Monomers forming donor-acceptor complexes

When a monomeric donor of suitable structure is encountered by a monomeric acceptor, a donor-acceptor (DA) complex is formed. It is characterized by a reorganized electron structure accompanied by charge shift. Therefore such formation are also designated as charge-transfer (CT) complexes. Until recently, the tendency to the formation of alternating copolymers has been ascribed to a special kind of monomer reactivity and of the resulting 

Yoshimura et al. [183] have used a similar scheme for the quantitative 

An electron acceptor such as maleic anhydride forms complexes with many donors, amongst which is vinyl acetate [80, 189, 190], Its existence at 363 K was proved by UV spectroscopy, ll NMR and by the formation of an alternating copolymer [80]. The complex is not formed above 363 K. From the two monomers, a statistical copolymer is formed, its composition depending on the ratio of initial monomer concentrations. 

Further monomers with which maleic anhydride produces donor-acceptor complexes are conjugated dienes [191-193], vinyl ethers [194a], furan, thiophene, indole [195, 196], / -isopropenylnaphthalene [197], 4-vinylpy-ridine [194b], 2-vinyl-1,3-dioxolane [198], cycloalkenes [199] and other complicated vinylic or acrylic monomers [200]. Maleic anhydride complexes have even been observed in some terpolymerizations [201], 

Donor-acceptor complexes of p-methoxystyrene with trisubstituted ethy-Ienes 

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The picture is more complicated when the Lewis acids are used in combinations with donor-acceptor monomers. The donor-acceptor complexes are believed to form first and then react with Lewis acids according to the following scheme ... 

The living polymerisations differ from the transfer-dominated ones in that the latter are mostly initiated by oxo-acids only, whereas to achieve living character, the ester needs to form a donor-acceptor complex with a third component, the modifier . One function of the modifiers is to prevent the chain-breaking transfer of a P-proton from the growing end to a base, such as the monomer or solvent. 

Copolymerization, on the other hand, is very easy with maleic anhydride. It copolymerizes by a free-radical reaction with a wide variety of monomers and many of the copolymers are perfectly alternating. This tendency of MA to form alternating copolymers derives from the participation of a donor-acceptor complex formed by the two reacting monomers. The term is used to describe... 

An interesting possibility of development in ionic polymerizations was described by Salamone et al. [307, 308], An ion pair is formed from a pair of suitable monomers, probably by way of a donor-acceptor complex as intermediate. Both the cationic and the anionic components of the pair can polymerize individually... 

Henrici-Olive and Olive were the first to put forward the hypothesis that complexes are sometimes formed between the active centre and the monomer and or/solvent [45], As only the complex with monomer is capable of propagation, part of the centres is inhibited and the polymerization rate is reduced. This theory was found to be valid with styrene [46], but not with MMA [47]. Burnett called attention to the important circumstance that radicals solvated in various ways may react differently, or at least at different rates [47]. His conclusions were based on kinetic studies of MMA polymerization in various halogenated aromatics. In the copolymerization of butyl vinyl ether with methacrylates, complex formation between the active centre and condensed aromatics prior to monomer addition was observed by Shaik-hudinov et al. [48], The growing polymer forms a stable donor-acceptor complex with naphthalene, described by the formula. 

When donor—acceptor complexes are formed from the monomers, they can take part in copolymerization. When the equilibrium constants of complex formation are not extremely high, both complexes and monomers coexist and compete with active centres in the reaction. In addition, the reverse case may occur when one part of the active centres forms complexes with some component of the medium, the reactivity of the complexed centers is, of course, different from that of the free centres. The situation is formally similar to that of the preceding paragraph. 

There exist many alternating copolymerizations ethylene or propene with alkyl acrylates [244], vinyl acetate with maleic anhydride [245], styrene with acrylonitrile [246], styrene with fumaronitrile [247], vinyl carbazol with fumaronitrile, vinyl ferrocenne with diethylfumarate [248], and further pairs or systems of three monomers [238, 249-253]. External conditions can support or hinder alternation. At not too high temperatures, vinyl acetate forms a donor—acceptor complex with maleic anhydride. Under these conditions (and in the presence of a radical initiator), an alternating copolymer is formed. The concentration of the complex decreases with increasing temperature above 363 K the complex cannot exist. Under these conditions, copolymerization yields a statistical copolymer whose composition depends on the composition of the monomer mixture [245]. 

The Monomer-Monomer Complex Participation (MCP) Model was first suggested by Bartlett and Nozaki (16), later developed by Seiner and Litt (17), and refined by Cais and co-workers (18). In this model, it is assumed that the two monomers can form a 1 1 donor-acceptor complex and add to the propagating chain as a single unit in either direction. Assuming that the terminal unit is the only unit affecting radical reactivity, eight addition reactions and an equilibrium constant are required to describe the system. 

In this paper, we report efforts to find donor/acceptor systems, comprised of at least one multifunctional monomer, capable of sustaining rapid free-radical polymerization without the need for external photoinitiators. Although we will include in this report comonomer systems which form ground state CT complexes, we stress that the primary mechanism for generating free-radical in each case may not be via excitation of ground state CT complexes. 

Thus, among possible binary complexes that can be formed from these monomers, the H3N- HF (best donor, best acceptor) complex is expected to be strongest, whereas the inverted HF- -HNH2 (worst donor, worst acceptor) complex should be weakest. These extreme differences in donor-acceptor strength are consistent with the wide disparity in H-bond energies (5.35a) and (5.35b). 

More generally, we can recognize that an acceptor orbital of unusual size or shape may demand an unusual Lewis base to offer a suitable matching donor orbital. The CT complexes formed by a monomer therefore provide a direct reflection of the shapes, sizes, and energies of its filled and unfilled valence orbitals. The rich diversity of donor-acceptor chemistry can be largely attributed to the richly variegated forms of donor and acceptor orbitals, which is consistent with the strongly quantum-mechanical character of donor-acceptor phenomena. 

The positive modifiers are exemplified by Higashimura s system of monomer + HX (equivalent to HMX) + ZnX2, where the C-X bond of the ester (organic halide) is activated by the metal halide. The transition state (II) for this reaction has been given above. Evidently, the active species PnX.ZnX2 is a reversibly formed donor (PnX) - acceptor (ZnX2) complex. 

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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