The ring-opening polymerization of bicyclo[2.2.1]-hept-2-ene (norbornene, NBE) is carried out with the metathesis catalysts WCl6/RLi, where R = Me-, n-Bu-, sec-Bu-, tert-Bu-, sec-Bu-x-C-60(x-), sec-Bu-CH2-C(Ph)(2)(-), PS-. The bulkiness of the R chain of the co-catalyst, combined with the coordination of side substituents of R chain on the metal, increases the degree of cis-stereoselectivity of poly(NBE), beyond those of the systems based on WCl6 which are of intermediate cis-content. The range of the fraction of cis-double bonds varies from sigma(c) = 0.47 to an all-cis-polymer (sigma(c) = 0.96), and all the polymers derived exhibited a blocky distribution (r(c)r(t) > 1) of the double-bond dyads. In case of R = PS- an all-cis-poly(NBE) was formed. The observed order for the cis-stereoselectivity is: PS- > sec-Bu-CH2-C(Ph)(2)(-) > sec-Bu- > tert-Bu- > n-Bu- > sec-Bu-x-C-60(x-) > Me-(C) 2002 Elsevier Science B.V. All rights reserved.