Hydrogenation of cis‐1,4‐polyisoprene catalyzed by Ru(CHCH(Ph))Cl(CO)(PCy3)2

Hydrogenation is a useful method which has been used to improve oxidative and thermal degradation resistance of diene‐based polymers. The quantitative hydrogenation of cis‐1,4‐polyisoprene which leads to an alternating ethylene–propylene copolymer was studied in the present investigation. To examine the influence of key factors on the reaction, such as catalyst concentration, polymer concentration, hydrogen pressure, and temperature, a detailed study of the hydrogenation of cis‐1,4‐polyisoprene catalyzed by the Ru complex, Ru(CH?CH(Ph))Cl(CO)(PCy₃)₂ was carried out by monitoring the amount of hydrogen consumed. Infrared and ¹H‐NMR spectroscopic measurements confirmed the final degree of hydrogenation. The hydrogenation of cis‐1,4‐polyisoprene followed pseudo‐first‐order kinetics in double‐bond concentration up to high conversions of double bond, under all sets of conditions studied. The kinetic results suggested a first‐order behavior with respect to total catalyst concentration as well as with respect to hydrogen pressure. The apparent activation energy for the hydrogenation process, obtained from an Arrhenius plot, was 51.1 kJ mol^?1 over the temperature range of 130 to 180°C. Mechanistic aspects of the catalytic process are discussed. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3259–3273, 2004     

Hydrogenation of natural rubber in the presence of OsHCL(CO)(O2)(PCY3)2: Kinetics and mechanism

The homogeneous catalyst precursor, OsHCl(CO)(O₂)(PCy₃)₂, was utilized for the hydrogenation of natural rubber to convert the unsaturated structure to a saturated form, providing an alternating ethylene‐propylene copolymer. A detailed kinetic investigation was carried out by monitoring the amount of hydrogen consumption during the reaction using a gas‐uptake apparatus. ¹H NMR spectroscopy was used to determine the final olefin conversion to the hydrogenated product. Kinetic data, collected according to a statistical design framework, defined the influence of catalyst and polymer concentration, hydrogen pressure, and reaction temperature on the catalytic activity. The kinetic results indicated that the hydrogenation rate exhibited a first‐ shifted to zero‐order dependence on hydrogen at lower hydrogen pressure, which then decreased toward an inverse behavior at pressures higher than 41.4 bar. The hydrogenation was also observed to be first‐order with respect to catalyst concentration, and an apparent inverse dependence on rubber concentration was observed due to the impurities in the rubber. The hydrogenation rate was dependent on reaction temperature, and the apparent activation energy over the temperature range of 125–145°C was found to be 122.76 kJ/mol. Mechanistic aspects of the hydrogenation of natural rubber in the presence of OsHCl(CO)(O₂)(PCy₃)₂ were proposed on the basis of the observed kinetic results. The addition of some acids and certain nitrogen containing materials showed an effect on the hydrogenation rate. The thermal properties of hydrogenated natural rubber indicated that the thermal stability increased with increasing % hydrogenation of the rubber. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4499–4514, 2006     

Hydrogenation of natural rubber latex in the presence of OsHCl(CO)(O2)(PCy3)2

Hydrogenation is an important method of chemical modification, which improves the physical, chemical, and thermal properties of diene elastomers. Natural rubber latex (NRL) can be quantitatively hydrogenated to provide a strictly alternating ethylene–propylene copolymer using a homogeneous osmium catalyst OsHCl(CO)(O₂)(PCy₃)₂. A detailed kinetic investigation was carried out by monitoring the amount of hydrogen consumption during the reaction using a gas‐uptake apparatus. The kinetic results of NRL hydrogenation indicated that this system had a second‐order dependence of the hydrogenation rate on hydrogen pressure and then decreased toward a zero‐order dependence for hydrogen pressures above 13.8 bar. The hydrogenation was also observed to be first‐order with respect to catalyst concentration and inverse first‐order on rubber concentration due to impurities present in the rubber latex. Additions of a controlled amount of acid demonstrated a beneficial effect on the hydrogenation rate of NRL. The temperature dependence of the hydrogenation rate was investigated and an apparent activation energy (over the range of 120–160°C) was calculated as 57.6 kJ/mol. Mechanistic aspects of this catalytic process are discussed on the basis of kinetic results. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 640–655, 2006     

