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This article reveals the effects of mixed activators on ethylene polymerization and ethylene/1-hexene copolymerization over MgCl₂/SiO₂-supported Ziegler-Natta (ZN) catalysts. First, the conventional ZN catalyst was prepared with SiO₂ addition. Then, the catalyst was tested for ethylene polymerization and ethylene/1-hexene (E/H) co-polymerization using different activators. Triethylaluminum (TEA), tri-n-hexyl aluminum (TnHA) and diethyl aluminum chloride (DEAC), TEA+DEAC, TEA+TnHA, TnHA+ DEAC, TEA+DEAC+TnHA mixtures, were used as activators in this study. It was found that in the case of ethylene polymerization with a sole activator, TnHA exhibited the highest activity among other activators due to increased size of the alkyl group. Further investigation was focused on the use of mixed activators. The activity can be enhanced by a factor of three when the mixed activators were employed and the activity of ethylene polymerization apparently increased in the order of TEA+ DEAC+TnHA > TEA+DEAC > TEA+TnHA. Both the copolymerization activity and crystallinity of the synthesized copolymers were strongly changed when the activators were changed from TEA to TEA+DEAC+TnHA mixtures or pure TnHA and pure DEAC. As for ethylene/1-hexene copolymerization the activity apparently increased in the order of TEA+DEAC+TnHA > TEA+TnHA > TEA+DEAC > TnHA+DEAC > TEA > TnHA > DEAC. Considering the properties of the copolymer obtained with the mixed TEA+DEAC+TnHA, its crystallinity decreased due to the presence of TnHA in the mixed activator. The activators thus exerted a strong influence on copolymer structure. An increased molecular weight distribution (MWD) was observed, without significant change in polymer morphology.

Two commercial nanoclays were used here as catalytic fillers for production of polyethylene (PE) and linear low-density polyethylene (LLDPE) nanocomposites via in situ polymerization with zirconocene/MAO catalyst. It was found that both types of nanoclays designated as clay A and clay B can improve thermal stability to the host polymers as observed from a thermal gravimetric analysis (TGA). The distribution of the clays inside the polymer matrices appeared good due to the in situ polymerization system into which the clays were introduced during the polymer forming reaction. Upon investigating the clays by X-ray diffractometer (XRD) and Fourier transform infrared spectroscopy (FTIR), it was observed that the crucial differences between the two clays are the crystallite sizes (A < B) and the amounts of amine group (A < B). The higher amount of amine group in clay B was supposed to be a major reason for the lower catalytic activity of the polymerization systems compared to clay A resulting from its deactivating effect on zirconocene catalyst. However, for both clays, increasing their contents in the polymerization systems reduced the catalytic activity due to the higher steric hindrance occurring. [ABSTRACT FROM AUTHOR]

MgCl2-SiO2/TiCl4 Ziegler-Natta catalysts for ethylene polymerization were prepared by impregnation of MgCl2 on SiO2 in heptane and further treatment with TiCl4. MgCl2·nEtOH adduct solutions were prepared with various EtOH/MgCl2 molar ratios for preparation of the MgCl2-supported and MgCl2-SiO2-supported catalysts in order to investigate the effect on polymerization performance of both catalyst systems. The catalytic activities for ethylene polymerization decreased markedly with increased molar ratios of [EtOH]/[MgCl2] for the MgCl2-supported catalysts, while for the bi-supported catalysts, the activities only decreased slightly. The MgCl2-SiO2-supported catalyst had relatively constant activity, independent of the [EtOH]/[MgCl2] ratio. The lower [EtOH]/[MgCl2] in MgCl2-supported catalyst exhibited better catalytic activity. However, for the MgCl2-SiO2-supported catalyst, MgCl2 can agglomerate on the SiO2 surface at low [EtOH]/[MgCl2] thus not being not suitable for TiCl4 loading. It was found that the optimized [EtOH]/[MgCl2] value for preparation of bi-supported catalysts having high activity and good spherical morphology with little agglomerated MgCl2 was 7. Morphological studies indicated that MgCl2-SiO2-supported catalysts have good morphology with spherical shapes that retain the morphology of SiO2. The BET measurement revealed that pore size is the key parameter dictating polymerization activity. The TGA profiles of the bi-supported catalyst also confirmed that it was more stable than the mono-supported catalyst, especially in the ethanol removal region. [ABSTRACT FROM AUTHOR]

The present research focuses on investigation of the catalytic behaviors of MgCl2-SiO2/TiCl4/THF Ziegler-Natta (ZN) catalysts_ with fumed SiO2 variously treated with silane compounds. The non-treated silica (NTS) and other silicas treated with dimethylsilicone fluid (TSDMSF), dimethyldichlorosilane (TSDMDCS), and hexamethyldisilazane (TSHMDS) were employed. It was found that the Cat-TSDMDCS and Cat- TSHMDS exhibited remarkably high activity, even with a similar bulk Ti content as the others. Thus, the more powerful technique of XPS analysis was used to determine the Ti content at the catalyst surface. It was evident that the surface concentrations of Ti could play important role on the catalyst activity. As the result, the increased activity is proportional to the surface concentration of Ti. It was mentioned that the change in surface concentration of Ti with different treated silica can be attributed to the effect of silane spacer group and steric hindrance. The distribution of Ti on the external surface can be also proven by means of EDX mapping, which matched the results obtained by XPS analysis. The treated silica also resulted in narrower molecular weight distribution (MWD) due to the more uniform active sites produced. There was no significant change in polymer morphology upon treatment of the silica. [ABSTRACT FROM AUTHOR]

