紫外-可见光谱对β-二酮锆烯烃催化活性中心的研究

UV/Visible Spectroscopic Study of the β-Diketonate Zirconium Active Species for Olefin Polymerization

采用测试条件更接近实际聚合情况的紫外-可见吸收光谱法,研究了烯烃聚合催化剂β-二酮锆配合物(dbm)2ZrCl2分别与两种不同助催化剂一氯二乙基铝(AlEt2Cl)、甲基铝氧烷(MAO)在室温下的作用情况。结果发现,催化剂金属中心的最大吸收峰与配体向金属的电荷转移(LMCT)是相关的,并且随助催化剂的加入量而不同。无论助催化剂是AlEt2Cl还是MAO,少量的助催化剂即低铝锆比下,催化剂上的一个吸电子基氯首先被助催化剂上具有供电性的一个烷基所取代,发生单烷基化反应,催化剂金属中心的最大吸收峰波长发生蓝移。而当助催化剂量进一步增加时,金属中心的最大吸收峰波长呈红移,表明了单烷基取代的催化剂被大量的助催化剂夺去电子成为缺电子的阳离子活性中心,以便烯烃单体的配位插入。

The method of UV/Visible absorption spectroscopy for olefin catalytic system was introduced in this paper,whose testing condition was much closer to the polymerization conditions.The actions of olefin polymerization catalyst （dbm）2ZrCl2 with cocatalyst AlEt2Cl （or MAO） were investigated by UV/visible absorption spectroscopy at atmosphere temperature.It was shown that the UV/Visible main absorption band of the zirconocenium,which can be related to the ligand to metal charge transfer bands （LMCT）,varies greatly upon incremental addition of AlEt2Cl or MAO.For the low molar ratios of Al/Zr in the catalytic system,there was the substitution of an electron withdrawing chlorine atom by a donating alkyl group.Then a hypsochromic shift of the initial catalyst absorption band,corresponding to the monomethylation of the catalyst,was observed in each catalytic system （dbm）2ZrCl2/AlEt2Cl （or （dbm）2ZrCl2/MAO）.On the contrary,further addition of AlEt2Cl （or MAO） was accompanied by a continuous bathochromic shift of the maximal wavelength,which corresponding to the formation of more dissociated ionic active species.Then,there would be a coordination of monomer to the ionic active species.