A general new route to well-defined polymeric Lewis acids via borylation of silylated polymers is reported. Trimethylsilylated polystyrene (PS-Si) of controlled molecular weight and low polydispersity (PDI < 1.15) was obtained via atom transfer radical polymerization (ATRP) of 4-(trimethylsilyl)styrene. The functional polymer PS-Si was quantitatively borylated using BBr3 to give poly(4-dibromoborylstyrene)