A catalyst composition and process for preparing norbornene-type homopolymers or copolymers. The norbornene-type homopolymers or copolymers can be prepared using an in-situ catalyst composition including: (a) a transition metal compound of ML4, wherein M is a Group 10 metal and L is a neutral electron donor ligand; (b) a hydrocarbyl halogen containing a double bond or a triple bond; and (c) a salt of a non or weakly coordinative anion that can replace a halogen bonded to a metal. The catalyst composition is in a mixing state of components (a), (b), and (c), or in a mixing state of a reaction product of components (a) and (b) together with component (c).