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Abstract

A process for maintaining a property P of a polyisobutene around a target value V during a continuous polymerisation of isobutene in a reactor containing a boiling liquid reaction phase in equilibrium with a gas phase above the liquid phase. The process comprises continuously introducing, into the reactor, a catalyst and a C4 hydrocarbon feed and continuously withdrawing, from the reactor, the liquid reaction phase which is subsequently subjected, continuously, to at least one purification step to isolate the polyisobutene. The property P is chosen amongst the viscosity and the average molecular mass of the polyisobutene. The property P is maintained around the target value V (i) by measuring the isobutene partial pressure (PiC4) in the gas phase of the reactor, (ii) by calculating the isobutene concentration (CiC4.sub.R2) in the boiling liquid reaction phase by means of a first model predicting the value of CiC4.sub.R2 from the measured value of PiC4 and being based on liquid/vapor equilibrium law, (iii) by calculating an instantaneous value Pinst of the property P by means of a second model predicting the value of Pinst from the calculated value of CiC4.sub.R2 and including at least one function of the isobutene polymerisation kinetics, and (iv) by varying at least one process parameter so as to make the instantaneous value Pinst tend towards the target value V.

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