  In this article we study a one dimensional model for a polymer in a poor
solvent: the random walk on $\mathbb{Z}$ penalized by its range. More
precisely, we consider a Gibbs transformation of the law of the simple
symmmetric random walk by a weight $\exp(-h_n|R_n|)$, with $|R_n|$ the number
of visited sites and $h_n$ a size-dependent positive parameter. We use
gambler's ruin estimates to obtain exact asymptotics for the partition
function, that enables us to obtain a precise description of trajectories, in
particular scaling limits for the center and the amplitude of the range. A
phase transition for the fluctuations around an optimal amplitude is identified
at $h_n \approx n^{1/4}$ , inherent to the underlying lattice structure.
