2,2,6,6-Tetramethylpiperidine-1-oxyl-mediated living mini-emulsion polymerization of styrene under ambient pressure in a semi-batch process: effect of ascorbic acid

Abstract

2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO)-mediated living mini-emulsion polymerization of styrene with feeding of an ascorbic acid aqueous solution throughout the polymerization was performed at 90 °C under ambient pressure. The concentrations of sodium dodecylbenzenesulfonate (SDBS) and ascorbic acid were varied to study the shell polymerization mechanism of latex particles and evolution of growing chains. Interactions between SDBS and ascorbic acid and incompatibility between ascorbic acid and styrene were evident from UV-visible analyses. High hydrophilicity of ascorbic acid in the aqueous phase was proved using a gravimetric method. Accordingly, the formation of a surface barrier on particles was proposed because of the interactions between SDBS and ascorbic acid. For higher SDBS concentration, the surface barrier on the particles was denser. Therefore, the polymerization rate decreased with increasing SDBS concentration. However, the polymerization rate increased with increasing ascorbic acid concentration. This was due to a higher consumption rate of TEMPO by ascorbic acid. Free TEMPO tended to reside in surface zones of the particles because of the surface activity between the aqueous and oil phases. The surface zones were thus the main loci where TEMPO was consumed by ascorbic acid. The estimated number-average molecular weight (Mn) of growing chains increased in a linear fashion with conversion. This indicated that the growing chains were produced via living mini-emulsion polymerization. For these growing chains, the estimated Mn and final polydispersity increased with increasing SDBS concentration. This was caused by a decrease in TEMPO concentration in the surface zones of particles with increasing SDBS concentration. The ‘livingness’ of polystyrene was identified by conducting bulk polymerization of chain extension. Based on the results obtained, a shell polymerization mechanism of latex particles was proposed, and living mini-emulsion polymerization was limited to the surface zones of particles. Copyright © 2010 Society of Chemical Industry