On the structure of polybutadiene: 4. 13C n.m.r. spectrum of polybutadienes with cis-1,4-, trans-1,4- and 1,2-units

¹³C n.m.r. spectra of polybutadienes with different contents of cis-1,4- and 1,2-units are assigned in the resonance region of the cis and trans carbon double bond. The observed signals are assigned to configurational triads.     

Icosahedron cage occupancy of structure-H hydrate helped by Xe -1,1-, cis -1,2-, trans-1,2-, and cis-1,4-dimethylcyclohexanes

Four mixtures of 1,1-, cis-1,2-, trans-1,2-, and cis-1,4-dimethylcyclohexanes (hereafter abbreviated DMCH) including H₂O and Xe have been investigated in a temperature range over 274.5 K and a pressure range up to 2.7 MPa. The 1,1-DMCH and cis-1,2-DMCH generate the structure-H hydrate in the temperature range up to 295.2 and 280.2 K, respectively. Especially, very large depression of equilibrium pressure has been observed in the structure-H 1,1-DMCH hydrate system. On the other hand, neither trans-1,2-DMCH nor cis-1,4-DMCH generates the structure-H hydrate in the present temperature range. It is an important finding that the cis-1,4-DMCH does not generate the structure-H hydrate in the presence of Xe, while the mixture of cis-1,4-DMCH and methane generates the structure-H hydrate.     

Synthesis of spherical trans-1,4-polyisoprene/trans-1,4-poly(butadiene-co-isoprene) rubber alloys within reactor

Trans-polydiene rubber family as high-performance tire stock possessed excellent dynamic properties, including excellent anti fatigue, low rolling resistance, low heat buildup, good green strength, and low abrasion loss. Here, Reactor Granule Technology (RGT) was introduced into the field of synthetic rubber for the first time to produce trans-1.4-polyisoprene/trans-1,4-poly(butadiene-co-isoprene) rubber alloy, which showed significant improvement in rubber synthetic technologies and great development in abundance of trans-rubber family. A series of trans-polyisoprene alloys with excellent spherical morphology were successfully synthesized by using sequential multistage polymerization. The alloy was fractionated into four fractions by temperature-gradient fractionation, and the fractionation was analyzed by ¹H NMR, ¹³C NMR, DSC and WAXD.     

Synthesis and properties of polyimides derived from cis- and trans-1,2,3,4-cyclohexanetetracarboxylic dianhydrides

Cis-1,2,3,4-cyclohexanetetracarboxylic dianhydride (cis-1,2,3,4-CHDA) was synthesized. It was found that under such conditions as heating or boiling in acetic anhydride, cis-1,2,3,4-CHDA could be converted to its trans-isomer. The process of thermal isomerization was monitored by ¹H NMR spectra and the mechanism of conversion was proposed. Their absolute structures of cis- and trans-1,2,3,4-CHDAs were elucidated by single crystal X-ray diffraction. The polycondensations of cis- and trans-1,2,3,4-CHDAs with aromatic diamines such as 4,4′-oxydianiline (ODA), 4,4′-methylenedianiline (MDA), 4,4′-diamino-3,3′-dimethyldiphenylmethane (DMMDA), 4,4′-bis(4-aminophenoxy)benzene (TPEQ), 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) were studied. It is easy to obtain higher molecular weight polyimides from trans-1,2,3,4-CHDA using conventional one-step or two-step methods. However, higher molecular weight polyimides derived from cis-1,2,3,4-CHDA could not be prepared by the usual methods (solid content ca. 10%) owing to the trend of forming cyclic oligomers. Increasing the concentration of monomers could give higher molecular weight cis-polymers. All of the cis-polyimides were soluble at room temperature in aprotic polar solvents and phenolic solvents and some of them even soluble in chloroform and tetrahydrofuran, while the corresponding trans-polymers showed poor solubility as compared to cis-polymers. All of the polyimides showed good thermal stability with the 5% weight loss temperatures in air over 415 °C. Furthermore, polyimides derived from cis-1,2,3,4-CHDA have higher glass transition temperatures (T_gs) than corresponding trans-polyimides. The flexible polyimide films possessed a tensile modulus range of 2.1-3.6 GPa, a tensile strength range of 42-116 MPa, an elongation at break of 4-18%. These polyimides exhibited cutoffs at wavelengths around 270 nm and were entirely colorless. All the polyimides showed amorphous pattern according to Wide angle X-ray diffraction measurements. The differences of polymerization and properties were explained by the structural changes resulted from isomerism.     

