Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

We have firstly examined how the presence of ethyl benzoate (EB) close to the Ti species affects its stability and charge density on single crystal MgCl₂ (110) and (100) surfaces by periodic density functional calculations. The energetic modification upon the coadsorption of EB and the Ti species was quite slight on both the (110) and (100) surfaces, which implies the random placement of these adsorbents on the support. The electron transfer from EB to the support enhanced the electron negativity of the Ti species on the (110) surface. In contrast, the electron that was donated from EB to the support never moved into the Ti species on the (100) surface. These results suggest that the addition of EB into the catalyst system introduces coadsorption-induced active sites selectively on the (110) surface.

The relationship between the stereospecificity of active sites and hydrogen effects on propylene polymerization was investigated for MgCl₂-supported TiCl₃ catalysts at ultra-low Ti contents. Hydrogen had no effect on the catalyst activities for isospecific sites and for aspecific sites. The efficiency of hydrogen as a chain-transfer agent was found to depend on the stereospecificity of the active sites. Hydrogen was effective for isospecific sites but was not effective for aspecific sites. From the viewpoint of hydrogen dissociation, isospecific sites should have hydrogen dissociation sites in their surroundings. Therefore, isospecific sites may be in the island to be affected by hydrogen and isolated sites may be aspecific or have low isospecificity.

Summary: Temperature rising elution fractionation (TREF) has been regarded as a powerful technique for study of semicrystalline polymers. In this paper, two examples of unique applications of TREF were introduced. One was the study on the influence of extraction of internal donor on the variation of isospecific active sites of a MgCl₂- supported Ziegler catalyst, and the other was the estimation of the relationship between polymer micro-tacticity and degradation rate of isotactic polypropylene (iPP). The former example revealed the conversion from high to low isospecific site by the extraction of internal donors, whereas the latter showed a negative correlation between the level of isotacticity and the degradation rate. These results demonstrated that TREF was useful in these research applications.

Hydrogen effects for propylene polymerization were investigated with ultra low TiCl₃ loading MgCl₂-supported catalysts in which the electric states of Ti species can be almost uniform. Hydrogen did not affect the catalyst activity, while the efficiency of hydrogen as a chain transfer agent was found to depend on the Ti content of the catalyst and the stereospecificity of the polymerization sites: Hydrogen was effective for isospecific sites independent of Ti contents, but inert for aspecific sites only at the extremely low Ti content. These results were explained within the island model, where isospecific sites may be located in the islands with other Ti species in their surroundings acting, as a steric hindrance for isospecific polymerization and as hydrogen dissociation sites after deactivation. Most of the aspecific sites should be isolated only at the extremely low Ti content. These isolated sites have no other Ti species in their surroundings, i.e. no hydrogen dissociation sites, and are inert to hydrogen.

Possible coadsorption states of Ti mononuclear species and ethyl benzoate (EB) and their interaction on MgCl₂ (110) and (100) surfaces were investigated with periodic density functional calculations in order to obtain the microscopic understanding about how EB affects the steric and electronic natures of the Ti species. EB was unlikely to be attached to the TiCl₄ species on both the MgCl₂ (110) and (100) surfaces. The coadsorption of EB at Mg²⁺ ions near the Ti species was as favorable as the separate adsorption, which implied the random placement of these adsorbants in the final catalyst. The charge redistributions upon coadsorption among the Ti species, EB and the support were found to be dependent on the surface structures: the electron density of the Ti speceies was rather decreased by the coadsorption on the (100) surface, while that of the Ti species was enhanced due to the support-mediated electron transfer from EB on the (110) surface. It was suggested that the presence of EB close to the Ti species should generate donor-related active sites selectively on the (110) surface.

Polypropylene (PP) impact copolymer is one of the heterophasic PP systems that is improved by rubber modification. Because the copolymer is a complicated polymer blend, which mainly consists of PP and ethylene–propylene rubber (EPR) components, the degradation behavior has hardly been studied. In this study, the thermal degradation of the copolymer was studied through direct observation by atomic force microscopy, which is a powerful tool for observing a local domain in a polymer blend. The degradation behavior was visually captured by the mapping of topological changes. Although the EPR phase was hardly degraded, the neighboring PP matrix was degraded selectively. The degradation behavior of the copolymer was found to be heterogeneous. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1831–1835, 2006

Summary: The stopped-flow polymerization of propylene catalyzed by a Ziegler-Natta catalyst in the presence of internal and external electron donors was carried out. The influence of the electron donors on the active sites of the catalyst was investigated using a multiple active site model. The variation of each group of active sites, such as the concentration and k_p of each site at various polymerization times, can be obtained from this model. It was found that an electron donor mainly affects the aspecific active site by reducing its concentration and increasing its k_p. The isospecific site is only slightly altered in terms of concentration and its k_p because of steric hindrance.

