Penultimate group effect

It is possible to define average propagation rate constants for copolymerization subject to a penultimate group effect as follows. 

There is also clear evidence that penultimate group effects are important in determining the stereochemistry of addition in many homopolymerizations and copolymerizations. This is made evident from the fact that most homopolymers have tacticity (i.e. P(/ i)f0.5, Section 4.2). Indeed, for some homopolyinerizations there is evidence that the configuration of the pcnpcnultimatc unit may also influence the stereochemistry of addition/9 If penpen- and penultimate units... 

Penultimate Group Effects Copolymerization Model. This model represents an extension of the Mayo-Lewis model in which the next to last or penultimate group is assumed to affect the reaction rate. Under this assumption the eight reactions represented by the following equations are of importance ( ) ... 

The comparisons of the reactivities of monomers towards the radicals Me2C(CN) and Me CHPh have contributed to a re-assessment of a fundamental assumption in the usual treatment of radical copolymerization, namely the absence of so-called penultimate group effects in growth reactions. The reactivity of a growing radical is supposed to be governed solely by the nature of the monomeric unit last added, i.e. there is no influence of units further from the reactive site. 

Recognizing all the above, Dodgson and Ebdon " initiated a comprehensive study to test whether a mechanism involving a 1 1 styrene-MA CTC propagation could satisfactorily account for the observed dependence of copolymer composition upon monomer feed, which was previously attributed to penultimate group effects. Kinetics of the free-radical copolymerization of the pair were studied both neat and in several solvents at 60°C and to low conversions. The results and conclusions of this study follow. 

There may be other equilibrium constants to consider if penultimate group effects are significant (there is theoretical data and some experimental evidence to indicate that this is the case). There are also a further series of reactions that need to be considered that involve initiator radical-derived... 

The effect of the penultimate groups or even more distant groups is probably of no great importance. A special case when such interactions should be considered will be discussed later. 

The influence of the penultimate group of a growing chain is not limited, however, to the case of these polar monomers, but has been shown to arise in styrene polymerizations in cyclohexane and more recently in styrene copolymerizations with l,l-diphenylethylene. ° Thus in reactions of 1,1-diphenylalkyl-lithium compounds, these alkyls with aromatic substituents representing effectively a styryl penultimate group react approximately an order of magnitude more slowly than a species with an unsubstituted alkyl chain, while at the same time the latter ion pair is less dissociated. This has been interpreted in terms of a through space interaction between the penultimate group and the terminal one (19). Partial... 

Carbon-13 NMR spectroscopy may be used for detailed studies of the microstructure to test the predictions of the sequence lengths from the simple copolymer equation. Refinements to the equation include the consideration of the effect of the penultimate group in the chain on the reactivity of the terminal radical with the monomers (Tirrell, 1989). These studies do not provide information regarding the molecule-to-molecule variation in composition that may occur, since again these sequence distributions are the average over the whole population. 

Penultimate unit effects were observed in the ESR spectra of model dimeric radicals. The ESR spectra of dimeric radicals with acrylate terminal units will be discussed first. The effect of a methacrylate penultimate unit (due to the presence of a methyl group at the y-position) is seen by comparing the spectrum of H-EMA-rBA with... 

Of all the MA copolymerizations studied, the MA-styrene pair has perhaps received the greatest attention in an attempt to understand the mechanism of alternating copolymerization.For this pair, the understanding is complicated by the fact that styrene-rich copolymers can be prepared from styrene-rich feeds. Bamford and Barb studied the copolymerization of the styrene-MA pair in several solvents and Barb interpreted the kinetics of the reaction in terms of a penultimate group (i.e., unit adjacent to the radical end group) effect operating on polymer free radicals with a terminal styrene unit. Enomoto et after studying the same reaction... 

Extensive kinetic studies of propagation led to many interesting results. They allowed us to determine not only the rate constants of propagation for a variety of monomers but also, after some modification of the technique, they permitted us to evaluate directly the rate constants, k p 21 cross-propagation and to assess the effects of penultimate groups upon the rate of copolymerization . ... 

In writing Eqs. (6.3.1a)-(6.3.1j), it was assumed that the effect of penultimate groups on the reactivity is negligible. Suppose that this effect cannot be neglected. In that case, we define species Pj(AA), Qi(BA), P2(AB), and Q2(BB). Now, write the kinetic mechanism of copolymerization involving these species. 

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