EFFICIENCY OF MERCAPTAN CHAIN TRANSFER AGENTS IN EMULSION COPOLYMERIZATIONS - I - INFLUENCE ON KINETICS AND MICROSTRUCTURE - MODELING OF RADICAL DESORPTION

The effect of chain transfer agents (CTA) on the emulsion copolymeriza tion of styrene and butyl acrylate was studied in a bench scale 7 L re actor. On-line estimates of conversion were obtained through the joint use of calorimetric measurements and fast gravimetric data. Off-line measurements of partial conversions, molecular weight distribution (MW D), glass transition temperature (T-g), and particle diameter were als o performed in order to investigate the effect of two mercaptans (tert -butanethiol and n-dodecanethiol) on both the kinetics of the polymeri zation process and the microstructure-dependent properties of the copo lymer. The obtained experimental results were interpreted in terms of radical desorption and diffusive limitations of the CTA between the oi l droplets and the particles. A model has been derived to compute the kinetic constants, the number of radicals per particle, and both the G PC/SEC diagrams and DSC thermograms related to MWD and T-g measurement s, respectively. Several batch and semibatch examples are proposed to show that these important variables are satisfactorily fit by the mode l. (C) 1998 John Wiley & Sons, Inc.