Phillips catalyst has been contributing to about 40% of world high-density polyethylene production because of its ability to give products with unique microstructures like broad molecular weight distribution as well as short and long chain branches. Even after 50 years' effort, some crucial problems concerning the nature of active sites, polymerization, and branching mechanisms are still kept mysterious. In this work, ethylene and 1-hexene copolymerization with Phillips catalyst prereduced by CO was carried out in the presence of triethyl aluminum (TEA) cocatalyst. The microstructures of polymers were investigated by ¹³C NMR and gel permeation chromatography (GPC) methods. A hybrid-type kinetics was found for both homo- and copolymerization kinetics, which indicated that there existed two types of active sites namely site A and site B. Site A with instant activation, high activity, and fast decay was transformed from a metathesis site, namely Cr(II) site, coordinated with CO or CO₂ through desorption of CO or CO₂ by TEA, which contributed to the formation of short chain branches, especially methyl branches. Site B with slow activation, low activity, and slow decay was generated from reduction of residual chromate (VI) by TEA. Both 1-hexene and TEA can decrease the molecular weight of polyethylene as well as enhance short chain branching. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4632–4641, 2005

Investigation of propene polymerization by a modified stopped-flow technique using TiCl₄/ethylbenzoate(EB)/MgCl₂ Ziegler–Natta catalyst with or without pretreating the catalyst with triethylaluminium (TEA) within an ultra-short period (ca 1 s) was conducted to gain new understanding of the nature of active sites related to TEA in the early stage of polymerization. When the catalyst was pretreated by a cocatalyst, deactivation behaviour was clearly observed, even within an extremely short pretreatment period. In contrast, without pretreatment, the deactivation of active sites can be neglected within the polymerization period indicating that the activated Ti species might be protected from deactivation by TEA when monomer is present in the system. A plausible guard effect on the active sites by coordinating monomer and growing polymer chains in the initial stage of polymerization is proposed to account for this phenomenon. Copyright © 2004 Society of Chemical Industry

The stereospecific nature of active sites on various MgCl₂-supported Ziegler-Natta catalysts was investigated by stopped-flow technique combined with temperature rising elution fractionation (TREF) method. A modified three-sites model with precise description of the stereospecific nature of various types of active sites stemmed from surface titanium species, Al-alkyl compounds, Mg-compounds and electron donors has been proposed. It was demonstrated that the isospecificity of active sites strongly depends on the bulkiness of the ligands situated at the two most important ligand positions for construction of asymmetry and chirality of the active sites with steric hindrance. In general, there may exist both monometallic and bimetallic sites in heterogeneous Ziegler-Natta catalyst system. The kinds of active titanium species with different chemical structures on this catalyst system should be limited, whereas, the non-discrete distribution of isospecificity of active sites could be considered to generate from the numerous types of steric and electronic effects from the surroundings of active titanium species as well as large number of reversible and dynamic transformation reactions simultaneously occurred on the heterogeneous catalyst surface.

In this work, a CO-prereduced Phillips CrO_x/SiO₂ catalyst (PC600/CO) was characterized by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption with mass spectrometer (TPD-MS) in comparison with a calcined catalyst (PC600). It was found that surface chromate Cr(VI) species had not been completely reduced into Cr(II) species, meanwhile, CO and CO₂ still strongly adsorbed on the PC600/CO catalyst. Subsequently, the unique real-time profiles of ethylene polymerization rate using PC600/CO in the presence of TEA indicated the existence of two types of active sites: the first type derived from the desorption of CO or CO₂ from Cr(II) species by alkyl-Al and featured with fast activation, higher activity and fast decay; the second type generated from the further reduction of residual chromate Cr(VI) species by alkyl-Al and featured with slow activation, lower activity and slow decay.

Abstract

High resolution transmission electron microscopy (HRTEM) was used to observe the structure of α-TiCl₃ in order to investigate the nature of active sites in Ziegler–Natta catalyst. The crystalline structures of the α-TiCl₃ surface were directly observed in the HRTEM image and the electron diffraction pattern. The ordered arrangement of the crystal lattices in α-TiCl₃ particles was clearly revealed. The rapid deterioration of the crystalline structure of the α-TiCl₃ due to the exposure in ambient conditions was also observed on the atomic scale, reflecting its ultra-high sensitivity to the impurities in the air.

