| 简介： |
|
Qi Wang, Zhiqiang Fan, Huaxiang Yang, Lidong LiDepartment Of Polymer Science And Engineering, Zhejiang University, Hangzhou, 310027, China
Olefin coordination polymerization catalyzed by transition metal complex in combination with cocatalyst/activator has been intensively explored for several decades. From heterogeneous Ziegler-Natta catalyst, homogeneous metallocene catalyst to late transition metal complex, great progress has been made in the efficient and controllable synthesis of polyolefin products.
In the study of bicomponent catalyst system, much attention has been paid to the design and synthesis of new main catalysts, namely, the transition metal complexes. For traditional Ziegler-Natta catalyst, the influence of structure of titanium halide and electron donor on the microstructure of polyolefin has been intensively investigated. In the study on “single-site” catalysts, it has been found that variation of ligand structure of metallocene allows good control over polyolefin microstructure. In the new generation “non-metallocene” catalyst systems, the structure of polyolefin can also be regulated by modifying the ligand in organometallic complexes. On the other hand, discovery of new and more effective cocatalysts/activators1 has contributed significantly to scientific understanding and technological developments in this field. Aluminoxane has marked the metallocene as the new generation of catalyst for olefin coordination polymerization and it has attracted much attention as well. Understanding of these catalytic systems should be partly attributed to the studies on cocatalysts/activators. Up to now, many chemicals, such as aluminum alkyls, aluminoxanes and boranes have been employed as cocatalyst or activator for transition metal catalyst system. In recent years we have carried out extensive investigations on the roles of cocatalyst in the olefin polymerization systems, and found that the cocatalyst can affect both the polymerization kinetics and plymer chain structure. Actually, the cocatalyst may act as an effective regulator in the polymerization system. In the study of metallocene systems, a series of aluminoxanes as cocatalyst were compared, including the well known methylaluminoxane (MAO), ethylaluminoxane (EAO), isobutylaluminoxane (BAO), and a new kind of mixed aluminoxane, ethyl-isobutylaluminoxane (EBAO). The study on such aluminoxanes has bring us the following facts: (1) the cocatalytic activity in olefin polymerization of EBAO is much higher than either EAO or BAO2. Such effect has also been proved by the better properties of MMAO over MAO. (2) The ability of metallocene to become cationic active center and the ability of cocatalyst to stabilize the active center are all important factors for high catalyst activity, but gaining on one side can compensate for losing on the other side3,4. That is to say, when the metallocene can be easily transferred to cation, a cocatalyst with only medium ability to stabilize the active center (like EBAO) will be enough for achieving high activity. (3) In copolymerization of ethylene with bulky comonomers5, more comonomer can be incorporated when EBAO was used as activator, indicating that the size of alkyl in aluminoxane will affect the tightness of etallocenium-aluminoxane ion pair. Solid evidence was obtained from the UV-visible spectra of different metallocene/ aluminoxane pairs 6. These facts show us more choices in regulating the polymerization behaviors of metallocene catalysts. In manycases MAO is not the best choice of cocatalyst. Careful investigation on the structure and properties of cocatalyst7 for metallocene could lead to discoveries of new and cheap cocatalyst, and accelerate the wide application of metallocene catalyst in industry scale. Series of late transition metal (e.g. Ni, Pd, Fe, Co) catalysts reported by Brookhart and Gibson are highly active for olefin polymerization. The lower oxophilicity and greater functional-group tolerance of late transition metals favor copolymerization of α-olefin and polar monomers. Intensive researches indicate that both the central metal and the ligand of late transition metal complexes affect the polymerization activity and the structure of resulted polymers. However, great effects of cocatalyst on polymerization behavior were also found in these systems. The influence of activator on ethylene polymerization catalyzed by some nickel and ferrous complexes was studied. Alkyl aluminums,tetraalkylaluminoxane and EBAO was employed as activator for nickel8 and iron complexes9. Cocatalyst effect on the iron complex for ethylene polymerization is quite interesting. Both the polymerization activity and the polymer molecular weight and its distribution were markedly affected by varying the activator. Some activators behave much differently with MAO when combined with iron complex: (1) Et2AlOAlEt2 is a more efficient cocatalyst than MAO9, (2) Polyethylene with narrow MWD can be obtained when Bu2AlOAlBu2 was used as activator.9 (3) Bimodal molecular weight distribution of polyethylene prepared by iron catalyst could be controlled and regulated by variation of activator.
According to our study on cocatalyst for transition metal complex, it is proposed that cocatalyst as a component of the catalyst system plays an important role in both the generation of active species and the polymerization reaction. It affects the polymerization activity, the microstructure of polymer, and the molecular weight and MWD of the final product. Further research on the cocatalyst can bring us new opportunities of improving the performance of existed catalyst and new ways of regulating olefin polymerization. Supports from the National Natural Science Foundation of China and SINOPEC (grant no. 29734144) and the Special Funds for Major State Basic Research Projects (G1999064801) are gratefully acknowledged.
References:(1) Chen, E. Y. X.; Marks, T. J. Chem. Rev. 2000, 100, 1391-1434.(2) Wang, Q.; Weng, J. H.; Fan, Z. Q.; Feng, L. X. Eur. Polym. J. 2000, 36, 1265-1270.(3) Fan, Z. Q.; Weng, J. H.; Wang, Q.; Feng, L. X. Chem. J. Chin. Univ.-Chin. 1998, 19, 1178-1180.(4) Wang, Q.; Hong, J.; Fan, Z. Q.; Tao, R. Y. J Polym Sci Part A: Polym Chem 2003, 14, 998-1001.(5) Wang, Q.; Weng, J. H.; Fan, Z. Q.; Feng, L. X. Macromol. Rapid Commun. 1997, 18, 1101-1107.(6) Wang, Q.; Song, L. X.; Zhao, Y.; Feng, L. X. Macromol. Rapid Commun. 2001, 22, 1030-1034.(7) Wang, Q.; Zhao, Y.; Song, L. X.; Fan, Z. Q.; Feng, L. X. Macromol. Chem. Phys. 2001, 202, 448-452.(8) Yang, H. X.; Wang, Q.; Fan, Z. Q.; Lou, W. B.; Feng, L. X. Eur. Polym. J. 2003, 39, 275-279.(9) Wang, Q.; Yang, H. X.; Fan, Z. Q. Macromol. Rapid Commun. 2002, 23, 639-642.
论文来源：International Symposium on Polymer Chemistry,June,2004 |
|
