Adsorption of polyethylene standards from decalin on liquid chromatography column packings

Linear polyethylene and isotactic polypropylene standards were injected into columns which contained MFI (SH-300 and silicalite) or faujasite (CBV-780) type zeolites. 1,2,4-Trichlorobenzene, cyclohexanone, 2-ethyl-hexanol, decalin and tetralin were used as mobile phases at 140 degrees C. It was found that polyethylene is fully retained on zeolite SH-300 when decalin is used as a mobile phase. Moreover, polyethylene is partially retained on zeolite SH-300 from tetralin and from 1,2,4-tichlorobenzene, on silicalite from decalin and in a very small extent on zeolite CBV-780 from decalin. Using all other solvents, polyethylene and polypropylene were not retained in any of the columns tested. This is the first experimental observation of polyethylene adsorption from a solvent on a chromatographic stationary phase.     

Elution behavior of polyethylene in polar mobile phases on a non-polar sorbent

Linear polyethylene standards in the range of 1-500 kg/mol, dissolved in 1,2,4-trichlorobenzene, were injected into a column packed with oligo(dimethylsiloxane) modified silica gel. Fifteen polar solvents (cyclohexanone, cyclohexylacetate, cyclohexanol, nonylalcohol, dimethylformamide, dimethyl sulfoxide, ethylene- and diethylene glycol monobutyl ether, benzylalcohol, hexylacetate, bis(2-ethyl-hexyl)phthalate, N,N-dimethylacetamide, propylene carbonate, dipropylene glycol and N-methyl-pyrrolidone) were evaluated as mobile phases. Depending on the type of mobile phase evaluated, different elution behaviors are observed for polyethylene: (1) polyethylene was eluted in the size exclusion mode, (2) polyethylene was eluted together with the sample solvent peak at constant elution volume, (3) polyethylene was partially or fully retained on the column. The retained polymer was easily removed from the column by injecting a small volume of trichlorobenzene. The use of ethylene glycol monobutyl ether as the mobile phase enabled separation of the polyethylene from polypropylene. In this case polypropylene is eluted in the size exclusion mode, while polyethylene is eluted at a constant elution volume or remains in the column.     

Elution behavior of polyethylene and polypropylene standards on carbon sorbents

The elution behavior of linear polyethylene and isotactic, atactic and syndiotactic polypropylene was tested using three different carbon column packings: porous graphite (Hypercarb), porous zirconium oxide covered with carbon (ZirChrom-CARB), and activated carbon TA 95. Several polar solvents with boiling points above 150°C were selected as mobile phases: 2-ethyl-1-hexanol, n-decanol, cyclohexylacetate, hexylacetate, cyclohexanone, ethylene glycol monobutyl ether and one non-polar solvent, n-decane. Polyethylene standards were completely or partially adsorbed in all tested sorbent/solvent systems. Polypropylene standards were partially adsorbed on Hypercarb and carbon TA95, but did not adsorb on ZirChrom-CARB. ZirChrom-CARB retained polyethylene pronouncedly when 2-ethyl-1-hexanol, cyclohexylacetate or hexylacetate were used as mobile phases at temperature 150 or 160°C, while all three basic stereoisomers of polypropylene eluted in size exclusion mode in these sorbent/solvent pairs. This is very different from the system Hypercarb/1-decanol, which separated polypropylene according to its tacticity. The opposite elution behavior of polyethylene and polypropylene in system ZirChrom-CARB/2-ethyl-1-hexanol (polypropylene eluted, polyethylene fully adsorbed) enabled to realize separation of blends of polyethylene and polypropylene. Ethylene/1-hexene copolymers were separated according to their chemical composition using system Hypercarb/2-ethyl-1-hexanol/1,2,4-trichlorobenzene.     

