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)­(Me2Pyr)­(OAr) (Me2Pyr = 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)­(Me2Pyr)­(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.