  We prove that for any euclidean ring R and n at least 6, Gamma=SL_n(R) has no
unbounded quasi-homomorphisms. From Bavard's duality theorem, this means that
the stable commutator length vanishes on Gamma. The result is particularly
interesting for R = F[x] for a certain field F (such as the field C of complex
numbers, because in this case the commutator length on Gamma is known to be
unbounded. This answers a question of M. Ab\'ert and N. Monod for n at least 6.
