Copolymerization Of 1-hexene Research Articles

N-heterocyclic Carbene Scandium Complexes: Synthesis, Structure, and Catalytic Performance for α-Olefin Polymerization and Copolymerization with 1,5-Hexadiene

The N-heterocyclic carbene ligated scandium trialkyl complexes [{2,6-C6H3R2NCH}2CSc(CH2SiMe3)3] (R = Me (1); R = iPr (2)) were synthesized by the reaction of 1 equiv of [Sc(CH2SiMe3)3(THF)2] with the corresponding ligand [{2,6-C6H3R2NCH}2C]. Their molecular structures have been established by single-crystal X-ray diffraction. Upon activation with 2 equiv of [Ph3C][B(C6F5)4], complexes 1 and 2 exhibited high activities up to 2520 (kg of polymer) (mol of Sc)−1 h–1 for 1-hexene homopolymerization, affording polymers with moderate molecular weight (18–46 kg mol–1) at room temperature. The use of a low reaction temperature of −30 °C resulted in high molecular weight (up to 631 kg mol–1). The catalyst systems based on complex 1 or 2 also showed excellent activities for 1-octene or 1-decene polymerization. Moreover, in the presence of 2 equiv of [Ph3C][B(C6F5)4], complex 1 proved to be an excellent catalyst for copolymerization of 1-hexene (H) with 1,5-hexadiene (HD) to give random atactic H/HD copolymers with a...

Free Volume from Pressure and Temperature Dependent Viscosity and from PVT Measurements for Homo- and Copolymers

Abstract Pressure and temperature dependency of shear and elongation viscosity and the thermal expansivity and the compressibility determined from PVT data were investigated for propylene homo- and propylene-1-pentene, -1-hexene, 1-heptene and -nonene copolymers and ethylene homopolymer and ethylene-1-butene, 1-pentene and 1-hexene copolymers. The short branching degree dependence of thermal sensitivity and pressure coefficient and the thermal expansivity and the compressibility has been determined. The fractional free volumes were calculated from the viscosity and PVT curves and the thermal expansion coefficient and compressibility factor of fractional free volume were determined. The temperature, pressure and stress dependence of fractional free volume was investigated. The fractional free volume calculated from viscosity data were compared from values comes from PVT measurement. A conversion equation was suggested.

Ethylene/1‐Hexene Copolymers Produced with MAO/(nBuCp)2ZrCl2 Supported on SBA‐15 Materials with Different Pore Sizes

AbstractThe influence of the support architecture (structure and pore size) on the activity of MAO/(nBuCp)2ZrCl2 heterogeneous catalysts in the polymerization of ethylene and 1‐hexene is studied through the characterization of the polymers' microstructure. SBA‐15 silica‐based mesostructured materials are synthesized with different pore sizes and compared with commercial silica. All SBA‐15 supported catalysts lead to higher catalytic activities than amorphous silica and give rise to ethylene/1‐hexene copolymers with bimodal CCD, which suggests that the carrier structure plays an important role not only in the global rate of the process but also in the copolymer composition. The structures and pore sizes of the support show a marked influence on the polymer CCD shape. magnified image

Time-resolved FTIR spectroscopic study of the evolution of helical structure during isothermal crystallization of propylene 1-hexene copolymers. Identification of regularity bands associated with the trigonal polymorph

Two different types of regularity bands are identified in a real time FTIR crystallization of a series of random propylene 1-hexene copolymers. The first is akin to the bands observed in the homopolymer, those associated with 3 1 helices of isotactic sequences of different n length ( n, number of monomer units). The second type corresponds to vibrational coupling of short sequences of the chain that include the 1-hexene comonomer. Among the latter are absorbances at 910 and 1025 cm −1 which are markers for the formation of a trigonal phase in these copolymers. They remain unchanged prior to and during crystallization in copolymers with the 1-hexene units rejected from the crystallites (<13 mol% 1-hexene) and increase in intensity when the comonomer is an integral part of the crystallites (>13 mol% 1-hexene). Analysis of the real time evolution of IR regularity bands during isothermal crystallization of these copolymers confirms the beginning of crystallization at a critical helical sequence length ( n∗) of ∼12 isotactic units (841 cm −1), and enables details of the early and final stages of crystallization. In the homopolymer and copolymers, the intensity of regularity bands with n ≤ 10 is constant in the initial undercooled melt, and increases simultaneously with the appearance of helices with n ≥12, in support of a classical crystallization mechanism of nucleation and growth. Due to density fluctuations in the initial melt, the short helices eventually collapse in aggregates or precursors that spontaneously (within the experimental macroscopic time frame) extend to stable nuclei ( n ≥ 10). Stable nuclei further extend and grow cooperatively dragging additional short sequences as inferred by the simultaneous temporal evolution of helices with n = 10 and greater. The intensity of the 998 cm −1 ( n = 10) band prior to nucleation, correlates directly with the isotactic sequence length of the copolymer and is independent of the final structure that evolves, either monoclinic or trigonal. This feature infers a nucleation event driven preferentially by the initial steady-state content of short helices in iPP and iPP-based copolymers. The temporal evolution of the 841 cm −1 band is an excellent avenue to study the crystallization kinetics of copolymers, including those with very low crystallinities. Via FTIR, the mechanism of the formation of mesomorphic crystallites in copolymers with ∼10 mol% 1-hexene at low temperatures is contrasted with the formation of alpha crystallites at higher temperatures in the nucleation driven range. The intensity of the 841 cm −1 band ( n = 12) at the end of the transformation correlates linearly with the degree of crystallinity obtained by WAXD.

