We present a new approach to calculate the precipitation of polymer nanoparticles in confined impinging jets reactors (CIJIts) obtained by coupling together the computational fluid dynamics (CFD) model, accounting for turbulent mixing, nucleation, growth and aggregation, and the molecular dynamics (MD) description of polymer molecules. A new expression for the nucleation rate is adopted and the MD simulations are used to identify all the missing parameters involved. Different operating conditions are investigated in order to assess the reliability of the model. Its predictions, in terms of mean particles size, are compared with experiments, resulting in good agreement. (C) 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.