Average Termination Rate Coefficients in Emulsion Polymerization: Effect of Compartmentalization on Free- Radical Lifetimes STUART W. PRESCOTT, 1–3 * MATHEW J. BALLARD, 2 ROBERT G. GILBERT 1 1 Key Centre for Polymer Colloids, School of Chemistry, University of Sydney NSW 2006, Australia 2 CSIRO Molecular Science, Private Bag 10, Clayton South VIC 3169, Australia 3 Cooperative Research Centre for Polymers, 32 Business Park Drive, Notting Hill VIC 3168, Australia Received 4 August 2004; accepted 4 October 2004 DOI: 10.1002/pola.20580 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: A method is presented by which the time-dependent average termination rate coefficient in an emulsion polymerization may be calculated as an appropriate average of the chain-length-dependent termination rate coefficients. The method takes advantage of the fact that the overall termination rate is dominated by terminations between rapidly moving short radicals and much slower long ones. This termination rate coefficient is suitable for use in the Smith–Ewart equations describing the com- partmentalization of radicals in an emulsion polymerization. Rate data in emulsion polymerizations can be quantitatively interpreted if the kinetics fall into one of two categories: zero– one (showing compartmentalization; intraparticle termination is not rate-determining) or pseudo-bulk (no compartmentalization; intraparticle termination is rate-determining). The new method can be used to interpret rate data for systems falling between these categories and also can be used to find termination rate coeffi- cients from Monte Carlo simulations of termination kinetics. The latter is especially useful for predicting and understanding kinetics in controlled radical polymerizations in disperse media. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1076 –1089, 2005 Keywords: chain-length-dependent termination; emulsion polymerization; free-rad- ical polymerization; kinetics (polym.); modeling INTRODUCTION In describing the kinetics of an emulsion polymer- ization, we must include not only the processes of radical creation, propagation, transfer, and termi- nation but also radical phase-transfer processes such as the entry and exit of radicals from parti- cles. The effects of the compartmentalization of radicals into particles, by which the radicals in one particle are unable to react with the radicals in another particle, may be quite strong. An ex- ample of strong compartmentalization of radicals can be observed in the emulsion polymerization of styrene; for particles less than 70 nm in swollen radius, there may be only zero radicals or one radical in a particle with typical radical fluxes because termination is sufficiently fast to allow *Present address: Particulate Fluids Processing Centre, School of Chemistry, University of Melbourne, VIC 3010 Aus- tralia Correspondence to: R. G. Gilbert (E-mail: gilbert@ chem.usyd.edu.au) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 43, 1076 –1089 (2005) © 2005 Wiley Periodicals, Inc. 1076