Diffusion-controlled first contact of the ends of a polymer: crossover between two scaling regimes.

We report on Monte Carlo simulations of loop formation of an ideal flexible polymer consisting of N bonds with two reactive ends. We determine the first-passage time associated with chain looping that yields a conformation in which the end monomers are separated by a distance a--the reaction radius. In particular, our numerical results demonstrate how this time scale crosses over from tau(first) approximately N(3/2)/a to the a-independent tau(first) approximately N2 as N is increased. The existence and characteristics, of the two scaling regimes and the crossover between the two, are further illuminated by a scaling argument.