Improved chemical composition separation of ethylene–propylene random copolymers by high-temperature solvent gradient interaction chromatography

High-temperature solvent gradient interaction chromatography (HT-SGIC) is a fast and efficient fractionation technique for the chemical composition analysis of olefin copolymers. The separation of ethylene–propylene random copolymers (EPRs) was achieved on a graphitic stationary phase, Hypercarb, at 160 °C by using linear solvent gradient elution from 1-decanol to 1,2,4-trichlorobenzene (TCB). In the present work, the solvent gradient profile was modified to improve the chromatographic separation of EPRs. With the aim to obtain a better resolution in separation, a slow increase in the volume fraction of TCB was applied. This allowed for a relatively large retention region for linear polyethylene (PE) chains on the column; thereby, a broader elution volume zone between the start of the gradient and the PE elution was achieved. The efficiency of this new gradient profile was demonstrated by analysing two fully amorphous EPR samples. Clear differences in the chemical composition of these EPR samples with similar ethylene contents have been proven by using this modified solvent gradient. The comprehensive chemical composition and microstructure analysis of the SGIC-separated fractions by FTIR revealed that ethylene/propylene (EP) copolymer chains were eluted according to their ethylene/propylene contents and E or P sequence lengths, even though they are distributed in a random manner. These results showed that the solvent composition is an important factor to affect the interactive adsorption or desorption behaviour of EP chains on Hypercarb. In this way, for the first time, the determination of the complex composition and chain structure of EPR samples was achieved within short analysis time, which is not possible till now using other fractionation techniques reported.

HPLC Study of the Kinetics of Intramolecular Cyclization of Two New Phenyldiazene Molecules. Analytical Properties of the Resulting Liquid Crystals

A group of zeolites and a 3D nanoporous metal-organic material RPM-1 were tested as column packings for adsorption of isotactic polypropylene and linear polyethylene from dilute solutions. It was found that polyethylene is fully or partially retained from thermodynamically good solvents (1,1,2,2-tetrachloroethylene, 1,4-dimethylbenzene, diphenylether, 1,2-dichlorobenzene and 1,3-dichlorobenzene) at temperatures of 115 °C or 140 °C, when a specific type of zeolite with pore sizes 5–6 Å has been used as the column packing. Polypropylene was fully retained in another type of zeolite with pores of 7–12 Å, when diphenylether was used as the mobile phase. As far as known, this is the first system sorbent - mobile phase, where adsorption of polypropylene was observed.     

Adsorption of polyethylene from thermodynamically good solvents on a zeolite stationary phase

Adsorption of linear polyethylene and isotactic polypropylene on columns packed with zeolites ZSM‐5, Y, and silicalite was studied using high‐temperature liquid phase chromatography. Linear polyethylene was fully retained on a column packed with ZSM‐5 zeolite from non‐polar solvents, such as 1,1,2,2‐tetrachloroethane and 1,3,5‐trimethylbenzene at a temperature of 140°C. Partial adsorption on ZSM‐5 zeolite was found for polyethylene in 1,2,4‐trichlorobenzene and on silicalite from 1,2,4‐trichlorobenzene and 1,3,5‐trimethylbenzene. On the other hand, adsorption of polyethylene was not found from polar liquids, such as 2‐ethylhexyl acetate, cyclohexyl acetate, and cyclohexanone. Isotactic polypropylene was not adsorbed on any tested sorbent.     

Oxidation of polyethylene,polypropylene, and copolymers

This investigation of the autoxidation of ethylene–propylene copolymers and polyethylene–polypropylene mixtures was undertaken to determine whether reactivity is a linear function of composition. The copolymers and the mixtures were autoxidized in a trichlorobenzene solution at 100°C in the presence of 1,1′-azodicyclohexanecarbonitrile, and the rates of oxygen absorption were determined. The reactivity of the copolymers and the mixtures, after the underlying absorption of oxygen by initiator radicals is accounted for, is a nearly linear function of composition; however, the polymer mixtures and copolymers oxidized somewhat less readily than predicted by a straight line relationship. Several additional oxidations were performed on solutions of polypropylene so that the effects of initiation rate and substrate concentration could be evaluated. The oxidation kinetics of polypropylene even in dilute solution, are complex; titratable hydroperoxide yields are low. Further work will be required to specify the mechanism of oxidation.     

