Crossed molecular beams experiments were conducted to investigate the chemical dynamics of the reaction of dicarbon molecules, C-2(X-1 Sigma(+)(g)/a(3)II(u)), with diacetylene, C4H2(X-1 Sigma(+)(g)) at two collision energies of 12.1 and 32.8 kJmol(-1). The dynamics were found to be indirect, involved C6H2 intermediates, and were dictated by an initial addition of the dicarbon molecule to the carbon-carbon triple bond of diacetylene. The initial collision complexes could isomerize. On the singlet surface, the resulting linear triacetylene molecule (C6H2(X-1 Sigma(+)(g))) decomposed without an exit barrier to form the linear 1,3,5-hexatriynyl radical (C6H((XII)-I-2)). On the triplet surface, the dynamics suggested at least a tight exit transition state involved in the fragmentation of a triplet C6H2 intermediate to yield the 1,3,5-hexatriynyl radical (C6H((XII)-I-2)) plus atomic hydrogen. On the basis of the experimental data, we recommend an experimentally determined enthalpy of formation of the 1,3,5-hexatriynyl radical of 1014 +/- 27 kJmol(-1) at 0 K. Our experimental results and the derived reaction mechanisms gain full support from electronic structure calculations on the singlet and triplet C6H2 potential-energy surfaces. The identification of the 1,3,5-hexatriynyl radical under single collision conditions implies that the neutral-neutral reaction of dicarbon with diacetylene can lead to the formation of 1,3,5-hexatriynyl radicals in the interstellar medium and possibly in the hydrocarbon-rich atmospheres of planets and their moons such as Saturn's satellite Titan.