Cononsolvency is a rare phenomenon that the solubility of a polymer decreases or even vanishes in the mixture of good solvents [Macromolecules 1991, 24, 948]. Although PDEA is thought to be soluble in water/organic solvent mixtures, our recent small angle neutron scattering (SANS) measurement in Sans2d shows that it also shows re-entrance behavior in water/ethanol mixtures, i.e., coil-to-globule-to-coil, at 27 degree C. The origin of cononsolvency has been argued for decades. Basically, there are two kinds of explanations now, i.e., “competitive adsorption” [PRL 2008, 101, 028302], and the formation of a stoichiometric compound between the solvent molecules which is a poor solvent for the polymer [Phys. Rev. Lett. 2001, 86, 822]. Through the UK-China Research and Innovation Partnership Fund, we propose to use NIMROD and inverse Monte-Carlo Simulation to reveal the its origin.