Tandem Catalysis: Three Mechanistically Distinct Reactions from a Single Ruthenium Complex Christopher W. Bielawski, Janis Louie, and Robert H. Grubbs* Arnold and Mabel Beckman Laboratories of Chemical Synthesis, DiVision of Chemistry and Chemical Engineering California Institute of Technology, Pasadena, California 91125 ReceiVed May 17, 2000 Organometallic catalysts are traditionally designed and opti- mized to mediate a single reaction. 1 As the number of applications that require combinatorial and other high-speed synthetic protocols increases, 2 it will become desirable for catalysts to mediate multiple, mechanistically distinct transformations directly or upon simple modification. As an example of such a system, we demonstrate the ability of a single component precatalyst to mediate three different reactions to form well-defined block copolymers. The preparation of block copolymers composed of segments that cannot be prepared by the same polymerization mechanism remains a challenge in synthetic polymer chemistry. 3 Thus, many new strategies have emerged which are based on using substrates that are capable of initiating more than one type of polymerization. In general, various “controlled”/living radical polymerization methods 4 have been combined with ionic or ring-opening po- lymerization. 5 However, while a few of these protocols permit the combination of all the desired monomers at the beginning of the polymerization, the majority require timed additions (i.e., one polymerization must finish before another can begin). 6 Further- more, in addition to the initiator, a number of organometallic complexes and cocatalysts must be included to control the polymerizations. Ultimately, it would be desirable to have the necessary catalyst(s) and initiator(s) in a single component system, therefore requiring only the addition of desired monomers (preferably at the same time) to form block copolymers. 7 The ruthenium-based catalyst, Cl 2 (PCy 3 ) 2 RudCHPh (1), is effective for initiating the ring-opening metathesis polymerization (ROMP) of a variety of cyclic olefins. 8 Recently, Noels and co- workers demonstrated that 1 is also an effective catalyst for the atom-transfer radical polymerization (ATRP) of methyl meth- acrylate. 9 We proposed that a difunctional complex that incor- porates both a ROMP and an ATRP initiator could mediate both polymerizations simultaneously. 10 A complex that meets these requirements (2) was conveniently prepared from commercially available allyl 2-bromo-2-methylpropionate 11 and 1 in 75% isolated yield using previously reported methods. 8a Furthermore, at the conclusion of the aforementioned polymerizations, we reasoned that the residual ruthenium species could be transformed into a catalyst capable of hydrogenating the unsaturation in the polymer backbone (formed during the ROMP of the cyclic olefin). 12 As shown in Scheme 1, initial investigations confirmed that 2 initiated both ROMP and ATRP independently. For example, the ROMP of 1,5-cyclooctadiene (COD) in solution or bulk afforded poly(cyclooctadiene), equivalent to poly(butadiene) (PBD), in yields ranging from 85 to 95% and with polydispersity indices (PDIs) near two (Table 1). As expected, these results were similar to those obtained when Cl 2 (PCy 3 ) 2 RudCHPh (1) was used as the ROMP initiator. 8 Similarly, addition of MMA to a solution of 2 in toluene afforded poly(methyl methacrylate) (PMMA) after 18 h at 65 °C (75% yield, Table 1). In addition, a linear relationship between monomer conversion and polymer molecular weight was observed which suggested 2 effectively controlled the polymer- ization. However, as observed in other ruthenium-based ATRP systems, the molecular weights were higher than expected which may be related to the initiation efficiency. 