Acceptor Complexes in Copolymerization

If the reaction mixture is irradiated with high energy radiation, like gamma ravs, instead of being heated, an alternating copolymer forms. The complex converts to a diradical that homopolym- 

Charge-transfer complexes are also claimed to be the intermediates in free-radical alternating copolymerization of dioxene or vinyl ethers with maleic anhydride  

a third monomer can be included to interpolymerize with the complex that acts as a unit. The product is a terpolymer. A diradical intermediate was also postulated in sulfur dioxide copolymerizations and terpolymerizations with bicycloheptene and other third monomers. These third monomers enter the copolymer chain as block segments, while the donor-acceptor pairs enter the chains in a one-to-one molar ratio. This one-to-one molar ratio of the pairs is maintained, regardless of the overall nature of the monomer mixtures. 

The propagation and termination steps in the above reactions are claimed to be related. As stated, an interaction and coupling between two diradicals is a propagation step. When such interactions result in disproportionations, however, they are termination steps. This means the charge-transfer mechanisms are different from conventional free-radical polymerizations. They involve not only interactions between growing polymer radicals and monomers, but also between polymer radicals and complexes. In addition, they involve interactions between the polymer radicals themselves. 

The stability of charge-transfer complexes depends upon internal resonance stabilization. This degree of stabilization determines how easily the diradical opens up. Consequently, this stability also determines how the copolymerization occurs. It can occur spontaneously, or under the influence of light or heat, or because of an attack by an initiating free radical. 

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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.-... 

However, these observations are not proof of the role of a donor-acceptor complex in the copolymerization mechanism. Even with the availability of sequence information it is often not possible to discriminate between the complex model, the penultimate model (Seetion 7.3.1.2) and other, higher order, models. A further problem in analyzing the kinetics of these copolymerizations is that many donor-acceptor systems also give spontaneous initiation (Section 3.3.6.3). 

The above examples of successful copolymerization reactions illustrate the importance of coulombic and n donor-71 acceptor interactions in copolymerization of 4-vinylpyridinium ions. Thus, electrostatic attraction and repulsion or CT complexation offer potentially useful means for control of composition and sequence of monomers in copolymers. 

Currently this model is one of the most commonly used in the theory of free-radical copolymerization. The formation of a donor-acceptor complex Ma... iVlbetween monomers Ma and in some systems is responsible for a number of peculiarities absent in the case of the ideal model. Such peculiarities are due to the fact that besides the single monomer addition to a propagating radical, a possibility also exists of monomer addition in pairs as a complex. Here the role of kinetically independent elements is played by ultimate units Ma of growing chains as well as by free (M ) and complex-bound (M ) monomers, whose constants of the rate of addition to the macroradical with a-th ultimate unit will be... 

Lewis acids have long been known to influence free radical polymerizations [117]. They have been particularly important in copolymerizations of hydrocarbon olefins with electron-poor monomers such as acrylates or acrylonitriles. In this way strictly alternating copolymers can be synthesized from monomer pairs which in the absence of Lewis acids would give more random copolymers. The Lewis acid complexes with the electron pair of the acceptor group of the acrylate or acrylonitrile to form the more electrophilic complexed monomer, which then copolymerizes in alternating fashion with the electron-rich hydrocarbon olefin. 

The second type of nonideal models takes into account the possible formation of donor-acceptor complexes between monomers. Essentially, along with individual entry of these latter into a polymer chain, the possibility arises for their addition to this chain as a binary complex. A theoretical analysis of copolymerization in the framework of this model revealed (Korolev and Kuchanov, 1982) that the statistics of the succession of units in macromolecules is not Markovian even at fixed monomer mixture composition in a reactor. Nevertheless, an approach based on the "labeling-erasing" procedure has been developed (Kuchanov et al., 1984), enabling the calculation of any statistical characteristics of such non-Markovian copolymers. 

Donor-Acceptor Molecular Complexes in Alternating Copolymerization and in the Polymerization of Metal Halide-Complexed Vinyl Monomers... 

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). 

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 formation of the donor-acceptor complexes Mt. .. M2 between the monomers M, and M2 is regarded as being an additional important factor responsible for the deviations of some certain systems from the classic copolymerization model. Also it should be noted that besides the single monomer entrance into the polymer chain a possibility of the monomer addition in pairs as a complex also exists. The corresponding kinetic scheme of the propagation reaction parallel with reactions (2.1) involves four additional ones [36] ... 

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]. 

In copolymerization of acrylonitrile and styrene, 1-5% of added nitrogen base may promote MW reduction.297 Several bases including pyridine, triethylamine, and DABCO convert CCT catalysts into Cl catalysts in the polymerization of MMA,298 suggesting that the bases are proton acceptors converting LCo-H or LCo-R into a 7r-complex of LCo1 with monomer. Figure 9 shows the dependence of the... 

Currently this model is one of the most commonly used in the theory of free-radical copolymerization. The formation of a donor-acceptor complex Ma... 

Charge transfer complexes of the monomers were studied in the terpolymer-ization of neutral monomers (N) with electron-donor (D) and electron-acceptor (A) monomers [18a]. For example, norbomene as (D) monomer, SO2 as (A) monomer, and acrylonitrile as (N) molecules were studied. Thus acrylonitrile may not be effective in copolymerization but can be terpolymerized with SO2 [18a]. 

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