Electron transfer from donor to monomer

In aromatic hydrocarbons, some substituted alkenes, dienes, substituted acetylenes and ketones, one half of the n orbitals are empty and an electron can easily be placed in these antibonding orbitals. The capture of an electron by the acceptor molecule is an exothermic process because the energy of the antibonding orbitals lies below the level of the ionization potential of the acceptor radical anion. Many radical anions formed from unsaturated molecules are themselves stable they do not decompose and may exist indefinitely under suitable experimental conditions [182a],On the other hand, they react easily with other molecules. 

Electron transfer from donor to monomer is a frequently used process initiating anionic polymerizations. An electropositive metal, mostly a member of Group IA in the periodic table, is usually the donor. The acceptor is either the monomer direct or an aromatic molecule which is used to mediate the electron transfer from the metal to the monomer. For initiation by electron transfer, a more polar medium is typical (e. g. THF, dimethoxyeth-ane, etc.). The solvation energy represents a major part of the overall balance of the driving force of the process. 

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A special case of donor-acceptor interactions occurs when two monomers come into contact such that one exhibits a signicant donor and the other an acceptor character. The donor-acceptor complex then has the structure of a zwitterion resulting from the combination of radical ions after electron transfer from donor to acceptor... 

The transformations of SbClj caused by a one-electron transfer from an aromatic compound have been described earlier. If the pure Lewis acid SbClj is used, its reactivity is very difficult to control, and single-electron oxidation as well as chlorination of various aromatic donors can occur readily (Mori et al. 1998). Meanwhile, in the case of EtjO SbClg", the slow release of the active monomer SbClj occurs. In the case of SbClj as such, the 2SbCl5 — C Sb—CI2—SbCl4 dimerization occurs (Cotton and Wilkinson 1988, p. 395). The dimeric form may lead to the following electrophilic chlorination ... 

In contrast, the donor monomer-acceptor monomer interaction involves a one-electron transfer from the donor monomer to the acceptor monomer to form a charge transfer complex. The latter undergoes homopolymerization through a radical mechanism to give an alternating copolymer. 

The charge transfer complex resulting from the one-electron transfer from the electron donor monomer to the electron acceptor monomer has a stability which varies as a function of the internal resonance stabilization. The degree of stabilization apparently determines the ease with which the diradical complex opens, and consequently the stability of the complex determines whether the copolymerization occurs spontaneously or under the influence of heat, light, or free radical attack. 

Figure 4. HOMO/LUMO scheme for operation of a proposed molecular shift register [48]. a) The clock cycle is initiated by photoexcitation of the donor moiety, resulting in the electronic configuration shown. Decay pathways from this excited state are forward electron transfer within the same monomer unit (solid), back electron transfer to the adjacent monomer unit (dash-dot), and fluorescence (dot), b) Electronic configuration resulting from successive forward electron transfer steps. The charge-separated state [D -Ai-A2 ] can recombine charge within a single monomer unit (dot-dash) or with the adjacent monomer unit (solid).

Polymerization initiation can proceed by electron transfer from an electron donor to the monomer. For this, the monomers must possess a sufficient electron affinity in addition, the polymerization must be carried out... 

In the light-driven primary charge separation processes of photosynthesis, the initial step involves irreversible electron transfer from the primary electron donor to an acceptor within 10 picoseconds [50]. In bacterial photosynthetic systems, it has been established that a BChl special pair acts as the primary electron donor, and that BChl and BPheo monomers act as acceptors [51]. Moreover, electron transfer in green plant photosynthetic systems is in generally known to involve Chi and Pheo moieties [41]. 

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