Well defined ABA triblock copolymer comprising a biodegradable poly(epsilon-caprolactone) (PCL) middle block and two pH responsive poly(acrylic acid) (PAA) outer blocks was synthesized by atom transfer radical polymerization of tert-butyl acrylate, initiated by PCL-based macroinitiator, followed by selective hydrolysis of the poly(tert-butyl acrylate) blocks. The cooperative self-assembly of the synthesized poly(acrylic acid)-block-poly(epsilon-caprolactone)-block-poly(acrylic acid) (PAA(22)PCL(26)PAA(22)) copolymer with a temperature-responsive poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO26PPO40PEO26, Pluronic P85) triblock copolymer at different compositions in aqueous media was investigated. Based on experimental data, copolymer properties and composition, formation of nano-sized aggregates comprising a mixed PCL/PPO core and a mixed PEO/PAA corona is suggested. The binary mixture of PAA(22)PCL(26)PAA(22):PEO26PPO40PEO26 copolymers at molar ratio 3:1 favors the formation of mixed aggregates only, while at higher PEO26PPO40PEO26 content the aggregates coexist with pure PEO26PPO40PEO26 micelles.