Synthesis of polyethylene with bimodal molecular weight distribution by supported iron‐based catalyst

Through immobilization of two iron‐based complexes, [((2,6‐MePh)N = C(Me))₂C₅H₃N]FeCl₂ ( 1 ) and [((2,6‐iPrPh)N = C(Me))₂C₅H₃N]FeCl₂ ( 2 ), on SiO₂ pretreated with tetraethylaluminoxane (TEAO), two supported iron‐based catalysts, 1 /TEAO/SiO₂ ( 3 ) and 2 /TEAO/SiO₂ ( 4 ), were prepared. These two supported catalysts 3 and 4 could be used to catalyze ethylene polymerization with moderate polymerization activity and prepare linear high‐density polyethylene with bimodal molecular weight distribution (MWD). It was demonstrated that immobilization of catalyst could significantly improve molecular weight (MW) of high‐MW fraction of the resultant polyethylene, as well as maintain bimodal MWD of polyethylene produced by the corresponding homogeneous catalysts. Such bimodal MWD of polyethylene produced by supported iron‐based catalysts could be well tailored by varying polymerization conditions, such as ethylene pressure and molar ratio of Al to Fe. It has been proven that TEAO is an efficient activator for both homogeneous and heterogeneous iron‐based catalysts for producing polyethylene with bimodal MWD. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5662–5669, 2004     

DC electrorheological response of polyethylene/organically modified layered silicate nanocomposites

The present work reports the electrorheological (ER) response of high‐density polyethylene (HDPE)/organically modified silicate layers nanocomposites based on four commercially available HDPE matrices. Two single‐site catalyzed bimodal resins, one single‐site catalyzed unimodal resin and one Ziegler–Natta catalyzed unimodal resin are studied. It is revealed that the distinct separation of the two modes of the bimodal HDPE resins significantly enhances the ER response. It is proposed that the slower structural relaxation modes introduced by higher molecular weight species in the bimodal HDPE matrices enhance the ER response of the nanocomposites. This is ascribed to the longer induction time for leaking current density, which is an indicator of mobility and release of immobilized cationic surfactants at the silicate layers surface induced by field exposure. It is found that the screening effect of prematurely released cationic surfactants leads to a weaker ER response in nanocomposites whose matrices have faster relaxation modes. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1298–1309     

Molecular weight distribution of emulsion-polymerized polyethylene

Emulsion polymerizations of ethylene were run using potassium persulfate as the initiator and either an anionic or a cationic surfactant, usually at five times the critical micellar concentration. The molecular weight distributions had at least two major peaks. The broadly distributed lower molecular weight component is formed in the early stages of polymerization. The higher molecular weight component has a narrow distribution characteristic of termination by the combination of chain radicals. The weight-average molecular weight of this component was about 400,000 when the surfactant was sodium laurate and 33,600,000 when the surfactant was dodecyl amine hydrochloride. © 1992 John Wiley & Sons, Inc.     

Preparation of bimodal HDPEs with metallocene on Cr‐montmorillonite support

Previously described Cr‐montmorillonite (Cr‐MMT) was found to retain reactivity in the ethylene polymerization reaction regardless of which alkyl‐metal was used for workup in the preparation process, as long as alkylaluminium was used as a cocatalyst in the actual polymerization reaction. Introduction of hydrogen pressure was found to regulate the polymerization to give the product polymer with a narrower weight distribution, albeit with a somewhat smaller average molecular weight. Supporting metallocene onto Cr‐MMT produced a binuclear catalyst system which gave rise to bimodal polyethylene (PE). Polymer composition of the produced high density polyethylenes (HDPEs) could be controlled by changing factors such as the polymerization conditions and the identity of the metallocene compounds. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3722–3728, 2010     

Copolymerization of ethylene and α‐olefins with combined metallocene catalysts. I. A formal criterion for molecular weight bimodality

Metallocene and other single‐site catalysts can be combined to produce polyolefins with broadened distributions of molecular weight, chemical composition, and long‐chain branching. These resins are finding increasing applications because of their enhanced properties compared to ones made with conventional Ziegler–Natta catalysts. Resins with bimodal molecular weight distributions (MWDs) have especially attractive mechanical and rheological properties. Although the use of these resins is expected to increase, there are very few studies available to quantify MWD bimodality or to decide a priori which combinations of metallocene catalysts will lead to the formation of polyolefins with bimodal MWDs. In this article, a necessary condition for the production of polymer with bimodal MWD using two single‐site‐type catalysts is derived. Additionally, a bimodality index is defined to quantify MWD bimodality. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1408–1416, 2000     

Branched polyethylene prepared by in situ copolymerization of ethylene with monotitanocene and modified methylaluminoxane catalyst

