Q-e values

There have, however, been attempts to correlate Q-e values and hence reactivity ratios to, for example, c NMR chemical shifts 50 or the results of MO calculations 51153 and to provide a better theoretical basis for the parameters. Most recently, Zhan and Dixon153 applied density functional theory to demonstrate that Q values could be correlated to calculated values of the relative free energy for the radical monomer reaction (PA + Mn — PA ). The e values were correlated to values of the electronegativities of monomer and radical. 

The NMR method of predicting Q-e values appears attractive since spectra can be measured under the particular reaction conditions (solvent, temperature, pH). Thus, it may be possible to predict the dependence of the Q-e values and reactivity ratios on the reaction medium. 10... 

LJ Young. Tabulation of Q-e values. In J Brandrup, EH Immergut, W McDowell, eds. Polymer Handbook. 2nd ed. New York Wiley-Interscience, 1975, pp 387-404. 

Obviously the precision of this procedure is not very great, since the assumptions underlying the calculations of Q and e values can be regarded at best as semiquantitative. However, it has been shown that when the reactivity ratios are back-calculated from the Q,e values, quite good agreement is obtained with the experimental values, so that it is possible to make useful predictions of reactivity... 

Figure 3 shows the correlation obtained by plotting logio (relative rate) as a function of log Q — 0.5 (e + 0.8) for the seven monomers. Over a range of oxidation rates varying by a factor of 100 the relation predicts the rate from Q,e values to less than a factor of 3. This is less precise than the correlation with excitation energies used for alkyl-subsituted ethylenes (18), but is probably all that can be expected, since the Q,e system is an empirical relation and the assumption of equal reactivities and termination rate constants for primary and secondary peroxy radicals is imprecise (9). 

Only a limited number of monomer pairs form block copolymers in this manner. Examples are conjugated dienes and vinyl aromatics that have similar Q-e values. The nature of the anionic initiator, i.e., the ionic character of the carbon-metal bond plays an important role in both the amount and sequence of block formation. For instance, when potassium or cesium initiators are used, styrene polymerizes first as can be seen in Figure 12. 

Some quantitative data (r1 r2, Q, e values) for the copolymerization of captodative olefins are available, mainly for the copolymerization with styrene. They are compiled in Tables 11-14. In Tables 11 and 12, the data for the copolymerization of captodative olefins with styrene are given for comparison together with the values for other 1,1-disubstituted olefins. However, these data have to be considered with caution because of the possibility that some propagation steps could be reversible and because of possible penultimate effects [95]. 

The monomer reactivity ratios could be calculated from Table A and other values by the method of Fineman and Ross (10), but owing to the narrow range of compositions studied only the value of r2 (referring to the styrene radical) was significant. A value of 0.7 was obtained which may be compared with 0.52 for styrene-methyl methacrylate, and a value of 0.41 calculated from the Q — e values for hydroxyethyl methacrylate supplied by Rohm and Haas (25). 

Extensive lists of Q-e values are given in handbooks and texts on copolymerization. Table 7-2 contains a brief list. As a general rule monomers with electron-rich double bonds have more negative e values and those which are extensively resonance stabilized have higher Q numbers. 

Rank the following monomers in order of their increased tendency to alternate in copolymerization with butadiene and explain your reasoning vinyl acetate, styrene, acrylonitrile, and methyl methacrylate, Hint Use Q-e values if reactivity ratios are not readily available.)... 

Monomer Reactivity Ratios and Q-e Values in Copolymerization of 2,3-Epoxy-1-propyl Methacrylate (Mi) with Vinylpyridines (Ma) Ma... 

TABLE 1.1 AJfrey Price Q, e Values For Selected Vinylcyclopentadienyl Metal-Contaiimig Monomers Versus Common Organic Monomers... 

S is taken as the reference monomer with =1.0 and e - -0.8. Values for other monomers are derived by regression analysis based on literature or measured reactivity ratios. The Q-e values for some common monomers as presented in the Polymer Ihwdbook are given in Table 7.7. The accuracy of Q-e parameters is limited by the quality of the reactivity ratio data and can also suffer from inappropriate statistical treatment employed in their derivation. A further problem is that the data analysis makes no allowance for the dependence of reactivity ratios on reaction conditions. Reactivity ratios can he dependent on solvent (Section 7.3.1.2), reaction temperature, pll, etc. It follows that values of e and perhaps Q for a given monomer should depend on the medium, the monomer ratio and the particular comonomer. This is especially true for monomers which contain ionizablc groups e.g. MAA, A A, vinyl pyridine) or arc capable of fomiing hydrogen bonds e.g. HEMA, HEA). 

There have, however, been attempts to correlate Q-e values and hence... 

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