Chain Straightening Research Articles

A hybrid steric ligand modification strategy promotes propylene (co)polymerization

Recently, a Hybrid Steric Ligand Modification (HSLM) strategy has been shown to significantly increase the polymerization activity and the branching degree of the resulting (co)polymers in the ethylene (co)polymerization via α-diimine catalysts. In this study, the HSLM strategy was employed for propylene (co)polymerization using a group of α-diimine Pd(II) catalysts with bulky hybrid ligands. The HSLM catalysts (Pd1-4) produced polypropylene with high molecular weights (45.4–156.9 kg/mol) and resulted in significantly high levels of activity (105 g mol−1 h−1). Additionally, nearly half of the propylene underwent chain straightening (1,3-enchainment = 43–46 %). Our findings suggest that Pd1-4 catalysts are more effective than the symmetrical catalysts Pd5-6 in producing high molecular weight polypropylene with high activity. Among all the catalysts tested, Pd4 with 4-tert-butylphenyl displayed the highest polymerization activity and molecular weight, although with lowest branching densities. It is suggested that the HSLM may provide a more open spatial configuration around the metal center, which facilitates the coordination and insertion of propylene molecules as well as chain walking. The hybrid Pd(II) catalysts showed moderate activity during the copolymerization of propylene with MA. The copolymers produced had low molecular weights (3.1–11.2 kg/mol), high branching densities (120–211/1000C), and high incorporation ratios (8.82–18.08 mol%). The copolymerization activity was highest with 4-tert-butylphenyl Pd4. The resulting P-MA copolymers had the highest molecular weight, the largest branching degree, and the lowest incorporation ratios.

Systematic investigations of ortho-aryl substitution effect on chain microstructure in nickel-catalyzed long chain α-olefin polymerization and copolymerization with methyl undecenoate

The tuning of branching density and branch-type distribution of the polyolefin through the ligand modification in “chain-walking”-type late transition metal catalysts is highly desired but remains a significant challenge in olefin polymerization. Herein, we performed a systematic investigation on the (co)polymerization of long chain α-olefin (1-octene and 1-decene) by a series of α-diimine Ni(II) catalysts with the varied ligand sterics arising from the number of the ortho-aryl (4-methylphenyl) substituents. The effects of structural variations, mainly including the steric tuning on the ortho-position of N-aryl ring and ligand backbone, steric distribution (ligand symmetry) tuning and electronic tuning, and polymerization conditions on catalytic activities, polymer molecular weight, polymer branching density, and branch-type distribution were described. The steric effect of ortho-methylphenyl substituents played a major role rather than the electronic effect in determining the preference of insertion fashion (1,2-insertion or 2,1-insertion) and chain walking in long chain α-olefin polymerization. As the bulk and number of ortho-methylphenyl substituents increased, the preferred 2,1-insertion and 1, ω-enchainment (chain straightening) were observed even at high monomer concentrations, thus leading to the decrease of branching density as well as the percentage of methyl branches, and eventually affording the semicrystalline “polyethylene-like” polymer with high Tm up to 121 °C and good mechanical properties. Moreover, the tuning of chain microstructure can be achieved over a very wide range through the modification of ligand sterics. Most importantly, these Ni(II) catalysts could copolymerize long chain α-olefin and polar monomer methyl undecenoate (MU) to generate the various functionalized semicrystalline copolymers with high incorporation ratios of MU up to 11% and Tm in a wide range of 5.6–93 °C.

