Metallocene-initiated polymerization kinetics

MAO is needed in large excess relative to the metallocene initiator, usually IO2 IO4 1, to achieve high activities and stable kinetic profiles. MAO is usually added first in a polymerization system, and a portion may actually serve the function of destroying deleterious impurities prior to the addition of the metallocene initiator. Otherwise, the impurities would destroy the metallocene if the metallocene were added first. 

After the synthesis and employment of thousands of metallocenes, a vast amount of information has been obtained regarding catalyst stmcture-polymer property relationships. This information has been adroitly dissected by a large number of scientists to compile a rather detailed mechanistic understanding of the metallocene-mediated polymerization mechanism. Metallocenes have made possible detailed studies on initiation, propagation, termination, kinetics, and stereochemical control. These studies have been integrated to make possible many novel polyolefins with highly engineered micro-stmctures to meet a wide variety of applications - most of which would probably amaze the founders of ZN polymerization. 

This interpretation explains why Ziegler, Ballard, and metallocene catalysts, which have a different initiation chemistry, can be supported on the same silicas, but display totally different kinetics (and much faster reactions) without a gradual rise in activity. It also explains how the kinetics of polymerization with a single Cr/silica catalyst can be so easily manipulated by the choice of the reaction temperature, the activation temperature, CO reduction (Figure 16), and incorporation of metal alkyls and poisons in the reaction mixture. 

That is, in terms of reaction rates, the molecular weight of polyolefins is given by the ratio between the overall rate of propagation (Rp) and the sum of all rates of chain release (Rr) reactions this means that the molecular weight is dependent on the type of catalyst and the kinetics of the process, that is, the polymerization conditions (polymerization temperature, monomer concentration, catalyst/cocatalyst ratio). Hence, understanding the details of the mechanisms of chain release reactions is the key to molecular weight control in metallocene-catalyzed olefin polymerization. Here, chain release reactions (usually referred to as termination or transfer reactions) are all those steps that cause release of the polymer chain from the active catalyst, with the formation of a new initiating species (see section... 

Rg. 9.8. Mixed-metallocene polymerization of ethylene in a semibatch reactor branching (constrained geometry) catalyst CGC-Ti linear catalyst Et[lnd]2ZrCl2. Reactor and kinetic data initial concentration CGC-Ti 8 x 10 kmol m initial concentration Et[lnd]2ZrCl2 3.2 X 10 kmol m monomer molar feed... 

Fig. 9.9. Mixed-metallocene polymerization of ethylene in a CSTR. Kinetic data are the same as in Figure 9.8. Residence time CSTR 300 s feed concentrations identical to initial concentrations in Figure 9.8. Bivariate chain length/number of branches distribution. Hydrogen present. Based on molecular weight distribution and branching distribution from...

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