the invention relates to compositions that may be used as catalysts for the preparation of polymers of carbon monoxide and at least one olefinically unsaturated organic compound . the invention also relates to a process using the catalyst compositions of the invention to prepare said polymers . the process comprises contacting the monomers in the presence of a catalyst obtained from the reaction of a group viii metal compound with a bidentate phosphorus , arsenic or antimony ligand , a compound selected from the group consisting of a main group metal salt having an anion of a non - hydrohalogenic acid with a pka less than 6 , and an ether .