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

Abstract

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 1H- and 13C 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)-(+)-α-methylbenzyl-5-norbornene-2,3-dicarboximide, 2,3-bis[(menthyloxy)carbonyl]norbornadiene, and methyl-N-(S)-(−)-α-methylbenzyl-2-azabicyclo[2.2.1]hept-5-ene-3-carboxylate. The cationic initiators [Mo(NAr)(5iPr)(CHCMe2Ph)(X)][B(ArF)4] (Ar = 2-tBuC6H4, 2,6-Me2C6H3, 2,6-iPr2C6H3; 5iPr = 1,3-diisopropylimidazol-2-ylidene; X = pyrrolide, O-2,6-(2,4,6-Me3C6H2)2C6H3; B(ArF)4 = B(3,5-(CF3)2C6H3)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.