We consider a simple random walk of length N , denoted by (Si) i∈{1,...,N} , and we define (wi) i≥1 a sequence of centered i.i.d. random variables. For K ∈ N we define ((γ −K i , . . . , γ K i )) i≥1 an i.i.d sequence of random vectors. We set β ∈ R, λ ≥ 0 and h ≥ 0, and transform the measure on the set of random walk trajectories with the HamiltonianThis transformed path measure describes an hydrophobic(philic) copolymer interacting with a layer of width 2K around an interface between oil and water.In the present article we prove the convergence in the limit of weak coupling (when λ, h and β tend to 0) of this discrete model towards its continuous counterpart. To that aim we further develop a technique of coarse graining introduced by Bolthausen and den Hollander in [6]. Our result shows, in particular, that the randomness of the pinning around the interface vanishes as the coupling becomes weaker.