Supramolecular association of acid-terminated poly(dimethylsiloxane)s. 2. Molecular weight distributions

Association through hydrogen bonding of benzoic acid-terminated poly(dimethylsiloxane) 1 in low-polarity solvents has been studied by FTIR spectroscopy and viscosimetry. At high concentrations, chain extension leads to high viscosity values, whereas at low concentrations, intramolecular hydrogen bonding is responsible for the formation of low-viscosity cyclic species. A quantitative model based on Jacobson-Stockmayer theory and describing the competition between chain extension and macrocyclization is presented. On the basis of this model, the molecular weight distributions of ring and chain fractions has been calculated as a function of concentration in three different solvents. The length df the siloxane backbone has been shown to have a large influence on the cyclic weight fraction and on the average molecular weight.