Sequence distribution of polyisoprenes

The ¹³C n.m.r. spectra of hydrogenated polyisoprenes were investigated. Polyisoprenes were prepared with n-BuLiEt₂O, Ti(OBu)₄AlEt₃, Col₂ROHPhMgBr, Alfin, and radical catalysts. These polymer were hydrogenated with p-toluenesulphonylhydrazide. ¹³C n.m.r. signals were assigned for triad sequences of

arising respectively from 1,4- and 3,4-units. Signals due to head-to-head and tail-to-tail linkages of 1,4- and 3,4-units were also assigned. It is disclosed that 1,4- and 3,4-units were randomly distributed in n-BuLiEt₂O, Col₂ROHPhMgBr, and radical catalysed polyisoprenes, rather alternating in Ti(OBu)₄AlEt₃ catalysed polyisoprene, and slightly in blocks in Alfin polyisoprene. It is also confirmed that Alfin, radical, and cobalt catalysed polyisoprenes contained significant amounts of head-to-head and tail-to-tail linkages of 1,4- and 3,4- units. Signals due to the tacticity of the polyads of 3,4-units were also discussed.     

Gas hydration of trans-1,2-dimethylcyclohexane with cooperative assistance of methane and cis-1,2-dimethylcyclohexane

It has been regarded that the limit of the largest cage occupancy for the structure-H hydrate is between the 1,2-dimethylcyclohexane stereo-isomers, because the cis-isomer is able to generate the structure-H hydrate in the presence of methane while the trans-isomer is not. In the present study, gas hydration of trans-1,2-dimethylcyclohexane in the presence of methane and cis-1,2-dimethylcyclohexane is found from stability boundaries for the structure-H hydrate system.     

Studies on the thermal degradation of cis-1,4-polyisoprene

cis-1,4-Polyisoprene is pyrolyzed under an inert atmosphere at different temperature ranges by using differential thermal analysis and pyrolysis-gas chromatography. Fourier transform-infrared is used for investigating the pyrolysis residue. The pyrolyzed gaseous products are analyzed by gas chromatography. The effect of temperature range on the relative yields of the major decomposition products is studied. Based on these data, the thermal degradation mechanism of cis-1,4-polyisoprene is also proposed.     

Transport of water in synthetic cis-1,4-polyisoprenes and natural rubber

The sorption, permeation and diffusion of water in two synthetic cis-1,4-polyisoprenes and natural rubber and of methanol in natural rubber have been measured in the range 25 to 60°C. At the higher relative pressures the diffusion coefficient in all systems decreases with concentration and activation energies for diffusion increase with concentration. The results are consistent with increased clustering of the sorbed penetrant at higher relative pressures which renders an increasing fraction of the penetrant relatively immobile. The effect of polar impurities on the sorption and diffusion process is discussed to account for differences in behaviour of the various samples at lower relative pressures.     

Metal dependent control of cis-/trans-1,4 regioselectivity in 1,3-butadiene polymerization catalyzed by transition metal complexes supported by 2,6-bis[1-(iminophenyl)ethyl]pyridine

Fe(III), Cr(III), Fe(II), Co(II) and Ni(II) chloride complexes supported by 2,6-bis[1-(iminophenyl)ethyl]pyridine have been synthesized and characterized along with single crystal X-ray diffraction. These complexes, in combination with MAO, have been examined in butadiene polymerization. The catalytic activity and regioselectivity are strongly controlled by metal center and cocatalyst (MAO/Co ratio dependent in the case of Co(II) complex). The activity decreases in the order of Fe(III) > Co(II) > Cr(III) ≈ Ni (II) complexes, in consistent with the space around the metal center. Polybutadiene with different microstructure content, from high trans-1,4 units (88-95% for iron(III) and Cr(III)), medium trans-1,4 and cis-1,4 units (55% and 35%, respectively, for iron(II)) to high cis-1,4 units 79% for Co(II) and 97% for Ni(II) can be easily achieved by varying of the metal center. In addition, mechanism speculation is also presented to elucidate the dependence of catalytic behaviors on metal and cocatalyst.     

