Selective Chain Transfer Reactions in Metallocene Catalyzed Copolymerization of Ethylene with Allylbenzene
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Intensive research on the polymerization of olefins with metallocene catalysts has added a great number of details to the knowledge of the mechanism of olefin insertion/termination reactions and the relationship between catalyst symmetry and polymer microstructure. An understanding of the chain termination is an important issue because it is critical for the control of molecular weight and chain end structure of polymers. It is known that two kinds of chain transfer reactions, â-H elimination and chain transfer to aluminum, prevail in olefin polymerization with metallocene/MAO catalysts. However, only a few publications that describe the predominant occurrence of chain transfer to aluminum during olefin polymerization have appeared in the scientific literature.1-5 Generally, chain termination via alkyl chain transfer to aluminum is described as a minor chain termination process for the metallocene-catalyzed olefin polymerization.6-11 Recently we found that chain transfer to aluminum became a major chain transfer reaction when ethylene was copolymerized with allylbenzene.12 In this communication, we report the copolymerization behavior of ethylene and allylbenzene with several metallocene catalysts having different ligand structures where chain transfer mode depends on the catalysts’ structures. The chain transfer behavior of ethyleneallylbenzene copolymerization was studied through analysis of the molecular weight and the chain end structures of the copolymers. Ethylene was copolymerized with allylbenzene at 80 °C in the presence of 50 × 10-6 mol/L zirconocene catalysts and MAO (Al/Zr ) 2000). Table 1 shows the results of catalytic activity, incorporation content of allylbenzene, and molecular weight of the copolymers. In the copolymerizations, the catalytic activity was increased with addition of allylbenzene compared to homopolymerization of ethylene. Comonomer effect on activity is frequently observed in olefin polymerization because of enhanced monomer diffusion into active sites or awakening of active site.13-15 Incorporation content of the allylbenzene into the polymeric chain was determined by 1H NMR spectroscopy. Table 1 showed that copolymer prepared with more hindered catalyst had a smaller amount of the allylbenzene unit. The incorporation of allylbenzene in the copolymer decreases in the following order: Cp2ZrCl2 > (n-BuCp)2ZrCl2 > (2-MeInd)2ZrCl2 . Cp*2ZrCl2. This effect can be explained in terms of accessibility of the bulky comonomer to the active center, which is effected by the steric hindrance of the substituted ligands.13 It is well-known that the molecular weight of copolymers is influenced by the incorporation of the comonomer13-18 which facilitates chain transfer reactions, â-H elimination, or chain transfer to aluminum. As expected, the average molecular weight of the copolymers is smaller than that of ethylene homopolymer. Also the copolymers containing more allylbenzene units have lower molecular weights except for the copolymers made with (2-MeInd)2ZrCl2 (runs 6 and 11).19 To understand the chain transfer process in the ethylene-allylbenzene copolymerization, the copolymer solution was treated with dry air (oxidative workup) that converts aluminum terminal groups formed through chain transfer to aluminum to hydroxy groups. Then, the chain end structure of the copolymers was analyzed with NMR spectroscopy. Table 2 shows the data for the chain end structure of the copolymer obtained with oxidative workup. The percentages of chain transfer to aluminum were calculated from the hydroxy group contents in Table 2. Under the polymerization condition investigated, significant amount of vinylidene end group was formed by â-H elimination in the copolymerization with Cp2ZrCl2/MAO and (n-BuCp)2ZrCl2/MAO, while chain transfer to aluminum is highly preferred in the copolymerization with (2-MeInd)2ZrCl2/MAO and Cp*2ZrCl2/MAO. The preference for â-H elimination in the copolymerization can be rationalized in terms of the chain conformation for â-agostic interaction, which is believed to stabilize the metal center.20-22 The typical polymer end groups, found in ethylene-R-olefin copolymer prepared by ordinary metallocene catalysts, are the vinylidene end group (Scheme 1g,h) generated by â-H elimination and the initial end group (methyl) (Scheme 1e,f) formed by insertion of monomer into the metal hydride. When the chain transfer to aluminum occurs, the polymer end groups become the alkyl-aluminum group. Unambiguous evidence for the chain transfer to aluminum in the copolymerization can be obtained from oxidative workup.1,23 Oxidative workup of the aluminumterminated polymer affords hydroxy-terminated polymers (Scheme 1c,d) of which 1H and 13C NMR spectra are shown in Figure 1 and Figure 2. The resonance at 3.5 ppm in the 1H NMR spectra and at 65.4 ppm in the 13C NMR spectra are the peaks of the hydroxymethyl end group obtained from structure 3 in Scheme l. Also, the resonance at 3.6 ppm in the 1H NMR spectra and at 63.0 ppm in the 13C NMR spectra correspond to the hydroxymethyl end group obtained from structure 4 in Scheme l. The two peaks at 4.7 ppm in the 1H NMR spectra are corresponding to the vinylidene group formed by â-hydride elimination after allylbenzene insertion (Scheme 1g). The vinylidene end group is detected predominantly in the case of using the catalysts having a less substituted Cp-type ligand, while the more hindered zirconocenes with substituted Cp-type ligands prefer chain transfer to aluminum to â-hydride elimination. These results suggest that the preference for â-hydride elimination vs aluminum transfer in metallocene-catalyzed * To whom all correspondence should be addressed. Telephone: 82-42-869-2834. Fax: 82-42-869-2810. E-mail: kimsy@kaist.ac.kr. 1921 Macromolecules 2000, 33, 1921-1923