HIGHLIGHT Iniferter Concept and Living Radical Polymerization TAKAYUKI OTSU 1-3-6, Gakuen-Midorigaoka, Nara 631-0026, Japan Received 13 March 2000; accepted 15 March 2000 ABSTRACT: Iniferters are initia- merizations proceed by a living rad- functional, trifunctional, or poly- tors that induce radical polymeriza- ical polymerization mechanism in a functional iniferters; monomer or tion that proceeds via initiation, homogeneous system. In these macromonomer iniferters; and so propagation, primary radical termi- cases, the iniferters (COS bond) are forth. These lead to the synthesis of nation, and transfer to initiator. Be- considered a dormant species of the various monofunctional, telechelic, cause bimolecular termination and initiating and propagating radicals. block, graft, star, and crosslinked other transfer reactions are negligi- In this article, I describe the history, polymers. The relations between this ble, these polymerizations are per- ideas, and some characteristics of work and other recent studies are formed by the insertion of the mono- iniferters and living radical polymer- discussed. © 2000 John Wiley & Sons, Inc. mer molecules into the iniferter ization with some iniferters that con- J Polym Sci A: Polym Chem 38: 2121–2136, bond, leading to polymers with two tain dithiocarbamate groups as pho- 2000 iniferter fragments at the chain ends. toiniferters and several compounds The use of well-designed iniferters as thermal iniferters. From the view- Keywords: iniferter; living radi- would give polymers or oligomers point of controlled polymer synthe- cal polymerization; dithiocarbam- bearing controlled end groups. If the sis, iniferters can be classified into ate; thiuram disulfide; block and end groups of the polymers obtained several types: thermal or photo- graft copolymers; photopolymer- by a suitable iniferter serve further iniferters; monomeric, polymeric, or ization; controlled synthesis as a polymeric iniferter, these poly- gel iniferters; monofunctional, di- Dr. Takayuki Otsu is a Professor Emeritus, Osaka City University. He was born in Osaka in 1929 and received his B.Sc. degree from the Osaka Institute of Science and Technology in 1951. He then was appointed as an instructor at Osaka City University and started his research work on radical polymerization under the late Professor Minoru Imoto. In 1956, he found that polymers derived from thiuram disulfides could induce pho- topolymerization to give block and graft copolymers. This discovery became the foundation for this highlight. For his work, he was awarded a D.Sc. degree from Osaka University in 1959 and went to the United States of America to work as a research associate with Professor Carl S. Marvel at the University of Illinois for a year. Then, he returned to Osaka TAKAYUKI OTSU City University and was appointed Associate Professor. In 1965, he accepted the position of Full Professor of Polymer Chemistry of the Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 38, 2121–2136 (2000) © 2000 John Wiley & Sons, Inc. 2121 2122 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 38 (2000) Department of Applied Chemistry, Faculty of Engineering, Osaka City University, and worked there for 26 years. In the meantime, he served as a senator for Osaka City University from 1977 to 1979. After his retirement in 1992, he moved to Kinki University, Wakayama, as Pro- fessor until his retirement in 1999. The main topics of his research are the various fields of radical polymerization: basic studies of rates and mechanisms, new monomers and initiators, monomer structure–reactivity relationships, syntheses of head-to-head polymers, controlled polymer syntheses with the iniferter and living radical polymerization techniques, and syntheses of new acrylic polymers from maleic, fumaric, and itaconic derivatives. He is the author or coauthor of more than 550 original papers, 120 review articles, 10 books, 30 book chapters, and more than 100 patents. Dr. Otsu was invited abroad to more than 40 International Symposiums, universities, and laboratories to give various lectures. He has served as a member of the organizing committee of the International Symposiums held in Japan. He was also a chairman of the Japanese side of the Japan–China Sym- posium on Radical Polymerization (now the Asia Polymer Symposium) from 1980 to 1990. He has served on the editorial boards of several journals and edited a number of monograph series on polymer and experimental sciences. Dr. Otsu also served as Vice President of the Society of Polymer Science, Japan from 1986 to 1988. He was given the Chemical Society of Japan Award for Young Chemists in 1961, and he was also the recipient of the Award for Distinguished Service in the Advancement of Polymer Science in 1990 from the