Charge-transfer complex comonomers

The free radical copolymerization of methyl methacrylate or acrylonitrile in the presence of zinc chloride with allylic compounds such as allyl alcohol, allyl acetate, and allyl chloride or butene isomers such as isobutylene, 1-butene, and 2-butene is characterized by the incorporation of greater amounts of comonomer than is noted in the absence of zinc chloride (35). Analogous to the radical homopolymerization of allylic monomers in the presence of zince chloride, the increase in the electron-accepting capability of the methyl methacrylate or acrylonitrile as a result of complexation results in the formation of a charge transfer complex which undergoes homopolymerization and/or copolymerization with a polar monomer-complexed polar monomer complex. 

Free radical copolymerization is initiated upon UV irradiation of mixtures of isobutyl vinyl ether and acrylonitrile (7), presumably as a result of photoexcitation of the comonomer charge transfer complexes. The excited complexes dissociate into ion-radicals which initiate radical propagated copolymerization. 

If the excited ethylene dimer exists as an ion radical pair, propagation may incorporate both monomeric units of the dimer, analogous to the behavior of comonomer charge transfer complexes such as butadiene-maleic anhydride, or only one unit, as noted in the homopolymerization of N-vinylcarbazole in the presence of electron accepting monomers such as acrylonitrile and maleic anhydride. 

In the presence of styrene as a comonomer, the grafting reaction seems to be predominant and therefore, the chain degradation by B -scission is reduced. This was confirmed by GPC analysis. Cross-propagation of styrene and maleic anhydride, both in solvent or in bulk, is well known. This behaviour is attributed to a charge-transfer complex (CTC) between maleic anhydride and styrene and can lead, in solution, to spontaneous copolymerisation (34,35). 

The formation of alternating copolymer is attributed to the homopolymerization of a comonomer charge-transfer complex. The latter is formed spontaneously, subject to equilibrium considerations, with the interaction of a strong donor monomer and a strong acceptor monomer. 

Although the formation of the ground-state comonomer charge-transfer complex occurs spontaneously, it has been suggested (2, 13) that polymerization involves the excited-state complex (exciplex). 

The failure to incorporate moieties arising from radical attack on the solvent into the alternating copolymers, coupled with the virtual absence of catalyst residues in the copolymer when the copolymerization of styrene and maleic anhydride is initiated by AIBN (3, 4), indicates that radical species may initiate the polymerization of comonomer charge-transfer complexes, but they are not incorporated into the polymer chain. 

The molecular weight of an alternating copolymer depends on the concentration of polymerizable species, the comonomer charge-transfer complex, which in turn depends on the reaction temperature. At elevated temperatures, the copolymerization rate is rapid and the molecular weight of the alternating copolymer is low. When the polymerization is carried out under those conditions in the presence of a polymer, the grafted alternating copolymer is present as multiple short branches. 

Cellulose-water may act as a matrix and promote the development of arrays of comonomer charge transfer complexes (19). The cellulose acts not only as a substrate for such alignment but also as a complexing agent. The matrix of complexes may be represented as shown in I (styrene-methyl methacrylate) and II (butadiene-acrylonitrile). The radical-, thermal-, and radiation-induced graft polymerizations involve homopolymerization of comonomer complexes rather than copolymerization of uncomplexed monomers. 

The retrograde dissociation of the Diels-Alder adduct to generate the diene and dienophile, followed by the polymerization of the comonomer charge transfer complex, has previously been proposed (12-15) in the polymerization of the CPD-MAH adduct, in the presence of peroxides having short half-lives at the elevated polymerization temperatures. Under these conditions, the ground state complex, presumably involved in endo-exo isomerization, is converted to the excited state complex which undergoes the indicated polymerization. 

Butler and Campus [40] undertook a study to provide further evidence of the formation of the charge-transfer complex between the comonomers and of its participation in the cyclocopolymerization. The 1,4-diene used was divinyl ether (DVE) and the monoolefins were maleic anhydride (MA) and fumaronitrile (FN). The results of the determination of the composition of the charge-transfer complex formed between DVE-MA are shown in Figure 5. Acrylonitrile (AN) was used as the third monomer in the terpolymerization experiments. For comparison, the results of a complex study of styrene-maleic anhydride and ethyl vinyl ether (EVE)-maleic anhydride were also reported. The results of determination of the equilibrium constants for the DVE-MA and styrene-MA complexes by the NMR method are shown in Figure 6. The results... 

The most widely accepted explanation for these surprising results is the formation of an charge-transfer complex between the electron acceptor MAH and an electron donor comonomer that exclusively or predominantly participates in the chain growth process [973,974]. 

The existence of charge transfer complexes of MAH in solution. Equilibrium constants in the range 3 to 60 x lO cm /mol were found at 20 to 30 °C in CHCI3 or hexane for comonomers such as styrene, a-methylstyrene, vinyl acetate, vinyl ethers, or vinyl sulfides [21-23]. 

The equilibrium constants for the charge-transfer complexes of MA with benzofuran (BF), benzothiophene (BT), and indole (table in appendix to this chapter) follow the order MA-BT > MA-indole MA-BF. This suggests that the copolymerization rates should follow the same order. This is not the case time-conversion curves show copolymerization rates of the three monomer pairs to be MA-BF > MA-BT > MA-indole, The same sequence was also found for maximum conversions at infinite time. The results suggest clearly that the reactivity of the comonomers to form copolymers with MA is predominately governed by resonance stabilization of the various monomer pairs. However, Goethals et suggest that the copolymeriz-... 

While attempting to establish the existence and role of the CTC in alternating copolymerizations, many authors have determined the equilibrium constant K) for a large number of MA-comonomer pairs (table in appendix to this chapter). Other charge-transfer complexes of MA have also been extensively investigated,by the use of ultravioletand spectroscopic methods (table in appendix to this chapter). Identification of the CTC is centered on the concentration and temperature dependence of the absorption in the visible and ultraviolet spectra. In NMR, the acceptor or donor protons undergo a chemical shift when the CTC is formed and this variation is used to determine the K values. Calorimetric methods have also been used to determine equilibrium constants. 

Equilibrium Constants K) for Charge-Transfer Complexes of Maleic Anhydride with Various Comonomers " ... 

When two comonomers vary widely in their electron affinity, a charge-transfer (CT) complex may be formed between them, and this complex may participate in copolymerization. The existence of a CT complex could be detected by many methods such as PMR,... 

Composition constancy regardless of the monomer ratio in the reaction mixture allowed to suppose, that copolymerization of DAAH with SO2 proceed via formation of the complexes. Investigations showed simple mixing of the comonomers at 20-80°C to result in momentary formation of a viscous amber-coloured adduct. A new band of a charge transfer with Amax=263 nm (Figure 1) was deteeted in UV-spectra of DAAH and SO2 mixture, recorded in chloroform. Formation of the eomplex of DAAH with SO2, becoming apparent as a result of a deviation from additivity of absorbances of DAAH, SO2 and their mixture (Figure 2), was also detected in DMSO and aqueous solutions. It is seen, that deviation maximum is observed at equimolar ratio of the monomers. 

A newer theory of the formation of alternating copolymers assumes the formation of complexes between comonomer molecules, associated with a charge transfer. 

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