The present research focuses on investigation of the catalytic behaviors of MgCl2-SiO2/TiCl4/THF Ziegler-Natta (ZN) catalysts_ with fumed SiO2 variously treated with silane compounds. The non-treated silica (NTS) and other silicas treated with dimethylsilicone fluid (TSDMSF), dimethyldichlorosilane (TSDMDCS), and hexamethyldisilazane (TSHMDS) were employed. It was found that the Cat-TSDMDCS and Cat- TSHMDS exhibited remarkably high activity, even with a similar bulk Ti content as the others. Thus, the more powerful technique of XPS analysis was used to determine the Ti content at the catalyst surface. It was evident that the surface concentrations of Ti could play important role on the catalyst activity. As the result, the increased activity is proportional to the surface concentration of Ti. It was mentioned that the change in surface concentration of Ti with different treated silica can be attributed to the effect of silane spacer group and steric hindrance. The distribution of Ti on the external surface can be also proven by means of EDX mapping, which matched the results obtained by XPS analysis. The treated silica also resulted in narrower molecular weight distribution (MWD) due to the more uniform active sites produced. There was no significant change in polymer morphology upon treatment of the silica. [ABSTRACT FROM AUTHOR]

The study revealed enhancement (four to seven times) of catalytic activities for ethylene/1-octene copolymerization via the Zr-modified SiO2-supported the metallocene/MAO catalyst. Increased activity can be attributed to an increase in absorption ability of MAO on the modified support. In addition, the strong interaction between MAO and the support was also considered. [ABSTRACT FROM AUTHOR]

The bimodal polyethylene obtained from TiO2-supported zirconocene/MAO catalyst was observed during polymerization of ethylene and ethylene/1-hexene. By means of XPS, it revealed that TiO2 consisted of Ti3+ (BE = 462.6 eV) and Ti4+ (BE = 464.9 eV). The dual catalytic sites were attributed to the presence of Ti3+ and Ti4+ in TiO2. [ABSTRACT FROM AUTHOR]

Effect of SiCl4-modified silica/MAO-supported Et[Ind]2ZrCl2 metallocene catalyst on copolymerization of ethylene with α-olefins was investigated. Effect of SiCl4 on activities was diminished with higher α-olefins. Molecular weights of copolymers decreased with SiCl4 modification. SiCl4 modification also resulted in a lower molecular weight distribution. 13C NMR showed that ethylene incorporation in all systems gave copolymers with similar triad distribution. In addition, a narrow branching distribution can be achieved with SiCl4 modification. [Copyright &y& Elsevier]

Studies the linear low-density polyethylene (LLDPE)/nano-silica composites synthesized through in situ polymerization of ethylene/1-hexene with methylaluminoxane (MAO)/metallocene catalyst. Applications of LLDPE polyolefin; Drawbacks of polyolefins; Preparation of LLDPE/nano-composites.

Polyethylene-cellulose biocomposites were synthesized here via the ethylene polymerization with metallocene as a catalyst along with methylaluminoxane (MAO) as a cocatalyst. The immobilization method in which the catalyst or cocatalyst is fixed onto the catalytic filler (cellulose) can be classified into 3 methods according to the active components fixed onto the filler surface: 1) only metallocene catalyst (Cellulose/Zr), 2) only MAO cocatalyst (Cellulose/MAO) and 3) mixture of metallocene and MAO (Cellulose/(Zr+MAO)). It was found that the different immobilization methods or different fillers altered the properties of the obtained composites and also the catalytic activity of the polymerization systems. It was found that Cellulose/MAO provided the highest catalytic activity among all fillers due to a crown-alumoxane complex, which caused the heterogeneous system with this filler behaved similarly to the homogeneous system. The different fillers also produced the biocomposites with some different properties such as crystallinity which Cellulose/Zr provided the highest crystallinity compared with other fillers as observed by a thermal gravimetric analysis-differential scanning calorimetry (TGADSC). Nevertheless, the main crystal structure indicated to the typical polyethylene was still observed for all obtained biocomposites with different fillers as observed by an X-ray diffractometer (XRD). [ABSTRACT FROM AUTHOR]

Abstract: Three fluorinated bis(phenoxy-imine)titanium complexes bearing ortho-halide substituents on the phenoxy rings [Cl (1), Br (2), I (3)] were synthesized, characterized and evaluated as precatalysts for the polymerization of ethylene in conjunction with methylaluminoxane (MAO) under atmospheric pressure. These C2-symmetric catalysts exhibit high activity toward ethylene polymerization and produce ultra high molecular weight polyethylene (UHMWPE) (M W > 3,000,000). The living polymerization was presented as evidenced by the narrow molecular weight distribution. The living nature of these catalysts is attributed to the interaction of a fluorine atom adjacent to the imine nitrogen and a β-hydrogen of a polymer chain resulting in the suppression of β-hydrogen transfer. The highest activity at 11.65 kg PE/mmol cat h was observed for the polymerization using complex 2. In comparison with complex 1, the more bulky bromo substituents can induce an effective ion-pair separation between the cationic active species and an anionic cocatalyst yielding a higher polymerization rate. However, in case of complex 3 with iodo substituents, the enhancement of the steric congestion at the metal center results in the decreased polymerization activity. [Copyright &y& Elsevier]