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Seeded and ab initio emulsion polumerizations of isoprene using redox initiation systems were investigated and suitable reaction conditions determined to prepare polyisoprene latexes with minimal cross-linking. Polymerizations initiated with the potassium persulfate/sodium bisulfite (KPS/SBS) redox couple had a significant inhibition period and low yield. Polymerizations initiated with the 'tert'-butyl hydroperoxide/tetraethylene-pentamine redox couple showed reasonable yields and no apparent inhibition. It is postulated that the lipophyllic nature of the t-butyl group plays a favorable role in the entry of hydroperoxide-initiated oligomeric radicals, while persulfate-initiated radicals are more likely to undergo aqueous phase termination before entry. The cross-linking reaction by benzoyl peroxide (BPO) at 70°C was investigated using this lightly cross-linked polyisoprene latex. ¹H NMR and gel permeation chromatography results were consistent with a reaction mechanism in which the radicals formed by the decomposition of BPO react exclusively with polyisoprene to abstract a hydrogen atom, and the resulting radicals react by termination to form cross-links. No loss of double bonds was found, suggesting that radical formation is overwhelmingly achieved by hydrogen abstraction and cross-linking occurs by termination across two radicals. Cross-linking was accompanied by chain scission, which was observed only at the beginning of the reaction. At low weight fractions of polymer, the rate of cross-linking was dependent on the concentrations of BPO and abstractable hydrogens in a manner consistent with the postulated mechanism. |
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