Oligomers, radical cation dimers

Oxidative Polymerization Reactions. Clays can initiate polymerization of unsaturated compounds through free radical mechanisms. A free radical R", which may be formed by loss of a proton and electron transfer from the organic compound to the Lewis acid site of the clay or, alternatively, a free radical cation, R+, which may be formed by electron transfer of an electron from the organic compound to the Lewis acid site of the clay, can attack a double bond or an aromatic ring in the same manner as an electrophile. The intermediate formed is relatively stable because of resonance, but can react with another aromatic ring to form a larger, but chemically very similar, species. Repetition of the process can produce oligomers (dimers, trimers) and, eventually, polymers. 

Anderson SM and Krinsky NI (1973) Protective action of carotenoid pigments against photodynamic damage to Uposomes. Photochem Photobiol 18 403 08 Badger B and Brocklehurst B (1969) Absorption spectra of dimer cations. Trans Faraday Soc 65 2576-2581 Bally T, Roth K, Tang W, Schrock RR, Knoll K and Park LY (1992) Stable polarons in polyacetylene oligomers Optical spectra of long polyene radical cations. J Amer Chem Soc 114 2440-2446... 

There is no evidence of rt-dimer formation or dimerization to an oligomer w ith double chain length as in solution (compare Section 4.2.2), not even for a2T and a3T. After one day the original spectra of the undoped materials are revealed. Apparently the radical cations remain nearly immobile on the surface. 

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