Influence of Reaction Conditions on Catalyst Behavior during the Early Stages of Gas Phase Ethylene Homo- and Copolymerization

A packed bed stopped flow minireactor (3 mL) suitable for performing gas phase polymerizations of olefins has been used to study the initial phases of ethylene homo- and copolymerization with two supported metallocene catalysts. The reactor can be used to perform gas phase polymerizations at times as short as 100 ms under industrially relevant conditions. It has been used to follow the evolution of the rate of polymerization, the gas phase temperature (and indirectly the particle temperature), and the polymer properties (molecular weight distribution, melting temperature, and crystallinity) for the two catalysts. It is shown that polymerization activity during the first 2-5 s of reaction can be up to 20 times higher than what is measured at longer polymerization times. The main consequence is the release of a significant amount of heat due to the rapid reaction that has to be efficiently evacuated in order to avoid particle overheating and melting. It has been seen that insufficient heat removal can strongly influence the behavior of the active sites, eventually leading to uncontrolled transfer reactions and polymers with unusually broad molecular weight distributions (MWD). It is also observed that the kinetic behavior of the two types of catalyst is similar at short times. Finally, some influence of particle size on reaction rate and molecular weight is observed between the largest and smallest catalyst particle cuts.