Adsorption of polypropylene from dilute solutions on a zeolite column packing

Faujasite type zeolite CBV-780 was tested as adsorbent for isotactic polypropylene by liquid chromatography. When cyclohexane, cyclohexanol, n-decanol, n-dodecanol, diphenylmethane, or methylcyclohexane was used as mobile phase, polypropylene was fully or partially retained within the column packing. This is the first series of sorbent-solvent systems to show a pronounced retention of isotactic polypropylene. According to the hydrodynamic volumes of polypropylene in solution, macromolecules of polypropylene should be fully excluded from the pore volume of the sorbent. Sizes of polypropylene macromolecules in linear conformations, however, correlate with the pore size of the column packing used. It is presumed that the polypropylene chains partially penetrate into the pores and are retained due to the high adsorption potential in the narrow pores.     

Aging and degradation of polyolefins. III. Polyethylene and ethylene–propylene copolymers

Rates of oxygen absorption and formation of oxidation products were determined in γ-initiated oxidations of thin films of high- and low-density polyethylene, atactic and isotactic polypropylene, and of three ethylene–propylene copolymers. Radiation yields G for O₂ absorbed and formation of hydroperoxides depend on dose rates and decrease sharply with increasing ethylene content of the copolymers and moderately with increasing crystallinity of any base polymer. G values for dialkyl peroxide and carbonyl formation, and therefore for chain initiation and termination, do not change much with polymer composition and crystallinity and not at all with dose rates. A few experiments with atactic polypropylene and an amorphous ethylene–propylene copolymer, initiated by di-tert-butylperoxy oxalate, indicate that 37 mole-% of ethylene in the polymer increases the efficiency of initiation and the tendency toward crosslinking.     

New materials from new catalysts

The polyolefins, especially polypropylene and polyethylene, industry of today is very different from that of 10 years ago. The development of highly active and stereospecific catalysts, represented by Ti/Mg supported catalysts, have made the gas-phase polymerization process practical. The trend in catalyst development is shifting from an emphasis on improving the stereospecificity and activity toward improving the polymer physical properties, processability and morphology. Many hybrid thermoplastic olefins, such as high-impact copolymers, propylene–ethylene–butene terpolymers, and very low density polyethylene, have already been developed by utilizing the features of the gas-phase polymerization process. These hybrid thermoplastic olefins cover a very broad range of products. They cannot be clearly identified as polyethylenes, polypropylenes or elastomers. Incidentally, metallocene catalysts for polyolefins have been under development for the past 15 years, and are now in the early stage of commercialization. These catalysts differ significantly from the conventional heterogeneous catalysts. They can polymerize not only ethylene, propylene and other linear α-olefins, but also styrene, cycloolefins and functional monomers In addition, they can control the microstructure of polymer molecules by varying the transition metals and the cyclopentadienyl ligands. Because of these features, we have to be confident that the development of metallocene catalysts, or more widely homogeneous catalysts, may be a dominant force throughout the 1990s in the polyolefin industries.     

Reactions of polyolefins with strong lewis acids

Treatment of a cyclohexane solution of low density polyethylene and polystyrene with anhydrous aluminum chloride causes chemical reaction between the two polymers which results in the formation of a graft copolymer. The initial copolymer-forming reaction is very rapid, and prolonged contact of the polymers with aluminum chloride causes subsequent degradation in molecular weight. Treatment of separate solutions of polyethylene, isotactic polypropylene, and ethylene–propylene copolymers with aluminum chloride was studied as a function of time. The intrinsic viscosities of the polymers dropped from initial values of 2.4–6.5 to 0.55–0.85 in 5 min, followed by a slower decline over the next 2 hr. In the case of polypropylene, the low molecular weight fragments largely retained the isotactic structure, which demonstrates that stereochemical isomerization is not a major reaction.     

Liquid chromatography at critical conditions in ternary mobile phases: Gradient elution along the critical line

In ternary mobile phases consisting of acetone, methanol, and water, the retention of PEG on reversed‐phase columns is independent on molar mass at certain compositions of the mobile phase. Along this critical adsorption line, the retention of polypropylene glycol varies quite strongly, which can be utilized in the separation of block copolymers. Gradient elution along the critical line allows a baseline separation of all oligomers in polypropylene glycol up to approximately 25 propylene oxide units. The same resolution can be achieved in the separation of ethylene oxide‐propylene oxide block copolymers, regardless of the length of the ethylene oxide block.     

