Phosphorylated polymers are versatile materials for a broad range of applications from flame-retardant coatings to bioactive scaffolds. Traditionally, they are synthesized in solution using corrosive concentrated phosphoric acid and energy-intensive drying techniques. In the past decade, mechanochemistry has proven to be a valuable tool for green chemists to conduct new transformations, with minimal waste, often solvent-free. This work presents the phosphorylation of cellulose nanocrystals, poly­(ethylene glycol), poly­(vinyl alcohol), poly­(vinyl chloride), and lignin through mechanochemical processes with phosphorus pentoxide to produce reproducible phosphorylation for potential flame-retardant applications. Through 31P magic angle spinning (MAS) NMR, loadings of up to 3300 mmol/kg were determined for cellulose nanocrystals, far superior to loadings in solution around 1600 mmol/kg, and loadings of up to 4375 mmol/kg were obtained for synthetic polymers such as poly­(vinyl alcohol).