Griselda B. Galland

Influence of the comonomer content on the thermal and dynamic mechanical properties of metallocene ethylene/1-octene copolymers

Studies of ethylene/1-octene copolymers prepared by a metallocene catalyst were carried out through dynamic-mechanical experiments (DMTA), differential scanning calorimetry (DSC), Raman spectroscopy and gel permeation chromatography (GPC). The influence of comonomer content on the ethylene/1-octene properties, specially those related to the dynamic-mechanical behaviour were studied. It was observed that the intensity of the β-transition increases as the comonomer content increases and decreases crystallinity.

Nickel α‐Keto‐β‐Diimine Initiators for Olefin Polymerization

Late transition metal initiators for olefin polymerization and oligomerization are of interest owing to their low oxophilicity and high functional group tolerance. Nickeland palladiumbased initiators provide several benefits. These include control over molecular weight characterisitics, backbone stereochemistry, and comonomer incorporation. 4] “Chain walking” reactions, in which the metal center migrates along the growing polymer chain through a series of b-hydride eliminations and reinsertions, allow for the generation of a wide variety of novel materials. New ligand structures continue to appear and offer further optimization of reactivity and polymer properties. 4b, 5c,6] Scheme 1 shows representative cationic and zwitterionic nickel active sites. Example 1a (charge compensating anion is not shown) corresponds to the broadly studied class of adiimine complexes, for which the resting state is a cationic alkyl olefin species. These species can be generated from a dihalide precursor in the presence of olefin and an appropriate activator, such as methylaluminoxane (MAO). Also shown are two prototypical zwitterionic initiators (1b and 1c ; important resonance contributions are not shown). A relevant perspective for understanding the reactivity of zwitterionic species is that Lewis acid complexation onto a basic ligand site reduces the electron density of the metal center in a neutral complex. This concept is more easily appreciated by examination of 1c. Bulky substituents that occupy axial sites are important structural components for minimizing chain-growth termination in both cationic and zwitterionic systems: for example, 1b produces high-molecular-weight polyethylene (PE), whereas 1c yields a distribution of 1-alkenes. Herein, we report on the synthesis, characterization, and reactivity of a ligand–metal combination that was designed to be cationic and also to benefit from removal of electron density from the metal center by the action of a Lewis acid on the ligand framework. We targeted a nickel complex containing a bulky a-keto-b-diimine ligand with 2,6-diisopropylphenyl substituents (2, Scheme 2). The exocyclic carbonyl functionality was anticipated to provide an electronically delocalized conduit extending from the potentially cationic metal center to a site of Lewis acid interaction through the oxygen lone pairs. The previously reported b-diimine complex Scheme 1. Examples of cationic and zwitterionic polymerization/oligomerization active sites. R = alkyl.

Linear low-density polyethylene synthesis promoted by homogeneous and supported catalysts

Linear low-density polyethylenes (LLDPEs) were obtained through the copolymerization of ethylene with 1-hexene using (nBuCp) 2 ZrCl 2 co-catalyzed by methylaluminoxane (MAO). For comparative reasons, the same metallocene was supported on silica (0.85 wt%Zr/SiO 2 ) by grafting. The copolymerizations were performed in toluene, at 1.6 bar of ethylene, 60°C and in an Al/Zr molar ratio of 2500. The 1-hexene concentration varied from 0 to 0.50 mol L -1 . The resulting copolymers were characterized by GPC, DSC and 13 C NMR. The catalytic activities remained close to 2 × 10 7 and 7 × 10 6 (g pol) mol Zr -1 bar -1 h -1 for the homogeneous and supported systems, respectively. For the homogeneous system, the catalytic activity and the comonomer incorporation increased with the 1-hexene concentration up to 0.30 mol l -1 . Higher comonomer concentrations led to a decrease in catalytic activity in the case of the homogeneous system. The highest comonomer incorporation (6.3mol%) was achieved with 0.50 mol l -1 of 1-hexene in the reaction medium. The supported system afforded lower comonomer incorporations (maximum 4.0 mol%). The effect of the 1-hexene incorporation can also be evaluated through the polymer properties, namely, crystallinity, melting temperature, molecular weight and polydispersity.

Synthesis and characterization of copolymers of ethylene and 1-octadecene using the rac-Et(Ind)2ZrCl2/MAO catalyst system

The copolymerization of ethylene and 1-octadecene using a bridged metallocene was studied in order to observe the effect of the comonomer on the catalytic activity. A noticeable increase in activity is seen as the concentration of 1-octadecene in the reaction medium increases. 13 C NMR analysis shows 6.4 mol-% incorporation of comonomer at the highest 1-octadecene concentration in the feed used here. The molecular weight of the copolymers shows a drastic decrease that may be attributed to chain termination by transfer or β-elimination of the comonomer. As to the molecular weight distribution, it remains within a narrow range, as expected with metallocene catalysts. The melting temperature and the enthalpy of melting of the copolymers show a decrease with increasing comonomer content. As usual for ethylene copolymers, the X-ray crystallinities are higher than those determined from the enthalpy of melting.