Branched polyethylene was synthesized in heptane used as a polymerization medium with monotitanocene catalyst composed of η⁵‐pentamethylcyclopentadienyl tribenzyloxy titanium and modified methylaluminoxane (mMAO) that contained different amounts of residual trimethylaluminum (TMA). The residual TMA more strongly reduced Ti(IV) complexes to Ti(III) and Ti(II) ones, and Ti(IV) active species were suggested to be more effective for ethylene polymerization. Influences of the polymerization temperature and Al/Ti molar ratio on the catalytic activity and the degree of branching, branch length, and molecular weight of polyethylene were investigated. The obtained polymers were confirmed by ¹³C NMR to be higher molecular weight polyethylene containing significant amounts of isolated ethyl branches, butyl branches, or both. Branched polyethylene was prepared by the in situ copolymerization of ethylene with 1‐butene and 1‐hexene, which were formed through a proposed mechanism including metallcycloheptane and metallcyclopentane intermediates of Ti(II) species that were produced by the reaction of Ti(IV) complexes with TMA coexisting in mMAO. There was a remarkable increase in the chance of 1‐butene being produced from metallcyclopentane of Ti(II) intermediates with an increase in the polymerization temperature. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4258–4263, 2000     

Controlling molecular weight distributions of polyethylene by combining soluble metallocene/MAO catalysts

A critical look at the possibility of controlling the molecular weight distribution (MWD) of polyolefins by combining metallocene/methylalumoxane (MAO) catalysts is offered. Catalysts investigated were bis(cyclopentadienyl)zirconium dichloride (Cp₂ZrCl₂), its titanium and hafnium analogues (Cp₂TiCl₂ and Cp₂HfCl₂), as well as rac-ethylenebis(indenyl)zirconium dichloride (Et(Ind)₂ZrCl₂). As observed by other researchers, the MWD of polyethylene can be manipulated by combining soluble catalysts, which on their own produce polymer with narrow MWD but with different average molecular weights. Combined in slurry polymerization reactors, the catalysts in consideration produce ethylene homopolymer just as they would independently. Unimodal or bimodal MWDs can be obtained. This effect can be mimicked by blending polymers produced by the individual catalysts. We demonstrate how a variability in catalyst activity translates into a variability in MWD when mixing soluble catalysts in polymerization. Such a variability in MWD must be considered when setting goals for MWD control. We introduce a more quantitative approach to controlling the MWD using this method. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 831–840, 1998     

Morphology and physical properties of polyethylene/silicate nanocomposite prepared by melt intercalation

Maleated polyethylene (PEMA)/silicate nanocomposites with a different aspect ratio of silicate and maleated PEMA/SiO₂ composite were prepared by melt intercalation. The nanocomposites with a high aspect ratio silicate (montmorillonite) showed a faster decrease in the terminal slope of the storage modulus and a steeper increase in complex viscosity than those with a low aspect ratio silicate (laponite) and SiO₂. The addition of montmorillonite increases the crystallization and the melting temperature of PEMA but decreases above 3 vol % of the silicate content because of the increased viscosity. The nanocomposite with montmorillonite showed the highest yield strength and secant modulus among the composites because of the highest aspect ratio of the filler. It also revealed strong interfacial adhesion with the matrix and orientation during tensile deformation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1454–1463, 2002     

Synthesis of tailor‐made polyethylene through the control of polymerization conditions using selectively combined metallocene catalysts in a supported system

Tailoring of the molecular weight distribution (MWD) in ethylene polymerization was attempted by selectively combining different types of metallocene catalysts onto a single support. The catalyst produced by supporting Et[Ind]₂ZrCl₂ and Cp₂HfCl₂ onto a single MAO pretreated silica support was able to produce polymers with unimodal or bimodal MWD's. This approach permits the synthesis of polyethylene with different MWD's using the same catalyst as a function of the polymerization conditions. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 331–339, 1999     

Aminosilylene-bridged ansa-zirconocenes for branched polyethylenes with bimodal molecular weight distributions

Reactions of the dilithium salt of aminosilylene-bridged ligands with (Me₂N)₂ZrCl₂(THF)₂ followed by the treatment of Me₃SiCl are found to be an efficient synthetic route to aminosilylene-bridged ansa-zirconocenes, R₂N(Me)Si(η⁵-C₅H₄)₂ZrCl₂ (R = Me (1), Et (2)) and Me₂N(Me)Si(η⁵-C₅H₄)(η⁵-C₅Me₄)ZrCl₂ (3). Crystal structure of 3 determined by X-ray diffraction study reveals the presence of π-bonding interaction between N and Si atoms, which is further supported by DFT calculation results. These complexes are very active (>1 × 10³ Kg/(mol Cat.·atm·h)) for homopolymerization of ethylene in the presence of methylalumoxane (MAO) cocatalyst, generating polyethylenes that contain branches as well as bimodal molecular weight distribution (MWD). Methyl, ethyl, butyl, and other longer branches (n ≥ 6) are observed in the resulting polyethylenes. The polyethylenes from 1, 2 and 3/MAO show a broad MWD range (6.3-42.2, 3.5-4.0 and 2.6-3.4, respectively).     

