A highly efficient majority-rules effect of poly(quinoxaline-2,3-diyl)s (PQXs) bearing 2-butoxymethyl chiral side chains at the 6- and 7-positions was established and attributed to large ΔG(h) values (0.22-0.41 kJ mol(-1)), which are defined as the energy difference between P- and M-helical conformations per chiral unit. A PQX copolymer prepared from a monomer derived from (R)-2-octanol (23% ee) and a monomer bearing a PPh2 group adopted a single-handed helical structure (>99%) and could be used as a highly enantioselective chiral ligand in palladium-catalyzed asymmetric reactions (products formed with up to 94% ee), in which the enantioselectivity could be switched by solvent-dependent inversion of the helical PQX backbone.

Nothing ROMP with that: Unsymmetrical double‐stranded ladderphanes are obtained by sequential ring‐opening metathesis polymerization and Glaser oxidation of norbornene appended with bisalkyne moieties. Hydrolysis of these ladderphanes gives substituted poly(m‐phenylene butadiynylene)s with narrow polymer dispersity index (PDI) and well‐controlled degree of polymerization.

Ring-opening metathesis polymerization (ROMP) of cyclobutenes fused with N-arylpyrrolidene with Schrock−Hoveyda catalyst containing a racemic biphenolate ligand [Mo(N-2,6-i-Pr 2 C 6 H 3 )(CHCMe 2 Ph)(biphenolate)] gives polycyclobutenes with homogeneous tacticity and predominantly double bonds in Zconfiguration. Reactions of the same substrates with the first-generation Grubbs catalyst [(Cy 3 P) 2 Cl 2 RuCHPh] or Schrock molybdenum carbene with monodendate alkoxy ligands [Mo(N-2,6-i-Pr 2 C 6 H 3 )(CHCMe 2 Ph)(OCMe(CF 3 ) 2 ] yield the corresponding polycyclobutene containing a mixture of Z-and E-double bonds. Upon diimide reduction, all these polycyclobutenes give the same tactic hydrogenated polymers, indicating that the stereochemistry at the asymmetric carbons remains the same in all these reactions. The stereoselectivities of ROMP with cyclobutenes and with norbornenes are compared, and the plausible mechanisms are proposed. ■ INTRODUCTIONMetal−carbene-catalyzed ring-opening metathesis polymerization (ROMP) 1 of strained cycloalkenes has been extensively used in synthesizing different kinds of polymers for 193 nm photoresists, 2 nonlinear optical properties, 3 brush polymers for optoelectronic applications, 4 assembly of double-stranded ladderphanes, 5 and membrane transporting carriers. 6 Bicyclo[2.2.1]heptene systems have been extensively studied, and the reaction can proceed stereoselectively depending on the nature of the substrates and catalysts. Thus, syndiotacticcis, 7 isotactic-trans-, 8 and isotactic-cis-polynorbornenes 9 are readily obtained. Relatively speaking, ROMPs of cyclobutene derivatives have received much less attention, and the reactions in general are nonselective, polymers with a mixture of Z-and E-double bonds being obtained, 10 unless specially designed substrates are employed. 11 We have previously shown that, upon treatment with the first-generation Grubbs catalyst 1a, 12 ROMPs of norbornenes fused with N-aryl-endo-pyrrolidine give the corresponding polynorbornenes having isotactic stereochemistry with trans double bonds. 5,8 It is believed that the endo-fused substituents in 2 provide a bulky environment so that the stereoselectivity of the reaction may be directed. More recently, treatment of 2 with a Schrock−Hoveyda molybdenum−carbene catalyst 13 containing a bidendate ligand 3a yields polynorbornenes in cis, isotactic selectivity. 9a The span for each of the monomeric species in polynorbornenes is about 5.5 Å, and interactions between adjacent pendants in polynorbornenes or linkers in ladderphanes have been shown to be prominent. 5,8 The spacing occupied by each of the monomeric units in polycyclobutenes would be expected to be somewhat shorter because the bridging methylene groups are missing in polycyclobutenes. The distance separating adjacent pendants in polycyclobutenes would therefore be shorter than that in polynorbornenes. Accordingly, interactions between pendants in polycyclobutenes would be different from those in polynorbornenes. In this paper, we describe a syst...