Charge transfer complexes monomers

Photoinduced copolymerization of donor-acceptor monomer pairs (Scheme 1) can be either self initiated by excitation of the charge transfer complex (charge transfer initiation) or by polymerization of the charge transfer complex/monomer equilibrium... 

They also argued that the polymerization of the adduct proceeded only when the retrograde reaction giving back the two monomers was occurring, i.e. at higher temperatures and thus the real monomer was the charge-transfer complex. 

Stoicescu and Dimonie103 studied the polymerization of 2-vinylfuran with iodine in methylene chloride between 20 and 50 °C. The time-conversion curves were not analysed for internal orders but external orders with respect to catalyst and monomer were both unity. Together with an overall activation energy of 2.5 kcal/mole for the polymerization process, these were the only data obtained. Observations about the low DP s of the products, their dark colour, their lack of bound iodine and the presence of furan rings in the oligomers, inferred by infrared spectra (not reported), completed the experimental evidence. The authors proposed a linear, vinylic structure for the polymer, and a true cationic mechanism for its formation and discussed the occurrence of an initial charge-transfer complex on the... 

Equilibrium constants for complex formation (A") have been measured for many donor-acceptor pairs. Donor-acceptor interaction can lead to formation of highly colored charge-transfer complexes and the appearance of new absorption bands in the UV-visible spectrum may be observed. More often spectroscopic evidence for complex formation takes the font) of small chemical shift differences in NMR spectra or shifts in the positions of the UV absorption maxima. In analyzing these systems it is important to take into account that some solvents might also interact with donor or acceptor monomers. 

The interaction of the template with monomer and/or the propagating radical may involve solely Van der Waals forces or it may involve charge transfer complexation, hydrogen bonding, or ionic forces (Section 8.3.5.1). In other cases, the monomer is attached to the template through formal covalent bonds (Section 8.3.5.2). 

The charge transfer complex (CTC) of dioxygen with the monomer is very probable as a precursor of this reaction [48],... 

Let us briefly mention some other binary A- B charge-transfer complexes involving neutral monomers A and B chosen rather arbitrarily from the large number of possible species of this type. These examples serve to illustrate interesting aspects of the general CT phenomenon and exhibit the strong commonality with donor-acceptor interactions considered elsewhere in this book. 

Another very instructive case concerns the alleged initiation of a cationic polymerisation by a charge-transfer complex formed by the compound chloranil (2,3,5,6-tetrachloroquinone) with the monomer N-vinyl-carb-azole. It was shown (Natsuume et al., 1969 1970) that this compound is not an initiator, but that the polymerisations were caused by a hydrolysis product, 2-hydroxy-3,5,6-trichloroquinone, which is a strong acid. One has learnt from this finding to be extremely suspicious of any claims for charge-transfer catalysis and to test one s suspicions by appropriate experiments involving progressive purification of the putative catalyst. 

Further evidence for the participation of the charge transfer complex in these terpolymerization systems was obtained by dilution experiments (12). The effect of dilution with various solvents on the AN content of the terpolymer is shown in Figure 4. Except for chloroform, the AN content of the copolymer increases with dilution. This suggests a higher order dependence of monomer consumption on monomer concentration... 

Spontaneous copolymerizations are encountered much more frequently, particularly when monomers of opposite polarity are mixed [9-10]. Early workers noticed that, upon mixing of certain electron-rich and electron-poor olefins, spontaneous polymerizations occurred without added initiator [99, 124 128]. Mixing electron-rich olefins with electron-poor olefins almost always results in brightly colored solutions. The colors are due to the CT excitation (hvCT) of the electron-donor-acceptor (EDA) complex [129], Theories for these spontaneous polymerizations mostly center around the charge-transfer complexes (CT or EDA complexes) [128]. 

Le. those which form a charge-transfer complex with the monomer units of the transporting polymer. The charge transfer complex then becomes the carrier generating species and can also contribute to the transport. The electron acceptor can also be chemically attached to the polymer. In such a case the polymer may become intrinsically photoconductive. 

All the above examples involve charge transfer complexes between monomer units of the polymer and an added electron acceptor. There have been several cases reported in which both donor and acceptor groups become parts of the structure of the same polymer. For example, strong electron acceptor groups such as dinitro-phenyl and chloranil were chemically attached to copolymers of N-vinylcarbazole with vinylacetate or acryloyl chloride158) ... 

Notable work in the area of photopolymerizations of donor monomers initiated by acceptor initiators was done by Shirota [5-7] in his study of polymerization of N-vinylcarbazole (VCZ) and by Hayashi and Irie [8] on the polymerization of a-methylstyrene (a-MSt). The initiation mechanism was proposed to proceed via the charge-transfer complex between VCZ (or a-MSt) and the acceptor, which then yields two kinds of ion-radicals D and A ... 

Electron donor-acceptor monomer pairs form charge-transfer complexes (CTC), which collapse to the tetramethylene intermediates through the bond-formation between the p-carbons ... 

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