Unprecedented living olefin polymerization derived from an attractive interaction between a ligand and a growing polymer chain

Ti complexes incorporating fluorine-containing phenoxy-imine chelate ligands (fluorinated Ti-FI catalysts) have been demonstrated to induce an unprecedented living polymerization effect with both ethylene and propylene, through an attractive interaction between the fluorine atom in the ligand and a beta-hydrogen atom on the growing polymer chain. With the aid of this attractive interaction, highly controlled living ethylene polymerization, highly-syndiospecific living propylene polymerization, the synthesis of unique block copolymers from ethylene and propylene, and the catalytic production of monodisperse polyethylene and Zn-terminated polyethylene have been realized. The attractive interaction provides a conceptually new strategy for the achievement of controlled living olefin polymerization.     

Free-volume estimates in heterogeneous polymer systems. I. Diffusion in crystalline ethylene–propylene copolymers

Apparent thermodynamic diffusion coefficients were obtained from carbon tetrachloride, benzene, and n-hexane sorption-desorption kinetics in crystalline and amorphous ethylene-propylene copolymers (with propylene content from 1 to 70 wt. %, and crystallinity from 0% to 77%), in high-density and low-density polyethylene, and in polypropylene. The dependence of the diffusion coefficient vs. concentration curves on crystallinity and propylene content in the copolymers is reported. The diffusion coefficient at zero penetrant concentration increases with decrease in crystallinity. The apparent diffusion activation energies in the temperature interval investigated (25 to 75°C) are independent of temperature and are constant for crystalline copolymers. A modified Fujita-like free-volume theory for diffusion in crystalline polymer systems is introduced, and the theoretical estimates of diffusion coefficients show satisfactory agreement with experiment.     

Novel polyolefin materials via catalysis and reactive processing

Recent advances in transition metal catalyzed olefin polymerization and melt processing stimulate the production of new polymers derived from old monomers. Modern polyolefin processes do not require polymer purification and give excellent control of molecular and supermolecular polyolefin architectures. Progress in catalyst design and preparation of tailor-made homo-and copolymers is highlighted for isotactic, syndiotactic, atactic and stereo-block polypropylene (PP), novel 1-olefin copolymers, and ethylene copolymers with polar monomers, e.g., CO and acrylics. Today polyethylene short-and long-chain-branching is controlled either by uniform ethylene copolymerization with 1-olefins using single-site” metallocene catalysts, or by migratory polyinsertion of ethylene, respectively. Stiff cycloaliphatic polymers expand the frontiers of polyolefins into engineering applications. New families of polyethylenes and EPM with pendent polypropylene chains are obtained via copolymerization of PP macromonomers or polymer-analoguous coupling of functionalized PP during melt processing.     

Structure of ethylene sulphide crystallites in block copolymers

Block copolymers, having the configuration ethylene sulphide-isoprene-styrene or ethylene sulphide-isoprene-ethylene sulphide, have been characterized to define the structure of the ethylene sulphide moiety. It is concluded that ethylene sulphide-isoprene-styrene block copolymers exist in solution and in bulk (above the styrene softening point) as radial aggregates held together by polyethylene sulphide crystallites. During polymerization, these crystallites are formed with a linear extended chain morphology which is retained in bulk in ethylene sulphide-isoprene-styrene copolymers: in the copolymers which require processing above the melting point of the ethylene sulphide crystallites, the linear morphology is destroyed during moulding.     

Unconstrained geometry complexes: Ethylene/α‐olefin copolymerizations with a tetramethyldisilyl‐bridged Cp‐amido titanium complex

A new disilyl‐bridged complex, [(N‐tert‐butylamido)(3‐indenyl)tetramethyldisilyl]titanium dichloride ( 3 ), was synthesized and activated with methylaluminoxane (MAO) for propylene homopolymerization and ethylene/propylene and ethylene/1‐hexene copolymerizations. A polypropylene with a slight isotactic enrichment was obtained. The number of regioerrors present in the polypropylene was somewhat smaller than that found in most polypropylenes made from monosilyl‐bridged [(N‐tert‐butylamido)(3‐indenyl)dimethylsilyl]titanium dichloride. The regioerrors detected in the copolymers obtained from 3 /MAO were on the order of the amounts observed in polymers made with the monosilyl‐bridged constrained geometry catalysts. Ethylene copolymers of propylene and 1‐hexene had random sequence distributions and showed significant comonomer incorporation. Because of the presence of regioerrors, a modified method for determining the monomer composition and sequence distribution was developed from the direct measurement of the monomer content from the number of methylene and methine carbons per polymer chain, regardless of propylene inversion. An estimate of the error in the copolymerization reactivity ratio determination for regioirregular ethylene/α‐olefin copolymers was obtained by the calculation of the reactivity ratios from monomer dyad sequences, with consideration given to the contribution of major regioirregular sequences. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3840–3851, 2005     

