Monomer reactivity ratio resonance effects

All the above factors controlling monomer and radical reactivities contribute to the rate of polymerization, but in a manner which makes it difficult to distinguish the magnitude of each effect. Attempts to correlate copolymerization tendencies based on these factors are thus mainly of a semiempirical nature and can, at best, be treated as useful approximations rather than rigorous relations. However, a generally useful scheme was proposed by Alfrey and Price [23] to provide a quantitative description of the behavior of diferent monomers in radical polymerization, with the aid of two parameters, for each monomer rather than for a monomer pair. These parameters are denoted by Q and e and the method has been called the Q — e scheme. It allows calculation of monomer reactivity ratios r and T2 from properties of monomers irrespective of which pair is used. The scheme assumes that each radical or monomer can be classified according to its reactivity or resonance effect and its polarity so that the rate constant... 

Copolymerization. Acrylic and methacrylic acids readily copolymerize free radically with many vinyl monomers. This versatility results from a combination of their highly reactive double bonds and their miscibility with a wide variety of water- and solvent-soluble monomers. Reactivity ratios derived from copolymerizations with many monomers are tabulated in many books on polymerization, for example in Wiley s Polymer Handbook (14) (see also Wiley s Database of Polymer Properties). Q and e values are parameters that may be established for a monomer based on a large number of reactivity ratios with other monomers. These parameters are associated with interactions between the monomer and the growing chain via resonance (Q) and polar effects (e). 

The results of the Alfrey-Price Q-e parameters [76] (where Q and e take into account the stabilization by resonance and the polar effects of the monomer) are commonly used to predict the monomer reactivity ratios. The Alfrey-Price parameters are known for AN, MAN, MVCN, and MATRIF [68]. The calculated values of the reactivity ratios, and r2, and the product (rj x rj) are summarized in Table 20.2. 

Alfrey and Price proposed a means of predicting monomer reactivity in copolymerization from two parameters, (a measure of resonance) and e (a measure of polar effects) (8). These parameters have been related to the reactivity ratios by equations 15—17. 

The basis of the scheme developed particularly by Alfrey and Price is the assumption that the activation energies of the propagation reactions, and hence the related rate constants and reactivity ratios, are governed primarily by resonance effects and by the interaction of the charges on the double bonds of the monomers with those in the active radicals. Accordingly, the rate constant of the reaction between a radical and a monomer is represented by ... 

On the other hand presence of B -N dyad in the polymer could only arise from a transesterification reaction where the B was inserted between the -BN-. As shown in Figure 3, NMR displays a small but distinct peak (ca.14%) at the resonance position corresponding to B -N diad. Since the polymerization was run at the same temperature of 245°C forl70 min and to the same degree of polymerization (M n 2252) as the earlier experiment on the reactivity ratios, one can conclude that the role of interchain transesterification is relatively small and that the monomers have approximately equivalent reactivity ratios. The reaction of B with the HBA-HNA dimer was also examined at 225° and 285°C to determine the possibility of a temperature effect The times of reaction were 20 hrs and one hour, respectively,... 

Ionic copolymerizations are more complicated than free-radical ones. Various complicating factors arise from effects of the counterions and from influences of the solvents. These affect the reactivity ratios. In addition, monomer reactivity is affected by the substituents. They influence the electron densities of the double bonds and, in cationic polymerizations, the resonance stabilization of the resultant carbon cations. Yet, the effects of the counterions, the solvents, and even the reaction temperatures can be even greater than that of the substituents in cationic polymerizations. There are only a few studies reported in the literature, where the reactivity ratios were determined for different monomers, using the same temperature, solvent, and counterion. One such study was carried out on cationic copolymerizations of styrene with two substituted styrenes. These were a-methylstyrene, and with chlorostyrene. " The relative reactivity ratios of these substituted styrenes were correlated with Hammett pa values. The effect of the substituents on reactivity of styrene fall in the following order ... 

The inhibiting effect of radical acceptors, however, suggests that polymerization proceeds by a radical mechanism. The yield of polymer from such reactions depends on the tendency of the different monomers to be polymerized by ionic and radical catalysts. The reactivity ratio on grinding for styrene plus methyl methacrylate is similar to that for peroxy-initiated polymerization. Nuclear magnetic resonance data, however, show a different stereochemical configuration for the copolymers [200]. 

Steric hindrance and electronic (inductive and resonance) effects are involved in the intrinsic reactivity of R . The two same effects also determine the reactivity of the monomer (M). To evaluate the proper reactivity of M irrespective of that of R, it can be measured by what is called methyl affinity. By convention, this affinity (a) is taken equal to the ratio of the rate constant of addition (ki) onto the monomer double bond to the rate constant of a reference reaction, which is the transfer reaction (k -) of CH3 to isooctane (abstraction reaction) ... 

Normally reactivity ratios lie between 0 and 1 (Table 2.9) and so there is usually a tendency toward alternation in most copolymerization reactions. It is found that for the same pair of monomer molecules the reactivity ratios can differ greatly depending upon the nature of the chain carrier used (i.e. free radical, cationic or anionic). Obviously the rate constants fcii, k 2, ki2 and k2 will be affected in different ways by the nature of the active centre and it is found that the relative reactivity of different monomers can be correlated with resonance stability, polarity and steric effects. Such correlations are beyond the scope of this book and the reader is directed towards more advanced texts. 

Most copolymerizations in the presence of a free radical initiator obey the simple copolymerization equation. Equation (22-22). Consequently, the copolymerization parameters calculated from this equation can be interpreted directly as the ratios of two rate constants. Since they are relative reactivities, they must be influenced by the polarity, the resonance stabilization, and the steric effects of the monomers. In these cases, resonance stabilization effects are generally stronger than polarity influences, and these, in turn are greater than effects due to steric hindrance. 

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