Ethylene Polymerization by Metallocene Catalysts Supported over Siliceous Materials with Bimodal Pore Size Distribution

AbstractSummary: As known, the pore structure of the catalytic support plays a decisive role during the polymerization reactions determining intra‐particle mass and heat transport phenomena. In this work, several ethylene polymerizations have been carried out by using as catalytic support different mesostructured materials with unimodal and bimodal pore size distributions in order to evaluate the influence of this pore size distribution on the catalytic behavior. Calcined mesoporous materials were impregnated with the catalytic system MAO/(nBuCp)2ZrCl2 and used for ethylene polymerization and ethylene/1‐hexene copolymerizations, at 70 °C and 5 bar of ethylene pressure. Polyethylenes obtained were characterized by GPC, DSC and Crystaf. Results indicate that porous structure of the support has a significant influence on polymerization activity and polymer properties. Despite the catalyst bimodal pore size distribution, only ethylene/1‐hexene copolymers presented a bimodal chemical composition distribution.

The Influence of Comonomer on Ethylene/α-Olefin Copolymers Prepared Using [Bis(N-(3-tert butylsalicylidene)anilinato)] Titanium (IV) Dichloride Complex

We describe the synthesis of [bis(N-(3-tert-butylsalicylidene)anilinato)] titanium (IV) dichloride (Ti-FI complex) and examine the effects of comonomer (feed concentration and type) on its catalytic performance and properties of the resulting polymers. Ethylene/1-hexene and ethylene/1-octene copolymers were prepared through copolymerization using Ti-FI catalyst, activated by MAO cocatalyst at 323 K and 50 psi ethylene pressure at various initial comonomer concentrations. The obtained copolymers were characterized by DSC, GPC and 13C-NMR. The results indicate that Ti-FI complex performs as a high potential catalyst, as evidenced by high activity and high molecular weight and uniform molecular weight distribution of its products. Nevertheless, the bulky structure of FI catalyst seems to hinder the insertion of α-olefin comonomer, contributing to the pretty low comonomer incorporation into the polymer chain. The catalytic activity was enhanced with the comonomer feed concentration, but the molecular weight and melting temperature decreased. By comparison both sets of catalytic systems, namely ethylene/1-hexene and ethylene/1-octene copolymerization, the first one afforded better activity by reason of easier insertion of short chain comonomer. Although 1-hexene copolymers also exhibited higher molecular weight than 1-octene, no significant difference in both melting temperature and crystallinity can be noticed between these comonomers.

Computer modeling of the crystallization process of single‐chain ethylene/1‐hexene copolymers from dilute solutions

AbstractLangevin molecular dynamics (LMD) simulations have been performed to understand the role of the short chain branches (SCB) on the formation of ordered domains by cooling dilute solutions of ethylene/α‐olefins copolymer models. Three different long single‐chain models (C2000) with 0, 5, and 10 branches each 1000 carbons were selected. These models were equilibrated at high reduced temperature (T* = 13.3) and cooling in steps of 0.45 until the final temperature (T* = 6.2) by running a total of 35 × 106 LMD steps. During the cooling process, global order parameter, torsion distribution, position of the branches, and local‐bond order parameter were calculated and monitored. The peaks of crystallization for each model were calculated by differentiating the global order parameter with temperature. The Tc (crystallization temperature) decreases as the number of branches increases as has been experimentally reported. The formation of order in the copolymers is affected by the amount of the SCB in the backbone of the polymer chain. Initially, the SCB move to the folding surface. Once the SCB are located near the folding surface the order starts to grow. In all cases here shown, the C4 branches are excluded from the ordered domains. To take into account, the influence of the branch distribution, a different branch distribution model has been considered for the two‐branched systems. The crystallization fraction (α) and the density of the amorphous and ordered fractions was defined using the local‐bond order parameter. Both magnitudes decrease as the number of branches increases. These facts fairly agree with experimental literature data. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Effect of the Sequence Length Distribution on the Lamellar Crystal Thickness and Thickness Distribution of Polyethylene: Perfectly Equisequential ADMET Polyethylene vs Ethylene/α-Olefin Copolymer.

