Abstract
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.
Highlights
Progress in catalyst technology has lead to the synthesis of a rich set of new polymers with different structures and performances to meet the progressive demands of modern industry and life [1,2,3,4,5,6,7,8,9,10,11,12].Recently, branched polyethylenes such as linear low-density polyethylene (LLDPE) have grown in importance in industry because of the specific properties that can be obtained by varying comonomer content and polymerization conditions
The ethylene/α-olefin copolymers obtained by metallocene catalysts show homogeneous comonomer distribution and narrow molecular weight distributions in comparison with those obtained with traditional Ziegler–Natta (ZN) catalysts [14]
(4.5) > tri-n-hexyl aluminum (TnHA) (4.3) > TnHA+diethyl aluminum chloride (DEAC) (3.8) = TnHA+DEAC (3.8) > TEA (3.7) > DEAC (3.6). These results showed no relationship among the Mw, molecular weight distribution (MWD), reducing power and activity in copolymerization system
Summary
Progress in catalyst technology has lead to the synthesis of a rich set of new polymers with different structures and performances to meet the progressive demands of modern industry and life [1,2,3,4,5,6,7,8,9,10,11,12]. Rapid exchange between the alkyl groups in mixtures of TEA with the iso-butyl group in TIBA may be an important reason for the increase in catalytic activity and yields of both the random copolymer and the segmented copolymer parts, which were close to the highest level in PE-PP copolymer In spite of these interesting results, the effect of mixed alkyl aluminum on catalytic activity of ethylene polymerization and ethylene/1-hexene copolymerization, and polymer properties has received little attention, even though it could be of crucial importance to successfully design and operate industrial polymerization processes. The effect of various activator mixtures on activity, product morphology and molecular weight distribution of polyethylene and ethylene/1-hexene copolymere synthesized by the MgCl2/SiO2/TiCl4/THF-ZN catalyst was investigated. The obtained polymers were characterized by means of X-ray diffraction (XRD), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and nuclear magnetic resonance (13C-NMR) techniques
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