Stepwise fatigue crack propagation in polyethylene resins of different molecular structure

Stepwise fatigue crack propagation in a range of polyethylene resins, some of which are candidates for use in pipes for natural gas distribution, was studied. Examination of the effect of molding conditions on fatigue crack propagation in a pipe resin indicated that fast cooling under pressure produced specimens with the same crack resistance as specimens taken from a pipe extruded from this resin. The mechanism of stepwise crack propagation in fatigue was the same as reported previously for creep loading. Observations of the region ahead of the arrested crack revealed a complex damage zone that consisted of a thick membrane at the crack tip followed by a main craze with subsidiary shear crazes that emerged from the crack tip at an angle to the main craze. The effects of molecular parameters, such as molecular weight, comonomer content, and branch distribution, on the kinetics of fatigue crack propagation were examined. Correlation of creep and fatigue crack resistance made it possible to relate fatigue fracture toughness to molecular parameters by invoking concepts of craze fibril stability developed for creep. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2355–2369, 1998     

Polymerization Kinetics and Microstructure of Ethylene/1‐Hexene Copolymers Made with Dual Metallocenes

Mathematical models are developed to describe the polymerization of ethylene and 1‐hexene with a constrained geometry catalyst (CGC‐Ti) and with bis(cyclopentadienyl)‐zirconium (IV) dichloride (Cp₂ZrCl₂). Particle swarm optimization is used to fit these models to homo‐ and copolymerization data. The models are also used to describe copolymerizations with mixtures of CGC‐Ti and Cp₂ZrCl₂ to make copolymers with inverse short chain branching distribution. Copolymer molecular weight and short chain branch distributions, as well as polymerization rates with the dual metallocene system, are measured to test whether they agreed with model predictions. The results show that the two metallocenes do not interact strongly when used as a mixture to make ethylene/1‐hexene copolymers.     

Chain dynamics in partially cross-linked polyethylene by combined rheology and NMR-based molecular rheology

In recent years, ¹H double-quantum NMR (DQ NMR) was established as a suitable molecular-rheology technique to elucidate chain dynamics and to determine entanglement or crosslink densities in linear entangled polymer melts and permanent as well as transient networks. In this work, industrial grade high-density polyethylene, partially cross-linked via electron beam irradiation in the semicrystalline state, is probed in the melt state by low-field DQ NMR and shear rheology. The DQ NMR data is analyzed by two approaches, one assuming the presence of a permanent network and the other considering the potentially complex relaxation spectrum of the studied inhomogeneous systems. A correlation between the DQ NMR results and extent of cross-linking is found. By direct comparison of the rheological results and the NMR-based segmental orientation autocorrelation functions (OACF) via time–temperature superposition (TTS), qualitative consistency between the microscopic and macroscopic observables is established. In this way, the frequency range of shear rheology can be extended by about two decades into the 10 krad/s range. The NMR method is thus a valuable extension of the toolbox of characterization techniques, where gel content measurements by solvent extraction proved to be the least sensitive.     

Effects of different ultrahigh molecular weight polyethylene contents on the formation and evolution of hierarchical crystal structure of high-density polyethylene/ultrahigh molecular weight polyethylene blend fibers

In this paper, the blend fibers of ultrahigh molecular weight polyethylene (UHMWPE) and high-density polyethylene (HDPE) were prepared by solution blending and gel spinning process. The uniformity of the blend fibers has been confirmed by rheological data and thermodynamic unimodal curve. They were further characterized by single fiber strength test, scanning electron microscopy, wide-angle X-ray diffraction, small-angle X-ray scattering, and so forth, to explore the structural evolution mechanism with the change of UHMWPE content. The results showed that when the molar content of UHMWPE was only 2.9 mol%, entanglement appeared in the structure of shish-kebab, and when the proportion reached 20 mol%, an interlocking structure could be observed. With the increase of UHMWPE content, kebab began to be networked, and when the content reached 33 mol%, kebab's orientation reached its peak. After that, the interlocking network structure gradually improved. When the content reached 50 mol%, the shish's orientation reached saturation, and the shish-kebab network became perfect. In addition, with the increase of UHMWPE content, stress-induced recrystallization occurred on the wafer, some kebab would be converted into shish crystals, and when the content exceeded 50 mol%, the microfibers began to merge, and the wafer became denser, but still had entanglements. Our work has proposed a quantitative explanation for the evolution of hierarchical crystal structure of HDPE/UHMWPE blend fibers.     

