Towards Tailoring of Ethene/1-Hexene Copolymers by Dual-Site Metallocene Catalysis
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Mixing of different metallocene catalysts is a method that can be used to tailor the polymer properties in one reactor. This work deals with the tailoring of ethene/1-hexene copolymers by use of dual-site metallocene catalysts; mixtures of two different homogeneous metallocene catalysts. The behavior of the metallocene mixtures in ethene/1-hexene copolymerization is investigated and compared with the behavior of the individual catalysts. The studied metallocenes were: (1,2,4-Me3Cp)2ZrCl2, (Me5Cp)2ZrCl2 and Me4Si2(Me4Cp)2ZrCl2. The first dual-site system investigated was a mixture of (1,2,4- Me3Cp)2ZrCl2 and (Me5Cp)2ZrCl2 in a 1:1 molar ratio. In the other dual-site system studied, the unbridged (Me5Cp2ZrCl2 catalyst was exchanged with the disilylene bridged Me4Si2(Me4Cp)2ZrCl2 catalyst. Methylaluminoxane (MAO) was used as cocatalyst. Ethene homo- and copolymerizations with varying amounts of 1-hexene were performed in toluene solution at 80 °C and under a total pressure of 2 bar in the reactor. The initial 1-hexene concentration was varied in the range 0 – 0.72 mol/L for each of the catalysts. The other polymerization conditions were kept constant. The (1,2,4-Me3Cp)2ZrCl2 catalyst has a substantial comonomer incorporation, whereas the molecular weight is kept relatively high. The (Me5Cp)2ZrCl2 and Me4Si2(Me4Cp)2ZrCl2 catalysts both incorporate only low amounts of comonomer. Both dual-site systems can produce poly(ethene-co-1-hexene) with narrow molecular weight distribution (MWD) as a mixture of chains containing high and low comonomer contents. Narrow MWD was obtained since the molecular weight differences of the individual catalysts were not very different. The activities of the 1:1 mixtures for homo- and copolymerization were between those of the individual catalysts. In principle, the behavior of the dual-site systems can be predicted from the behavior of the individual catalysts, assuming there are no interactions between the catalysts in the mixture. However, the dual-site systems demonstrated a number of discrepancies from the expected behavior. First of all the copolymers made with the mixtures had lower incorporation than expected. Further, the melting endotherms and Crystaf profiles of copolymers made with the mixtures clearly showed that the influence of the polymer fraction made with the (Me5Cp)2ZrCl2- or the Me4Si(Me4Cp)2ZrCl2-site was much stronger than expected. These results demonstrated that the relative activities of the (Me5Cp)2ZrCl2 and the Me4Si2(Me4Cp)2ZrCl2 catalysts were enhanced in the mixtures with (1,2,4-Me3Cp)2ZrCl2. Vinylidene was the major type of unsaturation in copolymers made with the (1,2,4-Me3Cp)2ZrCl2 catalyst alone. Invstigations on the effect of ethene pressure on homopolymers made with (1,2,4-Me3Cp)2ZrCl2, showed that b-hydrogen transfer to monomer is the dominating mechanism for chain termination during ethene homopolymerization. Further, b-hydrogen elimination after 1,2-insertion of 1-hexene was found to be the most important termination mechanism during ethene/1-hexene copolymerization with this catalyst. The (Me5Cp)2ZrCl2 catalyst, on the other hand, produced copolymers with vinyl and trans-vinylene as the dominating types of end groups. The (Me5Cp)2ZrCl2 and Me4Si2(Me4Cp)2ZrCl2 catalysts had similar 1-hexene incorporations, but their termination mechanisms were different; the latter catalyst terminated more frequently after 1,2-insertion of 1-hexene, but less frequently after ethene insertion than (Me5Cp)2ZrCl2. At the same time both of these catalysts had a significant termination by chain transfer to trimethylaluminum (TMA). For the dual-site (1,2,4-Me3Cp)2ZrCl2 and (Me5Cp)2ZrCl2 catalyst, it was astonishing that the concentrations of the vinyl and trans-vinylene unsaturations approached the (Me5Cp)2ZrCl2 values for high 1-hexene concentrations. Again, an indication of relative activity enhancement of the (Me5Cp)2ZrCl2 catalyst in the mixture was seen. Chain transfer to TMA was important for homopolymerization with the mixture, but became less important as the 1-hexene concentration increased. The differences in the termination mechanisms for the (Me5Cp)2ZrCl2 and Me4Si2(Me4Cp)2ZrCl2 catalysts were also reflected in the dual-site catalysts. For the mixture of (1,2,4-Me3Cp)2ZrCl2 and Me4Si2(Me4Cp)2ZrCl2, chain transfer to TMA also became less important with increasing 1-hexene concentration. Astonishingly, the copolymers made with the dual-site (1,2,4-Me3Cp)2ZrCl2 and (Me5Cp)2ZrCl2 catalyst had higher molecular weights than the copolymers made with any of the individual catalysts. This effect was also observed for the dual-site (1,2,4- Me3Cp)2ZrCl2 and Me4Si2(Me4Cp)2ZrCl2 catalyst at intermediate 1-hexene concentrations. Investigations on the effect of different amounts of TMA on ethene/1-hexene copolymerization with (1,2,4-Me3Cp)2ZrCl2, and also with the mixture of (1,2,4- Me3Cp)2ZrCl2 and (Me5Cp)2ZrCl2 revealed interesting observations concerning the termination mechanisms. For both systems the 1-hexene incorporation was constant with the TMA concentration. Surprisingly, the vinylidene content decreased almost linearly with the TMA concentration. It was proposed that for the (1,2,4-Me3Cp)2ZrCl2 catalyst, TMA can coordinate to the catalytic site after 1,2-insertion of 1-hexene and rotation to the b-agostic state. Accordingly, the standard termination reactions after 1-hexene insertion giving vinylidene end groups are suppressed and more or less taken over by chain transfer to TMA. This effect of TMA was proposed to the possible key to the observed discrepancies in the dual-site systems.