Cationic chain polymerization free ions

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. 

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. 

The chain carriers in chain-growth polymerization may be anions or cations rather than free radicals. Such ionic polymerization shares many features with free-radical polymerization, but differs in one important respect Since ions of the same charge sign repel one another, spontaneous binary termination by reaction of two chain carriers with one another cannot occur. In fact, the reaction may run out of monomer with chain carriers still intact. 

Termination will occur when the carbocation undergoes reaction with nucleophilic species other than monomer to produce a dead chain and no re-initiation. Since cationic polymerizations are carried out with high-purity reagents and under rigorous conditions this reaction is much less likely than chain transfer to monomer. The mutual repulsion of the charged polymerization sites ensures that bimolecular termination cannot occur (unlike in free-radical polymerization, where this is the most probable termination route). Recombination of the cation with the counter-ion will occur, and these termination reactions are often very specific to the chemistry of the initiator. 

In chain polymerization initiated by free radicals, as in the previous example, the reactive center, located at the growing end of the molecule, is a free radical. As mentioned previously, chain polymerizations may also be initiated by ionic systems. In such cases, the reactive center is ionic, i.e., a carbonium ion (in cationic initiation) or a carbanion (in anionic initiation). Regardless of the chain initiation mechanism—free radical, cationic, or anionic—once a reactive center is produced it adds many more molecules in a chain reaction and grows quite large extremely rapidly, usually within a few seconds or less. (However, the relative slowness of the initiation stage causes the overall rate of reaction to be slow and the conver-... 

In general, these anions are associated with a counterion, typically an alkali metal cation. The exact nature of the anion can be quite varied depending on the structure of the anion, counterion, solvent, and temperature [3-5]. The range of possible propagating species in anionic polymerization is depicted in terms of a Winstein spectrum of structures as shown in Equation 7.2 for a carbanionic chain end (R ) [3, 6]. In addition to the aggregated (associated) (I) and unaggregated (unassociated) (2) species, it is necessary to consider the intervention of free ions (5), contact... 

In principle, the appearance of electron donor groups near the double bond of the monomer leads to a cationic mechanism, while positive groups that withdraw electrons mostly lead to the anionic process. An increase of temperature usually leads to a decrease in rate of reaction or length of the chain. Polymerization always occurs in solution, wherein the solvent acts as separator of the ion-pair, and quite often as a transfer reagent. As already mentioned, most monomers undergo polymerization via free radicals, mainly when conjugated double bonds are present or substitute groups that withdraw electrons. 

In these chain addition reactions, the active species is cationic in nature, initiated by strong acids, either of the protic or Lewis variety [21,104-111], Since most of these ionic polymerizations are carried out in nonaqueous solvents with low dielectric constants [109], it is unlikely that the active species is a free ion, analogous to a free radical. A multiplicity of active species may be involved as propagating species as shown below by the spectrum of cationic species, one or more of which may be involved as active propagating species, especially in more polar solvents [112], Unfortunately, very little information... 

An important characteristic of ionic polymerization is that the propagation rate coefficients are several orders of magnitude higher than for free-radical polymerization. In the equation fct[X] is the bimolecular termination rate coefficient multiplied by the impurity concentration. This equation shows that the rate of polymerization is proportional to the first power of initiation rate, i.e., to the first power of dose rate. Water is a common chain breaker of cationic polymerization since it replaces the cation by a hydroxonium ion. As a proton donor it also inhibits the anionic polymerization... 

The chemical mechanism of the preparation of ZnS nanoparticles in PVA matrix is shown in Figure 2. From the preparation method used here, there is a high probability of free ions at the polymeric chain without S ion counterparts. Zinc accetate dissociate into zinc ions (Zn +) and acetate (Ac ) ions in aqeous solution. Similarly, HjS dissociate into its respective cations and anions (Manzoor et al., 2003 Sharma... 

The polarity of the solvent affects chain propagation in cationic polymerization. The higher the polarity, the stronger the equilibrium shift toward separated ion pairs and free ions and the faster the addition of monomers to these ions. For the polymerization of styrene under HCIO4 in CCI4—CICH2CH2CI mixtures with different... 

As described above, the radiolysis of olefinic monomers results in the formation of cations, anions, and free radicals. It is possible for these species to initiate chain polymerizations. Whether radiation-induced polymerization is initiated by radicals, cations, or anions depends on the monomer and the reaction conditions. However, in most radiation-initiated polymerizations, initiating species are radicals [4]. It is usually only at low temperatures that ions are stable enough to react with a monomer [5]. At ambient temperatures... 

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