Transfer to MAO

Chain Transfer to MAO If MAO is used to activate catalyst, the growing polymer chain- attached to an active center can exchange with the methyl group of a MAO molecule, forming the Al-terminated polymer chain and the M-CH3 bond in the active center  

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Co-catalyst effects. The effect of different cocatalysts on various polymerization parameters is very remarkable. The activity of MAO-activated systems shows only a small dependence on the Al/M ratio, Figure 47. Similarly, molecular mass is also substantially independent of the Al/M ratio, which suggests that transfer to MAO is not the dominant... 

Chien s kinetic model [48,49], unlike Ewen s model described above, is for the systems in which more than one active species is present. The model assumes the presence of multiple active center types, chain transfer to MAO, chain transfer by /3-H elimination (see p. 801), and first-order deactivation reactions of active centers. Chien applied the model in the study of ethylene polymerization with Cp2ZrCl2/MAO catalyst and propylene polymerization with Et(Ind)2ZrCl2/MAO and Et(H4lnd)2ZrCl2/MAO catalysts. 

Besides chain transfer to MAO, chain transfer by elimination is another main termination process. These two rates are expressed as follows ... 

Based on the analyses of experimental results, some researchers have suggested that there are at least two types of active species, which have diflferent activity and stereospecificity, formed in the homogeneous catalyst systems. Chien [49] indicated that in the case of the Et(H4lnd)2ZrCl2/MAO system, two types of active species (see Table 9.8) coexist in about equal amounts one has higher selectivity, 10-20 times greater rate constant of propagation, and a factor of 5-15 times faster chain transfer to MAO than the second type of active species. Metallocene complexes with different... 

Problem 9.14 Besides chain transfer to MAO, the chain transfer by -H elimination (see... 

Molecular Weight Control. Chain transfer under the usual sPS polymerization conditions occin-s primarily via -hydride elimination or chain transfer to aluminum alkyl (Fig. 5). For the (C5H5)Ti(OC4H9)3/MAO catalyst system at low monomer concentration, it has been reported that the rate of )3-hydride elimination is 76 times faster than chain transfer to MAO (84). Additional evidence for -hydride elimination as the main mechanism of chain transfer has been reported from analysis of the tert-butylcyclopentadienyltitanium trichloride catalyst... 

A kinetic investigation with the CpTi(0- -Bu)3/MAO system by Chien and coworkers showed that at high styrene concentration ([S] = 1.4 M), the rates of P-H elimination and chain transfer to monomer were comparable. Chain transfer to monomer became slower at lower styrene concentrations ([S] < 1.4 M). Meanwhile, the rate of P-H elimination was independent of styrene concentration and was about 76 times faster than chain transfer to MAO. ... 

Problem 9.14 Besides chain transfer to MAO, the chain transfer by j8-H elimination (see Section 9.7.4.3) is another main termination process in Chien s model for polymerization with metallocene/MAO catalysts. Show how the rate constant k ) of chain transfer by /8-H elimination can be calculated from the polymer molecular weight data. 

The rates of chain transfer to MAO and chain transfer by yS-H elimination (1 ) are expressed as ... 

Chain transfer to the monomer is judged to be the major chain transfer mechanism with the indenyl bridged zirconium catalysts. This conclusion is based on the following two findings (1) vinyl selectivity is approximately one unsaturation in each chain, which suggests that chain transfer to MAO is neghgible, and (2) the is independent of ethylene concentration, which rules out p-H eliminatimi. 

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