Glass network polymerization critically influences rheological behavior and with it the kinetics and dynamics of nuclear waste immobilization. Molybdenum-bearing borosilicate melts may undergo unmixing and rheological changes, which is dominantly controlled by the associated chemical modifications of the melt network. Here, we obtain in-situ (500-940°C) Raman spectra to probe structural changes of a sodium-molybdenum borosilicate melt undergoing unmixing. The extraction of alkali and molybdenum to form droplets induces polymerization of the residual borosilicate network. Conversely, the opposite phenomenon is observed during droplet re-dissolution. This work provides new insights into the polymerization of a molybdenum-bearing borosilicate composed of two composition sets due to a miscibility gap and has direct contributions for the immobilization of nuclear wastes.