The composition is an adhesion promoter that includes a chlorinated polyolefin and an olefin-based block copolymer that has an olefin block that is substantially saturated and at least one (poly)ester or (poly)ether block. The olefin-based block copolymer can be prepared by reacting an hydroxyl-functional, saturated or substantially saturated olefin polymer with a chain-extension reagent that is reactive with hydroxyl groups and will polymerize in a head-to-tail arrangement of monomer units. The composition provides excellent adhesion to olefinic substrates like TPO. A method of making the adhesion promoter composition includes forming the olefin-based block copolymer, providing the copolymer at temperatures between 85 C. and 50 C. and adding, with agitation, chlorinated polyolefin to the copolymer at these temperatures. A coating is formed by combining the adhesion promoter with desired film-forming polymers, crosslinkers and coating additives.