CT Complexes—Polymerization

Since the 1940s, it has been recognized that MA forms alternating copolymers with various olefinic monomers such as styrene,allyl acetate,and p-dioxene. Recently, dienes, furan /V-vinyl compounds,and unsaturated sulfides have also been added to this list. 

It has been proposed that this perfectly alternating character of the polymer, no matter what the concentration of the individual components, may mean that polymerization is of the complexes. The mechanism has been discussed by Ratzsch. Furthermore, Tamura et have described the 

When rationalizing a reaction as involving charge-transfer intermediates, caution should be exercised. Complex formation and its slow disappearance during the reactions may result from side reactions and/or disappearance of one of the components, leading to unwarranted conclusions. 

Paul et have studied the basicities of some anhydrides in fluorosul- 

Lewis acids, such as aluminum chloride, boron trifluoride, and zinc chloride, also react with MA. Complexes like MA 2 acceptor are proposed to be intermediates for Friedel-Crafts acylation reactions.Some of these complexes have been identified by spectroscopic methods. 

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It can be concluded that the high conductivity and other physical properties of these completly organic composites can be controlled by the film casting conditions and composition, depending on the nature of the CT-complex, polymeric matrix, and the solvent used. 

A satisfactory explanation of all effects has not yet been achieved. But, to a first approximation, it appears that the kind of CT-complex polymerization occurring depends on the equilibrium constant for complex formation (Table 22-9). 

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. 

Nitrobenzene (NB) is such a weak acceptor that it is not capable of initiating polymerization of VCZ in the dark. Nitrobenzene has been used as a solvent for polymerization of VCZ in several instances. However, from a spectroscopic study of VCZ-NB mixture, the formation of a CT complex is confirmed (33). Although no new absorption peak is observed when VCZ and NB are mixed in benzene solution, a shift of absorption of NB is clearly detected. This CT complex is the contact charge transfer type as shown in Fig. 4. 

Interaction of 1-vinylindole with halogenated organic electron acceptors, even such weak acceptors as alkyl halides, results in the formation of polymeric CT complexes (28) of varying composition, all of which contain paramagnetic centers and display resistivities in the insulator range (74IZV1837). 

Preparation of addition polymers having the oxolene (dihydrofuran) functionality can be envisioned to occur in two possible ways (Scheme 13). Both, in fact, have been observed (77MI11102). Whereas furan (53) or its derivatives do not homopolymerize under free radical conditions, 1 1 alternating copolymers possessing the 1,4-structure are produced with maleic anhydride (50). Intermediate formation of a CT complex between monomers (50) and (53) is believed to be necessary before polymerization can occur. On the other hand, cationic polymerization is quite facile. The outcome is straightforward with benzo[f>]furan derivatives, producing 1,2-polymers. Optically active poly(benzofurans) are formed when the cationic polymerizations are conducted in the presence of a chiral anion. 

In another example of the ease of polycondensation of heterocyclic diesters, diethyl 4-pyranone-2,6-dicarboxylate (diethyl chelidonate) (130) has been reported to undergo rapid polymerization at room temperature when mixed with 1,6-hexanediamine (Scheme 38) (75MI11104). The polymerization is believed to occur subsequent to an initial CT complex formation, and was accelerated in the presence of direct sunlight. 

We have reported the first electroactivity of a thioketene dimer compound [116]. The CV measurement of 2,4-dibenzylidene-l,3-dithietane (31), which was prepared by a basic dimerization of phenylthioketene derived from ben-zyltriphenylphosphonium chloride, showed irreversible two-step oxidation peaks at 0.25 and 0.61 V vs Ag/Ag+, indicating that 31 acts as a stronger electron donor than 2,6-bisphenyl-l,4-dithiafulvene (30) and TTF (2). The dimer (31) can form a 1 1 CT complex with TCNQ in DMSO. Cycloaddition polymerization of bisthioketene derived from p-xylenebis(triphenylphosphoni-um chloride) gave a -conjugated polymer (32) with thioketene dimer unit in the main chain (Scheme 12). This polymer was the first polymer contain-... 

Another acceptor is fumaronitrile (FN). In spite of its lack of absorption above 300 nm, the partners or the CT complexes may absorb light and thus bring about polymerization. In studying 2-VN/FN system, Shirota obtained not only 1 1 alternating copolymer but also several cycloaddition products [26] ... 

A charge-transfer complex (CT complex or CTQ is involved in all thermal charge-transfer polymerizations and in most of the photo-induced charge-transfer copolymerizations. Mixing of a donor (D), such as VCZ, with an acceptor, such as tetracyanoethylene (TCNE), immediately produces color [31] due to the formation of a CT complex ... 

Compared to excitation of the CT complex, excitation of monomer is a more effective pathway for polymerization or cycloaddition. 