Hydrogenation of cis‐1,4‐poly(isoprene) catalyzed by OsHCl(CO)(O2)(PCy3)2

The quantitative hydrogenation of cis‐1,4‐poly(isoprene) (CPIP) provides an easy entry to the alternating copolymer of ethylene–propylene, which is difficult to prepare by conventional polymerization. The homogeneous hydrogenation of CPIP, in the presence of OsHCl(CO)(O₂)(PCy₃)₂ as catalyst, has been studied by monitoring the amount of hydrogen consumed during the reaction. The final degree of olefin conversion measured by computer‐controlled gas uptake apparatus was confirmed by infrared spectroscopy and ¹H nuclear magnetic resonance analysis. Kinetic experiments for CPIP hydrogenation in toluene solvent indicate that the hydrogenation rate is first order with respect to catalyst and carbon–carbon double bond concentration. A second‐order dependence on hydrogen concentration for low values and a zero‐order dependence for higher values of the hydrogen concentration was observed. The apparent activation energy for the hydrogenation of CPIP over the temperature range of 115–140°C was 109.3 kJ/mole. Mechanistic aspects of this catalytic process are discussed. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 142–152, 2003     

Preparation and graft‐copolymerization of hydrogenated natural rubber in latex stage

Preparation and graft‐copolymerization of hydrogenated natural rubber are performed in latex stage after removal of proteins from the rubber with urea and surfactant. Hydrogenation of deproteinized natural rubber (DPNR) latex is carried out with palladium catalyst under hydrogen atmosphere. The hydrogenated DPNR (HDPNR) is crosslinked with a peroxide followed by graft‐copolymerization of styrene (Sty) and acrylonitrile (AN) in latex stage in order to prepare a graft‐copolymer of crosslinked HDPNR with poly(Sty‐co‐AN) (HDPNR‐graft‐PSAN). Characterization of the products is performed by nuclear magnetic resonance spectroscopy. The conversion of hydrogenation is investigated with respect to the catalyst feed, acidity (pH), and dry rubber content. In the resulting HDPNR‐graft‐PSAN, mole fraction of AN and Sty is 1.4 and 5.8 mol %, respectively. The graft‐copolymer is used to improve properties of PSAN as an impact modifier. The Charpy impact strength of crosslinked HDPNR‐graft‐PSAN/PSAN is about eight times as high as that of PSAN. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42435.     

Density‐Dependent Formation of the Pure trans‐Isomer of the Unsaturated Alcohol by Selective Hydrogenation of Citral in Supercritical Carbon Dioxide

Selective hydrogenation of citral to unsaturated alcohol [geraniol (trans) + nerol (cis)] was carried out in supercritical carbon dioxide (scCO₂) using an MCM‐41 supported plantinum catalyst (∼1 wt% Pt). A remarkable rate of isomerization of the unsaturated alcohol [nerol (cis) to geraniol (trans)] during the hydrogenation of citral was achieved simply by tuning the density of CO₂. Optimum reaction conditions were developed to obtain only geraniol (trans) with a selectivity of 98.8% and citral conversion of 99.8%. A significant change in the cis:trans ratio of the product (1:82.3) from the substrate (1:1.3) was observed depending on the various reaction parameters like carbon dioxide and hydrogen pressure, reactant concentration, reaction time and, particularly, the total selectivity for unsaturated alcohol [geraniol (trans) +nerol (cis)]. It has been observed that the presence of hydrogen is necessary for isomerization. Our results were explained in terms of a density‐dependent, two‐step model. The kinetic behaviour shows that the rate of isomerization was higher in scCO₂ compared to other organic solvents and the pure form of geraniol (trans) was obtained exclusively. A probable reaction pathway was proposed in order to explain the isomerization during hydrogenation of citral in scCO₂ medium.     

New crosslinked polyurethane elastomers with various physical properties from natural rubber derivatives

New hydroxytelechelic cis‐1,4‐oligoisoprenes exhibiting variable values and distributions of the hydroxyl functionality were successfully prepared. The synthesis reactions involved chemical modifications of carbonyl telechelic cis‐1,4‐polyisoprene, which was obtained by controlled degradation of synthetic or natural rubber. These new oligomers were reacted with toluene diisocyanate to elaborate crosslinked polyurethane elastomers. The thermomechanical properties of the prepared polyurethanes were investigated. The results show a strong relationship between the chemical structures and properties. This work mainly shows the potentiality of making new crosslinking polyurethane materials with controlled and various properties from natural rubber, a renewable resource. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Synthesis and characterization of low relative molecular weight trans‐1,4‐poly(isoprene)

