Active centre cationic

A A, Aj, A2 AC Ac AIBN As A °p acac [al] acceptor dilatometric constant, conversion factor active centre cation, cationic part of an acid 2,2 -azo-bis-isobutyronitrile solvated anion radical excited acceptor surface area of a particle of volume V 2,4-pentanedione-3yl (acetylacetonate) concentration of a monomeric organoaluminium compound at the surface of a catalyst... 

In this relatively simple random walk model an ion (e.g., a cation) can move freely between two adjacent active centres on an electrode (e.g., cathode) with an equal probability A. The centres are separated by L characteristic length units. When the ion arrives at one of the centres, it will react (e.g., undergoes a cathodic reaction) and the random walk is terminated. The centres are, therefore absorbing states. For the sake of illustration, L = 4 is postulated, i.e., Si and s5 are the absorbing states, if 1 and 5 denote the positions of the active centres on the surface, and s2, s3, and s4 are intermediate states, or ion positions, LIA characteristic units apart. The transitional probabilities (n) = Pr[i-, —>, Sj in n steps] must add up to unity, but their individual values can be any number on the [0, 1] domain. 

In 1990 we reported the synthesis of new redox-responsive crown ether molecules that contain a conjugated link between the crown ether unit and a ferrocene redox-active centre (Beer et al., 1990a). Examples of some of the species synthesized are shown in Fig. 5. The electrochemical behaviour of these species was investigated and also the electrochemical behaviour of their analogues with a saturated link between the ferrocene unit and the crown ether. The changes in the CVs of [2a] upon addition of magnesium cations are shown in Fig. 6. The metal cation-induced anodic shifts of [2a], [2b] and also their saturated analogue [3] and vinyl derivatives [4a], [4b] are shown in Table 1. 

Compounds [54] and [55] have been shown to complex group 1 and 2 metal cations and also ammonium and alkylammonium cations by nmr and UV/Vis spectroscopies and also by a number of solid-state X-ray crystallographically determined structures. The quinone moieties in these molecules constitute not only the coordination site but also the redox-active centre. The complexation... 

If the proposed structure was true, then a solvent of higher solvating power, such as dme, should decrease the stability of the intramolecular complex. Using sodium as the counterion part of the active centres should exist in the form of externally solvated contact ion pairs or even solvent-separated ion pairs. With caesium as the counterion there should be little change, because this cation is only poorly solvated by both solvents and consequently the possibility of solvent-separated ion pairs to be found should be extremely small. 

Generally polymers prepared by cationic means have relatively broad distributions because of the variety of transfer and termination reactions of the active centres. This makes interpretation of molecular weights difficult, and also means that such data is of little use as an aid to understanding mechanism. 

An example of a grafting reaction via cationic active centres is the reaction of the allylic —Cl of polyvinylchloride (formed by partial loss of HC1 from the polymer) with A1R2C1, which leads to a carbocation along the polymer chain which, in the presence of a suitable monomer, can initiate a cationic polymerization 20). 

According to this mechanism, initiation includes reaction between the tertiary amine and epoxide, and the primary active centre is represented by a zwitterion with an alkoxide anion and an irreversibly bound amine in the form of an ammonium cation (Eq. (69)). This zwitterion reacts in the next step with the anhydride (Eq. (70)) yielding a carboxylate anion. The growth reactions (Eqs. (71) and (72)) include interactions of the carboxylate anion with epoxide, and of the alkoxide anion with the anhydride. 

During the cationic polymerisation, e.g. with sulfuric acid, the process is the following at the initial stage of initiation, when organocyclosiloxanes interact with sulfuric acid, the acid proton attacks the oxygen atom of the siloxane cycle. As a result of the redistribution of the electron density, the =Si-0 bond breaks, opening the cycle and forming an active centre at the end of the chain ... 

An important criterion for classification is the type of active centre and depending on its type we classify polymerizations as radical, ionic (which are further classified as anionic or cationic) and coordination. 

Non-stationary polymerization are complicated from the kinetic point of view. The changing concentrations of active centres, of monomer and possibly even of further components produce conditions unsuitable for an analysis of the process. Even technical and technological difficulties occur. Nevertheless, these have to be solved as most known coordination and cationic, and a considerable number of anionic, polymerizations are non-stationary. Information on the polymerization mechanisms of the more conventional monomers are summarized in Table 3. 

Most data were obtained from copolymerization studies. The copolymerization parameter r (see Chap. 5, Sect. 5.2) is the rate constant ratio for the addition of two different monomers to the same active centre. The inverse values of r j determined for the copolymerization of a series of monomers with the monomer M, define the relative reactivities of these monomers with the active centre from the first monomer, M°,. Thus it is possible to order monomers according to their reactivities in radical, anionic, cationic and coordination polymerizations from the tabulated values of copolymerization parameters [101-103]. 

Busson and van Beylen [205] studied the role of the cation and of the carbanionic part of the active centre during anionic polymerization in non polar media. They were interested in the problem of complex formation between the cation and the monomer double bond [206] and they therefore measured the reaction of various 1,1-diphenylethylenes with Li+, K+ and Cs+ salts of living polystyrene in benzene and cyclohexane at 297 K. Diphenylethy-lene derivatives were selected for two reasons. 

The initiators of cationic polymerizations must produce sufficiently reactive cations which are able to yield active centres with the monomer. It is therefore useful to discuss each of the two main initiator types separately. 

Molecules with two or more cationic active centres are very useful for certain macromolecular syntheses. The first dicationic initiator was used in our laboratory [251-253]. It is formed by hydrolysis and condensation of dimethyldichlorosilane in the presence of a strong acid, e. g. HC104. The condensation is an equilibrium process. When its volatile products are removed by evacuation, a siloxonium dication is finally formed. 

Similar to their anionic counterpart (see Sect. 2.4), even with cationic polymerizations the structure and size of the molecule to which the active centre is bound plays an important role. The required macromolecules with one or two active ends are formed by living polymerizations. Modern macro-molecular syntheses use them as agents, especially for the preparation of... 

Interaction of a cation with a monomer yields an active centre this is initiation in its most proper sense. The proton and the most simple carboca-tions are attached to the / atom of an alkene. This is documented both by experimental findings and by the corresponding calculations of the localization energies of the respective structures (Hiickel) and of the electron densities on the alkene carbon atoms [264]. 

The pH dependence of the polymerization rate of acrylic acid [54] in the presence of various neutralizing agents does not exhibit a course. Between pH 2 and 6, the rate drops abruptly afterwards it grows, depending on the degree of neutralization and the kind of neutralizing agent. The rate increase is connected with the presence of a cation which restricts the repulsion between the anions of the active centre and the monomer... 

So, for example, in the polymerization of 3-methyl-1-butene, monomer addition to the active centre yields a cation on the secondary carbon I, which is then isomerized to II. The energy change in the transition from I to II is about 40 kJ mol-1. 

To date these have not been very important as active centres of polymerizations, mainly because of the limited number of conventional monomer types suitable for building such a centre. Nitrogen cations are an exception. Cyclic imines polymerize on centres [148, 149]... 

Pticyna et al. measured the electrical conductivity of polyoxirane terminated by OEtO K+ active centres in tetrahydrofuran. At concentrations of > 10 3 mol dm-3 these centres associate. The dissociation constant of low-molecular-weight alcoholates increases with growing chain length. Polyoxyethylene) (in a similarly way to crowns and cryptands) solvates the cation and thus promotes charge separation [259]. Littlejohn et al. quantified EtOCH SbCl dissociation in MeCl2 by specific conductivity and permittivity determinations [260]. 

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