In this present work, we report the synthesis of UCST-thermoresponsive diblock copolymers using reversible addition fragmentation chain transfer (RAFT) polymerization in aqueous media. A water-soluble poly(N,N-dimethylacrylamide) macromolecular chain transfer agent (PDMAc macroRAFT) is used to promote and control the copolymerization of acrylamide and acrylonitrile in water and obtain PDMAc-b-P(AAm-co-AN) diblock copolymers. The fAN,0 and the length of the thermosensitive block (DPn) are systematically varied, in order to study their influence on the thermoresponsiveness of the block copolymers. A good blocking efficiency is generally evidenced by size exclusion chromatography. Remarkably, amphiphilic copolymer nanoparticles are formed in situ for the highest fAN,0. This is indeed the first time that such particles are produced by a polymerization-induced self-assembly (PISA) process. The morphology of the in situ formed nanoparticles and their behavior with temperature are studied by means of dynamic light scattering (DLS), (cryogenic) transmission electron microscopy ((cryo-)TEM) and turbidimetry. Spherical and worm-like nanoparticles are formed which exhibit unexpected properties, such as an unprecedented heating-induced worm-to-sphere morphological transition in water.