The copolymerization of ethylene and styrene can be efficiently carried out by using Cp＊TiCl2（N=CtBu2）/ MAO （Cp＊=η5-C5Me5） system, yielding the poly（ethylene-co-styrene）s with isolated styrene units. In order...The copolymerization of ethylene and styrene can be efficiently carried out by using Cp＊TiCl2（N=CtBu2）/ MAO （Cp＊=η5-C5Me5） system, yielding the poly（ethylene-co-styrene）s with isolated styrene units. In order to in- vestigate the reasons for formation of the structure, the mechanism of copolymerization, especially the selective in- sertion of ethylene and styrene, is studied in detail by density functional theory （DFT） method. At the initiation stage, insertion of ethylene is kinetically more favorable than insertion of styrene, and insertion of styrene kinet- ically and thermodynamically prefers 2,1-insertion. That is different from the conventional half-titanocene system, in which the 1,2-insertion is favorable. At chain propagation stage, the computational results suggest that the con- tinuous insertion of styrene is hard to occur at room temperature due to the high free energy barriers （28.90 and 35.04 kcal/mol for 1,2-insertion, and 29.15 and 34.00 kcal/mol for 2,1-insertion） and thermodynamically unfavora- ble factors in two different conditions. That is mainly attributed to the steric hindrance between the coming styrene and chain-end styrene or ketimide ligand. The computational results are in good agreement with the experimental data.展开更多
基金This work was supported by the National Natural Science Foundation of China （Nos. 21404018, 21503030） and the Fundamental Research Funds for the Central Universities （No. DUT16RC（4）79）. Gaohong He gratefully acknowledges Education Department of the Liaoning Province of China （No. LT2015007）, the Fundamental Research Funds for the Central Universi- ties （No. DUT16TD19） and the Chang Jiang Scholar Program （No. T2012049）.
文摘The copolymerization of ethylene and styrene can be efficiently carried out by using Cp＊TiCl2（N=CtBu2）/ MAO （Cp＊=η5-C5Me5） system, yielding the poly（ethylene-co-styrene）s with isolated styrene units. In order to in- vestigate the reasons for formation of the structure, the mechanism of copolymerization, especially the selective in- sertion of ethylene and styrene, is studied in detail by density functional theory （DFT） method. At the initiation stage, insertion of ethylene is kinetically more favorable than insertion of styrene, and insertion of styrene kinet- ically and thermodynamically prefers 2,1-insertion. That is different from the conventional half-titanocene system, in which the 1,2-insertion is favorable. At chain propagation stage, the computational results suggest that the con- tinuous insertion of styrene is hard to occur at room temperature due to the high free energy barriers （28.90 and 35.04 kcal/mol for 1,2-insertion, and 29.15 and 34.00 kcal/mol for 2,1-insertion） and thermodynamically unfavora- ble factors in two different conditions. That is mainly attributed to the steric hindrance between the coming styrene and chain-end styrene or ketimide ligand. The computational results are in good agreement with the experimental data.
