The invention relates to polymeric cobalt bearing compounds, in particular for use as metal-rubber adhesion promoters (RAPs) in tires, belts and hoses. Known active substances are a.o. cobalt stearates, naphthenates, resinates, decanoates, boro-decanoates and many other forms of acylates. While these substances appear to enhance the metal-rubber adhesion, all of them also come with significant drawbacks, a.o. linked to the relatively high bioavailability of cobalt in these substances. The invention concerns more particularly a polymer comprising Co-carboxylate sequences, with a Co content of at least 3% by weight, and with a mean molecular weight of more than 2000. Several synthesis methods, allowing to reach relatively high Co concentrations in the polymer, illustrate the invention. The invented polymers show a strongly reduced bioavailability of Co compared to currently available products, as demonstrated by aqueous leaching tests. As RAPs, they perform similarly to commercial products, as shown by pullout tests.