Microstructure Of Polyisoprene Research Articles

Synthesis, Characterization, and Catalytic Behaviors in Isoprene Polymerization of Pyridine-Oxazoline-Ligated Cobalt Complexes.

A family of pyridine-oxazoline-ligated cobalt complexes L2CoCl23a-h were synthesized and characterized. Determined via single-crystal X-ray diffraction, complexes 3a and 3d, ligated by two ligands, displayed a distorted tetrahedral coordination of a cobalt center. The X-ray structure indicated the pyridine-oxazoline ligands acted as unusual mono-dentate ligands by coordinating only to Noxazoline. Upon activation with AlEt2Cl (diethylaluminum chloride), these cobalt complexes all exhibited high catalytic activity (up to 2.5 × 106 g·molCo-1·h-1), affording cis-1,4-co-3,4-polyisoprene with molecular weights of 4.4-176 kg mol-1 and a narrow Ð of 1.79-3.42, suggesting a single-site nature of the active sites. The structure of cobalt catalysts and reaction parameters, especially co-catalysts and the reaction temperature, all have significant influence on the polymerization activity but not on the microstructure of polyisoprene.

Microstructure of Products of Isoprene Polymerization on Titanium–Magnesium Catalysts: an NMR Study

Polyisoprenes synthesized on titanium–magnesium catalysts were studied by 1H and 13C NMR spectroscopy (Larmor’s frequency for protons 700 MHz). The problems concerning the accuracy of determining the microstructure of polyisoprenes with high content of trans units are discussed. As demonstrated for a series of samples with varied configurational and isomeric composition, the cis and trans units are distributed in the macromolecules in the block (continuous) fashion, whereas the 3,4-units are distributed randomly.

Correlation of alkylaluminum cocatalyst in Nd-based ternary catalyst with the polymerization performance of isoprene

Nd-based ternary catalyst systems are widely used for rubber industry. The effect of alkylaluminum cocatalysts has been investigated for decades. However, there is a lack of structural evidence, especially the quantitative result to illustrate the inner reasons. In the present paper, the effect of different alkylaluminum cocatalysts to the polymerization performance of isoprene has been studied. Catalysts adopted in the present paper all show high 1,4-cis selectivity (above 96%) of polyisoprene and kind of alkylaluminum is not the crucial factor to affect the microstructure of polyisoprene. By comparing the polymerization performance of isoprene and the local atomic structure around Nd centers, it can be concluded that the short carbon-chain in the alkylaluminum cocatalyst is helpful to increase the percent conversion of isoprene. While the carbon-chain length and number in each alkylaluminum cocatalyst can be used to modulate the molecular weight of the synthesized polyisoprene. All the results in the present paper provide a clue to improve the Nd-based ternary catalyst for rubber synthesis.

Comprehensive structural characterization of polyisoprene synthesized via cationic mechanism

The microstructure of polyisoprene synthesized with tBuCl/TiCl4 initiating system is investigated using 1D and 2D (HSQC and HMBC) NMR spectroscopy. It is found that trans-1,4-units with regular (head-to-tail) and inverse (tail-to-tail) and (head-to-head) enchainments are predominant structures of unsaturated part of polymer chain, while 1,2- and 3,4-units are presented in minor amounts. The new methodology for the quantitative calculation of the content of different structural units in polyisoprene chain including both types of inverse trans-1,4-addition (tail-to-tail and head-to-head) is proposed. It is shown that head groups consist of tert-butyl group connected to trans-1,4-unit of polyisoprene chain. In addition, two types of chlorine-containing end groups are found (trans-4,1-Cl and 4,3-Cl), while conjugated double bonds at the chain end are totally absent. The methodology for the calculation of number-average functionality by tert-butyl head and chlorine end groups, respectively, is developed. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 2430–2442

Microstructure of polyisoprenes synthesized with titanium- and neodymium-containing catalytic systems

Polyisoprenes predominantly composed of cis-1,4-units are studied by the methods of NMR spectroscopy (600 MHz), GPC, and DSC. A comparative investigation of synthetic polyisoprene and natural rubbers shows that a small (up to 3–4 mol %) amount of polymer-chain disruptions, such as inverse additions of cis-1,4 units and units occurring in 3,4- and trans-1,4-configurations, has no effect on the local conformational structure of the polymer.

Characterisation of blends of polyisoprene and polystyrene by on‐line hyphenation of HPLC and 1H‐NMR: LC‐CC‐NMR at critical conditions of both homopolymers

Blends of polystyrene (PS) and polyisoprene (PI) were analysed by on-line hyphenation of LC at critical conditions and (1) H-NMR. Chromatography at critical conditions was established for both PS and PI. At both critical conditions, a perfect separation into the blend components was achieved. By operating at critical conditions of one blend component and size exclusion mode for the other it is possible to determine the molar mass using a suitable calibration. By using NMR as a detector, the microstructure of PI can be identified, quantified and the chemical composition of the blends can be calculated by monitoring the signal intensities of the olefinic protons of isoprene and the aromatic protons of PS.

