Aminomethylated polystyrene resins are prepared in-house from crosslinked polystyrenes by a direct aminomethylation reaction and are chemically defined, homogeneous and free from any additional crosslinking by the functionalization process. Aminomethylated polystyrene resins can easily be acylated with any appropriate carboxylic acid-containing linker using standard methods to generate supports for solid phase organic synthesis.