Coarse-grained modeling reveals the impact of supercoiling and loop length in DNA looping kinetics

Measurements of protein-mediated DNA looping reveal that in vivo conditions favor the formation of loops shorter than those that occur in vitro, yet the precise physical mechanisms underlying this shift remain unclear. To understand the extent to which in vivo supercoiling may explain these shifts, we develop a theoretical model based on coarse-grained molecular simulation and analytical transition state theory, enabling us to map out looping energetics and kinetics as a function of two key biophysical parameters: superhelical density and loop length. We show that loops on the scale of a persistence length respond to supercoiling over a much wider range of superhelical densities and to a larger extent than longer loops. This effect arises from a tendency for loops to be centered on the plectonemic end region, which bends progressively more tightly with superhelical density. This trend reveals a mechanism by which supercoiling favors shorter loop lengths. In addition, our model predicts a complex kinetic response to supercoiling for a given loop length, governed by a competition between an enhanced rate of looping due to torsional buckling and a reduction in looping rate due to chain straightening as the plectoneme tightens at higher superhelical densities. Together, these effects lead to a flattening of the kinetic response to supercoiling within the physiological range for all but the shortest loops. Using experimental estimates for in vivo superhelical densities, we discuss our model’s ability to explain available looping data, highlighting both the importance of supercoiling as a regulatory force in genetics and the additional complexities of looping phenomena in vivo.

Uniaxial Stress and Electron Irradiation Effects on Nanochains Straightening in Film Polymer Materials

Effects of uniaxial stress, temperatures and electron irradiation on nanochain straightening in film polymer materials (polytetrafluoroethylene and lavsan) have been studied comprehensively. Essential change in material behavior including chain straightening was found in strain (e) – stress (s) curves.

Understanding the Insertion Pathways and Chain Walking Mechanisms of α-Diimine Nickel Catalysts for α-Olefin Polymerization: A 13C NMR Spectroscopic Investigation

Nickel α-diimine catalysts have been previously shown to perform the chain straightening polymerization of α-olefins to produce materials with melting temperatures (Tm) similar to linear low density polyethylene (Tm = 100–113 °C). Branching defects due to mechanistic errors during the polymerization currently hinder access to high density polyethylene (Tm = 135 °C) from α-olefins. Understanding the intricacies of nickel α-diimine catalyzed α-olefin polymerization can lead to improved ligand designs that should allow production of chain-straightened polymers. We report a 13C NMR study of poly(α-olefins) produced from monomers with 13C-labeled carbons—specifically 1-decene with a 13C-label in the 2-position and 1-dodecene with a 13C-label in the ω-position—using a series of α-diimine nickel catalysts. Furthermore, we developed a mathematical model capable of quantifying the resulting 13C NMR data into eight unique insertion pathways: 2,1- or 1,2- insertion from the primary chain end position (1°), the penul...

Insights into traction-separation phenomena of graphene-cis-1,4-polyisoprene interface using molecular dynamics

Detailed investigations of graphene-cis-1,4-polyisoprene (PI) interfacial traction-separation (τ-δ) behavior and its causative phenomena in the molecular scale are addressed in this work, using Molecular Dynamics (MD) simulations. Configurations of dense amorphous cis-1,4-PI network have been generated and validated. Effects of temperature, separation rate and compressive load on τ-δ behavior are studied. The molecular level physics during interface separation in opening mode is explained using void dynamics and chain straightening. It is found that the evolution of voids and τ-δ behavior are strongly correlated at quasistatic separation rate. Interestingly, a viscous behavior is seen to develop at the interface as the separation rate increases. The magnitude of traction in the opening mode is higher than that in the sliding mode by nearly two orders of magnitude. It is also seen that the amount of polymer bound to graphene following complete separation in opening mode was independent of temperature, while it decreased with increase in separation rate.

Structural Biological Materials: Critical Mechanics-Materials Connections

Spider silk is extraordinarily strong, mollusk shells and bone are tough, and porcupine quills and feathers resist buckling. How are these notable properties achieved? The building blocks of the materials listed above are primarily minerals and biopolymers, mostly in combination; the first weak in tension and the second weak in compression. The intricate and ingenious hierarchical structures are responsible for the outstanding performance of each material. Toughness is conferred by the presence of controlled interfacial features (friction, hydrogen bonds, chain straightening and stretching); buckling resistance can be achieved by filling a slender column with a lightweight foam. Here, we present and interpret selected examples of these and other biological materials. Structural bio-inspired materials design makes use of the biological structures by inserting synthetic materials and processes that augment the structures' capability while retaining their essential features. In this Review, we explain this idea through some unusual concepts.

