Diradical, chain-length

Dicyanohexasulfane 8 Diradical, chain-length 91 Displacement reaction 91 Dissimilatory reduction 169 DLVO-theory 172 Dynamic light scattering 178... 

The polymerization is photosensitive, involves diradicals and leads to chain lengths that exceed 200000 S atoms at -I80"C before dropping slowly to "-1000 S at 400" and -100 S at 600". 

Liquid sulfur is almost entirely S8 up to 160° at higher temperatures equilibrium mixtures of S8 and polymeric forms predominate. Work by Gee (128) and paramagnetic resonance measurements by Gardner and Fraenkel (127) show that the polymers are probably diradical chains with a maximum average length of about 106 atoms at approximately 170°. Schenk (195) has recently reviewed the evidence for long chain molecules... 

Diradicals longer than 6 monomer units have not been observed in TS though asymmetric carbenes and stable oligomers have been reported with ten or more repeat units. Thus, it seems that in TS the diradicals are stable only for short chain lengths and at room temperature these are rapidly converted into carbene species C33]. Although similar results have been observed for some diacetylenes, these show differences in detail, in particular the annealing processes at higher temperatures vary markedly from compound to compound. 

Figure 6. Optical absorption energy versus chain length for diradicals (A), asymmetric carbenes (D) and stable oligomers (X) in TS. n is the number of chain repeat units, after (29).

The reduction of bipyridyls was discussed in Part I (see also Refs. 223-226). Paraquat dimer molecules (155) held together by a chain of methylene groups are reduced through the cation-radical and further to a diradical the splitting of the waves depends on the length of the methylene bridge.227 They have been considered as two-electron mediators in indirect electrode... 

The argument in favour of the latter was that the diradical should be lower in energy, because its formation requires disruption of only one carbon-carbon rc-bond instead of two in case of carbene formation. Both ESR work and optical spectroscopy have meanwhile confirmed the diradical mechanism for growth of oligomeric chains up to length of 5 repeat units. Upon further addition of monomers, the acetylenic structure becomes energetically more stable causing a cross-over to the carbene mechanism. For further discussion of this topic the reader is referred to the article by H. Sixl in this volume. 

It is possible to exactly identify and characterize the radical species and chain structures of the reaction intermediates, which are determined by their different reactive or unreactive chain ends. The reactive intermediates are best described by diradical (DR), asymmetric carbene (AC) and dicarbene (DC) oligomer molecules of different lengths. The respective singlet (S = 0), triplet (S = I) or quintet (S = 1) states and their roles in the polymerization process are investigated in detail by solid state spectroscopy. A one-dimensional electron gas model is successfully applied to the optical absorption series of the DR and AC intermediates as well as on the different stable oligomer SO molecules obtained after final chain termination reactions. 

From spectroscopic data, presented in the following, we conclude that the mechanism of polymerization is described by three series of intermediate states differing by the number of reactive radical or carbene chain ends these are the diradicals DR , the dicarbenes DC , and the asymmetric carbenes AC . Via a final chain termination reaction an additional series of reaction products is obtained. These are the stable oligomers SO with two unreactive chain ends. The schematic structures of the DR, DC, AC, and SO molecules are shown by example of the trimer in Table 2. The lengths of the dimer-, trimer-, tetramer-... units are characterized by the numbers n = 2, 3,4,... of the respective monomer molecules. The symbols and the schematic structures as well as the notation of the optical and the ESR absorption lines, are summarized in Table 2. 

Intramolecular photocycloaddition of 2-alkenylenones consists of reactions ofenones of structure 1. According to the rule of five , the initial attack of the enone on the alkene double bond gives 1,4-diradical intermediates of type A, B, C or D depending on the length of the connecting chain (n = 1-3)9. 

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