Rate constant cationic chain polymerization

When one compares the brutto polymerization rate constants, a measure of the reactivity of monomers during cationic homopolymerizations is obtained. It was found for p-substituted styrenes that lg kBr increased parallel to the reactivity, which the monomers show versus a constant acceptor 93). The reactivity graduation of the cationic chain ends is apparently overcomed by the structural influence on the monomers during the entire process of the cationic polymerization. The quantitative treatment of the substituent influences with the assistance of the LFE principle leads to the following Hammett-type equations for the brutto polymerization rate constants ... 

The determination of the various rate constants (ki, kp, kt, kts, ktr) for cationic chain polymerization is much more difficult than in radical chain polymerization (or in anionic chain polymerization). It is convenient to use Rp data from experiments under steady-state conditions, since the concentration of propagating species is not required. The Rp data from non-steady-state conditions can be used, but only when the concentration of the propagating species is known. For example, the value of kp is obtained directly from Eq. (8.143) from a determination of the polymerization rate when [M J is known. The literature contains too many instances where [M" "] is taken equal to the concentration of the initiator, [IB], in order to determine kp from measured Rp. (For two-component initiator-coinitiator systems, [M" ] is taken to be the initiator concentration [IB] when the coinitiator is in excess or the coinitiator concentration [L] when the initiator is in excess.) Such an assumption holds only if Ri > Rp and the initiator is active, i.e., efficiency is 100%. Using this assumption without experimental verification may thus lead to erroneous results. 

Rate Constants for Propagation and Termination. For cationic chain polymerizations, the rate of polymerization is generally accepted to be proportional... 

VEs do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. This is on account of the differences in reactivity, resulting in significant rate differentials. Consequendy, reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymeriza tion (50,51). Table 5 illustrates this situation for butyl vinyl ether (BVE) copolymerized with other VEs. The rate constants of polymerization (kp) can differ by one or two orders of magnitude, resulting in homopolymerization of each monomer or incorporation of the faster monomer, followed by the slower (assuming no chain transfer). 

The DPs obtained in cationic polymerizations are affected not only by the direct effect of the polarity of the solvent on the rate constants, but also by its effect on the degree of dissociation of the ion-pairs and, hence, on the relative abundance of free ions and ion-pairs, and thus the relative importance of unimolecular and bimolecular chain-breaking reactions between ions of opposite charge (see Section 6). Furthermore, in addition to polarity effects the chain-transfer activity of alkyl halide and aromatic solvents has a quite distinct effect on the DP. The smaller the propagation rate constant, the more important will these effects be. 

It is the author s hope that the foregoing detailed discussion has helped to clarify some features of cationic polymerizations. Many other aspects, such as co-polymerization and radiation polymerization, which I have not been able to discuss here, deserve equal attention. But perhaps the most urgent task, and one which is much more widely relevant, is the elucidation of details of reaction mechanism, and in particular the identification of the chain-carriers in many widely differing systems. The next problem then is to measure their concentration, its variation throughout the reaction, and, hence, the absolute rate constants. It is essential that the factors which decide whether a polymerization is ionic or pseudo-ionic be determined as soon as possible. 

For any specific type of initiation (i.e., radical, cationic, or anionic) the monomer reactivity ratios and therefore the copolymer composition equation are independent of many reaction parameters. Since termination and initiation rate constants are not involved, the copolymer composition is independent of differences in the rates of initiation and termination or of the absence or presence of inhibitors or chain-transfer agents. Under a wide range of conditions the copolymer composition is independent of the degree of polymerization. The only limitation on this generalization is that the copolymer be a high polymer. Further, the particular initiation system used in a radical copolymerization has no effect on copolymer composition. The same copolymer composition is obtained irrespective of whether initiation occurs by the thermal homolysis of initiators such as AIBN or peroxides, redox, photolysis, or radiolysis. Solvent effects on copolymer composition are found in some radical copolymerizations (Sec. 6-3a). Ionic copolymerizations usually show significant effects of solvent as well as counterion on copolymer composition (Sec. 6-4). 

The kinetics of radiation-induced polymerization of bulk nitroethylene was also studied at 10° C by the use of hydrogen bromide as an anion scavenger (27). The value of Gt (yield of the initiation by 100 eV energy absorbed) was found to be about 3, which was much larger than the value obtained for many radiation-induced cationic polymerizations. The propagation rate constant, kp, was estimated to be 4 x 107 M-1 sec-1. The large kp value was attributed to the concept that the propagating chain ends were free ions in contrast to the existence of counter ions in catalytic polymerization. 

Solvent polarity and temperature also influence ihe results. The dielectric constant and polarizability, however, are of little predictive value for the selection of solvents relative to polymerization rates and behavior. Evidently evety system has to he examined independently. In cationic polymerization of vinyl monomers, chain transfer is the most significant chain-breaking process. The activation energy of chain transfer is higher than that of propagation consequently, the molecular weight of the polymer increases with decreasing temperature. 

In cationic ring-opening polymerization, there are not too many examples of the systems in which ratios of kplk, are known. In the polymerization of substituted aziridines and substituted thietanes the ratios of rate constants of chain transfer to polymer to the rate constants of propagation have been measured and at least the value obtained for polymerization of N-/-butylaziridine (1.2-104) [260], indeed indicates the living character... 

The proportions of oxonium and carbenium ions in the chain growth, as well as their relative reactivities, may markedly depend on the polymerization coixlitions and vary with monomer structure. The high values of rate constants of the reaction between carbenium cations and ethers or acetals make questionable Okada s interpretation of the 1,3-dioxane-triethyloxonium salts system It is impossible to observe, at room temperature, the individual ROCHl species with excess of such nucleophiles as diethyl ether. We would rather suggest that the signal at 11—13 ppm 6 ( H-NMR) or 175—180 ppm 6( C-NMR) is due to some exchanging protonated species. 

Here we can draw an analogy with the equilibrium dissociation reaction, when the association rate constant in equilibrium is not limited by diffusion, regardless of the viscosity of the medium. In our opinion, this question requires at present a theoretical and experimental investigation. It is customary to assume that radical polymerization is characterized by a rather intensive chain termination reaction and a short time for the propagation of one chain, as compared to the time of polymerization. The existence of continuous processes ( living polymers) has been ascertained for anionic9 and cationic polymerization10, where there is no bimolecular interaction of active centers with one another. Let us now examine certain radical polymerization processes in which the chain termination reactions are considerably inhibited or almost excluded. 

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