Chemical Engineering Science 58 (2003) 1177–1190 www.elsevier.com/locate/ces Nitroxidemediatedlivingradicalpolymerizationofstyrene inminiemulsion—modellingpersulfate-initiatedsystems JohnW.Ma a ,MichaelF.Cunningham a ; * ,KimB.McAuley a , BarkevKeoshkerian b ,MichaelGeorges c a Department of Chemical Engineering, Queen’s University, Kingston, Ont., Canada, K7L 3N6 b Xerox Research Centre of Canada, Mississauga, Ont., Canada, L5K 2L1 c University of Toronto, Toronto, Ont., Canada, L5L 1C6 Received 29 January 2002; accepted 11 June 2002 Abstract Recentlywehaveconstructedamechanisticmodeldescribingthenitroxidemediatedminiemulsionpolymerization(NMMP)ofstyreneat 135 ◦ C,usingalkoxyamineinitiatorstocontrolpolymergrowth(Nitroxide-MediatedPolymerizationofStyreneinMiniemulsion.Modeling Studies of Alkoxyamine-Initiated Systems, 2001b). The model has since been expanded to describe styrene NMMP at 135 ◦ C using TEMPO and the free radical initiator, potassium persulfate (KPS). The model includes mechanisms describing reactions in the aqueous and organic phases, particle nucleation, the entry and exit of oligomeric radicals, and the partitioning of nitroxide and styrene between the aqueous and organic phases. Predicted monomer conversions, number average molecular weights and polydispersities were in agreement withexperimentallymeasuredvalues.ModelsimulationsrevealedthatforsystemsemployinghighratiosofTEMPO:KPS,theconsumption of TEMPO by polymer radicals derived from KPS decomposition and styrene thermal initiation (using the accepted literature kinetic rates) is not sucient to lower TEMPO concentrations to levels where polymer growth can occur. By accounting for the consumption of TEMPO by acid-catalyzed disproportionation, TEMPO concentrations are signicantly reduced, allowing for accurate model predictions of monomer conversion, number average molecular weight and polydispersity at every experimental condition considered. ? 2003 Elsevier Science Ltd. All rights reserved. Keywords: Emulsion; Mathematical; Modelling; Multiphase reactions; Polymers; Polymerization 1. Introduction Several “living” radical polymerization processes (e.g., nitroxide-mediated polymerization, ATRP, RAFT) have been developed in recent years that show characteristics of living systems, e.g., narrow molecular weight distributions (MWDs) and active polymer end-groups. Of particular interest to this study is nitroxide-mediated polymerization (NMP). Originally developed by Solomon, Rizzardo, and Cacioli (1985) and later advanced by Georges, Veregin, Kazmaier, and Hamer (1993), NMP employs a nitroxide, such as 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) to reversibly deactivate/trap growing polymer radicals to formdormantalkoxyamines(seeScheme 1).At125 ◦ C,the equilibrium coecient for radical deactivation, K eq = k a =k d , for TEMPO and polystyrene radicals greatly favors the ∗ Corresponding author. Tel.: +1-613-533-2782; fax: +1-613-533-6637. E-mail address: cunning@chee.queensu.ca (M. F. Cunningham). formation of the dormant alkoxyamine (K eq =2:1 × 10 -11 M -1 , Fukuda et al., 1996). As a result, polymer radicals are predominantly dormant in NMP, which reduces the likelihood of bimolecular ter- mination. Because monomer can only add to the growing polymer radical between successive activation and deacti- vation reactions, the rate of polymerization is slower than in conventional free radical polymerization. By initiating polymer radicals at nearly the same instant, polymers with narrow MWDs can be synthesized by NMP. Because the nal product consists of nitroxide-trapped polymer radi- cals, polymers made by NMP can be used to initiate subse- quent homo/copolymerizations to make well-dened block copolymers and polymers with complex architectures. Being a free radical process, NMP is relatively insensi- tive to impurities and can be performed without the need to purify solvents or reagents, unlike traditional living polymerization processes (e.g., ionic polymerization and group transfer polymerization). Additionally, NMP can be performed in commonly used heterogeneous systems, such 0009-2509/03/$-see front matter ? 2003 Elsevier Science Ltd. All rights reserved. PII:S0009-2509(02)00557-2