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

Association through hydrogen bonding of benzoic acid-terminated poly(dimeth ylsiloxane) 1 in low-polarity solvents has been studied by FTIR spectroscop y and viscosimetry. At high concentrations, chain extension leads to high v iscosity values, whereas at low concentrations, intramolecular hydrogen bon ding is responsible for the formation of low-viscosity cyclic species. A qu antitative model based on Jacobson-Stockmayer theory and describing the com petition between chain extension and macrocyclization is presented. On the basis of this model, the molecular weight distributions of ring and chain f ractions has been calculated as a function of concentration in three differ ent 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.