Cp_2 TiCl_2/坡缕石黏土负载型催化剂催化乙烯聚合的特性

Characteristics of palygorskite supported Cp_2 TiCl_2 catalysts for ethylene polymerization

利用浸渍法制备了两种二氯二茂钛(Cp2TiCl2)/坡缕石黏土负载型催化剂Cat-A和Cat-B,并进行了乙烯淤浆聚合评价。对热活化黏土进行表征的结果表明,坡缕石黏土在热活化过程中由于配位结晶水的脱除表面主要由Lewis酸性位占据。黏土载体所具有的表面酸性使其具有完全不同于硅胶载体的负载茂金属催化剂聚合行为。在相同的聚合条件下,直接负载型催化剂的活性高于载体化学修饰型催化剂,甚至高于均相Cp2TiCl2催化剂。直接负载催化剂所得聚合产物的分子量和熔点低于载体化学修饰催化剂,且其产物性质受温度影响更为显著。以硅胶负载型茂金属催化剂作为对比,分析了表面具有较强Lewis酸性的载体活性中心性质,以此解释了直接负载型催化剂的乙烯聚合特性。对直接负载型催化剂不同时间段的聚合产物形态进行了扫描电镜观察,发现最终聚合产物中聚合物“纤维”和“纤维”聚集体形态的形成,并进一步分析了聚合物形态演化过程的特点。

Palygorskite clay supported Cp2TiCl2 catalyst system was developed and evaluated by ethylene slurry polymerization. The inorganic carrier was thermally activated at 500℃ to generate surface Lewis acidity, which was confirmed by pyridine adsorption experiment and subsequent FTIR spectra analysis. Two types of supported catalysts were prepared： immobilizing the metallocene catalyst directly on the carrier （Cat-A） and also on the carrier with methylaluminoxane （MAO） modification （Cat-B）. Polymerization was carried out in toluene at 40℃ and 60℃ with the supported Cp2TiCl2 catalyst and homogeneous analogue. Cat-A gave higher catalyst activity compared with Cat-B, and was even more robust than corresponding homogeneous catalyst. This specifically high activity of Cat-A was attributed to the surface Lewis acidity of the thermally activated clay. Models of active surface species were discussed in comparison with silica supported metallocene catalyst. With the directly supported catalyst, molecular weights and melting points of the polymer products were lower than those of the polymer products obtained with Cat-B, and the polymer properties were much more affected by the polymerization temperature.By analyzing the SEM photographs of polymers at different polymerization stages, morphological replication of the single fiber of palygorskite was revealed and a final polyethylene ＂fiber cluster＂ structure was formed. This interesting evolution of polymer morphology was further discussed in relation to polyethylene/clay composite and the method of improving the final polymer morphology.