Monomer reactivity ratio steric effects

The effect of temperature on the monomer reactivity ratio is fairly small. In those few cases examined with sufficient accuracy,the ratio nearly always changes toward unity as the temperature increases —a clear indication that a difference in activation energy is responsible, in part at least, for the difference in rate of the competing reactions. In fact, the difference in energy of activation seems to be the dominant factor in these reactions differences in entropy of activation usually are small, which suggests that steric effects ordinarily are of minor importance only. 

The anionic copolymerization of styrene and l-(4-dimethylaminophenyl)-1-phenylethylene in benzene has been investigated [188]. As discussed previously in Sect. 5, Yuki and coworkers [125, 126, 129, 133-136] have developed the formalism for analyzing the kinetics of copolymerization of 1,1-diphenylethylene (M2) with styrene and diene monomers (Mi). It was assumed that the 1,1-diphenylethylene derivative, M2, does not add to itself due to steric effects, i.e., k22=0, as discussed previously in Sect 5. Thus, the monomer reactivity ratio for M2 is zero, i.e., r2- 22l ii- - It was also assumed that the styrene monomer is completely consumed at the end of the polymerization... 

We use the term constitution to describe the way in which the monomeric units, or constitutional units, are linked together. A knowledge of copolymer constitution thus requires a study of the distribution of the constitutional sequences (more briefly, sequences) of both monomers. As a general rule, the constitution is quantitatively characterized by the product of the reactivity ratios, the parameter of the terminal copolymerization model. The presence of non-ideal constitutional units is not accounted for by this model small numbers of inversions of C3 units or steric effects must be regarded as a perturbation in this approximation. 

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. 

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. 

The scheme above belies the complexity of the reaction see Twitchett s discussion for the detailed mechanism [2]. The overall reaction between formaldehyde and two moles of aniline produces the 4,4 -, 2,4 -, and 2,2 -isomers of methylenedianiline. These diamine isomers will react to form the higher methylene bridged polyphenylene polyamines. It has been stated that 2,4 - and 2,2 -methylenedianiline are preferentially consumed for the production of the higher polyamines [4,5]. The author is unaware of any published work to this effect, but such a preferential reactivity is expected from the steric influence on electrophilic aromatic substitution. It is certainly true that under typical industrial conditions the resulting monomer fraction is mostly 4,4 -methylenedianiline, with 2 to 7% of the 2,4 -isomer and lesser quantities of the 2,2 -isomer. The 2,4 - and 2,2 -isomer levels may be dramatically increased with elevated reaction pressures and heterogeneous catalysis [6,7]. Otherwise, the composition of the polyamine mixture can be altered through several variables including the aniline to formaldehyde ratio, the aniline to HQ ratio, and temperature [2,5]. 

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) ... 

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