Evolution in concepts concerning the mechanism of plasticity in solid polymers after the 1950s

The evolution of fundamental concepts concerning the mechanisms of plastic deformation in solid polymers, crystals (both glassy and semicrystalline) over the recent five decades has been reviewed. The most effective approach to the description of cold plastic deformation is a gradual drift away from the chain straightening mechanism as a limiting kinetic stage of the process (or, in other words, from the picture of the rubbery character of deformation) to concepts that are characteristic of the deformation physics of nonpolymer solids, primarily metals. Various nonpolymeric mechanisms, such as nucleation and growth of dislocations in polymer crystals, formation and evolution of short-scale undeveloped shear transformations in glasses, and mutual deformation constraints of amorphous and crystalline phases in semicrystalline polymers, are introduced in polymer science. An analysis of the published data shows that deformation in solid polymers is undoubtedly accompanied by chain straightening, but, in most cases, this process does not control the overall kinetics of plasticity, at least at small and moderate strains. In glassy polymers, conformational chain rearrangements are not going directly but proceed as shape fitting of macromolecule coils to short-scale shear transformations that serve as active sites of plasticity. The chain character and flexibility of macromolecules have a slight effect on the nucleation of the above transformations. The development of local shear at the early stages of loading is the thorniest stage of deformation. Problems concerning the joint development of polymer and nonpolymer processes during deformation of macromolecular solids and the specific features related to the polymer structure of macromolecules are discussed.

Effect of Chain Straightening on Plateau Modulus and Entanglement Molecular Weight of Ni‐diimine Poly(1‐hexene)s

AbstractSummary: In this communication, we report the first rheological study on the chain‐straightened Ni‐diimine poly(1‐hexene)s and investigate the unique effect of chain straightening on plateau modulus and entanglement molecular weight of this series of polymers. Two Ni‐diimine poly(1‐hexene) samples having different levels of chain straightening were prepared with a chain‐walking Ni‐diimine catalyst, (ArNC(An)C(An)NAr)NiBr2 (An = acenaphthene, Ar = 2,6‐(i‐Pr)2C6H3) at two different temperatures. Rheological analyses show that the chain‐straightened polymers exhibit significantly enhanced plateau modulus and reduced entanglement molecular weight compared to regular poly(1‐hexene)s by metallocene catalysis. Such an effect becomes more pronounced with an increase in the level of chain straightening.Loss moduli G″(ω) versus reduced angular frequency in a linear, natural logarithm plot for the three polymers at the reference temperature of 100 °C.imageLoss moduli G″(ω) versus reduced angular frequency in a linear, natural logarithm plot for the three polymers at the reference temperature of 100 °C.

Chain Straightening During Constant-Strain Relaxation in Melt-Crystallized Polyethylene Films

The low-frequency Raman study of the chain-straightening process during stress relaxation in melt-crystallized polyethylene with molecular weight 105 is presented. The application of tensile stress to low- and mid-drawn films causes the formation of “short” (as compared with crystal cores) straight-chain-segments (SCS). During the subsequent stress relaxation, while keeping the strain constant, the amount of SCS in the λ = 7 sample (λ is the draw ratio) gradually increases, while in the λ = 22 sample, a fraction of “short” SCS diminishes to zero within 3 days. This difference in the behavior of the low- and mid-drawn samples was ascribed to the difference in the contribution of the chain-breaking process. In the ultimately drawn sample (λ = 31), the length of the newly formed SCS is close to the crystal size, and they are situated, mainly, in the axially ordered interfibrillar phase. The formation of taut-tie molecules was not observed.