Kinetics of graft polymerization of styrene on cis-1,4-polybutadiene

By measurements of polymerization rate, grafting efficiency and number-average molecular weight of free and grafted polystyrene, the α-dicumyl peroxide initiated polymerization of styrene on cis-1,4-polybutadiene at 100°C and at low extents of reaction was studied. The polymerization rate and the polystyrene molecular weights decreased with the rubber content of the solution. The grafting efficiency was found to be substantially independent of the peroxide concentration, but to increase with rubber content. A rigorous mathematical model of the reaction was developed, from which it has been possible to confirm the proposed mechanism and to establish the value of many kinetic constants.     

Vibrational dynamics of trans-1,4-polyisoprene (β-form)

The earlier work on the vibrational dynamics of trans-1,4-polyisoprene (β-TPI) suffered from several infirmities. This work has been redone with all infirmities removed. It not only leads to a few different assignments but also better values of interactive constants. Two significant outcomes of the present work are; one change in the profile of the dispersion curves and repulsion between the two acoustic modes at the phase values away from the zone center. The present work leads to a fuller interpretation of observed spectra. Several observed unassigned modes have been accounted for due to the presence of regions of high density-of-states such as von Hove type singularities. Predictive values of heat capacity as a function of temperature are reported.     

The melting process and the rigid amorphous fraction of cis-1,4-polybutadiene

A thorough analysis of the melting behavior of cis-1,4-polybutadiene (cis-PBD) is detailed in this contribution. Isothermal crystallization at −26 °C, followed by cooling, provides a three-phase structure composed of a mobile amorphous fraction equal to 0.41₃, a crystallinity of 0.27₇, and a rigid amorphous fraction of 0.31₀. Similar to many other polymers, cis-PBD displays multiple melting after isothermal crystallization, and up to three main endotherms can be evidenced by calorimetry, in dependence of the scanning rate. The results of conventional and temperature-modulated calorimetry analyses presented in this contribution suggest a link between multiple melting and devitrification of the rigid amorphous fraction in cis-PBD. The small endotherm located a few degrees above the crystallization temperature appears to be caused by concurrent partial mobilization of both the crystal and the rigid amorphous portions. Additional partial mobilization of rigid amorphous segments seems to take place at around −11 °C, and it is only above this temperature that large reorganization of the crystal phase becomes possible, allowing partial melting and recrystallization/annealing/crystal perfection.     

Linear and nonlinear optical properties of trans- and cis-poly(1-ethynylpyrene) based sonogel hybrid materials

The synthesis of sol-gel materials induced by ultrasonic action (sonolysis) is implemented as an alternative method for the fabrication of highly pure organic-inorganic composites with good monolithic and optical properties. The resulting SiO₂ glass exhibits high porosity and allows the inclusion of organic compounds in the colloidal sol-state. In this work, optical properties of trans-poly(1-ethynylpyrene) (trans-PEP) and cis-poly(1-ethynylpyrene) (cis-PEP) (M_w=24,000 g/mol) incorporated in this kind of gels were studied by absorption and fluorescence spectroscopies. Absorption spectra of the polymers showed that trans-PEP possesses a higher degree of conjugation than its homologue cis-PEP in sol-gel. Intramolecular interactions occur between adjacent pendant pyrene units (associated pyrenes) present in each polymer, giving rise to static excimer emissions, strongest in cis-PEP because of the shorter distances between aromatic rings. The results were compared to those previously reported for these polymers in solution. Besides, trans- and cis-PEP exhibited nonlinear optical properties like third harmonic generation (THG), which were measured in sol-gel phase for spin-coated film samples.     

Highly efficient cis-1,4 polymerisation of isoprene using simple homoleptic amido rare earth-based catalysts

Polymerisation of isoprene was performed by means of ternary catalytic systems made of Ln[N(SiMe₃)₂]₃ (Ln = Nd (1), Y (2))/borate/Al(ⁱBu)₃ combinations (borate = [HNMe₂Ph][B(C₆F₅)₄] (HNB), [CPh₃][B(C₆F₅)₄] (TB) or B(C₆F₅)₃ (Barf)). With borate activator B(C₆F₅)₃, highly active (up to 4700 kg mol⁻¹h⁻¹) and selective catalysts are obtained with both neodymium and yttrium metals, leading up to 96% cis-1,4-polyisoprene.     

