Intrachain reactions of a pair of reactive groups attached to polymer ends. 3. Intrachain charge-transfer complex on...

Polysarcosine having a terminal p-dimethylaminoanilide group and a terminal 3,5-dinitrobenzoyl group was synthesized. The number-average degree of polymerization n was varied from 6 to 25. In chloroform solution this polymer showed a distinct absorption band around 455 nm, which was attributed mostly to intramolecular charge-transfer interactions. The extinction coefficient of the charge-transfer complex was determined for low molecular weight model compounds. Using the same extinction coefficient, the fraction of polymers forming intrachain charge-transfer complex was evaluated at infinite dilution. The fraction was about 0.15 for n = 6 and decreased asymptotically with increasing n finally to 0.03 for n = 25. These values are almost 20-100 times as large as those estimated from the Monte Carlo calculation and on the basis of the intramolecularly catalyzed hydrolysis of polysarcosine chain. This indicates that the cyclized conformations of polysarcosine chain are greatly stabilized by the formation of intrachain charge-transfer complex. The fraction of the cyclized polymer was decreased with increasing temperature for short chains. The thermodynamic parameters characterizing the conformational change required for cyclization in chloroform were obtained and compared with those for the same reaction in ethanol solution, as well as those for the intramolecularly catalyzed hydrolysis on polysarcosine chain in aqueous solution.