Kinetics of Propylene Polymerization Using Bis(2-phenylindenyl)zirconium Dichloride/Methylaluminoxane

Abstract

The kinetics of propylene polymerization in toluene solution by bis(2-phenylindenyl)zirconium dichloride/methylaluminoxane at 20 °C were investigated. As the structure and properties of elastomeric polypropylenes produced by these catalysts depend sensitively on the reaction conditions, a detailed study of the kinetics was carried out to evaluate the influence of these parameters on the polymerization behavior. Studies of the solubilities and mass-transfer rates reveal that dissolved atactic polypropylene has little effect on propylene solubility but influences the mass-transfer rate of propylene into solution. The rates of propylene polymerization reach a maximum after 10−20 min and then decrease. The decrease in rate over time is faster at higher monomer concentrations. Catalyst activity was negligible at [Al]/[Zr] = 1000 but constant from [Al]/[Zr] = 2500 to [Al]/[Zr] = 10 000. Analysis of molecular weights as a function of monomer concentration reveal β-hydride elimination to be the primary chain-transfer mechanism. Narrow molecular weight distributions (M w/M n = 2.0−2.6) were obtained. The increase of the isotactic dyads and pentads ([m] and [mmmm]) with increasing monomer concentration reveals an additional kinetic event which competes with the stereodifferentiating olefin insertion step. Modeling studies are more consistent with a mechanism involving interconversion of the catalyst between isospecific and aspecific states than a mechanism involving epimerization of the stereogenic centers of the growing polymer chain.

Propylene Polymerization with Sterically Hindered Unbridged 2-Arylindene Metallocenes

“…Molecular weight distributions were broader (M w /M n ) 2.8-3.8) than those of polypropylenes synthesized by 1/MMAO where M w /M n ) 2.0-2.6. 7 The microstructure of the polypropylenes produced with metallocene 2 is quite sensitive to monomer concentration, a characteristic of other metallocenes of this class. 7 As shown in Table 2 and Figure 2 (Table 2).…”

“…7 The microstructure of the polypropylenes produced with metallocene 2 is quite sensitive to monomer concentration, a characteristic of other metallocenes of this class. 7 As shown in Table 2 and Figure 2 (Table 2). DSC thermograms of the polymers in Table 2 reveal very broad melting endotherms for polymers exhibiting crystallinity, another feature of polypropylenes synthesized by unbridged 2-arylindene catalysts.…”

“…24,34 Kinetic modeling implies that for 1 the rates of insertion relative to isomerization differ only by a factor of 10 (depending on the monomer concentration). 7,36 If there is a distribution of relative rates for these two processes, it is possible that these metallocenes also isomerize only between the enantiomeric forms A and B and that the atactic stereosequences in the polymers are due to populations of kinetic states where the rates of olefin insertion are slower than the rate of isomerization (Curtin-Hammett conditions). Although we have no direct evidence at this time to differentiate between the contribution of a syn rotamer and the performance of the catalyst under Curtin-Hammett conditions, the similarity (at pentad resolution) between the 13 C NMR spectra of two polypropylenes of comparable tacticity, one synthesized with 1 in liquid propylene ([mmmm] ) Polymerizations with (2-(3,5-t-Bu2Ph)Ind)2ZrMe2 a (3 (2-(3,5-t-Bu2Ph)Ind) 44%, entry 1, Table 1) and one made with 2 at low monomer concentration ([mmmm] ) 40%, entry 8, Table 2), reveals that lower tacticity elastomeric polypropylenes are accessible from either the unsubstituted 1 or the tert-butyl-substituted 2 under appropriate conditions.…”

“…Procedures for determination of propylene solubility in toluene at 40 and 60°C as well as for conducting polymerizations were identical to the experimental procedures previously described. 7 13 C NMR of the polymers synthesized were collected using a 75 MHz Varian Inova spectrometer. Samples were prepared using C2H2Cl4/C2D2Cl4 and data collected at 100°C.…”

Synthesis of Ultrahigh-Molecular-Weight Ethylene-1-Hexene Copolymers with High Hexene Content via Living Polymerization with Fluorinated Bis(phenoxy-imine) Titanium(IV)

“…[ 22 ] High viscosity hindered mass transfer of ethylene from gas to liquid. The decrease in ethylene mass transfer would also increase [H]/[E] ratio in the solution, favoring high 1-hexene content in the copolymers.…”