Anionic polymerization of octamethylcyclotetrasiloxane in miniemulsion II. Molar mass analyses and mechanism scheme

Abstract

Hydroxyl α,ω-functionalized polydimethylsiloxane (PDMS) standards have been synthesized by ring-opening anionic polymerization of octamethylcyclotetrasiloxane (D4)11The notation for the siloxane oligomers throughout the paper is the following: M holds for (CH3)3SiO1/2–, Mv for CH2CHSi(CH3)2O1/2–, MOH for HOSi(CH3)2O1/2– and D for –Si(CH3)2O–. in miniemulsion. Their absolute molar masses as well as Mark–Houwink–Sakurada parameters were then estimated in toluene at 30°C by triple detection size exclusion chromatography (SEC3). MALDI-TOF confirmed the SEC3 results in addition to providing several clues on chain microstructure and evidence of redistribution reactions. Trimethyl and vinyl α,ω-terminated PDMS oligomers were similarly synthesized in miniemulsion in the presence of functional transfer agents and used together with small dihydroxylated PDMS standards to compare the effect of substituents on the SEC calibration curve. Precise measurement of average molecular weights showed that the existing kinetics model could not account solely for mass control below 70% conversion. A thermodynamic explanation is then proposed where the molar mass control is exerted by the surface activity of the chains which drives their reactivity. Small chains rapidly propagate and backbite at the interface. Once the chains reach a critical DP corresponding to their loss of surface tension activity, they penetrate into the particles where side reactions such as redistribution and condensation hardly occur.