The use of reactive solvents provides a unique opportunity to extend the processing characteristics of both intractable and standard (tractable) polymers beyond existing limits. The polymer to be processed is dissolved in the reactive solvent (monomer) and the solution is transferred into a mould. Upon polymerisation, phase separation and phase inversion occur and the originally dissolved polymer becomes the continuous (matrix) phase, whereas the reactive solvent is dispersed as a particulate (thermoset or thermoplastic) phase, hence there is no need for solvent removal. In the present study, the structure development of the model system polyethylene/styrene will be discussed. In situ X-ray studies, SAXS and WAXS, combined with Raman spectroscopy to follow the conversion from monomer to polymer, revealed the importance of liquid-liquid (L-L) versus liquid-solid (L-S) phase transformations upon the structure development.