This work investigates a batch and unseeded emulsion polymerization of isoprene at 10 degrees C with n-dodecylmercaptan as chain transfer agent (CTA). The obtained polyisoprene (PI) was analyzed by size exclusion chromatography (SEC) with on-line viscometry. A global polymerization model was adjusted to the measurements of conversion, average particle size, and average molecular weights. The CTA concentration strongly affects the average Molecular weights but has a negligible effect on conversion, average particle diameter, and average branching. Due to the combined effects of chain transfers to the polymer and to the CTA, the final molar mass distributions exhibited dispersity indexes higher than 10. The polymerization model predictions on the average degree 0 of branching was combined with an ideal SEC model for adjusting the branching exponent of P1 in tetrahydrofuran at 25 degrees C that resulted epsilon = 2.5. A sensitivity analysis showed that +20%, errors in (M-omega) over bar induced variations in epsilon lower than -10%. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 116: 590-601, 2010