymerization for a sequential distribution of comonomers, Macromole-cules, 41 (4) 1104—1 111, 2008. 

Anomalies in the course of copolymerizations which have been ascribed to penultimate unit effects can alternatively be explained by the reversibility of one or several addition types. 

The simple copolymer model is a first-order Markov chain in which the probability of reaction of a given monomer and a macroradical depends only on the terminal unit in the radical. This involves consideration of four propagation rate constants in binary copolymerizations, Eqs. (7-2)-(7-4). The mechanism can be extended by including a penultimate unit effect in the macroradical. This involves eight rate constants. A third-order case includes antepenultimate units and 16 rate coefficients. A true test of this model is not provided by fitting experimental and predicted copolymer compositions, since a match must be obtained sooner or later if the number of data points is not saturated by the adjustable reactivity ratios. 

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). 

Experimental studies on models of the propagating radicals in S-AN copolymerization " and a few other systems provide support for an explicit penultimate unit effect. Of particular interest is the data of Tirrcll and coworkers. They investigated the relative reactivity of S and AN towards various y-substituted propyl radicals (Scheme 7.3 and Table 7.2). They found that ... 

If the terminal model adequately explains the copolymer composition, as is often the case, the terminal model is usually assumed to apply. Even where statistical tests show that the penultimate model does not provide a significantly better fit to experimental data than the tenninal model, this should not be construed as evidence that penultimate unit effects are unimportant. It is necessary to test for model discrimination, rather than merely for fit to a given model. In this context, it is important to remember that composition data are of very low power when it comes to model discrimination. For MMA-S copolymerization, even though experimental precision is high, the penultimate model confidence intervals are quite large 0.4[Pg.348]

The majority of copolymerization systems studied so far can by represented well by the implicit penultimate unit effect (IPUE) model, where the two radical reactivity ratios, Si = kau/km and S2 - ki22/ 222. are introduced as additional parameters, to account for the influence of the penultimate unit on homopropagation. Within the IPUE model, no penultimate unit effect is considered for the reactivity ratios r = r2i and r2i = rx2- Despite the remarkable... 

Roberts, G.E., Coote, ML., Heuts, J.P.A., Morris, L.M., and Davis, X.P. (1999) Radical ring-opening copolymerization of 2-methylene 1,3-dioxepane and methyl methacrylate experiments originally designed to probe the origin of the penultimate unit effect. Macromolecules,... 

However, recent experimental and theoretical work has shown that the assumptions of the implicit penultimate model are unlikely to be applicable to the majority of copol5mierization systems. We have recently published a review of this evidence (37,42), which draws on direct experimental and theoretical measures of reactivity ratios, model testing in a range of copolymerization systems, and other tests of the mechanism of the propagation step via, for example, the examination of solvent effects on reactivity ratios. These studies provide strong evidence for penultimate imit effects but, in all cases where penultimate unit effects have been measured directly, effects on radical selectivity have been shown to be significant. In other words all available evidence contradicts the assumption of the implicit penultimate model that the penultimate unit affects reactivity but not selectivity. 

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. 

The ATRP is based on a sequence of ATRA reactions and ATRA can be used to prepare various dimeric species that model the expected chain ends in a copolymerization reaction. An ESR study of radicals generated from these mixed dimers can be used to clarify the penultimate unit effects relevant to copolymerization. The ESR spectra of monomeric radicals of (meth)acrylates were previously investigated by Fischer et Gilbert et al., and Matsumoto et al. However, no ESR... 

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. 

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