Based on our previous study on the donor-free TiCl₄/MgCl₂ catalyst, the stereospecific nature of active sites on this catalyst after the addition of an internal donor, ethylbenzoate (EB), was investigated by stopped-flow propylene polymerization combined with temperature rising elution fractionation (TREF) and gel permeation chromatography (GPC) methods. The addition of donor EB was observed to significantly restrain the formation of aspecific sites (AS sites), to drastically promote formation of sites with the second highest isospecificity (IS₂ sites) and to construct some new active sites with the highest isospecificity (IS₃ sites). It was first observed that the formation of IS₃ sites still strongly depends on the interaction between the catalyst and cocatalyst (up to 10 s of pretreatment) even in the presence of internal donor. Based on Busico and co-workers' three sites model and our previous report, a plausible mechanism for the transformation of stereospecific active sites through some secondary bimetallic complexation reactions was proposed. The extraction of EB from the catalyst by the cocatalyst was found to initiate after 10 s of pretreatment resulting in partial transformation of IS₃ sites into AS sites and IS₁ sites.

During propylene polymerization with the Cr(acetylacetonate)₃/MgCl₂–Et₂AlCl–ethylbenzoate catalyst system which shows high isospecificity for propylene polymerization, it was found that the chain transfer and termination reactions can be neglected. Based on this result, some stepwise polymerizations of propylene and ethylene were carried out with the same catalyst system varying polymerization conditions. The resulting copolymers were separated by temperature rising elution fractionation (TREF) and afforded two fractions which were eluted at different temperatures. From the ¹³C NMR, DSC and GPC analyses of each fraction, it was considered that the corresponding block copolymer existed in the fraction eluted at higher temperature.

© 2003 Society of Chemical Industry

The effect of the incorporation of ethylene on the photostability of isotactic poly(propylene) (iPP) was studied with the aim of improving the photostability. iPP was prepared with a random ethylene sequence (ethylene–propylene random copolymer, rPP), and the photooxidative degradation behavior was compared with that of homogeneous iPP. Both samples were thermally post-treated under vacuum to ensure the same crystallinity. The degradation behavior was studied by infrared spectroscopy (IR), gel permeation chromatography (GPC), and temperature rising elution fractionation (TREF) measurements. The rates of hydroperoxide and carbonyl formation in the irradiated iPP increased with irradiation time for > 192 h, whereas those in the irradiated rPP are almost constant after 96 h. The change in molecular weight with the irradiation time showed similar behavior, suggesting that the degradation reaction in the irradiated rPP was suppressed after 96 h. The degradation behavior of rPP was thought to be due to the dissociation of the methyl group, which leads to the termination of degradation. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1863–1867, 2002

The formation, deactivation and transformation of stereospecific active sites on the donor-free TiCl₄/MgCl₂ catalyst induced by short time reaction with triethylaluminium (TEA) cocatalyst (pretreatment time: 0 ∼ 60 s) were investigated by stopped-flow propylene polymerization combined with temperature rising elution fractionation (TREF) and gel permeation chromatography (GPC) methods. We got the first report that the origin of isospecificity of active sites derived from the catalyst substrate and cocatalyst were individually observed without and with TEA pretreatment, respectively. Based on the three-sites model of Corradini and Busico et al., a plausible mechanism of the formation and transformation of stereospecific active sites was proposed using active sites models in terms of specified roles of Al-alkyl cocatalyst. It was demonstrated that the introduction of bulky alkyl groups instead of chlorine atoms into the neighborhood of active sites seems to be crucial for the generation of active sites with the highest isospecificity.

Block copolymerization of propene and 1,5-hexadiene was carried out by a modified stopped-flow polymerization method with an MgCl₂-supported Ziegler catalyst. The resulting polymer, polypropene-block-poly(methylene-1,3-cyclopentane-co-propene) (PP-b-(PMCP-co-PP)), in which the crystallizable PP part was linked with the non-crystallizable PMCP-co-PP part, was characterized by optical microscopy, differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and tensile testing. The block copolymer having a chemical linkage between PP and PMCP-co-PP showed properties different from those of homopolymer, random copolymer and blend polymer.

© 2001 Society of Chemical Industry

In situ propene polymerization was observed using CP MAS (cross polarization with magic angle spinning) ¹³C NMR spectroscopy under the conditions of continuously flowing propene through a supported Ziegler catalyst treated with triethylaluminium. The signals at 44, 26, and 22 ppm attributed to the CH₂, CH, and CH₃ groups of polypropene, respectively, were found to appear upon exposure to the propene flow (0.1 L/min). An increase in intensities of these lines was observed for 30 min, then they remained approximately constant. The cross-polarization parameters of nascent polypropene, measured using standard techniques, were close to those of typical crystalline polypropene. The technique has a great potential as an effective tool for the investigation of the polypropene formation on the Ziegler catalyst.