Original carboxylic acid-based copolymers have been developed for the complexation of actinides. For such purpose, ring-opening copolymerizations of α-benzyl carboxylate-ε-caprolactone (BzCL) and ε-caprolactone (εCL) have been carried out with poly(ethylene glycol) (PEG) as macro-initiator and tin(II) octanoate, as catalyst, to afford poly(εCL-st-αBzεCL)-b-PEG-b-poly(εCL-st-αBzεCL) (PB) copolymers. Three different εCL/BzCL ratios were targeted (90/10, 75/25 and 50/50), leading to PB10%, PB25% and PB50%, respectively. Then, hydrogenation of the prepared copolymers allowed the deprotection of benzyl esters to carboxylic acids groups, leading to poly(εCL-st-αCOOHεCL)-b-PEG-b-poly(εCL-st-αCOOHεCL) copolymers (PA10%, PA25% and PA50%). All materials were fully characterized by 1H NMR, 13C NMR and size exclusion chromatography. Experimental ratios were found to be close from theoretical ones as equal to 92/08, 75/25, and 60/40. Molecular weight was the same for all copolymers (4000 g.mol−1). Complexing properties of the different copolymers were studied by Isothermal Titration Calorimetry (ITC) with neodymium (Nd(III)) and cerium (Ce(III)), used as actinide surrogates. ITC enabled the determination of the full thermodynamic profile (ΔG, ΔH, TΔS, Ka, and stoichiometry). The results showed that PA25% was the polymer with the highest sorption capacity (13.6 mg.g−1 for Nd(III) and 13.7 mg.g−1 for Ce(III)) and the highest binding constant (8500 M−1 and 5500 M−1 for Nd(III) and Ce(III), respectively). This demonstrated that by increasing the amount of complexing carboxylic acid functions, the complexing capacity did not necessarily increase as well, reaching a maximum with PA25%. In a more general manner, all developed copolymers are very promising for cation complexation.