Penultimate group effects copolymerization

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 ( ) ... 

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... 

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. 

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... 

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. 

Clearly, this approach is straightforward only for ultimate group or terminal group copolymerization. If more than penultimate effects are required, the equations become unwieldy. Using the concepts and mathematics of Markovian processes, Ham (9) generalized an extended form of the selectivity equation. Later, Price (20, 21) formalized the theory of Markov chains as applied to... 

Recently, the Research Group on NMR, SPSJ, assessed reliability of copolymer analysis by NMR using three samples of radically prepared copolymers of MMA and acrylonitrile with different compositions. 1H and 13C NMR spectra of the copolymers were collected from 46 NMR spectrometers (90 500 MHz) and the composition and sequence distribution were determined.232 Table 14 summarizes the monomer reactivity ratios determined by 13C NMR analysis. The large difference between rxx and r2X indicates the presence of a penultimate effect in this radical copolymerization, as previously reported.233 The values of riy, especially rxx, depended on the comonomer feed ratio, suggesting higher order of neighbouring unit effect on the reactivity of chain-end radicals. 

On the other hand, for a reason why the intermolecular ht addition may be equilibrated, the electrostatic repulsion between the highly polar anhydride units may be considered similar to the well known case in the radical copolymerization of vinyl monomers carrying carbonyl(34) or nitrile group(35) in which the penultimate effect is involved. That is, the polymer(D) obtained via intramolecular hh and intermolecular tt additions in which five-membered anhydride units are separated by two methylene units is sterically and/or electrostatically favorable compared with the polymer(E) formed via intramolecular ht and intermolecular ht additions in which six-membered anhydride units are separated by only one methylene unit. 

This influence by the penultimate chain end can considerably modify the reactivity ratio (Table 22-4). The effect is particularly strong with monomers containing very polar groups near the main chain, and it is probably also responsible for the fact that the product of the conventionally determined reactivity ratios in the free radical copolymerization of ethylene with different monomers is sometimes greater than one (Table 22-5). 

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