The present invention provides a process for the preparation of long chain hydrocarbyl substituted mono- or dicarboxylic acid materials in high active ingredient concentrations which comprises reacting a long chain hydrocarbon polymer, generally a polyolefin, and an monounsaturated carboxylic reactant such as maleic anhydride in the presence of chlorine, wherein at least about 60 mol% of the chlorine is added to the reaction mixture containing said polyolefin and monounsaturated carboxylic reactant at a reaction temperature not greater than about 170°C while imparting to the reaction mixture a total mixing energy of at least about 4 HP/1000 gal. This process selectivity produces more monofunctionalized long chain hydrocarbyl substituted mono- or dicarboxylic acid material product and substantially reduces the amount of oxidized by-product material, giving less sediment to be removed from the product.