Low relative molecular weight trans‐1,4‐polyisoprene oligomers were synthesized successfully by bulk precipitation and solution polymerization with supported titanium catalyst using hydrogen as relative molecular weight modifier. The effects of polymerization conditions on intrinsic viscosity ([η]), catalyst efficiency (CE) and structure of polymer were studied. Increasing the hydrogen pressure resulted in the decrease of [η] of the polymer. With the increasing of hydrogen pressure and reaction temperature, CE decreased but still maintained above 2500 g polymer/g Ti. The percentage composition of (trans‐1, 4‐unit) in the polymer was over 90% in all results. The crystallinity of polymer was about 50–60% with T_m being about 60°C. The relative molecular weight distribution index (MWD) was quite difference according to the polymerization method. While number average molecular weight (M_n) exceeded 860, polymer turned from viscous materials to fragile wax materials, and then to toughness materials at 1800. Dynamic property testing showed that the additional of this oligomer could increase the wet‐skid resistance of the rubber. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Synthesis of cis‐1,4‐polyisoprene with Al‐Ti catalysts modified by different electron donor reagent

The preparation of the high cis ?1,4‐polyisoprene by Ziegler‐Natta catalysis system was studied. The effect of Al‐Ti catalysts modified by ethers with different structures which are different electron donor reagent on polymerization of isoprene has been mainly investigated. By the measurement method of the monomer conversion, FTIR, and ¹H NMR spectroscopy, the influence of, respectively, added diphenyl ether, anisole, dibutyl ether, or methyl tert ‐butyl methyl ether as a third active component on the heterogeneous TiCl₄‐Al(i ‐Bu)₃‐ether catalyst activity and microstructure of synthetic polyisoprene was analyzed. By the adding of diphenyl ether or dibutyl ether, the process of prefabricated heterogeneous catalyst is quickly and catalyst particle quantity is large. The polymerization conversion is high and the microstructure cis ?1,4 content of the resulting polymer can reach 92%. But Al(i ‐Bu)₃ added anisole or methyl tert ‐butyl methyl ether hard to cooperate with TiCl₄. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133 , 44357.     

Preparation of cis‐1,4‐Polyisoprene Electrospun Microfibers

cis‐1,4‐Polyisoprene, a significant industrial elastomer, is electrospun into different nanostrucutures. Cis‐1,4‐polyisoprene electrospun fibers are prepared from cis‐1,4‐polyisoprene solutions in dichloromethane or chloroform and characterized by environmental scanning electron microscope and Fourier‐transform infrared spectroscopy. ESEM observation reveals that the cis‐1,4‐polyisoprene fibers show a bamboo‐like morphology with a nearly constant node distance, a diameter of 20–60 µm and a length of about 300 µm. In addition, within the individual nodes parallel grooves are clearly seen, which is very promising for their use in microprinting in the field of microelectronics. Smooth cis‐1,4‐polyisoprene fibers with a diameter of 5–8 µm can be obtained via electrospinning its chloroform solutions. In contrast to most polymers, the jet of cis‐1,4‐polyisoprene does not split during the electrospinning processes, which facilitates the collection of highly aligned fibers by using a rotating mandrel as a ground target.

     

Enantioselective Synthesis of (−)‐CP‐55940 via Ruthenium‐ Catalyzed Asymmetric Hydrogenation of Ketones

A new and efficient catalytic asymmetric synthesis of the potent cannabinoid receptor agonist (−)‐CP‐55940 has been developed by using ruthenium‐catalyzed asymmetric hydrogenation of racemic α‐aryl ketones via dynamic kinetic resolution (DKR) as a key step. With RuCl₂‐SDPs/diamine [SDPs=7,7′‐bis(diarylphophino)‐1,1′‐spirobiindane] catalysts the asymmetric hydrogenation of racemic α‐arylcyclohexanones via DKR provided the corresponding cis‐β‐arylcyclohexanols in high yields with up to 99.3% ee and >99:1 cis‐selectivities. Both ethylene ketal group at the cyclohexane ring and ortho‐methoxy group at the phenyl ring of the substrates 6 have little effect on the selectivity and reactivity of the hydrogenations. Based on this highly efficient asymmetric ketone hydrogenation, (−)‐CP‐55940 was synthesized in 13 steps (the longest linear steps) in 14.6% overall yield starting from commercially available 3‐methoxybenzaldehyde and 1,4‐cyclohexenedione monoethylene acetal.     

