A general runaway criterion based on the divergence of the system has recently been developed. In this work we have applied the criterion to polymerization reactions. The runaway limits (or parametric sensitive regions) have been found and compared with previous criteria. The results show that the new criterion is able to distinguish between runaway and non-runaway behaviour for these types of systems. A considerable advantage over existing criteria is that it can be calculated on-line using only temperature measurements and, hence, it constitutes the core of an early warning runaway detection system we are developing.