Society of Polymer Science, Japan. In 1951, I started to work under Professor M. Imoto at In 1954, we began to examine the initiating ability of the newly established Institute of Polytechnics, Osaka these compounds in radical polymerization of St and City University, on the radical polymerization of vinyl MMA and found in 1956 that various sulfides and dis- chloride (VC) with benzoyl peroxide (BPO)/dimethyla- ulfides (e.g., phenyl, benzoyl, benzothiazoyl, xantho- niline as an initiator, and I published two articles1 on the gene, thiuram, and dithiocarbamate derivatives) could initiation mechanism in 1955. In those days, two new serve as efficient photoinitiators.7,8 Among thiuram dis- 9 ϭ polymers, high-density polyethylene and isotactic ulfides, tetraethylthiuram disulfide (1;R C2H5) was polypropylene, were discovered with novel initiators, the most excellent photo- and thermal initiator. However, and the initiation mechanism for radical polymerization for other monomers such as vinyl acetate (VAc), acry- was also discussed by several workers.2 In 1953, the lonitrile, and VC, 1 served as a weak initiator or termi- definition of block and graft copolymers was clarified by nator, depending on the reactivities of the monomers.8,10 Mark,3 who emphasized that some physical properties From kinetic studies,10–12 1 was found to act not only were different from those of random and alternating as an initiator but also as a retarder, terminator, and copolymers. After that, the synthesis of these polymers transfer agent. Data on sulfur analysis indicated that the has been attempted with several methods4 in which the resulting polymers had nearly two initiator fragments (5 block copolymer of styrene (St) and methyl methacrylate in eq 4) at the chain ends, as is shown in Table I.13 (MMA) is prepared with polymeric radical initiators5 and Therefore, the polymerization of the monomer (M) with the living polymer discovered by Szwarc6 in 1956. 1 proceeds via the dissociation of 1, initiation, propaga- This being the case, I had an interest in new initiators tion, primary radical (PR) termination, and chain transfer and their mechanisms, and I focused my attention on the (CT) to 1, according to eqs 1–5,12 respectively: unique reaction behavior of organic sulfur compounds, ⅐ which have been used as a thiyl radical source, an ac- RЈ2NOCOSOSOCONR2 ^ 2RЈ2NOCOS celerator, a modifier, a terminator for vinyl or diene ʈʈ ʈ polymerization, and an accelerator for mastication and SS S vulcanization in the rubber industry. 1 2 (1) HIGHLIGHT 2123 Table I. Polymerization of St with 1a S b ϫ Ϫ4 f g 1 (g) Yield (%) Mn 10 (%) N Reference 1.0 20 3.3d 0.40 2.2 13 0.15 25 1.6d 0.78 1.7 13 0.20 32 1.2d 0.78 1.5 13 0.25 38 0.96d 0.74 2.2 13 0.15 28 8.2e — 2.0 22 0.30 35 5.3e — 2.0 22 045 39 4.2e — 2.3 22 0.10 23c 6.4e — 1.7 (2.1)h 22 0.18 10c 4.5e — 1.7 (2.1)h 22 0.25 18c 5.9e — 1.7 (2.1)h 22 a 10 mL of St, 4 h, 80 °C. b 27 h (St) and 20 h (MMA). c MMA was used. d Determined by viscometry. e Determined by membrane osmometry in toluene. f Determined by the Carius method. g O Number of (C2H5)2NC(S)S end groups per polymer determined by S (%) or UV spectrometry and Mn. h Calculated with Mn determined by viscometry. ⅐ 2 ϩ M ¡ RЈ2NOCOSOM After purification by reprecipitation twice, polymer 5, ʈ obtained from St, was found in 1957 to initiate the 13,14 S photopolymerization of MMA and VAc to give 3 (2) block copolymers (12 in eq 9). Alkaline hydrolysis of the latter provided a new block copolymer consisting of a ϩ ¡ Ј O O O ⅐ hydrophobic poly(St) and a hydrophilic poly(vinyl alco- 3 nM R2N C S–(M)–n M 15 ʈ hol). Moreover, when the polymers of St were allowed to S react with 1 in the presence or absence of BPO in 4 (3) benzene at 60–100 °C, an appreciable amount of frag- ments of 1 was introduced. These polymers also induced ϩ ¡ Ј O OS–(M)–OSO O Ј 4 (or 3) 2 R2N C nϩ1 C NR2 the photopolymerization of MMA, leading to graft co- ʈʈ polymers.14 By this method, several graft copolymers SS were prepared.16 At that time, however, the idea of 5 (4) polymer design was not taken into consideration, but this method has been applied for the preparation of various 4 (or 3) ϩ 1 ¡ 5 ϩ 2 (5) block and graft copolymers. After we published these results in several journals, The ordinary bimolecular termination is neglected be- the study was interrupted because I worked with Profes- cause PR 2 is less reactive for initiation and more reac- sor Carl S. Marvel at the University of Illinois (Urbana, tive for PR termination. The CT to 1 also occurs because Illinois). At that time, a new thermally stable polymer, the CT constants are high values,22 0.72 for St and 0.48 polybenzimidazole, was prepared by Vogel17 in the next for MMA at 60 °C, yielding a relatively low molecular room.