Dimerization and Polymerization

It appears from the description of radical ions in Sects. 1 and 3 that redox reactions can significantly change the chemical and physical properties of conjugated 7r-systems. Whether the extended jc-species are treated within molecular orbital theory or within band-structure theory, the inherent assumption in these concepts is that an electron transfer is reversible and does not promote subsequent chemical reactions. While inspection of cyclic voltammetric waves and the spectroscopic characterization of the redox species provide reliable criteria for the reversibility of an electron transfer and the maintenance of an intact (T-frame, it is generally accepted that electron transfer, depending on the nature of the substrate and on the experimental conditions, can also initiate chemical reactions under formation or cleavage of er-bonds [244, 245], 

While in many cases reactions of radical anions and radical cations are unwanted side effects, there are many examples in which electron-transfer induced reactions of unsaturated systems are of chemical and physical use. A 

There has been a prolonged discussion on whether the initial coupling step implies the dimerization of two radical cations or the reaction between the radical cation and the neutral substrate [284-287], 

It has been mentioned already that polypyrrole (25) and polythiophene (26) play an important role as electrical conductors and polymeric anodes in battery cells [2,47,226]. Since the charging and discharging of the conjugated polymer is accompanied by the incorporation and removal of counterions it is clear that the material can also act as a carrier of chemically different anions which influence the physical, chemical and physiological properties of the material [292]. With regard to the full structural elucidation of the polymers it must be added, however, that the electropolymerization process of pyrrole and thiophene does not provide a clean coupling of the heterocycles in the 2,5-positions. Instead, the 3- and 4-position can also be involved giving rise to further fusion processes under formation of complex polycyclic structures [47]. 

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While both 2- and 3-vinylindole have been synthesized and characterized[l,2], they arc quite reactive and susceptible to polymerization. This is also true for simple l-alkyl derivatives which readily undergo acid-catalysed dimerization and polymerization[3]. For this reason, except for certain cases where in situ generation of the vinylindoles is practical, most synthetic applications of vinylindoles involve derivatives stabilized by EW-nitrogen substituents[4]. 

Dimerization and polymerization can be initiated by electrophilic attack on an oxirane by a protonated or Lewis acid coordinated oxirane (Section 5.04.3.2.2). 

Let us consider the conditions which favor the formation and survival of the dimeric and polymeric radical ions. This might be achieved by keeping the concentration of monomer high, the concentration of monomer" ions low and by removing the radical ions as rapidly as possible from the zone containing the primary electron donors. Moreover, since the radical ions dimerize, their average life time increases as their concentration decreases. The following experiment should probably produce the best results. 

A stream of monomer (or mixture of monomers) is made to flow rapidly over the surface of an alkali metal. If the reaction with the metal is sufficiently slow, a low concentration of monomer" ions will result. In view of the high concentration of the monomer, the monomeric" ions would add further monomer to form the dimeric and polymeric radical ions. Of course, the final product is not a radical, but it would result from a polymerization which took place to some extent on the radical ends. The mixture of monomers may be recirculated many times to increase the conversion and a solvent may be added to the system when necessary. 

The solid-state structures of several benzylic carbanion salts have been elucidated by X-ray analysis9 depending on the nature of the benzylic part, the cation, and the additives, the structures range from er-bonded organometallic compounds to delocalized ion pairs, from monomeric to dimeric and polymeric aggregates. Some compounds are listed together with leading references ... 

Difuryleeryloyl Peroxide. See Vol 5, D1260-R to D1261-L Ethylideneperoxide, Dimeric and Polymeric. See Vol 6, E306... 

Alkyl 2-(hydroxymethyl)acrylates are versatile functionalized monomers and synthetic building blocks. Conventional preparations employ the Baylis-Hillman reaction which involves the addition of formaldehyde to the parent acrylate ester, catalyzed by l,4-diazabicyclo[2.2.2]octane (DABCO). These reactions typically take several days at room temperature, but can be achieved within minutes in the CMR and MBR (Scheme 2.4). Rapid heating under pressure prevents loss of formaldehyde. Subsequent cooling limits hydrolysis of the product, as well as dimerization and polymerization [33],... 

Radialene (2) can be stored at —78°C, but undergoes dimerization and polymerization reactions in solution at room temperature neat 2 may even start to burn at this temperature39. It is also very sensitive towards oxygen exposure to air leads to an intractable material containing up to 40% oxygen. 

Using FSOsH-SbFs at low temperatures Olah el al. (1966) recently obtained U. V. spectra of several alkylaryl- and cycloalkylaryl-carbonium ions. The data are summarized in Table 12. These data were obtained in concentrated solutions (10 to 10 m) at low temperature ( — 50 to — 60°) imder the exact conditions for which the N.M.R. spectra had demonstrated the existence of the ions. Earlier results of Williams (1962) and Grace and Symons (1959) reported absorption maxima above 400 mja for a number of alkylmonoarylcarbonium ions. It must be suggested that in this earlier work, done in H2SO4, dimeric and polymeric ions must have actually been the absorbing species, since it has now been shown by N.M.R. spectroscopy that such ions are not stable in sulphuric acid. 

Kureshy, R. I. Prathap, K. J. Singh, S. Agrawal, S. Khan, N. H. Abdi, S. H.. R. Jasra, R. V. (2007) Chiral recyclable dimeric and polymeric Cr(III) salen complexes catalyzed aminolytic kinetic resolution of trans-aromatic epoxides under microwave irradiation., Chirality, 19 809-815. ... 

Wilkes, J. S., and Zaworotko, M. J., Manifestations of noncovalent interactions in the solid state. Dimeric and polymeric self-assembly in imidazolium salts via face-to-face cation-cation Il-stacking, Supramolec. Chem., 1,191-193, 1993. 

We have found a rare reaction where two topochemical processes occur competitively in a single crystal, that is, a competitive photocyclo-dimerization and -polymerization in the crystal of 1,4-dicinnamoylbenzene 3 (15). On photoirradiation with a mercury lamp (100W) at 20°C for 9 h, the crystals of 3 (2.00g), dispersed in heptane (400 ml), are transformed into amorphous substances consisting of a tricyclic dimer, 21,22,23,24-tetraphenyl-1,4,11,14-tetraoxo-2 (3), 12 (13) -diethano(4,4Jparacyclophane, 4 (isolated yield 57%) (16), a mixture of oligomers (ca. 30%) and unreacted 3 (7%). 

A large variety of pentaammine, tetraammine, and triammine complexes have been prepared and characterized in recent years using the synthetic procedures outlined in Section II,B,4. The various classes of pentaammine complexes that have been prepared are summarized in the appropriate sections (Table I) (2, 47, 54-60, 66-71, 74, 75, 80, 85, 90, 118-120, 167-169, 171-181, 194-201). Dimeric and polymeric complexes are described in Section II,D. 

The dimerization and polymerization of the resulting quinones will be discussed more extensively later in relation to the formation of compounds in which the aromatic structure increases successively during the reactions. 

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