The present invention provides methods by which carbon nanotubes can be functionalized via Diels-Alder reactions under solvent free conditions. Such methods include reacting carbon nanotubes with Diels-Alder dienes or dienophiles to obtain adducts that includes the diene or dienophile moiety bound to the carbon nanotubes. Functionalized carbon nanotubes and dispersions containing functionalized carbon nanotubes are provided. The present invention provides functionalization methods of carbon nanotubes through gas phase, liquid phase, or solid phase reactions without any solvents other than the reactants. Such processes are also amenable to a wide variety of chemical reactions that use other functionalizing agents. Additionally, such methods are cost effective, easily scalable and can provide for functionalized CNTs in large, industrial-scale quantities.