NITROXIDE-MEDIATED POLYMERIZATION

Macromolecules 1993,26, 2987-2988 2987 Narrow Molecular Weight Resins by a Table I. Polymerization of Styrene (TEMPO/BPO = 13) Free-Radical Polymerization Process sample convn(%) rmtime(h) M.(lW) M,(lW3) PD I 20 21 1.7 2.2 1.28 I1 51 29 3.2 4.1 1.27 Michael K. George#.' Richard P. N. Veregin, I11 76 45 6.8 8.2 1.21 Peter M. Kazmaier, and Gordon K. Hamer Xerox Research Centre of Canada, 2660 Speakman Drive, IV 90 69 7.8 10.0 1.27 Mississauga, Ontario, Canada L5K 2Ll Received January 11,1993 TEMPODPO Ratio Table 11. Polymerization of Styrene as a Function of the sample TEMPO/BPO convn(%) Mn(lW) Mw(lW) PD I 0.5 86 45.6 71.7 1.57 n 1.5 74 33.1 41.1 1.24 iniferters to provide "living" free-radical propagating I11 3.0 71 18.2 21.7 1.19 A primary goal of free-radical polymerization reactions has recently been achieved with the introduction of chains.' However, the goal of synthesizing narrow poly- dispersity resins by a free-radical polymerization process stable free radicals, such as nitroxides, would yield adducts remainselusive.2 Wereport, herein, thatnarrow molecular having weaker bonds with a propagating styrene chain weight resins have been synthesized by a new free-radical than, for example, the sulfur free radicals formed by the polymerization process with polydispersities (PDs) com- dissociation of iniferters. It was, therefore, anticipated parable to those that can be obtained by anionic polym- that stable free radicals, like nitroxides, could function to erization processes and below the theoretical limiting form thermdy transient adducts in a similar manner to polydispersity of 1.5 for a conventional free-radical PO- the iniferterse butwithsome addedadvantages. Nitroxide lymerization pro~ess .~ stable freeradicals arewell-knownasfree-radicalinhibitors Narrow polydispersity resins, in principle, may be and are not known to initiate polymerization.? Conse- obtained by a free-radieal polymerization process if the quently, there would be little concern with the stable free processproceedsbyalivingmechanism,withnopremature radicals initiating new chains late in the polymerization termination, and if all the propagating chains are initiated process as they reversibly react with a propagating chain. at about thesame time, similar to what occurs inan anionic Furthermore, nitroxide stable free radicals have been polymerization process. While we considered using in- shown to promote the dissociation of peroxide initiators8 iferters to provide a living free-radical polymerization and, therefore, could contribute to enabling all the system: molecular orbital calculations5 indicated that polymeric chains to initiate at the -e time. The new process comprises heating a mixture of *a monomer or monomers, free-radical initiator, andastable free radical. Reaction of styrene with benzoyl peroxide ?B (BPO) and 2,2,6,6-tetramethyl-l-piperidinyloxy (TEM- ?" PO), under argon, at 95 O C for 3.5 h, followed by heating at 123 OC for 69 h yielded a polystyrene (sample IV, Table I) with a polydispersity of 1.26 (Figure 1). Samples were extracted from the reaction mixture over the course of the reaction at the times indicated. The gel permeation chromatography (GPC) results and the percent conversion narrow polydispersity was obtained early in the reaction, an indication that the polymeric chains were all initiated .Y I D H / " Y L C I NumbLC r,ucr.9* 707, "e ight Auecag. 9989 1 , 1 6 ~ , ~ 1 for each sample are summarized in Table I. Note that a Fcak lolccular "e ight ,0271 DISperrity z '"9 , H w g 1.11&911 Figure 1. GPC of polystyrene showing narrow polydispersity. _ . . . im IV 111 II I persent 8 8 . € 0 ' 4 n . 2 8 ' B no1 Ut Figure 2. GPCs of polystyrene samples I-IV, from Table I showing the incremental increase of molecular weight with time with no concomitant broadening of the distribution. 0024-9297/93/2226-2987$04.00/0 0 1993 American Chemical Society 2988 Communications to the Editor Macromolecules, Vol. 26, No. 11 , 1993 188 Percent 38 60 48 28 I . I , , , . , I , I I , , , , I , , . , , " w e e 1 l8H8R 'lBeR no1 ut Figure 3. GPCs of poly(styrene-co-butadiene) prepared by a suspension process: (a) a narrow polydispersity resin prepared using TEMPO to reversibly cap the propagating chains; (b) a control resin prepared by a conventional suspension free-radical polymerization process. at about the same time. The polydispersity remains constant over the entire course of the reaction, suggesting the reaction is proceeding via a type of living chain mechanism. The living nature of the propagating chains is further evident by the incremental increase in molecular weight with time (Figure 2). The narrow polydispersity is maintained even at very high monomer to polymer conversions. Similar results are obtained if the initial heating at 95 "C for 3.5 h, as described above, is eliminated and the reagents are simply reacted together at temper- atures between 125 and 150 OC. Molecular weights as high as 150 OOO have been achieved while maintaining polydispersities of 1.3 or less. A suspension copolymerization of styrene (86% by weight) and butadiene with BPO and TEMPO yielded a copolymer with a PD of 1.36. In contrast, a control reaction, with no TEMPO, gave a copolymer with a PD of 4.2L9 The molecular weight distributions of the two resins are compared in Figure 3. The molar ratio of TEMPO to BPO affects both the reaction rate and the polydispersity of the resins formed as illustrated in Table 11. The higher the ratio, the slower the reaction but the narrower the polydispersity. Lower molecular weight resins are also obtained with higher TEMPO/BPO ratios, an indication of increased initiator efficiency, a result that is under further investigation. In conclusion, narrow polydispersity resins have been synthesized by a free-radical polymerization process that can be performed in solution, bulk, or suspension. Various features of the reaction and a means for significantly increasing the reaction rate, while maintaining narrow polydispersities, will be described in subsequent publi- cations. Details describing the application of this process to the synthesis of block copolymers in which each block segment has a narrow polydispersity and a well-defined chain length are also forthcoming. Acknowledgment. The authors acknowledge the ex- cellent technical contribution of Randy Frank and R. Andrew Kee. References and Notes (I) Otau, T.; Yoshida, M. Makromol. Chem., Rapid Commun. 1982, 3,127. Otau, T.; Yoshida, M.; Tazaki, T. Makromol. Chem., Rapid Commun. 1982,3, 133. (2) Endo, K.; Murata, K.; Otau, T. Macromolecules 1992,25,5554. (3) Odian, G. G. Principles ofPolymerization, 2nd ed.; John Wiley & Sons: New York, 1981; pp 280 and 281. (4) We were concerned with the slowness of the reaction and other problems. Turner, S. R.; Blevins, R. W. Polym. Prepr. (Am. Chem. SOC., Div. Polym. Chem.) 1988, 29 (2), 6. (5) Kazmaier, P. M. Unpublished resulta. (6) Stable free radicals have been used to form adducts with monomer which can initiate "living" chains for the synthesis of oligomers. Soloman, D. H.; Rizardo, E.; Cacioli, P. U.S. Patent 4,581,429, March 27,1985. Rizzardo, E. Chem. A u t . 1987,54, 32. (7) Moad, G.; Rizzardo, E.; Soloman, D. H. Polym. Bull. 1982,6, 589 and references cited therein. (8) Moad, G.; Rizzardo, E.; Soloman, D. H. Tetrahedron Lett. 1981,22, 1165. (9) Alexandru, L.; Odell, P. G. U.S. Patent 4,558,108, Dec 10,1985.