One pot synthesis of bimodal UHMWPE/HDPE in‐reactor blends with Cr/V bimetallic catalysts

Bimodal ultra‐high‐molecular‐weight polyethylene (UHMWPE)/high‐density polyethylene (HDPE) in‐reactor blends (IRBs) are produced by the bimetallic catalysts, which are synthesized through co‐supporting silylchromate and vanadium‐oxide‐based catalysts on silica or alumina, zirconia and titania‐modified silica. After support modification, the activities of the catalysts for ethylene polymerization are substantially enhanced. The IRBs produced by the modified catalysts also contain more UHMWPE and low‐molecular‐weight polyethylene (LMWPE) fractions, and have much broader molecular weight distribution (MWD). The homogeneous nature of the IRBs is preliminarily confirmed by the differential scanning calorimetry melting curves, showing unique melting peak in both nascent and recrystallized states. The rheological results reflect that the viscosity of the IRBs is reduced more or less when compared with UHMWPE. The distinct elastic dominance of the IRBs is also observed, implying the UHMWPE characteristics in the IRBs. In addition, the intimate mixing of the IRBs is further verified by the similar slopes of Han curves for the polyethylene (PE) samples. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3404–3412     

Annealing behavior of ultrahigh molecular weight polyethylene investigated by confocal micro-Raman spectroscopy combined with two-dimensional correlation spectroscopy

Annealing was performed for ultrahigh molecular weight polyethylene (UHMWPE), including an isothermal process at 110.0°C and cooling process from 110.0 to 30.0°C. The processes were in situ investigated by confocal micro-Raman spectroscopy combined with two-dimensional correlation spectroscopy. Two phase transitions were directly observed in the annealing processes, i.e., from the amorphous phase to the intermediate phase and from the intermediate phase to the crystalline phase. The phase transitions derive from molecular chain segments sliding between different phases of UHMWPE and occur in different orders during the isothermal and cooling processes.     

Melting of nascent and thermally treated super-high molecular weight polyethylene

The melting of nascent and thermally treated super-high molecular weight polyethylene (SHMWPE) is investigated by means of differential scanning calorimetry (DSC). Higher melting temperatures and enthalpies of the nascent and annealed samples are observed. The melting temperatures and enthalpies of melt crystallized SHMWPE is lower and depends on the temperature of thermal treatment. All melting properties are explained by presuming that the lamellar structure contains a high concentration of entangled tie macromolecules in amorphous regions formed during polymerization. It was supposed that the concentration of the entanglements and the stressed tie molecules are changed with the temperature of the thermal pretreatment.     

Non-isothermal crystallization of polyethylene blends with bimodal molecular weight distribution

Non-isothermal crystallization of polyethylene (PE) blends with bimodal molecular weight distribution (MWD) was investigated by differential scanning calorimetry (DSC) at various scanning rates. The bimodal PE blends were prepared by blending two unimodal polyethylenes with large difference in molecular weigh in different ratio in xylene solution. Different kinetic parameters such as the half-time of crystallization (t_1/2), crystallization rate constant (Z_c), F(T) and the effective activation energy were determined. Some complicated relationships between these parameters and the average molecular weight were found. The crystallization rate first increased and reached a maximum then decreased, and also correlated with the MWD. The Avrami index under non-isothermal conditions was analyzed with a method developed by Harnisch and Muschik; the results indicated that homogeneous nucleation and spherulitic growth regimes were present in all samples studied.     

Simulation of gel permeation chromatography measurement for long chain branched metallocene polyethylene

The research about the polymerization reaction mechanism of long chain branched polymer provides a method to simulate the generation of LCB mPE (long chain branched metallocene polyethylene).^[1-3] In this work, after simulating the generation of one million LCB mPE molecules, we calculate the sizes (i.e. radii of gyration) of molecules in good solvents to obtain the molecular size distributions. Then we simulate the fractionation in GPC (gel permeation chromatography) measurement and the different GPC detector responses to obtain simulated GPC MWDs (molecular weight distributions). The simulated MWDs are compared to the real GPC results provided by the Dow Chemical Company.     

The critical molecular weight for resisting slow crack growth in a polyethylene

An ethylene-hexene copolymer was fractionated into five fractions and the density of short-chain branches was measured for each fraction. The slow crack growth behavior was measured on each fraction by sandwiching the small amount of fractionated resin of about 0.2 g between polyethylene grips. The resistance to slow crack growth was negligible for the three fractions whose M_w was less than 1.5 × 10⁵. For the fourth fraction with M_w greater than 1.5 × 10⁵, the resistance to slow crack growth was very high, being greater than that for the whole resin even though its density of short-chain branches was less than that of the whole resin. It is concluded that a molecular weight greater than 1.5 × 10⁵ is required to create the number of tie molecules that is necessary to produce a high resistance to slow crack growth in this particular copolymer. © 1996 John Wiley & Sons, Inc.