Abstract
Formations and reactivities of emulsifier radicals were investigated by means of electron spin resonance (ESR) to understand the chemical role of emulsifiers in emulsion polymerization. Formations of the emulsifier radicals were initiated by generation of hydroxyl radicals from a redox couple of TiCl3 and of H202 and the following results were obtained. All the ESR spectra from emulsifiers containing hydrophylic oxyethylene chains were the same and assigned to the radical, -CH2OCHCH2OCH2-, formed by hydrogen abstraction from oxyethylene, even if their hydrophobic groups are different. PVA radical was assigned to -CH2CH(OH), which was formed by chain scission followed by hydrogen abstraction from secondary alcohol, -CH(OH)-. No signals were observed from gum Arabic or hexadecyl trimethyl ammonium bromide. Addition of vinyl monomers retarded the formation of emulsifier radicals. Reactivities of the radicals from polyoxyethylene nonylphenol ether were extensively studied and the rate constants, k3, k5 and the rate constant ratio, k5/k3, were estimated to be ～106, ～102 in l/mol・sec and ～10-4, respectively, where k3 is the rate constant for the termination reaction between the emulsifier radicals and k5 is the rate constant for addition of the emulsifier radical to VAc. This addition reaction is significant especially, since it is the first step to result in the block copolymer, PVAc-b-polyoxyethylene nonylphenol ether, which is expected to act as an excellent stabilizer in PVAc emulsion.