Combination of TREF, high-temperature HPLC, FTIR and HPer DSC for the comprehensive analysis of complex polypropylene copolymers

A novel, powerful analytical technique, preparative temperature rising elution fractionation (prep TREF)/high-temperature (HT)-HPLC/Fourier transform infrared spectroscopy (FTIR)/high-performance differential scanning calorimetry (HPer DSC)), has been introduced to study the correlation between the polymer chain microstructure and the thermal behaviour of various components in a complex impact polypropylene copolymer (IPC). For the comprehensive analysis of this complex material, in a first step, prep TREF is used to produce less complex but still heterogeneous fractions. These chemically heterogeneous fractions are completely separated by using a highly selective chromatographic separation method—high-temperature solvent gradient HPLC. The detailed structural and thermal analysis of the HPLC fractions was conducted by offline coupling of HT-HPLC with FTIR spectroscopy and a novel DSC method—HPer DSC. Three chemically different components were identified in the mid-elution temperature TREF fractions. For the first component, identified as isotactic polypropylene homopolymer by FTIR, the macromolecular chain length is found to be an important factor affecting the melting and crystallisation behaviour. The second component relates to ethylene–propylene copolymer molecules with varying ethylene monomer distributions and propylene tacticity distributions. For the polyethylene component (last eluting component in all semi-crystalline TREF fractions), it was found that branching produced defects in the long crystallisable ethylene sequences that affected the thermal properties. The different species exhibit distinctively different melting and crystallisation behaviour, as documented by HPer DSC. Using this novel approach of hyphenated techniques, the chain structure and melting and crystallisation behaviour of different components in a complex copolymer were investigated systematically.

Separation of ethylene‐propylene copolymers and ethylene‐propylene‐diene terpolymers using high‐temperature interactive liquid chromatography

Ethylene‐propylene‐diene terpolymers (EPDM) are generally amorphous and, therefore, do not crystallize from solution. Consequently, fractionation techniques based on crystallization, such as crystallization analysis fractionation or temperature rising elution fractionation, cannot be used to analyze their chemical composition distribution. Moreover, no suitable chromatographic system was known, which would enable to separate them according to their chemical composition. In this study, two different sorbent/solvent systems are tested with regard to the capability to separate EPDM‐terpolymers and ethylene‐propylene (EP)‐copolymers according to chemical composition. While porous graphite/1‐decanol system is selective towards ethylene and ethylidene‐2‐norbornene, carbon coated zirconia/2‐ethyl‐1‐hexanol is preferentially selective towards ethylene. Consequently, the earlier system enables to separate both EP copolymers and EPDM according to the chemical composition and the latter mainly according to the ethylene content. The results prove that the chromatographic separation in both sorbent/solvent systems is not influenced by molar mass of a sample or by its long chain branching. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Carbon-13 nuclear magnetic resonance of ethylene–propylene copolymer and the determination of sequence distribution of monomeric units

The ¹³C-NMR spectra of ethylene–propylene copolymers and their model compounds were measured at 15.1 MHz. Assignments of the signals were carried out by using the equation of Grant and Paul and also by comparing the spectra with those of squalane, hydrogenated natural rubber, polyethylene, and atactic polypropylene. The accuracy and the precision of intensity measurements, that is, the deviation from the theoretical values and the scatter of the measurements, respectively, were checked for the spectra of squalane and hydrogenated natural rubber and were shown to be at most 12% for each of the signals. On the basis of these results the mole fractions of the four types of the dyad sequences, that is, the propylene–propylene (head-to-tail and head-to-head), the ethylene–propylene, and the ethylene–ethylene sequences, were determined together with the average sequence lengths of both monomer units.