Hydrogenation of dienes with cationic rhodium complexes

Methylcis-9,cis-12-octadecadienoate (methyl linoleate;c9,c12), itst10,t12 andt10,c12 isomers and methylcis-9-octadecenoate (methyl oleate;c9) were hydrogenated with rhodium complexes, the active species of which consisted of [RhL₂]⁺ and [RhL₂H₂]⁺ with ligands L=P(C₂H₅)₂C₆H₅ (catalyst A) P(i-C₄H₉)₃ (catalyst B) and P(CH₃)₃ (catalyst C). Using these catalysts the influence of steric effects on the reaction mechanism of hydrogenation of dienes was studied. The reactions were carried out in 2-propanol at atmospheric hydrogen pressure and ambient temperature. During hydrogenation ofc9 on catalysts A and B, geometrical isomerization mainly occurred, whereas on catalyst C some positional isomerization also took place.C9,c12 was almost exclusively hydrogenated via conjugated intermediates on catalyst A. On catalyst C, one of the double bonds was hydrogenated directly, in most cases. In the absence of hydrogen, catalysts A and B conjugatedc9,c12 very fast. The conjugation activity of catalyst C was much lower. Catalyst C showed a high 1,5-shift activity for the conjugatedcis, trans andtrans, cis intermediates during hydrogenation, in contrast to catalysts A and B, which showed a poor activity in this respect.T10,t12 was hydrogenated almost exclusively via 1,4-addition of hydrogen to thecisoid conformation, whereas only a slight preference was found in this mechanism over 1,2-addition for the hydrogenation oft10,c12. On the sterically unhindered catalysts A and C thetrans double bond int10,c12 was preferentially hydrogenated whereas on catalyst B, with its bulky ligands, thecis double bond was reduced faster than thetrans double bond.     

Effect of Isomerization on the Reversible Reaction of Hydrogenation‐Dehydrogenation of ortho‐Terphenyl on a Pt/C Catalyst

Reversible catalytic hydrogenation‐dehydrogenation reactions of ortho‐terphenyl (o‐terphenyl) as one of the most promising materials for hydrogen storage were investigated. The conversion of o‐terphenyl is 100 % and the selectivity to perhydro‐o‐terphenyl reaches 99 % in the first cycle of hydrogenation on a commercial Pt/C catalyst. The dehydrogenation process in a flow reactor is accompanied by the formation of isomerization products of o‐terphenyl, in particular, exhaustively and partially hydrogenated compounds formed from meta‐terphenyl (m‐terphenyl) and triphenylene. These substrates are subject to hydrogenation independently during the second hydrogenation cycle, which reduces the selectivity of recyclization of o‐terphenyl. Dodecahydrotriphenylene demonstrates a higher reactivity during the second cycle of dehydrogenation compared to perhydro‐o‐terphenyl. The amount of generated hydrogen is consistent with the kinetic data.     

Clay nanolayer reinforcement of cis‐1,4‐polyisoprene and epoxidized natural rubber

Conditions were established for dispersing clay nanolayers into both cis‐1,4‐polyisoprene (synthetic) natural rubber (NR) and epoxidized natural rubbers (ENR) having 25 or 50 mol % epoxide. The clay was a sodium montmorillonite and was used as a pristine layered silicate or as organically modified layered silicates to make the galleries more hydrophobic and thus more compatible with the elastomers. The chemical modifications were carried out using an ion‐exchange reaction with alkyl ammonium cations. Incorporation of the clays into the elastomers was achieved by mixing the components themselves in a standard internal blender or by mixing dispersions of them in toluene or methyl ethyl ketone. X‐ray diffraction results indicated intercalation of NR and ENR into the silicate interlayers, followed by exfoliation of the silicate layers into the elastomer matrices. Of primary interest was the effect of the intercalated and exfoliated clays on the mechanical properties of the elastomers. The reinforcing effects obtained were found to depend strongly on the extent of the dispersion of the silicate layers into the rubber matrices. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1391–1403, 2001     

Hydrogenation of palm oil in near‐critical and supercritical propane

Palm oil was hydrogenated in a single‐phase mixture with propane and hydrogen. This was done in a small (0.5 ml), continuous fixed‐bed reactor, using a 1% Pd/C catalyst. Temperature (65—135 °C), H₂/TG ratio (4—50 mol/mol) and residence time (0.2—2.0 s) were varied systematically to assess the iodine value (IV) as a function of these three variables. The substrate concentration was 1 wt‐%. The IV was dependent mainly on temperature and residence time. At 120 °C and a residence time of 2.0 s, full hydrogenation was achieved. The trends observed indicate that this is possible even at lower temperatures, if the residence time is increased further. Unexpectedly, the hydrogen concentration (i.e. the H₂/TG ratio) was of minor importance, which can be a sign of either H₂‐saturation of the catalyst or a phase‐split of the reaction mixture with resulting mass transport limitations for the hydrogen. Unfortunately, the catalyst showed strong signs of deactivation very early in the experiments, possibly due to impurities in the feedstock and/or to coke formation.     

