Mechanistic work on the olefin metathesis reaction by well-characterized ruthenium carbene complexes led to the rational design and synthesis of a modified, unsymmetrical "first-generation" catalyst, which, in contrast to either first-or second-generation systems with symmetrical ligands, converts a mixture of two cycloolefin monomers to a largely alternating copolymer. The mechanistic concept of a homogeneous catalyst that switches between more than one state at each turnover is general. The structures of the complexes, determined by mass spectrometry, NMR, and X-ray crystallography, reveal some unexpected features, which explain sequence errors in the copolymer. Experimental SectionGeneral Remarks. Unless otherwise stated, all manipulations were carried out under an argon atmosphere on a vacuum line using standard Schlenk techniques. The solvents were dried by distillation from the following drying agents prior to use and were transferred under N 2 : diethyl ether (Na/K), n-hexane (Na/K), THF (K), CH 2 Cl 2 (CaH 2 ), ethanol (Mg), methanol (Mg). Flash chromatography employed Fluka silica gel 60, type 60752 (230-400 mesh). TLC was done with Merck silica gel 60 F 254 plates and visualized by UV 254 light. Low-resolution ESI-MS measurements were done on a Finnigan MAT LCQ MS ion trap mass spectrometer, which were then used to set up high-resolution mass spectrometric measurements on a Finnigan MAT TSQ Quantum instrument with tetradodecylammonium bromide as an external standard for absolute mass calibration. NMR measurements are reported for a Varian Mercury XL 300 ( 1 H, 300 MHz; 13 C, 75 MHz; 31 P, 121 MHz) spectrometer. Chemical shifts (δ values) are reported in ppm with respect to Me 4 Si (δ ) 0 ppm), used as an internal standard for 13 C and 1 H NMR, and an 85% aqueous H 3 PO 4 solution, used as an external standard for 31 P NMR. Coupling constants (J) are given in Hz. 13 C NMR and 31 P NMR spectra were proton broad-banddecoupled. The multiplicities of peaks are denoted by the following abbreviations: s, singlet; d, doublet; t, triplet; m, multiplet; br, broad. Elemental analysis was performed by the Microanalytical Laboratory of the Laboratorium fu ¨r Organische Chemie, ETH-Zu ¨rich. Gel permeation chromatography (GPC) was carried out with a Polymer Laboratories PL-GPC 220 fitted with a refractive index and viscometry detector using 1,3,5-trichlorobenzene at 140 °C and a polystyrene reference.Ligand Syntheses. tert-Butylphenylchlorophosphine. A 0.95 M t-BuMgCl solution in Et 2 O was prepared by addition of 60 g (652 mmol) of t-BuCl to a suspension of 23.8 g (978 mmol) of magnesium turnings in 400 mL of Et 2 O. The suspension was stirred for 2 h at room temperature, filtered, and titrated. To a solution of 33 g (188 mmol) of phenyldichlorophosphine in 100 mL of Et 2 O at -50 °C was added 200 mL (188 mmol) of the 0.95 M t-BuMgCl solution over 1 h with vigorous stirring. The formed gray-white suspension was allowed to reach room temperature over an additional 2 h of stirring. Filtration, evaporation of the solv...