The self-assembly of a novel double hydrophilic block copolymer in water without the application of external triggers is described, namely pullulan-b-poly(2-ethyl-2-oxazoline) (Pull-b-PEtOx). The biomacromolecules, Pull (8-38 kgmol(-1)), is modified and conjugated to biocompatible PEtOx (22 kgmol(-1)) via modular conjugation. Moreover, the molecular weight of the Pull blocks are varied to investigate the effect of molecular weight on the self-assembly behavior. Spherical particles with sizes between 300 and 500 nm are formed in diluted aqueous solution (0.1-1.0 wt %) as observed via dynamic light scattering and static light scattering. Additionally, cryo scanning electron microscopy and laser scanning confocal microscopy are performed to support the finding from light scattering. The block ratio study shows an optimum ratio of Pull and PEtOx of 0.4/0.6 for self-assembly in water in the concentration range of 0.1-1.0 wt %. At higher concentrations of 20 wt %, vesicular structures with sizes above 1 mu m can be observed via optical microscopy. (C) 2017 Wiley Periodicals, Inc.