Initialization Behavior at Various Target Molecular Weight RAFT-Mediated Methyl Acrylate Polymerizations ERIC T. A. VAN DEN DUNGEN, HOWARD MATAHWA, JAMES B. MCLEARY, RONALD D. SANDERSON, BERT KLUMPERMAN Department of Chemistry and Polymer Science, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa Received 1 November 2007; accepted 16 December 2007 DOI: 10.1002/pola.22582 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: In the reversible addition-fragmentation chain transfer (RAFT) mediated polymerization of methyl acrylate, a selective reaction is observed in the early stages of the polymerization. This initialization process was earlier observed in in situ 1 H NMR spectroscopy experiments where extremely low target molar masses were cho- sen (around DP ¼ 5). Here, for the first time, the presence of the initialization pro- cess is identified as the cause of an induction period under typical conditions of a RAFT-mediated polymerization. V V C 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2500–2509, 2008 Keywords: inhibition/retardation; initialization period; kinetics (polym.); NMR; RAFT-mediated polymerization; radical polymerization INTRODUCTION During the last decade, interest in radical poly- merization has enormously increased by the invention of new living radical polymerization techniques, such as nitroxide-mediated polymer- ization 1 (NMP), atom transfer radical polymer- ization 2 (ATRP), and reversible addition-frag- mentation chain transfer (RAFT) mediated poly- merization. 3–5 RAFT-mediated polymerization is arguably the most versatile among the living radical polymerization techniques. 3–5 RAFT- mediated polymerizations can be performed under similar conditions to conventional radical polymerization, and it is possible to synthesize polymers of complex architecture, such as block, graft, star, and gradient copolymers. 6–8 In addi- tion, RAFT-mediated polymerizations can easily be combined with various pre- and post polymer- ization reactions. 9–12 The polymerization can be performed at room temperature using gamma or UV radiation as the initiating source. 13–16 With the detailed investigation of the RAFT mechanism (Scheme 1), a debate has arisen about induction period and rate retardation phe- nomena observed in RAFT-mediated polymeriza- tions. 17 Generally, the induction period is re- lated to the early stages of the polymerization, during which no conversion is observed. Rate re- tardation is the reduction in the rate of polymer- ization as compared to conventional radical poly- merization, when there is no RAFT agent pres- ent. It must be stated that the induction and rate retardation phenomena are system depend- ent. 18 Although there are some similarities, it is likely that induction period and rate retardation have largely different origins. In view of the ex- perimental work in this article, the focus in this section will be on the induction period. Perrier et al. concluded that slow fragmentation of the intermediate radical during the early stage of the polymerization, when the initial RAFT agent is converted into a macroRAFT agent, was the Correspondence to: B. Klumperman (E-mail: bklump@ sun.ac.za) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 2500–2509 (2008) V V C 2008 Wiley Periodicals, Inc. 2500