Free ions reactivity

Polymerization of P-propiolactone in a highly polar solvent (DMF) in the presence of a crown ether shows the unexpected feature that ion-pair reactivity decreases more slowly with decreasing temperature compared to free-ion reactivity [Penczek et al., 2000a,b Slomkowski, 1986]. Solvation of free ions becomes so extensive at 20°C that they are less reactive than ion pairs. 

This means that the active propagating sites such as free alkoxy ions and ion pairs are solvated with the hydroxyl groups. This must lead to an increase in the reactivity of the solvated ion pair as compared with that of the contact ion pair and decrease in the free ion reactivity, i.e. ultimately to the levelling off of the reactivity differences of these particles. 

The effect on ion pair propagation m ht be expected to be less than that on free ion reactivities, because the effective charges are smaller anyway. 

In catalytic polymerization the reactivity of the propagation center depends on the catalyst composition. Therefore, the dependence of the molecular structure of the polymer chain mainly on the catalyst composition, and less on the experimental conditions, is characteristic of catalytic polymerization. On the other hand, in polymerization by free-radical or free-ion mechanisms the structure of a polymer is determined by the polymerization conditions (primarily temperature) and does not depend on the type of initiator. 

These various structures show characteristic differences of the reactivity during the propagation step. When one observes cationic polymerizations, the propagation via free ions takes place from 10 to 100 times faster than that via ion pairs 1-2). This ratio should be valid for anions from Lewis acids as well as those from protic acids. 

During this, the electrons of the partial X—Z multiple bond are used. Experiments show that the ester can be further active in the polymerization. Its reactivity, however, is reduced in comparison with ion pairs. From a mechanistical point of view, the chain propagation should proceed in the manner of a SN2 reaction, that is with the monomer as nucleophile and the ester as substrate. With the assistance of quantum chemical calculations using the CNDO/2 method, the differences between covalent species and free ions should be examined. The following contains the three types of anions used ... 

Although it does not concern us here, it should be mentioned that organoboron and organoaluminum compounds exhibit anionoid (Grignard) and free radical reactivity as well as their behavior as carbonium ion analogs. 

Since both elementary theory and experiments in small-molecule kinetics show that the reactivities of paired and free ions can be very different, the ratio Up plays a dominant part in the kinetics of all ionic reactions and therefore we must start with a detailed consideration of this topic. 

The value of (feobsd caic) at a given concentration of bromide anion depends on the association constant for formation of the ion pair from free ions (Ai s = and on the relative reactivity of the ion-pair and free carbocation toward addition of solvent For example, if is small, then the concentration of the ion-pair... 

A cation arriving with a nncleophilic anion is another important factor. The nucleophile can attack the substrate in the form of a free ion or an ionic pair. As a rule, lithium salts are less reactive than sodium and potassium salts. Russell and Mndryk (1982) reported several examples of this. The sodium salt of ethyl acetylacetate reacts with 2-nitro-2-chloropropane in DMF yielding ethyl 2-(wo-propylidene) acetylacetate. Under the same conditions, the lithium salt does not react at all. Potassium diethyl phosphite interacts with l-methyl-l-nitro-l-(4-toluylsulfonyl)propane in THF and gives diethyl 1-methyl-l-nitro-l-phosphite. The lithinm salt of the same reactant does not react with the same substrate in the same solvent. 

Throughout the remainder of the chapter it should be understood that any rate constants that are presented are apparent rate constants unless otherwise indicated to be those for the free ion or ion pair. In general, the available data are used to point out certain trends (e.g., the effect of solvent on reactivity) without necessarily accepting the exact value of any reported rate constant as that for the ion pair or free ion. Comparison of data from different investigators should be done with caution. 

It is not an absolute necessity for LCP to have no free ions. If free ions are present, LCP is possible only if there are fast equilibria between free ions, ion pairs, and covalent species. If the equilibrium between free ions and ion pairs is slow, the result is a bimodal distribution. Further, to have any possibility of LCP with free ions present, the concentration and reactivity of the free ions should not be such that the reaction is too fast. 

Experimental data of kj versus 1 /T can be fitted to the preceding equations to yield values of the various activation and thermodynamic parameters pertinent to ion-pair propagation (Table 5-12). Corresponding values of the parameters for free-ion propagation are included in Table 5-12 for comparison. The solvent-separated ion pair is approximately half as reactive as the free ion, while the contact ion pair is more than 3 orders of magnitude... 

It has previously been shown that large changes can occur in the rate of a cationic polymerization by using a different solvent and/or different counterion (Sec. 5-2f). The monomer reactivity ratios are also affected by changes in the solvent or counterion. The effects are often complex and difficult to predict since changes in solvent or counterion often result in alterations in the relative amounts of the different types of propagating centers (free ion, ion pair, covalent), each of which may be differently affected by solvent. As many systems do not show an effect as do show an effect of solvent or counterion on r values [Kennedy and Marechal, 1983]. The dramatic effect that solvents can have on monomer reactivity ratios is illustrated by the data in Table 6-10 for isobutylene-p-chlorostyrene. The aluminum bromide-initiated copolymerization shows r — 1.01, r2 = 1.02 in n-hexane but... 

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