LAM Study of Chain Straightening During Stress Relaxation in Gel-Spun Polyethylene Fibers

The low-frequency Raman spectra of gel-spun polyethylene fibers were recorded in the frequency range of the longitudinal acoustic modes (LAM) localized on all-trans sequences, i.e., on straight-chain segments (SCS). During the experiment, the samples were stressed and held with their ends fixed. As soon as the stress was applied, there appeared a substantial amount of short (as compared to the size of the crystal) all-trans sequences randomly distributed in the amorphous phase. These primary SCS are not stable and decay (coil) within approximately half an hour. Further chain straightening continues in the interfacial regions between the crystalline and amorphous phases. In the course of this process, the individual all-trans sequences in the amorphous bulk are virtually replaced by the SCS adjacent to crystal cores. The final SCS length distribution in the relaxed sample is characterized by an increased fraction of the crystalline SCS penetrating deeply into the amorphous regions with formation of some quantity of taut-tie molecules.

Elastic behavior of nanoparticle chain aggregates: A hypothesis for polymer-filler behavior

Electron microscopy studies in our laboratory have shown that nanoparticle chain aggregates (NCAs) of inorganic oxides have elastic properties. Measurements were made with titania, alumina, and iron oxide NCAs generated by laser ablation. Primary particles were 5–10 nm in diameter, and the mobility diameters of the NCAs studied were about 0.5 μm. NCA stretching appeared to begin with the rotation and/or sliding of adjacent nanocrystals. This led to a small change in the NCA length but allowed for chain straightening. Most of the NCA lengthening resulted from the separation of kinked chain segments held together by weak, probably van der Waals (vdw), forces. NCA strains up to 90% were observed. Calculated values for NCA deformation energies per unit volume were compared with those for conventional polymers; under certain conditions, the two deformation energies were of the same order of magnitude. These results may help explain the remarkable effects that nanoparticle reinforcing fillers such as carbon black and silica have on commercial rubber. It may be possible to improve the properties of composites of molecular polymers and NCAs through the use of NCAs with prescribed primary particle sizes, vdw-bond numbers, chain lengths, and morphological properties. Synthesizing such NCAs will require the use of modern concepts of aerosol aggregate formation. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2658–2665, 2000

Chain flattening of spin-cast PMMMA on aluminum mirrors: Influence of polymer tacticity

Polymer tacticity has already been shown to be an influential parameter governing the behavior of polymer solutions and polymer blends, but not for polymer adsorption. However, using PMMA having a racemic fraction (ƒ r) ranging from 0.1 to 0.9, spin-cast on aluminum mirror surfaces, makes it possible to work out configuration differences in the adsorbed layer. Reflectance infra-red spectroscopy has been used to evaluate the fraction of surface bonded carbonyl groups (ƒ ab CO) and local conformations of both main and side chain. The ƒ ab CO value is 0.1 for highly syndiotactic PMMA and reaches 0.7 for mainly isotactic PMMA. This latter very high value of ƒ ab CO can only be explained by chain flattening resulting from a distortion from its Isotropic state. This distortion may be due to both strong interactions that may exist between PMMA and the aluminium substrate, and also to the spin-cast process that may lead to chain straightening and flattening. The assumption of conformational energy changes of both backbone and side chain is confirmed by changes in the FT-IR Av 2(1245 cm −1) Av 1(1265–1275 cm −1 ) ratio. Ellipsometric spectroscopy has provided information on the global conformation of adsorbed chains by precise thickness measurements. Thus, thickness ( h) increases for diluted solutions linearly from 10 to 70 A as the average syndiotactic sequence length ( n s ) increases. However, IR and ellipsometric results tend to prove that isotactic PMMA may adsorb on the aluminum surface in a more flattened way with longer chains than atactic and syndiotactic chains. This can be explained by the ability of chain segmental rearrangements ( meso tg state) that may allow isotactic PMMA to favor the establishment of interfacial acid-base bonds while the syndiotactic PMMA local stiffness of the tt state does not. The influence of the chain entropy in solution is also discussed as an important parameter.

