A sequence of long-chain alkene products is found to evolve from chemisorbed methyl groups on Cu(111) in temperature-programmed reaction. The results of low-energy electron diffraction measurements suggest the formation of 2D islands for the chemisorbed radicals. The close-packed adsorption geometry and the reaction kinetics to generate high mass species are closely correlated. From this and other studies, we infer that the tendency of the methyl radicals to aggregate even at submonolayer coverages is crucial in promoting the chain growth, and the observed islanding effect may be significant in other hydrocarbon catalytical systems.