Thermal chromic polymers

Sihcone products dominate the pressure-sensitive adhesive release paper market, but other materials such as Quilon (E.I. du Pont de Nemours Co., Inc.), a Werner-type chromium complex, stearato chromic chloride [12768-56-8] are also used. Various base papers are used, including polyethylene-coated kraft as well as polymer substrates such as polyethylene or polyester film. Sihcone coatings that cross-link to form a film and also bond to the cellulose are used in various forms, such as solvent and solventless dispersions and emulsions. Technical requirements for the coated papers include good release, no contamination of the adhesive being protected, no blocking in roUs, good solvent holdout with respect to adhesives appHed from solvent, and good thermal and dimensional stabiUty (see Silicon COMPOUNDS, silicones). 

When the substituent groups are large, aryl or branched alkyl, normally no batho-chromic shift takes place. This is observed for (n-alkylSiPh)n polymers, diarylpolysilanes and (c-HexSiMe)n. In some cases a slight hypsochromic shift is observed as the temperature is decreased, perhaps due to thermal contraction of the polymer molecules. 

Spontaneous Crosslinking. In the absence of TCPA, evaporated solutions of XVIII can be completely redissolved in fresh solvent in a few minutes. And solutions of 0.05 M XVIII containing 0.007 M TCPA in acetonitrile are stable indefinitely (at least three weeks) at room temperature. However, on evaporation of the solvent under vacuum, the mixture forms a very tough film that is insoluble in acetonitrile or hot DMF, does not melt up to 300°, and is resistant to attack by chromic acid. The IR spectrum is identical to that of XVIII crossiinked BF3 or by irradiation in the presence of TCPA. Apparently as the solution becomes highly concentrated, crosslinking occurs by thermal excitation of charge-transfer complexes of TCPA and XVIII. The IR evidence and the insolubility behavior appear to discount an alternative possibility of simple association of polymer chains facilitated by the presence of the acceptor compound. The spontaneous crossiinking is also observed with the acceptors chloranil and 1,3,5-trinitrobenzene, and presumably would occur with others. 

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