Synthesis of High Oxidation State Bimetallic Alkylidene Complexes for Controlled ROMP Synthesis of Triblock Copolymers Richard R. Schrock,* Andrea J. Gabert, Rojendra Singh, and Adam S. Hock Department of Chemistry 6-331, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received April 19, 2005 An X-ray study of [(THF)(R F6 O) 2 (ArN)ModCH] 2 (1,4-C 6 H 4 ) (OR F6 ) OCMe(CF 3 ) 2 ; Ar ) 2,6- diisopropylphenyl; THF ) tetrahydrofuran; 1b), which is closely related to known [(DME)- (R F6 O) 2 (ArN)ModCH] 2 (1,4-C 6 H 4 ) (DME ) 1,2-dimethoxyethane; 1a), showed it to be the expected bimetallic species in which each end is approximately a trigonal bipyramidal monoadduct of a syn alkylidene with the THF coordinated to the NOO face of the metal trans to the ModC bond. Treatment of 1a with lithium tert-butoxide yielded [(t-BuO) 2 (ArN)- ModCH] 2 (1,4-C 6 H 4 )(2). Addition of divinylferrocene to Mo(CHCMe 2 Ph)(NAr)(OR F6 ) 2 produced the bimetallic species {(R F6 O) 2 (ArN)Mo[dCH(C 5 H 4 )]} 2 Fe (3), which upon treatment with lithium tert-butoxide produced a related tert-butoxide complex (4). X-ray studies of 3 and 4 showed them to be “syn/syn” bimetallic species related to 1b. In solution two resonances can be observed in proton NMR spectra in the alkylidene region for the “syn/anti” isomer of 1a, 2, 3, and 4; the total amount varies between 4 and 20% of the total. Bimetallic species 1a, 2, 3, and 4 have been shown to initiate at both ends and to produce homopolymers of 4,5-dicarbomethoxynorbornadiene (DCMNBD) and methyltetracyclododecene (MTD) in a living fashion. MALDI-TOF mass spectra of ferrocene-containing species have been obtained that are consistent with the polymerization process being living. Triblock copolymers (poly[(MTD) x/2 (DCMNBD) y (MTD) x/2 ]) were prepared by adding y equivalents of DCMNBD to the bimetallic initiators followed (after consumption of DCMNBD) by x equivalents of MTD. These triblocks were shown to be of relatively high purity (free of homopolymer and diblock copolymer) and to have a relatively low PDI (e1.2). Introduction Ring-opening metathesis polymerization (ROMP) is catalyzed by a variety of alkylidene complexes. 1,2 In many cases the propagation step is essentially irrevers- ible (e.g., with various norbornene derivatives), and secondary metathesis reactions of double bonds in the resulting polymer chain can be slow. In these cases the polymerization becomes “living” under some specified set of conditions. In addition, if the rate of initiation and propagation are approximately the same order of mag- nitude or greater, then it is possible to control the average polymer length simply by controlling the num- ber of equivalents of monomer added. 3 Finally, with several catalysts, it is possible to control polymer structure in a systematic manner, i.e., to prepare all cis or all trans polymers and polymers that are isotactic or syndiotactic. 4,5 Well-defined high oxidation state imido alkylidene complexes of molybdenum and tungsten 6,7 have the added advantage of reacting in a Wittig-like fashion with benzaldehydes and other reactive alde- hydes (e.g., ferrocencarboxaldehydes 8,9 ), thereby install- ing a capping group on the end of the polymer. Imido alkylidene complexes of Mo or W that are living ROMP catalysts and that initiate at a rate comparable to the propagation rate are amenable to the synthesis of block copolymers. Some of the most interesting copolymers in terms of their physical properties are triblocks. 10,11 Triblocks (e.g., A x B y A z , where x, y, and z are the average numbers of monomers of type A or B in each block) have been synthesized via a sequential or linear method (addition of A to an initiator, then B, then A again, allowing each to be consumed) or by coupling of living homopolymers with a bifunctional central oligomer or polymer. However, such methods often have limitations, most seriously the presence of homopolymer or diblock copolymer due to (for example) incomplete coupling or catalyst degradation during the linear (1) Ivin, K. J.; C., M. J. Olefin Metathesis and Metathesis Polymer- ization; Academic Press: San Diego, 1997. (2) Grubbs, R. H., Ed. Handbook of Metathesis. Applications in Polymer Synthesis; Wiley-VCH: Weinheim, 2003; Vol. 3. (3) Gold, L. J. Chem. Phys. 1958, 28, 91. (4) O’Dell, R.; McConville, D. H.; Hofmeister, G. E.; Schrock, R. R. J. Am. Chem. Soc. 1994, 116, 3414. (5) McConville, D. H.; Wolf, J. R.; Schrock, R. R. J. Am. Chem. Soc. 1993, 115, 4413. (6) Schrock, R. R.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2003, 42, 4592. (7) Schrock, R. R. Acc. Chem. Res. 1990, 23, 158. (8) Albagli, D.; Bazan, G.; Schrock, R. R.; Wrighton, M. S. J. Phys. Chem. 1993, 97, 10211. (9) Albagli, D.; Bazan, G. C.; Wrighton, M. S.; Schrock, R. R. J. Am. Chem. Soc. 1992, 114, 4150. (10) Noshay, A.; McGrath, J. E. Block Copolymers; Academic: New York, 1977. (11) Hadjichristidis, N.; Pispas, S.; Floudas, G. A. Block Copolymers- Synthetic Strategies, Physical Properties, and Applications; John Wiley and Sons: New York, 2003. 5058 Organometallics 2005, 24, 5058-5066 10.1021/om058022n CCC: $30.25 © 2005 American Chemical Society Publication on Web 09/15/2005