The stereospecific ring-opening metathesis polymerization (ROMP) of endo,exo-2,3-dimethoxymethylnorborn-5-ene (DMMNBE) was accomplished using cationic tungsten imido alkylidene N-heterocyclic carbene (NHC) complexes as initiators. These even outperform a cationic molybdenum imido alkylidene NHC reference initiator and allow for high trans-isospecifity of up to 95%. Furthermore, tuning the steric demand of the ligands of the metal complexes allowed for a change in stereoselectivity. Thus, for the first time, cis-syndiotactic polymers were synthesized by the action of non-chelating cationic group 6 metal alkylidene NHC complexes. Both selectivities were not limited to DMMNBE, but also found for other monomers, namely endo,exo-2,3-dicarbomethoxynorborn-5-ene (DCMNBE), methyl-N-(S)-(−)-α-methylbenzyl-2-azabicyclo[2.2.1]hept-5-ene-3-carboxylate and 2,3-bis[(menthyloxy)carbonyl]norbornadiene.