Measurement and mathematical modeling of molecular weight and chemical composition distributions of ethylene/α‐olefin copolymers synthesized with a heterogeneous Ziegler‐Natta catalyst

The distributions of molecular weight and chemical composition of two linear low-density poly-ethylene resins synthesized with Ziegler-Natta catalysts were analyzed with several microstructural characterization techniques and the results were used to investigate the nature of the active site types present on the catalyst. Deconvolution of the molecular weight distribution measured by gel permeation chromatography and of the chemical composition distribution measured by either crystallization analysis fractionation or temperature rising elution fractionation or temperature rising elution fractionation leads to consistent results that indicate the presence of multiple active site types on the catalyst. This shows that these fractionation techniques, when combined with adequate mathematical models, are powerful tools to understand the behavior of Ziegler-Natta catalysts and structural properties of the produced resins.

Synthesis and characterization of ethylene-1-hexene copolymers using homogeneous Ziegler-Natta catalysts

SummaryThis study investigated the copolymerization of ethylene with 1-hexene using the homogeneous Et[Ind]2ZrCl2 and [Ind]2ZrCl2 catalysts. The Et[Ind]2ZrCl2 catalyst gave a higher catalytic activity than the [Ind]2ZrCl2 and also showed a better incorporation of 1-hexene for the same comonomer concentration in the feed. Thermal analysis (DSC) and viscosity measurements showed that an increase of the 1-hexene incorporated in the copolymer results in a decrease of the melting point, crystallinity and molecular weight of the polymer formed. The reactivity ratios for ethylene and 1-hexene confirmed the more successful incorporation of the comonomer for the polymerization catalyzed by Et[Ind]2ZrCl2.

Effects of Al/Zr ratio on ethylene-propylene copolymerization with supported-zirconocene catalysts

Abstract Effects of heterogenization parameters on compositional and catalytic properties of Et(Ind) 2 ZrCl 2 supported on silica modified with MAO were evaluated using data on metal loading, catalyst activity in ethylene–propylene copolymerization, and polymer properties. The supported catalysts were prepared according to a 2 3 factorial design for multivariate analysis. The parameters studied were: MAO concentration in impregnation, Et(Ind) 2 ZrCl 2 concentration in grafting, and immobilization temperature for both metallocene and organoaluminum. The grafting solution was monitored by UV–VIS spectroscopy, while the resulting catalyst systems were characterized by inductively-coupled plasma-optical emission spectroscopy (ICP-OES), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and X-ray photoelectron spectroscopy (XPS). At high statistical significance, both MAO and Et(Ind) 2 ZrCl 2 concentrations during preparation affected the determination of the final Al/Zr ratio on silica. The catalyst systems were tested in ethylene–propylene copolymerizations using external MAO as cocatalyst. Relationships were observed between (i) catalyst activity and the binding energy of Zr 3d 5/2 electrons and (ii) Al/Zr ratio on silica and the average ethylene incorporation. The polymer samples presented narrow molecular weight distribution, but according to differential scanning calorimetry (DSC), and cross-fractionation chromatography (CFC) data, catalyst systems with given Al/Zr ratios might yield different crystallites, suggesting a plural distribution of chemical composition.

Structure characterization of copolymers of ethylene and 1-octadecene

Four ethylene-1-octadecene copolymers and the corresponding polyethylene homopolymer, synthesized with a metallocene catalyst, have been analyzed by using three characterization techniques in the solid state: differential scanning calorimetry, wide-angle X-ray diffraction, and Raman spectroscopy. Very important annealing effects are observed in the copolymers with higher comonomer content while standing at room temperature, in such a way that the enthalpies of melting derived from the first and second melting are different. The X-ray diffractograms have been analyzed in terms of amorphous and crystalline components, determining both the crystallinity and the position of the different reflections. The variation of the unit cell parameters has been calculated from those reflections. No indication of a possible participation of the relatively long 1-octadecene branches in the crystallization can be deduced from the X-ray data. The degree of crystallinity has also been determined from the Raman spectra, following two procedures. The results indicate that the crystallinities deduced from the band at 1416 cm−1 are much lower than those derived from the other two characterization techniques. On the contrary, the data from the 1060 cm−1 band are practically coincident with the X-ray determinations.

Influence of the type and the comonomer contents on the mechanical behavior of ethylene/α-olefin copolymers

The influences of the type and concentration of α-olefin (1-hexene, 1-octene, 1-decene, 1-octadecene, 4-methyl-1-pentene) on the mechanical behavior and crystallinity degree of some ethylene/α-olefin copolymers obtained by metallocene catalysts were studied by means of stress/strain experiments. The crystallinity degree of these copolymers has been determined by X-ray measurements. It has been observed that the copolymers show less resistance to strain as the comonomer content increases and the crystallinity decreases. Most of the studied copolymers exhibit a significant increase in the crystallinity level after the stress/strain experiments.

Living polymerization of ethylene and alpha-olefins using a nickel alpha-keto-beta-diimine initiator.

A nickel alpha-keto-beta-diimine initiator can be activated using treated methylaluminoxane (MAO) to generate active sites capable of the polymerization of ethylene, propene and 1-hexene under living conditions.