Vinylcarbazole polymers and their CT complexes are widely known for their excellent photoconductive properties [115]. The CT processes in this polymer system have been extensively worked out by the conventional absorption spectroscopic techniques, but the application of the XPS technique to them has been rather limited [116]. For poly(Af-vinylcarbazole) (PVK)/perchlorate complex film synthesized via electrochemical polymerization and oxidation, it was shown that the NIs core-level spectrum exhibits a new high BE component shifted by about + 3.0 eV from the neutral carbazole nitrogen at about 400 eV. This new component was attributed to the positively charged carbazole nitrogen associated with CIO4 anion [117],... 

It is well known that cyano derivatives of anthracene form charge transfer (CT) complexes with certain aromatic compounds. It was reported [67] that the radical cations formed upon irradiation of these complexes played an important role in initiation of cationic polymerization of cyclic ethers. Pyridinium salts were also found [68] to form CT complexes with hexamethyl benzene and trimethoxy benzene which result in the formation of a new absorption band at longer wavelengths where both donor and acceptor molecules have no absorption. This way the light sensitivity of the pyridinium salts may be extended towards the visible range. According to the results obtained from the... 

Cyclic monomers such as cyclohexene oxide were readily polymerized upon irradiation of the CT complexes of pyridinium salts whereas spontaneous polymerizations were observed upon mixing with strong electron donating monomers such as butyl vinylether and A-vinyl carbazole. These monomers are known to form CT complexes themselves with electron acceptors which may interfere with the rapid polymerization observed. 

Charge transfer (CT) complexes, different from aromatic ketone/amine systems, such as quinoline-bromine, pyridine-bromine, tetrahydrofuran-bromine etc. have also been reported to behave as initiators of vinyl polymerization, particularly under photoactivation [65-68]. 

In this context, the polymerization kinetic of MMA, under light irradiation, using a CT complex between a polymeric donor such as poly(A -vinylcarbazole) [poly(NVC)] and bromine as the acceptor has been investigated [69]. A radical mechanism for the polymerization is suggested. Moreover, the presence of bromine atoms as end groiqjs in the obtained poly(MMA) and the lack of any evidence of poly(NVC) chemically linked to poly(MMA) have allowed the authors to propose that the radical generation process may be described as in Scheme 11 ... 

Whereas in the MMA photoinitiated polymerization by quinoline-bromine CT complex the formation of radicals is preceded by an instantaneous complexation reaction between the CT complex initiator and the monomer [68], no evidence of this occurrence is observed in the case of the poly(NVC)-Br2 CT complex, probably due to the steric hindrance provided by the polymeric chain. The behaviour of the above system should however be compared with that of the corresponding low-molecular-weight A-alkyl carbazole-Bra CT complex in order to clarify this point. 

Studies on ct-olefin polymerization with Cp-amido titanium complexes have been performed. 

It has been suggested that CT complexes may actually participate in the polymerization. For example, Butler and Campus (12) presented evidence for participation of the CT complex of MA and divinyl ether (DVE) in a terpolymerization study using fumaroni-trile, DVE and MA. Recently, a computer program (13) was published for evaluating the CT copolymerization modeTwhen operating in competition with the terminal model. 

A number of papers have been dedicated to homolytic degradation of TN under the action of ultraviolet or 7-rays. The reaction of TNM with bases (e. benzidine) beings with the formation of CT complexes [183] yielding radical anions which in turn are split into radicals (NO ) and anions (e.g. nitroformanion Irradiation with 7-rays at 77K yielded radicals N02 and C(N02)3 (184). As most nifro compounds TNT inhibits polymerization induced by radiati-[185, 186] and free radical polymerization [I8O- 191]. This is rationalized f the fact that TNM is a radicals acceptor. The higher the number of nitro groui in nitro alkanes the stronger the inhibition of polymerization (189). 

CT fraction. After the first precipitation, the supernatant is evaporated dry and taken up by a volume of methanol. The addition of 2 volumes of chloroform precipitates the condensed tannins (CT), consisting of difficult-to-separate, complex, polymerized procyanidins, with molecular weights between 2000 and 5000. These are, strictly speaking, the main components of tannins. 

Electrically conductive polymeric composites find many applications. They are usually obtained by mechanically mixing a polymer with particles of a conductive filler metal or carbon black. In such a case a high amount of filler is needed. Conductive charge transfer conplexes were also used to prepare conductive polymer composites, but in the early experiments also a high CT complex content - sometimes as high as 80% - was necessary to obtain a conductive material. 

The method discussed in the paper, is based on forming a continuous conductive network of the morphological elements of charge transfer (CT) complex which crystallizes during solidification of the polymeric matrix. 

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