The morphology of ADMET-synthesized polyethylene with n-butyl branches precisely spaced on every 39th carbon (EH39) was studied in comparison with an ethylene/1-hexene addition copolymer possessing the same branching probability, the goal being to elucidate the effect of the intramolecular sequence length heterogeneity on the lamella crystal thickness and its distribution. EH39 was found to have an orthorhombic crystalline polymorphism, which is normal for commercialized polyethylenes and different from that of the other ADMET polyethylenes with shorter CH2 spacing (C15, C21). EH39 exhibits a narrow lamella thickness distribution; the average thickness (lc,av.) corresponds exactly to the space length between two consecutive branches, suggesting the complete exclusion of n-butyl branches from the crystal stem. The average thickness, lc,av. mentioned above is also coincident with that obtained from WAXS and SAXS. On the other hand, the 1-hexene copolymer forms much thicker lamellae and a broader thickness d...

Rational Design of Silicon-Bridged Fluorenyl−Phenoxy Group 4 Metal Complexes as Catalysts for Producing High Molecular Weight Copolymers of Ethylene and 1-Hexene at Elevated Temperature

For producing high molecular weight copolymers of ethylene and 1-hexene, a series of silicon-bridged fluorenyl−phenoxy group 4 complexes R2Si(Flu)(3-tBu-5-Me-2-C6H2O)MCl2 (4a: M = Ti; R = Me; Flu = fluoren-9-yl; 4b: M = Ti; R = Et; Flu = fluoren-9-yl; 4c: M = Ti; R = Me; Flu =2,7-tBu2-fluoren-9-yl; 4d: M = Ti; R = Et; Flu =2,7-tBu2-fluoren-9-yl; 8: M = Zr; R = Me; Flu =2,7-tBu2-fluoren-9-yl; 9: M = Hf; R = Me; Flu =2,7-tBu2-fluoren-9-yl) were prepared by two methods. One was a reaction of MCl4 (M = Ti, Zr, and Hf) with the dilithium salts generated by treating corresponding ligands 1a−d with nBuLi in the presence of Et3N in toluene and the other used MCl2(NR′2)2 (6a: M = Ti; R′ = Me, 6b: M = Zr; R′ = Et, 6c: M = Hf; R′ = Me) instead of MCl4 followed by successive chlorination with Me3SiCl. X-ray crystallographic study of 4d indicated that the cyclopentadienyl moiety of the fluorenyl ligand is bound to the titanium atom in a fashion between η3 and η5. The newly prepared titanium complexes 4a−d exhibited high catalytic activities for copolymerization of ethylene and 1-hexene in the presence of iBu3Al/[Me2NHPh][B(C6F5)4] and afforded higher molecular weight copolymers than the parent tetramethyl-substituted cyclopentadienyl titanium complex Me2Si(η5-C5Me4)(3-tBu-5-Me-2-C6H2O)TiCl2 (10). In particular, the active species generated from complex 4d was thermally stable and exhibited high polymerization activities even at 210 °C and produced high molecular weight copolymers with good 1-hexene content.

Improving the design stresses of high density polyethylene pipes and vessels used in reverse osmosis desalination plants

Nanocomposite materials have been used on a wide scale in industrial and structural applications. The present work aims at studying the mechanical properties of high density polyethylene (HDPE) grade TR-401 hexene copolymer reinforced by montmorillonite nanoparticles (MMT), used to fabricate pipes and membranes vessels for reverse osmosis desalination plants. Different volume fractions and particle sizes of the MMT clay were used to investigate the effect of this filler on the mechanical properties of the produced composite. Mechanical properties tests were carried out and good improvements of the composite properties were obtained compared to the parent polymer. The test results showed a significant enhancement of the mechanical properties at low filler proportions. Pipe fabricated from these composites had many outstanding and desirable features. For example, by adding 4.75% MMT to the HDPE produced quality pipes and fittings with the highest design stress basis of any polyethylene. A significant increase in the modulus of elasticity observed, together with an unusual increase in the design stress, approved the HDPE/MMT composite for high pressure piping and membrane vessels used in reverse osmosis desalination plants.

