Monomer electron acceptor

Monomers that are strong electron donors may undergo spontaneous oupolymeri/.aliun with strong electron acceptor monomers by a radical mechanism. In certain cases homopolymers formed by an ionic mechanism accompany copolymer formation.312,j2s... 

While there is clear evidence for complex formation between certain electron donor and electron acceptor monomers, the evidence for participation of such complexes in copolymerization is often less compelling. One of the most studied systems is S-.V1 Al I copolymerization/8 75 However, the models have been applied to many copolymerizations of donor-acceptor pairs. Acceptor monomers have substituents such as carboxy, anhydride, ester, amide, imide or nitrile on the double bond. Donor monomers have substituents such as alkyl, vinyl, aryl, ether, sulfide and silane. A partial list of donor and acceptor monomers is provided in Table 7.6.65.-... 

Although it has been suggested that crosslinking in the presence of MAH involves coupling of appended MAH radicals with other appended MAH radicals or with polymer radicals (7) the former is improbable due to the tendency for disproportionation rather than coupling between radicals derived from strong electron acceptor monomers such as MAH. 

Some pairs of very strongly electron-donor and electron-acceptor monomers, such as p-methoxystyrene and dimethyl cyanofumarate, undergo spontaneous alternating copolymerizations without any added free-radical initiator, although heat may be required [Hall and Padias, 1997, 2001]. Initiation involves reaction of the comonomer pair to form a diradical,... 

As to the contributions of the ghost functions, those of s type are of the same behaviour than in the He2 dimer (see Table II, values are given for monomerl). The contributions of p functions, as expected, show a systematic trend it is to be emphasized that the contributions from monomerl in the SMOs for monomerl are usually higher than those from monomerl to monomerl. However, not so large difference can be noticed for the contributions in the corresponding CP-systems. These results also suggest that one cannot expect a similar correspondence for the correlation energy contributions in the SM- and CP-systems for the electron-donor and electron-acceptor monomers, respectively. 

In contrast to the radical-monomer interaction in the transition state proposed by Mayo and Walling (62, 63), the formation of a molecular complex between the electron donor monomer and the electron acceptor monomer—i.e., monomer-monomer interaction—has been proposed as the contributing factor in the free radical alternating copolymerization of styrene and maleic anhydride (8) as well as sulfur dioxide and mono-or diolefins (6, 9, 12, 13, 25, 41, 42, 43, 44, 61, 79, 80, 88). Walling and co-workers (83, 84) did note a relationship between the tendency to form molecular complexes and the alternating tendency and considered the possibility that alternation involved the attack of a radical on a molecular complex. However, it was the presence in the transition state of polar resonance forms resembling those in the colored molecular complexes which led to alternation in copolymerization (84). 

The significant distinction between these proposals lies in the nature of the components participating in the transition state. Thus, the radical-monomer interaction involves a one-electron transfer from the growing radical to the electron acceptor monomer. 

Iwatsuki and Yamashita (46, 48, 50, 52) have provided evidence for the participation of a charge transfer complex in the formation of alternating copolymers from the free radical copolymerization of p-dioxene or vinyl ethers with maleic anhydride. Terpolymerization of the monomer pairs which form alternating copolymers with a third monomer which had little interaction with either monomer of the pair, indicated that the polymerization was actually a copolymerization of the third monomer with the complex (45, 47, 51, 52). Similarly, copolymerization kinetics have been found to be applicable to the free radical polymerization of ternary mixtures of sulfur dioxide, an electron donor monomer, and an electron acceptor monomer (25, 44, 61, 88), as well as sulfur dioxide and two electron donor monomers (42, 80). 

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. 

Increasing the electron-accepting character of an electron acceptor monomer would result in a greater separation in the donor—acceptor relationship with a given electron donor monomer. As a result there would be an increased tendency for alternation in the copolymerization. 

Further examples for electron acceptor monomers are acrylonitrile [37], diethyl fumarate [39], fumaronitrile [29,30, 38], maleonitrile [38], N-carbethoxymaleimide [29], N,N-diethylaminoethyl methacrylate [39], nitroethylene [10] and iV-ethyl-maleimide [40], As electron donor monomem also are used vinyl alkyl ethers [38, 40], alkyl methacrylate [40], JV-vinyl pyrrolidone [40] and cyclohexene oxide [10]. 

Photoactivated Copolymerization. Although polymerization and copolymerization generally involve the addition of a monomer to a reactive chain end, the ground state charge transfer complex generated by the interaction of an electron donor monomer and a strong electron acceptor monomer, acts as a single unit and, upon excitation of the complex, both monomers enter the chain. 

The reaction of a conjugated diene such as butadiene or iso-prene and an electron acceptor monomer such as maleic anhydride proceeds through a ground state complex which undergoes cycliza-tion to yield the Diels-Alder adduct. However, under UV light, in air or in the presence of sensitizers, the adduct is accompanied by the equimolar, alternating copolymer which results from excitation of the ground state complex, followed by homopolymerization of the excited complex (13)> as shown in Eq. (I8). 

When the electron acceptor monomer is a relatively weak acceptor, e.g. acrylonitrile, the reaction with a conjugated diene such as butadiene, to yield the Diels-Alder adduct, proceeds slowly. However, in the presence of aluminum chloride, the acrylonitrile is converted to a stronger electron acceptor and the formation of the ground state complex and the adduct therefrom proceeds more rapidly. When the reaction is carried out under UV irradiation, the yield of adduct is greatly reduced and the equi-... 

This indicates that for any feed composition, the copolymer formed will have an equimolar regularly alternating composition. The chemistry behind these systems has been extensively discussed in the literature, and the charge-transfer complex concept was for a long time the prevailing theory to explain this behavior [73-75]. More recently, Hall and Padias [76, 77] have proposed an alternate explanation based on polar effects when an electron-donor and an electron-acceptor monomer interact. This last theory has the additional virtue of explaining the fact that many... 

As initiators nucleophilic substances are used like amines, phosphanes, alkoxides, carbanions, and Grignard reagents. Strong electron acceptor monomers like cyanoacrylate can be already initiated by water (used in... 

One proposed mechanism [171, 204] is that such charge-transfer polymerizations are in effect homopolymerizations of the charge-transfer complexes [D A. .. MeXJ. In other words, the metal halide is complexed with the electron acceptor monomer and acts as an acceptor component. 

There is also experimental evidence for the influence of radical-solvent complexes in small radical addition reactions. For instance, Busfield and co-workers used radical-solvent to explain solvent effects in reactions involving small radicals, such as t-butoxyl radicals towards various electron donor-electron acceptor monomer pairs. The observed solvent effects were interpreted in terms of complex formation between the t-butoxyl radical and the electron-acceptor monomer, possibly via a sharing of the lone pair on the t-butoxyl oxy-... 

Various mechanisms have been proposed to explain the initiation mechanism of self initiated copolymerizations of styrene (S) with electron acceptor monomers such as maleic anhydride (MA), acrylonitrile, vinyliden cyanide or dimethyl l,l-dicianoethane-2-2-dicarboxylate. They... 

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