A cohesive polymer matrix comprising a matrix polymer and a coated particulate fillter dispersed therein, characterized in that the matrix polymer is a cross-linked olefin/acrylic polymer, and the filler is coated at least in part with a composition comprising an organic coating polymer of number average molecular weight of up to 200,000, which contains an acidic group (or a precursor thereof) and an unsaturated group optionally linked to the matrix polymer and a process for its production, and precursors thereof for use in the process.