This work demonstrates how the thermodynamic loading of monodisperse polymeric single-stranded multi-tridentate receptors of variable lengths is controlled by the nature of the metallic carrier Ln(hfac)3 (Ln is La, Eu or Y, and hfac is hexafluoroacetylacetonate). Whereas the intrinsic affinity of the tridentate binding site is maximum for medium-sized Eu(3+) and decreases for Y(3+), the contraction of the hydrodynamic radius of the polymer upon metal loading induces positive allosteric cooperativity for the smaller cations. The origin of this behaviour is rationalized within the frame of intermetallic dipole-dipole interactions modulated by the solvation potential of dipolar solutes in dielectric materials. Positive cooperativity produces local high-density of metal ions along the ligand strands (metal clustering) with potential interest in energy migration and sensing processes.