Ethylene-hexene copolymer derived from [t-butylfluorenylsilyl-amido] dimethyl titanium complex

The copolymers of ethylene and 1-hexene were prepared with half-metallocene titanium complex ((t-BuNSiMe2Flu)TiMe2) and modified methylaluminoxane (MMAO). The initial concentrations of 1-hexene were varied to investigate how the different amounts of comonomer affect on the catalytic activity of copolymerization system and microstructure of the copolymers. It has been found that this catalytic system was not active for hexene polymerization, however, it can be active when ethylene was introduced to perform ethylene-hexene copolymerization. As comonomer, 1-hexene provides positive comonomer effect on the system although very high concentration of 1-hexene was introduced. However, the microstructures of the obtained copolymers, which were examined by 13 C-NMR need to be improved because with highly alternating sequence distribution of comonomer causing them losing some essential specific thermal properties.

Advanced Monte Carlo Algorithm for the Atomistic Simulation of Short- and Long-Chain Branched Polymers: Implementation for Model H-Shaped, A3AA3 Multiarm (Pom-Pom), and Short-Chain Branched Polyethylene Melts

We present a powerful connectivity-altering Monte Carlo (MC) algorithm for simulating atomistically detailed models of long- and short-chain branched polymers. Based on a mix of advanced and simple moves, the algorithm allows the robust simulation of chain systems with a variety of molecular architectures: H-shaped, A3AA3 multiarm (pom-pom), and short-chain branched (SCB) ones. For the H-shaped and A3AA3 architectures (A denotes the backbone and A the arm), in particular, the recently developed intermolecular double bridging (DB) move effecting a new bridging between the chain backbones or the branches of two different chains and the intramolecular double rebridging (IDR) move effecting a new bridging between two branches of the same chain [Karayiannis et al. J. Chem. Phys. 2003, 118, 2451] are shown to provide a robust sampling of their structural and conformational characteristics at the chain level. The double concerted rotation (d-CONROT) and H-shaped branch rebridging (H-BR) moves, on the other hand, which are important in relaxing internal degrees of freedom in the chain and the local structure around branch points, allow complete equilibration at the segmental level, which is prerequisite for the correct prediction of the volumetric and packing properties of the simulated systems. For the case of SCB polymers, a continuum configurational bias move for branched polymers (Br-CCB) has been designed capable of sampling changes in the atomic coordinates in the vicinity of branch points. Along with an effective implementation of the conventional end-bridging (EB) move, this has resulted in a powerful method for simulating SCB polymers with a prespecified distribution of branch points along the chain and given (i.e., fixed) number of carbon atoms per branch. To deal with polydispersity in the simulated systems, the algorithm is executed in a semigrand canonical ensemble by making use of the set of chain relative chemical potentials which for a bulk, linear system produces a uniform distribution of chain lengths [Pant and Theodorou Macromolecules 1995, 28, 7224]. Consistent with recent considerations by Ramos et al. [Macromolecules 2007, 40, 9640], two different chain length distributions should be simultaneously controlled in the course of the simulation for branched systems. The new, generalized MC algorithm is capable of thoroughly equilibrating model H-shaped, multiarm A3AA3, and SCB polymers at all length scales and adequately sampling fluctuations in their structural, volumetric, and conformational properties. Overall, we find that branched polymers are characterized by a stiffer conformation than linear analogues of the same total chain length. For the H-shaped and A3AA3 architectures, in particular, we have observed that each dangling arm exerts an entropically driven tensile force on the chain crossbar (the part of the chain between the two branch points), causing significant chain stretching. Our results extend the previous works of Karayiannis et al. [J. Chem. Phys. 2003, 118, 2451] on the simulation of H-shaped polymers to A3AA3 PE polymers and of Ramos et al. [Macromolecules 2007, 40, 9640] on the simulation of model ethylene−1-hexene copolymers to copolymers with longer α-olefinic side groups, such as the ethylene−1-octene and ethylene−1-decene ones.

