For the modelling and simulation of polymerization reactions the incorporation of thermodynamical properties and equilibrium computations puts particular challenges. The state equations can be more complex and the realization of a phase flash for full chain-length distributions is much more complicated than for standard chemicals, since hundreds of thousands of single species might be involved. The widely-used simulation package Predici has been extended by a CapeOpen interface recently. The interface has been designed to allow for all data and computations necessary within Predici simulations (i.e. some pure component data, phase computations). Special reaction modules for phase exchange ensure the realization of phases even within a complex polymer computation based upon open and user-defined reaction kinetics. The talk will present the general concept and the results and difficulties arising from tests with two thermodynamical property packages. In particular the new, more complicated standard 1.1 will be discussed.