A process is provided to graft functional moieties to base polymers by contacting the base polymer with functional group containing diazo compounds. The preferred function group containing diazo is ethyl diazoacetate. The base polymer is a copolymer or polymer having monomer units of alpha olefins, conjugated diolefins, vinyl aromatics, or hydrogenated conjugated diolefins. The process is performed in a melt and results in low levels of scissioning, coupling and degradation.