Tungsten Oxo Alkylidene Complexes as Initiators for the Stereoregular Polymerization of 2,3-Dicarbomethoxynorbornadiene

We have employed 2,3-dicarbomethoxynorbornadiene (DCMNBD) as a monomer to explore new tungsten oxo alkylidene complexes as initiators for stereoregular ROMP (ring-opening metathesis polymerization). The initiators include MAP (monoaryloxide pyrrolide) oxo alkylidene complexes with the general formula W(O)(CHCMe2Ph)(Me(2)Pyr)(OAr) (Me(2)Pyr = 2,5-dimethylpyrrolide, OAr = an aryloxide) and W(O)(CHCMe2Ph)(OR)(2) (OR = an aryloxide or OC(CF3)(3)), or PPh2Me or CH3CN adducts thereof. We have found that MAP initiators yield cis,syndiotactic-poly(DCMNBD) as a consequence of stereogenic metal control. In contrast, W(O)(CHCMe2Ph)(OR)(2)(L) initiators (where L = PPh2Me or acetonitrile) are strongly biased toward formation of cis,isotactic structures, while W(O)(CHCMe2Ph)(OR)(2) initiators are strongly biased toward formation of cis,syndiotactic structures. Addition of B(C6F5)(3) to W(O)(CHCMe2Ph)(Me(2)Pyr)(OR) species leads to a dramatic increase in the rate of polymerization and to an increase in the cis,syndiotacticity of the polymer (if not already high), while addition of B(C6F5)(3) to W(O)(CHCMe2Ph)(OR)(2) initiators leads to a dramatic increase in the rate of polymerization and to the formation of highly cis,syndiotactic polymers. All evidence supports the proposal that 16e W(O)(CHCMe2Ph)(OR)(2)(L) complexes can operate either through loss of L to yield 14e W(O)(CHCMe2Ph)(OR)(2) species (which yield largely cis,syndiotactic-poly(DCMNBD)) or by directly reacting with DCMNBD to yield an 18e intermediate and largely cis,isotactic-poly(DCMNBD). All polymerizations by W(O)(CHCMe2Ph)(OR)(2)(L) and W(O)(CHCMe2Ph)(OR)(2) initiators are proposed to operate through some version of chain end control.