Cationic deactivation processes

All these cationic deactivation processes were performed with oxolane as the monomer and with various initiators such as triethyloxonium tetrafluoroborate and benzoyl, acetyl or propionyl hexafluoroantimonate. Efficient difunctional cationic initiators such as adipoyl- or terephthaloyl hexafluoroantimonate) can also be used 42 to synthesize bifunctional macromonomers containing at both chain ends a polymerizable double bond. 

The lack of fluorescence of TAM dyes in solution is attributed to an extremely rapid, nomadiative deactivation process brought about by intramolecular rotation of the flexible aryl groups. Suppression of molecular rotation, by increasing the viscosity of the medium, by binding of the dye to a polymer or protein, or by selfassociation, diminishes this radiationless process. Coincident with this change in photophysical properties is often a dramatic increase in sensitivity toward photofading due to an increase in the quantum yield of the relatively long-lived and photochemically active triplet state [139-143]. The triplet state of the dye cation... 

Van Koten et al. have found that NCN-palladium complexes, for example, [(NCN)Pd-(H20)][BF4], (146) are active catalysts in various C-C bond formation reactions, such as aldol condensation and the Michael reaction. Recently, they have developed the attachment of palladium(II) complexes (146) to Cgo (Scheme 16.38) [45]. The corresponding cationic aqua complexes were evaluated as Lewis acid catalysts for the Michael addition reaction of (49a) and (64), and aldol condensation of (la) and methyl isocyanoacetate (64). In the case of Michael reactions catalyzed by methanofullerenes (18)-(20) are significantly slower than the reaction catalyzed by (146). On the other hand, no such deactivation process was observed using... 

Ziessel et al. developed ligand 52, in which two soft 6-carboxy-2,2 -bipyridine arms were directly tethered to a hard phenylphosphine oxide [133]. Its 1 1 complexes with Eu and Tb cations exhibited characteristic luminescence at neutral pH. Addition of hydrogen phosphate, ADP, and ATP anions increased the luminescence intensities and hfetimes of both Eu " " and Tb " " complexes, while AMP and NOs" anions induced no obvious change. ATP-induced luminescence enhancement was assigned to a diminution of the non-radiative deactivation processes due to the bound water molecules. 

The paper 6.5 [63] is particularly interesting historically, because the writer mentions explicitly that the esters involved in the propagation may be (or need to be) activated or deactivated. As we have seen, this idea was not developed to fruition until some 30 years later Other useful features in that paper are the examination of the evidence for the ionic nature of the propagators in the cationic polymerisations, and explanations of how difficult it was for polymer chemists to shake off the ideas taken over from the familiar radical polymerisations and to adapt their thinking to ionic processes. 

Excited states may be quenched as well via an electron transfer between the excited and quencher molecular entities. The electron can be transferred by two alternative ways, generating a radical anion and cation as a transient species (Figure 4.4). These then react thermally when the reaction leads to reproduction substrate AB and quencher Q in their ground states, the photophysical deactivation occurs when radical ions react with other medium components generating new species, the process belongs to photochemical redox reactions (see Chapter 6). 

The reaction of an unsaturated compound with an antagonist function located at the end of a polymer chain is still the most commonly used method to synthesize macromonomers. We have already mentioned some processes that can be used to introduce into the chain end of a macromolecule a functional group, e.g. by deactivation of living carbanionic sites and transfer reactions of various kinds in cationic polymerization. We have also described some methods used to link an active terminal double bond to the chain end originally bearing hydroxy groups. 

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