Role of stearic acid in the strain-induced crystallization of crosslinked natural rubber and synthetic cis-1,4-polyisoprene

Strain-induced crystallization of crosslinked natural rubber (NR) and its synthetic analogue, cis-1,4-polyisoprene (IR), both mixed with various amounts of stearic acid (SA), were investigated by time-resolved X-ray diffraction using a powerful synchrotron radiation source and simultaneous mechanical (tensile) measurement. No acceleration or retardation was observed on NR in spite of the increase of SA amount. Even the SA-free IR crystallized upon stretching, and the overall crystallization behavior of IR shifted to the larger strain ratio with increasing SA content. No difference due to the SA was detected in the deformation of crystal lattice by stress for both NR and IR. These results suggested that the extended network chains are effective for the initiation of crystallization upon stretching, while the role of SA is trivial. These behaviors are much different from their crystallization at low temperature by standing, where SA acts as a nucleating agent.     

Effect of pressure on phase transition and morphology of bulk poly(trans-1,4-butadiene)

The effect of pressure on the melting, solid-solid transition, and crystallization of poly(trans-1,4-butadiene) (PTBD) was investigated using the pressure range of 1–3000 kg/cm². D.t.a. measurements showed that, the melting and transition temperatures increase with increasing pressure, whose pressure coefficients are 38°C per 1000 kg/cm² and 22°C per 1000 kg/cm², respectively. These values were in fairly close agreement with those calculated from the Clausius-Clapeyron equation. Morphological studies using electron microscope and small-angle X-ray scattering method revealed that, the samples crystallized with relatively small supercoolings under normal or high pressure, are formed of distinct lamellae 400–800 Å thick. The lamellar thickness was inappreciably dependent on crystallization pressure. The significant effect of pressure on crystallization was recognized in a tendency of the crystallinity to increase, with increasing crystallization pressure. This pressure effect was explained by the mechanism that, the increased pressure might make the packing of molecular chains in liquid, more dense and that the secondary crystallization might be accelerated, to increase the lateral dimensions of lamellae.     

Thermal, optical, electrochemical properties and conductivity of trans- and cis-poly(1-ethynylpyrene): a comparative investigation

The thermal, optical and electrochemical properties of trans-poly(1-ethynylpyrene) (trans-PEP) and cis-poly(1-ethynylpyrene) (cis-PEP) have been studied as a function of polymer backbone configuration and internal stacking. Absorption spectra of the polymers showed that trans-PEP possesses a higher degree of conjugation than its homologue, cis-PEP. Intramolecular interactions occur between adjacent pendant pyrene units (associated pyrenes) present in each polymer, giving rise to static excimer emissions, strongest in cis-PEP because of the shorter distances between aromatic rings. Data resulting from excitation spectra and fluorescence decay profiles proved that such interactions take place in the ground state. Cyclic voltammetry of trans- and cis-PEP exhibited irreversible behaviors with different oxidation potentials as a result of their dissimilar geometry.     

Salt effect on cationic copolymerization between cis-and trans - ethyl propenyl ethers

In order to study the common-ion effect on monomer reactivity ratios, cationic copolymerization between cis- and trans-ethyl propenyl ethers was carried out at 0°C by using iodine or acetyl perchlorate as initiator. In toluene, the cis isomer was several times more reactive than the trans isomer regardless of the kind of initiators employed. When a common-ion salt (tetra-n-butylammonium iodide, triiodide, or perchlorate) was added to the copolymerization system in ethylene dichloride, the monomer reactivity ratios were changed approaching those observed in a non-polar solvent such as toluene. On the other hand, in nitroethane the monomer reactivity ratios were only slightly affected by the addition of common-ion salts. These noticeable common-ion effects on monomer reactivity ratios in the ‘cis-trans’ copolymerization were satisfactorily explained on the basis of the steric hindrance between the substituents of monomers and a bulky propagating chain end.     

Molecular dynamics simulation of cis-1,4-polybutadiene. 2. Chain motion and origin of the fast process

Molecular dynamics (MD) simulations of cis-1,4-polybutadiene in bulk amorphous state were performed to elucidate the origin of a fast relaxation process observed by quasielastic neutron scattering (QENS) measurements. The details of the torsional motion for each dihedral angle were investigated with the torsional auto- and cross-correlation functions for several temperatures in this study. Temperature dependence of the correlation between the torsional autocorrelation and cross-correlation functions is also evaluated. The origin of the fast process of cis-1,4-polybutadiene is found to be mainly the cooperative conformational transitions of two dihedral angles located at both sides of the CH₂-CH₂ bond when the bond is in the trans conformation. The cooperative conformational transitions exhibit even below the glass transition temperature of cis-1,4-polybutadiene. The cooperative motion appears at about 50 K below the glass transition temperature, corresponding to the Vogel-Fulcher temperature.