Copolymerization of sterically demanding phosphine-olefins and 1-hexene

The copolymerization of 1-hexene with t-Bu2P(CH2)3CH=CH2 (1) or t-Bu2P(CH2)9CH=CH2 (2) has been achieved using CpMe2Ti(NPt-Bu3) as the precatalyst and B(C6F5)3, [Ph3C][B(C6F5)4], or [PhNMe2H][B(C6F5)4] as the activator. The resulting polymers are shown to incorporate up to 9% of the phosphine comonomer, albeit with reduced catalyst activity and polymer molecular mass. The cause of the catalyst inhibition is also considered in the light of phosphine–activator interactions.

Separation of propene/1-alkene and ethylene/1-alkene copolymers by high-temperature adsorption liquid chromatography

A high performance liquid chromatography column (HPLC) Hypercarb® packed with porous graphite has proven to discriminate polyolefin molecules due to differences in their adsorption and desorption behaviour. While linear polyethylene (PE) and syndiotactic polypropylene (sPP) are adsorbed on the graphite packing, isotactic polypropylene (iPP) is not adsorbed. The column operates at 160°C with 1-decanol as sample solvent and mobile phase. We have now tested this HPLC system for separations of random propene/1-alkene and ethylene/1-hexene copolymers: While copolymers of propene with 1-butene, 1-hexene and 1-octene copolymers eluted in size exclusion mode without adsorption, propene/1-octadecene and ethylene/1-hexene copolymers are strongly retained and eluted only after application of a linear gradient starting from 1-decanol and ending with pure 1,2,4-trichlorobenzene. The retention of propene/1-alkene (>11 carbons in the side chain) copolymers increases with the concentration of comonomer, making this HPLC system suitable to separate these copolymers according to their chemical composition. In contrast, the retention of ethylene/1-hexene samples decreases with increasing 1-hexene content. Branching in this case shortens the length of continuous methylene sequences of the polymer backbone, which are expected to adsorb in a planar conformation to the graphite layers. This is the first report on the separation of short chain branched polyolefins by high-temperature adsorption liquid chromatography.

Study of Very Low Temperature Crystallization Process in Ethylene/α‐Olefin Copolymers

AbstractSummary: The crystallization behavior of Ziegler‐Natta (ZN) and single site (SS) based ethylene/1‐butene and ethylene/1‐hexene copolymers and SS copolymer fractionated by composition and molar mass (MM) has been studied by differential scanning calorimetry. It was observed that in addition to the high temperature crystallization peak (HTCP), and for ZN copolymers in addition also to low temperature crystallization peak (LTCP), a very‐low temperature crystallization peak (VLTCP) is present at temperatures in between 60–75 °C. Peak temperature of VLTCP, TVLTCP, decreases with increasing comonomer content (Ccomon) at fixed MM. If Ccomon is kept approximately constant, TVLTCP increases with increasing MM. It turns out that TVLTCP does not depend on the type of catalyst used. The degree of crystallinity calculated from the VLTCP is independent of the chemical nature of the comonomers present, but slightly changes with Ccomon. It also steeply increases with MM and levels off at MM around 50 kg/mol. It was found that the crystallinity as related to the area of the VLTCP is catalyst type dependent, and is higher for the SS catalyst used compared to the ZN catalyst.

Crystallinity in Ethene−1-Hexene Copolymers Determined by 1H and 13C NMR. A Comparative Study

The 13C DD-MAS spectra of six ethene/1-hexene (E/H) copolymers were fitted simultaneously to a sum of four pseudo Voigt functions in which two were assigned to the all-trans crystalline region. These two peaks possessed the same chemical shift but differed in both line width and line shape. Regarding the amorphous region, it was found to be represented by a sum of a Lorenzian function and a Gaussian function possessing different chemical shifts. The latter simply reflects the asymmetry of the spatial distribution of trans−gauche conformations. The crystallinity of the samples, as determined by DSC, was found to be in excellent agreement with the crystallinity determined from 13C DD-MAS spectra. Moreover, the crystalline part of the 1H-FID (acquired at 0.5 and 4.7 T) was fitted by using a Pake function and Abragamian function, respectively. The latter was found to give a crystallinity in quantitative agreement with the 13C DD-MAS and DSC. In contrast, the replacement of the Abragamian function with a Pake function (to represent the crystalline part of the FID) resulted often in unstable and poor model fits with rather unreliable estimate of the crystallinity. The inherent problem related to the necessary blanking of the receiver during the initial part of the FID (dead-time) is discussed.

