This work illustrates the use of basic statistical mechanics for rationalizing the loading of linear multitridentate polymers with trivalent lanthanides, Ln(III), and identifies the specific ionic sizes of europium and yttrium as promising candidates for the further design of organized heterometallic f-f' materials. Using [Ln(hfac)3] (hfac = hexafluoroacetylacetonate) as lanthanide carriers, the thermodynamically controlled formation of Wolf type-II lanthanidopolymers [{Ln(hfac)(3)}(m)(L4)] is modeled with the help of two simple microscopic descriptors: (i) the intrinsic affinity of Ln(III) for the tridentate binding sites f(N3)(Ln) and (ii) the intermetallic interactions Delta E-1-2(Ln,Ln) operating between two occupied adjacent sites. Selective complexation (f(N3)(La) < f(N3)(Y)) modulated by anticooperative interactions (Delta E-1-2(La,La) similar or equal to Delta E-1-2(Eu,Eu) > Delta E-1-2(Y,Y) approximate to 0) favors the fixation of Eu(III) in semiorganized lanthanidopolymers [{Eu(hfac)(3)}(m)(L4)] displaying exploitable light-downshifting.