Measurement of Propagation Rate Constants

In the mid-1960s the first measurements of propagation rate-constants for unsaturated monomers became available, from polymerisations initiated by y-radiation [5]. The circumstances of these experiments were such that it was immediately clear that these very high rate constants (106 to 108 1 mol"1 s 1) were those of unpaired cations, kp. All these reactions were carried out with bulk monomer, i.e., the polymerisations occurred in a medium of very low polarity (e c. 2 for hydrocarbons and 5 to 6 for alkylvinylethers). Unfortunately, the y-radiation method is not applicable to polymerisations in solution, especially in polar (usually alkyl halide) solvents. The methods which have been used to... 

The propagation reaction itself is of the first order with respect to the monomer concentration. This was demonstrated by measuring the propagation rate constants Kp at different monomer concentrations (98). 

The last experimental work was aimed at measuring the propagation rate-constant, kp, for various monomers under the ideal conditions which, it was hoped, would be provided by the solvent nitrobenzene. This was frustrated by the complexing of the propagating carbenium ions with the solvent but the mechanistic insight resulting from a detailed examination of the results provided useful new understanding of previously unexplained anomalies. 

This is the third report on attempts to measure the propagation rate constant, kp+, for the cationic polymerisation of various monomers in nitrobenzene by reaction calorimetry. The first two were concerned with acenaphthylene (ACN) [1, 2] and styrene [2]. The present work is concerned with attempts to extend the method to more rapidly polymerising monomers. With these we were working at the limits of the calorimetric technique [3] and therefore consistent kinetic results could be obtained only for indene and for phenyl vinyl ether (PhViE), the slowest of the vinyl ethers 2-chloroethyl vinyl ether (CEViE) proved to be so reactive that only a rough estimate of kp+ could be obtained. Most of our results were obtained with 4-chlorobenzoyl hexafluoroantimonate (1), and some with tris-(4-chlorophenyl)methyl hexafluorophosphate (2). A general discussion of the significance of all the kp values obtained in this work is presented. 

Propagation rate constants have been found to depend not only on the substrate but also on the nature of the attacking alkylperoxy radical. Thus, in order to obtain a meaningful correlation of propagation rate constants with C—H bond energies, the rate constants should be compared for the reactions of a series substrates RH with the same alkylperoxy radical. These rate constants can be measured experimentally by carrying out the autoxidations of the various substrates RH in the presence of moderate concentrations of an alkyl hydroperoxide R 02H. Under these conditions all of the alkylperoxy radicals derived from RH undergo chain transfer with the added hydroperoxide,... 

On the other hand, a certain dose of creative spirit is appropriate. When the requirements of modem research methods are respected, good reproducibility can be achieved, disturbing effects can be limited, and our knowledge can be promoted by a further step. The measured constants, even though only defined for a certain system, form an excellent basis for further discoveries. The values of propagation rate constants for some monomers in radical, ionic and coordination polymerizations are summarized in Table 9. 

Therefore further progress in this area depends on the measurement of equilibrium constants. At this stage I simply cannot say how much of the difference of two powers of 10 between the k+Bpl of the alkenes and the styrenes is to be attributed to an intrinsic difference in reactivity and how much to the existence of the P+ G complexes. The negative temperature coefficient of the rate constant for a-methyl styrene found by Chawla Huang (1975) is a strong indication in favour of my view that the propagation is not a simple bimolecular reaction. 

The list may yet be incomplete, but it involves nothing that is not familiar from other parts of chemistry and is free from any ad hoc inventions. It helps one to realise that most cationoid polymerisations under most conditions are likely to be eniedic and that consequently the rate equations will contain several terms. Therefore the corresponding propagation rate-constant is an apparent , composite quantity, difficult to define, and consequently any alleged measurements of this kpA or its components are likely to be doubtful thus one can understand the wide discrepancies in the reported values of alleged kp ... 

We report on the measurement of the propagation rate constants kp of styrene, indene, phenyl vinyl ether (PhViE) and 2-chloroethyl vinyl ether (CEViE) in nitrobenzene at (mostly) 298 K with 4-ClC6H4CO+SbF 6 as initiator. The dependence of the conductivity on the [4-ClC6H4CO+SbF"g] = c0 helped to establish that [Pn+] = c0 and thus to validate the foundation of this work. It is shown that most probably the propagating species are the uncomplexed, unpaired, solvated carbenium ions. Some new enthalpies of polymerisation have been found. 

The present author wanted to determine the propagation rate-constant, kp, for the cationic polymerisation of various alkenes (this term here includes vinyl ethers, VE) under such conditions that the interpretation of the measurements should be as unambiguous as possible. If there are no complications from the complexation of the propagating species with constituents of the reaction mixture, the rate of such polymerisations is given generally by equation (1) ... 

It is evident from the foregoing exposition that almost all the discussions in the literature about the propagation rate-constants of cationic polymerisations are defective, and that most attempts at their measurement are flawed in some more or less important respect. The present author notes with regret that his own earlier writings on the subject are no more than preliminary approaches, and he quite expects that even the present, much more profound, discussion may yet turn out to be defective in some essential respect. 

The propagation rate constant and the polymerization rate for anionic polymerization are dramatically affected by the nature of both the solvent and the counterion. Thus the data in Table 5-10 show the pronounced effect of solvent in the polymerization of styrene by sodium naphthalene (3 x 1CT3 M) at 25°C. The apparent propagation rate constant is increased by 2 and 3 orders of magnitude in tetrahydrofuran and 1,2-dimethoxyethane, respectively, compared to the rate constants in benzene and dioxane. The polymerization is much faster in the more polar solvents. That the dielectric constant is not a quantitative measure of solvating power is shown by the higher rate in 1,2-dimethoxyethane (DME) compared to tetrahydrofuran (THF). The faster rate in DME may be due to a specific solvation effect arising from the presence of two ether functions in the same molecule. 

The results obtained by liquid-phase oxidation or co-oxidation of various hydrocarbons are reviewed, and new results are reported for new kinds of compounds such as alkyl-aromatics, alcohols, and ethers, which were also systematically studied by co-oxidation. Gathering all kinetic data and discussing them in connection with data on absolute termination constants, obtained by other groups through physical measurements, enables us to estimate the termination and propagation rate constants for about 40 compounds and to present characteristic values for some new classes of compounds. Examples demonstrate that co-oxidation studies make it possible to explain the behavior of complex compounds reacting by different kinds of bonds, and more particularly the behavior of polymers oxidized in solution. 

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