Influences Of Activators On Ethylene Polymerization Catalyzed By Iron Complexes And Synthesis Of Polyethylene With Broad/Bimodal Molecular Weight Distribution

Activator is an important component of catalyst system for olefin coordination polymerization, which affects not only polymerization activity but also structure and property of resulting polymer. Compared to catalyst precursor, due to complexities of its composition and structure, study on activator is considerably limited. This dissertation systematically studied the influences of variation of composition and structure of activator on polymerization activity and property of resultant product through changing oligomeric degree and substituent groups of activator.Through reactions of alkylaluminiums with water or 4-fluorophenylboric acid, tetraalkylaluminoxanes, alkylaluminoxanes and their derivatives modified by 4-fluorophenylboric acid have been prepared, respectively. These products and alkylaluminiums were used as activators to catalyze ethylene polymerization in the presence of iron complexes [((2,6-R2Ph)N=C(Me))2C5H3N]FeCl2 (Fe.l, R=Me; Fe.2, R='Pr). The influence of activator on ethylene polymerization, especially on molecular weight (MW) and molecular weight distribution (MWD), was primarily studied. In addition, the mechanisms of ethylene polymerization catalyzed by various activators and iron complexes were carefully explored.The influences of various alkylaluminiums (AlEtn'Bu3.n, n=0-3) on ethylene polymerization catalyzed by iron complexes were firstly studied. When alkylaluminiums were used as activators, Fe.l had no activities for ethylene polymerization and Fe.2 afforded moderate polymerization activities. In the case of Fe.2, polymerization activities increased with the amount of Al-Et bonds in alkylaluminiums. Except Al'Bu3, Fe.2/alkylaluminium systems all prepared bimodal-MWD polyethylenes with predominant low-MW fractions, and the MWs of these polyethylenes reduced with increments of the amount of Al-Et bonds inalkylaluminiums. It has been demonstrated that the low-MW fractions of these bimodal MWDs are generated by chain transfer reaction to alklyaluminiums, and the increases of Al/Fe ratio and the amount of Al-Et bonds in alkylaluminiums favor such chain transfer reaction.When various alkylaluminoxanes, e.g. ethylaluminoxane (EAO) and ethylisobutylaluminoxane (EBAO), were used as activators, both Fe.l and Fe.2 could promote ethylene polymerization and polymerization activities were noticeably higher than those as alkylaluminiums were used as activators. It is suggested that the increase of oligomeric degree of activator might improve polymerization activity. In contrast to alkylaluminiums, alkylaluminoxanes exclusively prepared unimodal-MWD polyethylenes with higher MWs.The influence of tetraalkylaluminoxane on ethylene polymerization catalyzed by iron complexes has been intensively investigated. It was found that polymerization activity increased with the amount of Al-Et bonds in tetraalkylaluminoxanes. All tetraalkylaluminoxanes prepared polyethylenes with pronounced broad/bimodal MWDs. As the amount of Al-Et bonds in tetraalkylaluminoxanes increased, low-MW fractions of these broad/bimodal MWDs increased and high-MW fractions correspondingly reduced. As a result, the MWs of resulting polyethylenes were totally lowered and the MWDs were broadened. It has been approved that the low-MW fractions of broad/bimodal MWDs prepared by tetraalkylaluminoxanes are formed through chain transfer reaction to unreacted alkylaluminiums existing in tetraalkylaluminoxanes. The increments of the amount of Al-Et bonds in tetraalkylaluminoxanes, Al/Fe ratio and polymerization temperature favor such chain transfer reaction. Because Fe.l with less sterically crowded ligand prefers chain transfer reaction to aluminium compared with Fe.2, the bimodal MWDs prepared by Fe.l/tetraalkylaluminoxane systems are more pronounced than those prepared by Fe.2/ tetraalkylaluminoxane systems.The influences of aluminoxanes modified by 4-fluorophenylboric acid on ethylene polymerization varied with the kinds of iron complexes. When Fe.l was used as catalyst precursor, aluminoxanes modified by 4-fluorophenylboric acid (FB-TEAO and FB-E...