Propagation constants dienes

TABLE 9. Propagation rate constants (kprop) for the polymerization of selected alkenes and dienes at 60 °C6,22,23... 

It can be seen that the kinetic studies on dienes confirm that the propagation reaction has a first-order dependence on the monomer concentration. These observations are to be expected since in the absence of adventitious impurities the number of growing chains should remain constant, only the monomer concentration decreases. 

Monomeric alkyl lithium polymerizes isoprene through an anionic type propagating species. The transition between cis 1,4 and trans 1,4 polymerization is not clear since mono-ene polymerization also occurs in this region. Increased dielectric constant of the media, the addition of ethers, or the use of high lithium alkyl concentrations increased the character from that weakly anionic for the cis-diene polymerization to the slightly more anionic requirements for 3.4-monoolefinic polymerizations. 

The presence of electron-withdrawing substituents such as halogen on dienes substantially increases their propagation rate constants and reduces termination rate constants. As shown in Table 11, kc0 does not increase from isoprene to chloroprene so that Cc decreases. The trend continues for 2,3-dichloroduta-diene. Although the propagation rate constant for 1,2-dichlorobutadiene is not known, its Cc is less than... 

Equation 17 can then be plotted as a linear function of rate versus initiator concentration to yield k and k Kg. Furthermore, if Kg is determined, for example, from conducxivity measurements, then the absolute value of k is also available. A number of such measurements have been taken and yield rate constant values for various monomers (mainly styrenes and dienes) and various counterions and solvents (3 ). In general these data indicate that, although the free anions are only present in very small proportion (Kg lO ), they are responsible for most of the chain propagation because their rate constants (kp 10 -10 M sec ) are several orders greater than those of the ion pairs (k 10 M sec ). Hence, Reaction 14 seems to represent an adequate picture of the anionic mechanism in these systems. 

The reduction in the propagation rate constant under the effect of aromatic hydrocarbon is due to hydrocarbon and diene monomer competing for coordination at the active centre. The effect of slowing down the polymerisation reaction is due to the fact that aromatic hydrocarbon forms an arene complex with the lanthanide atom. The higher the 7t-electron-donating ability of the aromatic hydrocarbon, the lower the rate constant for polydiene chain propagation. Thus, the rate of polymerisation of dienes decreases in the series benzene > toluene > xylene [11, 14, 26, 28]. 

Equation (2.26) leads to a solution for from available knowledge of the rate R, the concentration of monomer in the monomer-polymer particles [M], and the number of particles, N. This method has been applied to several monomers and has been especially useful in the case of the dienes, where the classical method of photoinitiation poses difficulties. Some of these results are shown in Table 2.4 in the form of the usual kinetic parameters. The results obtained for styrene by photoinitiation techniques are included for comparison. It can be seen that the agreement is remarkably good, considering the widely different experimental methods used. Recent studies of the emulsion polymerization of butadiene have shown that the rate constant for propagation is even higher than previously estimated (see Table 2.1) (Weerts et al., 1991). 

The data in Table 2.4 provide evidence that the slow rates and low molecular weights obtained in homogeneous free radical polymerization of these dienes are not due to a low rate constant for propagation but rather must be caused by a high rate constant for termination (as indicated in Table 2.1) (Matheson et al., 1949,1951 Morton and Gibbs, 1963). Hence, under the special conditions of emulsion polymerizations, where the termination rate is controlled by the rate of entry of radicals into particles, it becomes possible to attain both faster rates and higher molecular weights. It is this phenomenon which led to the rise of the emulsion polymerization system for the production of diene-based synthetic rubbers. 

In this article, I have summarized applications of ESR spectroscopy to the radical polymerization of vinyl and diene compounds, with particular emphasis put on the advantages of the special cavities designed for enhancing the sensitivity of ESR spectrometers. These cavities allow determinations of the conformations of propagating radicals as well as the propagation rate constants for several monomers under conditions similar to usual radical polymerizations. However, I am afraid that the reliability of the kinetic data obtained with them is not yet high enough for precise determination of the rate constants because of experimental errors involved in the measurement of radical concentrations and polymerization rates. It is desirable that the ESR spectrometer be made about 10 times more sensitive for really reliable data... 

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