Copolymerization of Ethylene with tert-Butylethylene Using Nonbridged Half-Titanocene-Cocatalyst Systems

The copolymerization of ethylene with tert-butylethylene (TBE, 3,3-dimethyl-1-butene) using various half-titanocene [Cp′TiCl2X (X = O-2,6-iPr2C6H3, N═CtBu2)]-cocatalyst systems has been carried out. The aryloxo-containing half-titanocenes, Cp′TiCl2(O-2,6-iPr2C6H3) (Cp′ = Cp*, tBuC5H4, 1,2,4-Me3C5H2), displayed the capability to bring about TBE incorporation in the presence of MAO as a cocatalyst, while their ketimide analogues, Cp′TiCl2(N═CtBu2) (Cp′ = Cp, Cp*, tBuC5H4), could not accomplish TBE enchainment in ethylene/TBE copolymerization. Among aryloxo derivatives, those carrying tert-butyl and trimethyl substituents on the cyclopentadienyl moiety, Cp′TiCl2(O-2,6-iPr2C6H3) (Cp′ = tBuC5H4, 1,2,4-Me3C5H2), exhibited efficient TBE incorporation and moderate catalytic activity in the presence of MAO, affording ethylene/TBE copolymers with relatively high molecular weight and uniform molecular weight distribution. However, the copolymerization conducted by employing constrained geometry catalyst (linked half-titanocene), [Me2Si(C5Me4)(NtBu)]TiCl2, and zirconocene [Cp2ZrCl2]−MAO catalyst systems resulted in polyethylene without TBE incorporation. Furthermore, the copolymerization of 1-hexene (1-Hex) with TBE and vinyltrimethylsilane (VTMS), by (tBuC5H4)TiCl2(O-2,6-iPr2C6H3) and CpTiCl2(N═CtBu2)−MAO catalyst systems, indicated a considerably higher reactivity of VTMS. The 1-Hex/VTMS copolymerization has been elucidated to proceed via 1,2-insertion as manifested by the remarkably efficient VTMS enchainment, hence revealing the difference in the reactivity of TBE and VTMS to arise from that in their electronic and/or steric nature.

Application of a crystallization kinetics model to simulate the effect of operation conditions on Crystaf profiles and calibration curves

AbstractCrystallization analysis fractionation (Crystaf) is a polymer characterization technique used to estimate chemical composition distributions (CCDs) of semicrystalline copolymers. The Crystaf profile can be transformed into a CCD using a calibration curve that relates average comonomer content to peak crystallization temperature. The calibration curve depends on copolymer molecular properties and Crystaf operation conditions. In this investigation, we applied a crystallization kinetics model to simulate Crystaf calibration curves and to quantify how Crystaf calibration curves depend on these factors. We applied the model to estimate the CCDs of three ethylene/1‐hexene copolymers from Crystaf profiles measured at different cooling rates and showed that our predictions agree well with the CCDs described by Stockmayer's distribution. We have also used this new methodology to investigate the effects of cooling rate, molecular weight, and comonomer type on Crystaf profiles and calibration curves. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 866–876, 2009

Formation of the New Trigonal Polymorph in iPP−1-Hexene Copolymers. Competition with the Mesomorphic Phase

The polymorphic structure and the thermal transitions of several iPP−1-hexene copolymers have been analyzed by performing DSC and variable-temperature X-ray diffraction experiments. Special attention has been paid to the range of “intermediate” comonomer contents, between around 8 and 12 mol %, where the new trigonal form of iPP starts to be observed. It was found that the mesomorphic modification is also easily obtained in that comonomer range, so that the possibility of such mesomorphic phase acting as a transient structure of the trigonal form was entertained. However, the results have clearly shown that the mesomorphic modification is not acting as such transient structure, and rather it is a competitor (as well as the α crystals) for the formation of the trigonal phase.

Behaviour of the very-low-temperature crystallization peak of linear low-density polyethylene

The crystallization behaviour of Ziegler–Natta (ZN) and single-site catalyst based ethylene–1-butene and ethylene–1hexene copolymers with different comonomer content were studied by differential scanning calorimetry. In addition to a hightemperature crystallization peak, and for ZN copolymers in addition to a low-temperature crystallization peak, quite often a verylow-temperature crystallization peak (VLTCP) was observed at temperatures in between approximately 330 and 345 K. It was found that the VLTCP temperature decreased with increasing comonomer content and did not depend on the type of catalyst used. The fractional degree of crystallinity calculated from the VLTCP was independent of the chemical nature and content of the comonomers present as well as of the polydispersity of molar mass within the used range of magnitudes. However, crystallinity as related to the area of the VLTCP was strongly catalyst type dependent and was higher for the single-site catalyst used compared to the ZN catalyst used.