9,12 Nevertheless, nearly monodispersed polymers (PDI < 1.2) were obtained. To the best of our knowledge, 2 is the first example of a complete ATRP system containing both the transition metal mediator and the radical initiator, all in a single complex. (1) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals; Wiley: New York, 1988. (2) (a) Thompson, L. A.; Ellman, J. A. Chem. ReV. 1996, 96, 555. (b) Combinatorial Libraries-Synthesis, Screening, and Application Potential; Cortese, R., Ed.; Walter de Gruyter: Berlin, 1996. (c) Boger, D. L.; Chai, W.; Jin, Q. J. Am. Chem. Soc. 1998, 120, 7220. (3) (a) Noshay, A.; McGrath, J. E. Block Copolymers; Academic Press: New York, 1977. (b) Webster, O. W. Science 1991, 251, 887. (c) Bates, F. S. Science 1991, 251, 898. (4) (a) Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimure, T. Mac- romolecules 1995, 28, 1721-1723. (b) Patten, T. E.; Xia, J.; Abernathy, T.; Matyjaszewski, K. Science 1996, 272, 866. (c) Controlled Radical Polymer- ization; Matyjaszewski, K., Ed.; ACS Symp. Ser. No. 685; American Chemical Society: Washington, DC, 1998. (d) Benoit, D.; Chaplinski, V.; Braslau, R.; Hawker, C. J. J. Am. Chem. Soc. 1999, 121, 3904. (5) (a) Coca, S.; Paik, H. J.; Matyjaszewski K. Macromolecules 1997, 30, 6513. (b) Kajiwara, A.; Matyjaszewski, K. Macromolecules 1998, 31, 3489. (c) Matyjaszewski, K. Macromol. Symp. 1998, 132, 85. (d) Hedrick, J. L.; Trollas, M.; Hawker, C. J. Macromolecules 1998, 31, 8691. (e) Mecerreyes, D.; Trollas, M.; Hedrick, J. L. Macromolecules 1999, 32, 8753. (f) Xu, X. J.; Pan, C. Y. J. Polym. Sci. Polym. Chem. 2000, 38, 337. (g) Acar, M. H.; Matyjaszewski, K. Macromol. Chem. Phys. 1999, 200, 1094. (h) Mecerreyes, D.; Atthoff, B.; Boduch, K. A.; Trollas, M.; Hedrick, J. L. Macromolecules 1999, 32, 5175. (i) Stehling, U. M.; Malmstrom, E. E.; Waymouth, R. M.; Hawker, C. J. Macromolecules 1998, 31, 4396. (j) Bielawski, C. W.; Morita, T.; Grubbs, R. H. Macromolecules 2000, 33, 678. (6) (a) Mecerreyes, D.; Moineau, G.; Dubois, P.; Jerome, R.; Hedrick, J. L.; Hawker, C. J.; Malstrom, E. E.; Trollas, M. Angew. Chem., Int. Ed. 1998, 37, 1274. (b) Hawker, C. J.; Hedrick, J. L.; Malmstrom, E. E.; Trollas, M.; Mecerreyes, D.; Moineau, G.; Dubois, P.; Jerome, R. Macromolecules 1998, 31, 213. (c) Weimer, M. W.; Scherman, O. A.; Sogah, D. Y. Macromolecules 1998, 31, 8425. (d) Klaerner, G.; Trollas, M.; Heise, A.; Husemann, M.; Atthoff, B.; Hawker, C. J.; Hedrick, J. L.; Miller, R. D. Macromolecules 1999, 32, 8227. (7) A palladium complex was recently reported to mediate two distinct polymerizations. However, activation of the complex with carbon monoxide was required to initiate the second polymerization, see: Lim, N. K.; Arndtsen, B. A. Macromolecules 2000, 33, 2305. (8) (a) Schwab, P. E.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996, 118, 100. (b) Ivin, K. J.; Mol, J. C. Olefin Metathesis and Metathesis Polymerization; Academic Press: San Diego, CA, 1997. (9) (a) Simal, F.; Demonceau, A.; Noels, A. F. Tetrahedon Lett. 1999, 40, 5689. (b) Simal, F.; Demonceau, A.; Noels, A. F. Angew. Chem., Int. Ed. Engl. 1999, 38, 538. (10) The simultaneous ring-opening polymerization of ǫ-caprolactone and ATRP of methyl methacrylate was recently reported. 6a However, each polymerization was separately mediated by different organometallic catalysts. (11) Bromoisobutyrl esters are known ATRP initiators, see: Ando, T.; Kamigaito, M.; Sawamoto, M. Tetrahedron 1997, 53, 15445. (12) Other reports of tandem ROMP/hydrogenation with 1 have recently emerged, see: (a) McLain, S. J.; McCord, E. F.; Arthur, S. D.; Hauptman, A. E.; Feldman, J.; Nugent, W. A.; Johnson, L. K.; Mecking, S.; Brookhart, M. Proc. Am. Chem. Soc.; DiV. Polym. Mater. Sci. Eng. 1997, 76, 246. (b) Watson, M. D.; Wagener, K. B. Macromolecules 2000, 33, 3196. (c) Chen, Y.; Dujardin, R.; Pielartzik, H.; Franz, U. U.S. Patent 5,932,664, 1997. Scheme 1 12872 J. Am. Chem. Soc. 2000, 122, 12872-12873 10.1021/ja001698j CCC: $19.00 © 2000 American Chemical Society Published on Web 12/05/2000