Covalent species

The chemistry of Be is largely that of covalent species although ionic oxygen-co-ordinated species, e.g. [Be(H20>4]2 [Be3(OH)3p are... 

According to the Winstein-scheme, a spectrum of possible structures from covalent species to ion pairs to free ions can be formulated for the cationic chain end —C+... 

If anions, built during the initiation by means of HY/MtXn from Lewis acids, form a covalent species, a chain termination is the result according to ... 

The formation of covalent species from anions, which originate from protic acids, leads to an ester, according to ... 

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 ... 

AE the reaction energy for the recombination of the free ions leading to covalent species ... 

Aqc the alteration of the atomic charge at the C-atom of the original methyl group during the transfer into the covalent species as a measure of the charge transfer from the anion to the cationic centre (qc(CH3+) = 0.419) ... 

These are anions of very low nucleophilicity. The energy gain during the formation of the covalent species is strongly reduced in comparison with the halide ions. When... 

That the photoreactive species is the carbonium ion and not the corresponding alcohol is clearly indicated by the relative concentrations of the two species present. The calculated equilibrium constant for 5% aqueous sulfuric acid implies an alcohol content of 2 7 x 10 %, much too low to account for any detectable photoreaction from this covalent species. In addition, when the tropylium salt is irradiated in the absence of acid, neither 3 nor 4 is detected as a product, but rather ditropyl (5) and its photoisomer (6) are observed. 

A solution in which ionic, radical imd covalent species coexist in equilibria 209... 

A SOLUTION IN WHICH IONIC, RADICAL AND COVALENT SPECIES COEXIST IN EQUILIBRIA... 

OXIDATION OF INORGANIC COVALENT SPECIES 2.2.1 Halide ions... 

The problems (i)-(iii) are inextricably interrelated, because they all are, to some extent, of an analytical nature, and they have been solved satisfactorily only for relatively few systems. In particular, whether the propagating species are in fact ions, or whether they are highly polar covalent species, such as the esters of pseudo-cationic systems, has been clarified only for very few systems involving olefinic monomers although for many such our general background information on the chemistry of the species concerned and the pattern of the polymerisations themselves make an ionic propagating species much more likely [3, 4],... 

To 2.2.8 the Commentator s second sentence is not true. In an aromatic system p-X-C6-H4-CH=CH2, any changes in X which make the double bond more reactive than it is with X=H, will have the same effect on the reactivity of any derived covalent species, such as an ester, but it will have the opposite effect on the reactivity of the derived carbenium ion p-X-C6H4-C+HCH3. This is one of the most cogent arguments against the Commentator s views. 

For the cytochrome c-plastocyanin complex, the kinetic effects of cross-linking are much more drastic while the rate of the intracomplex transfer is equal to 1000 s in the noncovalent complex where the iron-to-copper distance is expected to be about 18 A, it is estimated to be lower than 0.2 s in the corresponding covalent complex [155]. This result is all the more remarkable in that the spectroscopic and thermodynamic properties of the two redox centers appear weakly affected by the cross-linking process, and suggests that an essential segment of the electron transfer path has been lost in the covalent complex. Another system in which such conformational effects could be studied is the physiological complex between tetraheme cytochrome and ferredoxin I from Desulfovibrio desulfuricans Norway the spectral and redox properties of the hemes and of the iron-sulfur cluster are found essentially identical in the covalent and noncovalent complexes and an intracomplex transfer, whose rate has not yet been measured, takes place in the covalent species [156]. 

This is clearly illustrated in the example given below for the reaction of the methoxymethyl cation with pivaldehyde. There are four possible reaction channels for the unimolecular dissociation of the initially formed adduct, which is presumed to be of the form of a covalent species resulting from the interaction of the carbocation center of the methoxymethyl cation with the carbonyl oxygen of the aldehyde, 2. For smaller carbonyl compounds this adduct has been demon-... 

The major species in most cationic polymerization systems are covalent, including halides and esters. There has been considerable controversy as to whether covalent species undergo propagation by direct monomer insertion into a covalent bond or only through ionization to... 

Similar effects are observed for reaction systems where covalent species are not present. Thus, BCI3 + H2O do not polymerize neat isobutylene, but polymerization occurs for... 

The major approach to extending the lifetime of propagating species involves reversible conversion of the active centers to dormant species such as covalent esters or halides by using initiation systems with Lewis acids that supply an appropriate nucleophilic counterion. The equilibrium betweem dormant covalent species and active ion pairs and free ions is driven further toward the dormant species by the common ion effect—by adding a salt that supplies the same counterion as supplied by the Lewis acid. Free ions are absent in most systems most of the species present are dormant covalent species with much smaller amounts of active ion pairs. Further, the components of the reaction system are chosen so that there is a dynamic fast equilibrium between active and dormant species, as the rates of deactivation and activation are faster than the propagation and transfer rates. The overall result is a slower but more controlled reaction with the important features of living polymerization (Sec. 3-15). 

In addition to the choice of Lewis acid, added common ion salt, and temperature, the fast equilibrium between active and dormant species can be fostered by including additional nucleophiles (separate from the nucleophilic counterion) in the reaction system and by variations in solvent polarity. Nucleophiles act by further driving of the dynamic equilibrium toward the covalent species and/or decreasing the reactivity of ion pairs. Nucleophilic counterions and added nucleophiles work best in nonpolar solvents such as toluene and hexane. Their action in polar solvents is weaker because the polar solvents interact with the nucleophiles and nucleophilic counterions, as well as the ion pairs. Polar solvents such as methylene... 

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. 

There is usually little or no difference in reactivity between free ions and ion pairs, similar to the situation in the polymerization of carbon-carbon double bond monomers [Penczek, 2000]. Covalent species are present in many systems as detected by NMR and other methods, but whether they are reactive or dormant is not always clear. When propagation by covalent species occurs, it is not covalent-to-covalent propagation in which monomer is inserted into a covalent bond to produce a new covalent propagating species containing an additional monomer unit [Penczek et al., 1995]. Covalent propagation is a two-step process. Monomer adds to the covalent species to form an ionic species (Eq. 7-22a), which collapses to (is in... 

More recently Teyssie determined the rate constants in the polymerization of e-caprolactone (eCL) initiated with aluminium alkoxides, believing that the covalent species are the only ones responsible for propagation [4]. For the same monomer Yamashita estimated tentatively rate coefficients of propagation using an anionic initiator [ ]. Lenz in his studies of substituted g-propiolactones (gPL) observed peculiar influence of structure on reactivity that can have its origin in the multiplicity of ionic structures involved [fi]. 

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