Hydrogenation of natural rubber using nickel 2-ethylhexanoate catalyst in combination with triisobutylaluminum

Kinetic studies for the homogeneous hydrogenation of natural rubber, in the presence of nickel 2-ethylhexanoate and triisobutylaluminum, have been carried out by monitoring the change in hydrogen pressure in a Parr reactor of fixed volume. ¹H-NMR spectroscopy provides the measurements of the extent of hydrogenation. The reaction kinetics, in the presence of a fixed amount of catalyst, showed an overall second-order kinetic with respect to [H₂] and [CC]. The reaction has a relatively low apparent activation energy of 26.0 kJ mol^-1 and is therefore suitable for the hydrogenation of natural rubber at ambient conditions to minimize side reactions. The impurities in commercial rubbers have a slight effect on the catalyst activity. © 1996 John Wiley & Sons, Inc.     

Kinetics of sulfur vulcanization of 1,4‐cis‐polyisoprene by differential scanning calorimetry

The vulcanization of 1,4‐cis‐polyisoprene was studied by differential scanning calorimetry under isothermal and nonisothermal conditions. On the basis of thermal characteristics obtained, the kinetic parameters of crosslinking (the induction period, maximum rate, and effective energy of activation) were determined. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 754–757, 2007     

Cationic polymerization of isoprene using zinc halides as co‐initiators: towards well‐defined oligo(isoprene)s under mild conditions

The activity of ZnX₂‐based initiating systems (X = Cl, Br, I) in the cationic polymerization of isoprene was studied. The highest activity was achieved when co‐initiator (ZnX₂) was solubilized in a minimal amount of strongly coordinating solvent such as diethyl ether or acetone and when trichloroacetic acid was used as an initiator. It is shown that the polymerization rate increased in the series ZnI₂ < ZnCl₂ < ZnBr₂. An increase of initiator concentration and temperature also led to an increase of the polymerization rate. The obtained polyisoprenes did not contain high‐molecular‐weight and insoluble fractions and were characterized by low number‐average molecular weight and relatively narrow molecular weight distribution. Unsaturation of polyisoprene decreased with an increase of monomer conversion and reaction temperature. The unsaturated part of the polyisoprene chain possessed predominantly 1,4‐trans microstructure with regular and inverse addition, whereas the 1,2‐ and 3,4‐isomers were present as minor components. It is shown that the synthesized low‐molecular‐weight polyisoprenes are effective plasticizers for rubber compounds in the manufacture of tyres. © 2012 Society of Chemical Industry     

1,3‐butadiene polymerization using Co/Nd‐based Ziegler/Natta catalyst: Microstructures and properties of butadiene rubber

Among Ziegler‐Natta catalysts used for 1,3‐butadiene (1,3‐BD) polymerization, the advantage of a neodymium (Nd)‐based catalyst is that it provides butadiene rubber (BR) with a high content of cis?1,4 configuration and a low amount of vinyl?1,2 units. Whereas, a cobalt (Co)‐based catalyst can produce BR with a low content of trans?1,4 configuration. Thus, this research was aimed to prepare BR containing a high content of cis?1,4 configuration with low amounts of both trans?1,4 and vinyl?1,2 units using a combination of Nd‐ and Co‐based Ziegler/Natta catalysts with triethyl aluminum (TEAL) and diethyl aluminum chloride (DEAC) acting as a co‐catalyst and a chlorinating agent, respectively. The effects of the molar Co/Nd ratio, TEAL concentration, DEAC loading, 1,3‐BD content, solvent type, and reaction temperature on % conversion, microstructures, molecular weight, and molecular weight distribution of the obtained BR (Co/Nd‐BR) were evaluated. The Co/Nd‐BR having >97% of cis?1,4 configuration, <2% of trans?1,4 structure, and <1% of vinyl?1,2 unit with >80% conversion was achieved when 3.01 M of 1,3‐BD concentration was treated in a toluene/cyclohexane mixture (7/3 [w/w]). The Co/Nd‐BR exhibited no gel formation with high mechanical performance, which was equivalent to commercial BR produced from a Nd‐based catalyst system. POLYM. ENG. SCI., 55:14–21, 2015. © 2014 Society of Plastics Engineers