A catalyst composition useful in the preparation of a polymer composition includes a Group VIII metal and a phosphorus bidentate ligand of the general formula (R.sub.2)(R.sub.4)P--R.sub.3 --P(R.sub.2)(R.sub.4) wherein R.sub.2 and R.sub.4 are optionally polar substituted alkyl groups, differing from each other in carbon number, and wherein R.sub.3 is a divalent organic bridging group which contains four atoms. Such catalyst compositions are particularly useful for preparing copolymers of carbon monoxide with .alpha.-olefins having at least three carbon atoms.