Emulsion polymerization of butadiene. 4. Effect of thiols

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Emulsion polymerization of butadiene. 4. Effect of thiols

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Emulsion polymerization of butadiene, 4. Effect of thiols Makromol. Chem. 192,2009-2019 (1991) 2009 Emulsion polymerization of butadiene, 4a) Effect of thiols Pierre A. Weerts, Jos L. M. van der Loos, Anton L. German* Laboratory of Polymer Chemistry, Eindhoven University of Technology, P. 0. Box 513, 5600 MB Eindhoven, The Netherlands (Date of receipt: November 6, 1990) SUMMARY The role of thiols of low water solubility, commonly used in the emulsion polymerization of butadiene, has been considered. The following effects have become apparent: (1) dodecanethiols act as efficient chain transfer agents in limiting the formation of heavily cross-linked polymer networks; (2) the monomer concentration within the particles is not influenced by such thiols; (3) C ,,-thiol radicals do not desorb because of their extremely low water solubility. The ‘promoting effect’ of thiols in emulsion polymerizations of diene-hydrocarbons is still poorly understood, but it appears to be related to impurities present in the emulsifier, as it was found completely absent in emulsifier-free polymerizations. Introduction In earlier papers the ab initio emulsion polymerization of butadiene with Dresinate 214 ’) and sodium dodecylsulfate’) as emulsifiers, both in the presence of tertiary dodecanethiol as chain transfer agent, were discussed. Chain transfer agents are routinally used in diene-polymerizations to limit the extent of branching and cross- linking of the polymer, and thus the amount of heavily cross-linked insoluble gel. A limited gel content greatly facilitates processability of the crude polymer, such as in the compounding of synthetic rubbers. However, chain transfer agents also exert other, sometimes less desirable effects in emulsion polymerization systems. It is well known they can promote radical desorption 3 3 4 ) , thus lowering the rate of polymerization. Furthermore, slightly water- soluble thiols were found to be essential to bring about reaction at

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