In the presence of an activator such as [Ph3C][B(C6F5)4] or (PhMe2NH][B(C6F5)(4)], the half-sandwich scandium dialkyl complexes Cp'Sc(CH2SiMe3)(2)(THF) (Cp' = C5Me4SiMe3 (1), C5Me5 (2), C5H3-(SiMe3)(2) (3)) showed excellent regio- and stereoselectivity for the polymerization of 1,3-cyclohexadiene (CHD) and its copolymerization with ethylene. The homopolymerization of CHD afforded soluble crystalline cis-1,4-linked poly(CHD)s with very high stereoselectivity (tacticity up to 99 mol %). The copolymerization of CHD with ethylene gave the corresponding copolymers with a wide range of CHD contents (10-67 mol %) in a regio- and stereospecific 1,4-cis fashion.