ChemInform Abstract: An Analysis of Factors That Influence Shape Selectivity in the Cracking of Long‐Chain Alkanes on Zeolite Catalysts

Abstract The method and equations previously developed for conformational analysis of linear alkanes are used to calculate the free energy differences between heavily coiled (four gauche bonds or more) and extended forms. Since a gauche bond is similar in its steric requirements to a methyl side chain, it follows that chain straightening is a required step (preequilibrium) for the reaction of linear alkanes inside the channels of small- and medium-pore zeolites. Any energy advantage from attractive van der Waals interactions with the walls of the channels can be gained only after the chain uncoils. For a 20-carbon chain the preequilibrium retards the reaction inside the pores relative to that on the external surface by at least four powers of ten. Two other factors acting in the same direction are a requirement for proper orientation to enter the channel (worth one order of magnitude for C 20 ) and the retarding effect of constraining a molecule inside the channel on the cracking reaction itself. The observed selectivities require either that the intrinsic activity of sites within channels be many orders of magnitude higher than that of sites on the external surface, or that the normal alkanes reacting in the coiled form have an intrinsically higher reactivity toward cracking than the branched isomers, and the reactions occur predominantly on the external surface of the zeolite crystal for both types of chains.

An analysis of factors that influence shape selectivity in the cracking of long-chain alkanes on zeolite catalysts

The method and equations previously developed for conformational analysis of linear alkanes are used to calculate the free energy differences between heavily coiled (four gauche bonds or more) and extended forms. Since a gauche bond is similar in its steric requirements to a methyl side chain, it follows that chain straightening is a required step (preequilibrium) for the reaction of linear alkanes inside the channels of small- and medium-pore zeolites. Any energy advantage from attractive van der Waals interactions with the walls of the channels can be gained only after the chain uncoils. For a 20-carbon chain the preequilibrium retards the reaction inside the pores relative to that on the external surface by at least four powers of ten. Two other factors acting in the same direction are a requirement for proper orientation to enter the channel (worth one order of magnitude for C 20) and the retarding effect of constraining a molecule inside the channel on the cracking reaction itself. The observed selectivities require either that the intrinsic activity of sites within channels be many orders of magnitude higher than that of sites on the external surface, or that the normal alkanes reacting in the coiled form have an intrinsically higher reactivity toward cracking than the branched isomers, and the reactions occur predominantly on the external surface of the zeolite crystal for both types of chains.

Effect of cholesterol on the solubilization site and the photoionization efficiency of chlorophyl a in dipalmitoylphosphatidylcholine vesicle solutions as studied by electron spin resonance and optical absorption spectroscopies

Electron spin resonance (ESR) and optical absorption studies are carried out to investigate the solubilization site and the photoionization efficiency of chlorophyll a (Chla) in DL-..cap alpha..-dipalmitoylphosphatidylcholine (DPPC) vesicles due to varying concentration of cholesterol. With an increase in the cholesterol concentration, the optical absorption intensity of Chla decreases in liquid vesicle solutions, indicating that the local light intensity inside the vesicle decreases. Photoionization efficiencies of Chla in frozen vesicle solutions with and without potassium ferricyanide, a lipophobic electron scavenger, decrease with the cholesterol addition, while no effect of added cholesterol is seen in electron-transfer efficiency from photoexcited Chla to p-benzoquinone, a lipophilic electron scavenger, in liquid vesicle solution. Paramagnetic dipolar interaction between photoproduced Chla cation and added ferricyanide ion becomes weaker with cholesterol addition in frozen vesicle solutions. It is concluded that both Chla and p-benzoquinone move deeper into the vesicle bilayer when cholesterol is added because of an increase in the internal fluidity or alkyl chain straightening of the bilayer.

Computer simulation of sintering in powder compacts

A computer simulation procedure has been applied to study the effects of packing geometry on the sintering behaviour of two- and three-dimensional powder compacts comprised of randomly packed equivalent spheres. The shrinkage rate of the compacts was found to be very sensitive to the average particle coordination number; both two- and three-dimensional ‘dense’ compacts shrank in a homogeneous fashion, white their more loosely-packed counterparts shrank inhomogeneously and at a much slower rate for a given ‘two-sphere’ behaviour. This was shown to be associated with the phenomenon of chain straightening in the loose packings. The grain structure that evolved during sintering was characterized by the Voronoi cell network associated with the panicles, from which the geometric parameters describing the microstructure were determined.