Synthesis of trans-Isotactic Poly(norbornene)s through Living Ring-Opening Metathesis Polymerization Initiated by Group VI Imido Alkylidene N-Heterocyclic Carbene Complexes

The selectivity of several molybdenum and tungsten imido alkylidene N-heterocyclic carbene (NHC) complexes in the ring opening metathesis polymerization (ROMP) of enantiomerically pure endo,exo-2,3-dicarbomethoxynorborn-5-ene (DCMNBE) was examined by H-1- and C-13 NMR spectroscopy. With one exception, all complexes showed a strong bias toward the formation of trans-isotactic polymers, some yielding polymers based on >98% trans-isotactic repeat units. This high selectivity was successfully extended to the ROMP of other monomers such as endo and exo-N-(R)-(+)-alpha-methylbenzyl-5-norbornene-2,3-dicarboximide, 2,3-bis[(menthyloxy)carbonyl]norbornadiene, and methyl-N-(S)-(-)-alpha-methylbenzyl-2-azabicyclo[2.2.1]hept-5-ene-3-carboxylate. The cationic initiators [Mo(NAr)(5(i)Pr)-(CHCMe2Ph)(X)][B(Ar-F)(4)] (Ar = 2-(BuC6H4)-Bu-t, 2,6-Me2C6H3, 2,6-(Pr2C6H3)-Pr-i; 5(i)Pr = 1,3-diisopropylimidazol-2-ylidene; X = pyrrolide, O-2,6-(2,4,6-Me3C6H2)(2)C6H3; B(ArF)(4) = B(3,5-(CF3)(2)C6H3)(4)) were found to be the most reactive ones, while also maintaining very high isoselectivity. Finally, a large imido ligand improved stereospecificity, while the choice of the NHC ligand had only a minor influence. Polymerizations can be terminated with 2-methoxystyrene, as evidenced by matrix-assisted laser-desorption time-of flight mass spectrometry.