Conversion of plastic wastes to fatty acids is an attractive means to supplement the sourcing of these high-value, high-volume chemicals. Herein, we report a method for transforming polyethylene (PE) and polypropylene (PP) at ~80% conversion to fatty acids with number average molar masses up to ~700 Da and 670 Da, respectively. The process is applicable to municipal PE and PP wastes and their mixtures. Temperature-gradient thermolysis is the key to controllably degrading PE and PP into waxes and inhibiting producing small molecules. The waxes are upcycled to fatty acids by oxidation over manganese stearate and subsequent saponification. PP ꞵ-scission produces more olefin wax and yields higher acid-number fatty acids than PE. We further convert the fatty acids to high-value, large-market-volume surfactants. Industrial-scale technoeconomic analysis suggests economic viability without subsidies.