Controlled Radical Polymerization of Styrene by Reverse Iodine Transfer Polymerization (RITP) in Miniemulsion: Use of Hydrogen Peroxide as Oxidant Jeff Tonnar, Patrick Lacroix-Desmazes,* and Bernard Boutevin Inge ´ nierie et Architectures Macromole ´ culaires, Institut Charles Gerhardt FR 1878-CNRS, UMR-CNRS 5076, Ecole Nationale Supe ´ rieure de Chimie de Montpellier, 8 Rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France ReceiVed July 21, 2006; ReVised Manuscript ReceiVed NoVember 6, 2006 ABSTRACT: The use of molecular iodine I 2 in controlled radical polymerization, called reverse iodine transfer polymerization, represents a new, straightforward way to prepare controlled macromolecular architectures. Herein, miniemulsion polymerization of styrene in the presence of molecular iodine has been successfully performed. The polymerization of styrene was initiated by bis(4-tert-butylcyclohexyl) peroxydicarbonate at T ) 60 °C with dodecyl sulfate sodium salt as surfactant and hexadecane as hydrophobe, yielding a stable and uncolored latex. A certain amount of iodine reacted with water to form hydroiodic acid, leading to an upward deviation of the experimental molecular weight from the theoretical value. However, when the iodine lost by hydrolysis was regenerated by continuous addition of hydrogen peroxide in acidic conditions, it led to the expected molecular weight (e.g., M n,SEC ) 7900 g mol -1 , M w /M n ) 1.46, styrene conversion ) 78%, M n,theoretical ) 7900 g mol -1 ). Hence, the molecular weight of the polymer chains could be accurately controlled by changing the concentration of iodine. Last, a chain extension was successfully performed in seeded emulsion polymerization, proving the living character of the polymerization. Introduction During recent years, controlled radical polymerization (CRP) 1-3 has provided several simple and robust routes to synthesize well- defined low-dispersity polymers and the fabrication of novel functional materials 4-8 like block, graft, or star copolymers. The main difference with free radical polymerization is that in CRP a reversible activation-deactivation process between dormant (or capped) chains and active chains (or propagating radicals) enables all polymer chains to grow at the same rate. Different CRP methods have been developed so far among which nitroxide-mediated polymerization (NMP), 6 atom transfer radi- cal polymerization (ATRP), 5 iodine transfer polymerization (ITP), 9-12 and reversible addition-fragmentation chain transfer polymerization (RAFT/MADIX) 13 are the most widely studied. Considering more specifically ITP in aqueous dispersed medium which is of industrial interest, promising results were obtained in miniemulsion polymerization of styrene 14-17 and seeded emulsion polymerization of n-butyl acrylate 16 leading to a PS- b-PBuA block copolymer. A new CRP method, which relies on molecular iodine I 2 to control the polymerization, was developed by our group 18-20 and patented. 21-23 The basic mechanism of reverse iodine transfer polymerization (RITP) is shown in Scheme 1. The initiator decomposes (e.g., thermally) to form radicals which react with iodine or add a few monomer units before reacting with iodine to form the iodinated transfer agents (A-I adduct or A-M n -I transfer agents) in situ. Once the whole free iodine has been consumed, the degenerative transfer mechanism establishes itself. One main difference between RITP and ITP is that in RITP the transfer agents are synthesized in situ. There is no need to synthesize and store transfer agents. One molecule of iodine can control two polymer chains and the targeted molecular weight is given by eq I. Recently, our group polymerized butyl acrylate by RITP in ab initio emulsion polymerization. 24,25 Although the molecular weight was efficiently tuned by varying the concentration of iodine, iodine hydrolysis was responsible for an upward deviation of the molecular weight from the theoretical value. Herein, we propose a major improvement of RITP in dispersed aqueous medium. Hydrogen peroxide is added as oxidant to counterbalance the side reactions of iodocompounds in water. This method is assessed in RITP of styrene in miniemulsion 26 and leads to a very good control of the molecular weight. Experimental Section Materials. Styrene (Acros, 99%) was purified by vacuum distillation before use. Dodecyl sulfate sodium salt (SDS, Aldrich, 98%, critical micelle concentration ) 2.6 g.L -1 ), bis(4-tert- * Corresponding author. Telephone: 33 4 67 14 72 05. Fax: 33 4 67 14 72 20. E-mail: patrick.lacroix-desmazes@enscm.fr. Scheme 1. Simplified Mechanism of Reverse Iodine Transfer Polymerization in Solution (A • , Radical from the Initiator; I2, Molecular Iodine; M, Monomer Unit; n, Number Average Degree of Polymerization) M n,targeted ) (mass of monomer)/(2 × n I2,initial ) + M A-I (I) 186 Macromolecules 2007, 40, 186-190 10.1021/ma061649c CCC: $37.00 © 2007 American Chemical Society Published on Web 12/22/2006