Microstructure of polyisoprene produced by cationic polymerization: NMR spectroscopic study

High-resolution 1H and 13C NMR spectroscopy, including two-dimensional heteronuclear experiments, has been used to study the microstructure of polyisoprene produced by cationic polymerization. It is shown that macromolecules resulting from both regular and inverse additions are predominantly composed of trans-1,4-units, while 1,2- and 3,4-units are present in small amounts. NMR spectra demonstrate the absence of cis-1,4-units in the polymer, whereas broad signals (pedestals) are related to the presence of saturated structures. It is proposed to determine the content of trans-1,4-, 1,2-, and 3,4-units in cationic polyisoprene via the combined measurements of intensities of signals in the olefinic regions of 1H and 13C NMR spectra.

Novel neodymium (III) isopropoxide‐methylaluminoxane catalyst for isoprene polymerization

AbstractA novel catalyst composed of neodymium (III) isopropoxide [Nd(OiPr)3] and methylaluminoxane (MAO) was examined in isoprene polymerization. The Nd(OiPr)3‐MAO catalyst proved to be highly effective in heptane even at low [Al]/[Nd] ratios (ca. 30) to give polyisoprene that possessed high cis‐1,4 stereoregularity (> ca. 90%), a high number‐average molecular weight (Mn ∼105), and relatively narrow molecular weight distributions (Mw/Mn = 1.9–2.8). The catalyst activity increased with an increasing [Al]/[Nd] ratio from 10 to 80 as well as temperature of aging and polymerization from 0 to 60 °C. The polymerization proceeded in the first order with respect to the monomer concentration. Aliphatic solvents (heptane and cyclohexane) achieved a higher yield and Mn of polymer than toluene as a solvent. The Mw/Mn ratio remained around 2.0, and the gel permeation chromatographic curve was always unimodal, indicating that this system is homogeneous and involves a single active site. The microstructure of polyisoprene was determined by IR, 1H NMR, and 13C NMR. The cis‐1,4 contents of the final polymers stayed in the range of 90–92%, regardless of reaction conditions, indicating the high stability of stereospecificity of the catalyst. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1838–1844, 2002

Classification of polar additives with respect to their influence on the microstructure in anionic polymerization of butadiene with butyllithium by transition energy measurements

A transition energy scale was established on the basis of di-Me indoaniline as org. dye for 23 mono- and bidentate polar additives. From polymn. expts. of isoprene with s-butyllithium in mixts. of n-hexane and the selected bases, the microstructure of polyisoprenes was detd. by 1H-NMR spectroscopy. The vinyl contents of these polymers could be satisfactorily correlated with the transition energy measurements. However, it turned out that esp. bidentate bases do not fit into this scheme. Obviously, besides their influence on the polarity of the reaction soln., they exhibit special interactions with the growing centers. The developed correlations allow the prediction of 1,2- and 3,4-linkages in polyisoprenes from anionic polymn. of isoprene with butyllithium in polar solvents in the presence of low concns., also of bases hitherto not investigated. The temp. dependence of the microstructure can likewise be estd. As in the case of butadiene, at least for weak bases, it was found that the vinyl contents depend primarily on the polarity of the reaction mixt. detd. by the base concn. and not on the molar ratio of base to initiator.

Anionic polymerization of isoprene by the complexes oligoisoprenyllithium/tertiary polyamines in cyclohexane: 3. Structural study of the polymers by 1H n.m.r. at 100 MHz

The microstructure of polyisoprene prepared anionically in the presence of catalytic amounts of N, N, N′, N′-tetramethylethylenediamine (TMEDA) and pentamethyldiethylenetriamine (PMDT) has been studied as a function of r = [ TMEDA] [ living ends] and r = [ PMDT] [ living ends] . No significant effect is observed for r ⩽ 0.5. The structure varies drastically for r > 0.5. The 4,3 addition increases mainly at the expense of the cis4,1 addition and a plateau is reached for r ⩾ 1.0. The results are discussed on the basis of the nature of the living species.

1H NMR Study of Polyisoprenes

Abstract 1H NMR spectra of polyisoprene were assigned using polymers of isoprene-1,1,4,4-d4, isoprene-1,1,5,5,5-d5, and isoprene-4,4-d2 polymerized with various catalysts. The methylene-proton signal at 2.1 ppm in cis-1,4- and trans-1,4-polyisoprenes was divided into H4- and H1-proton signals; H4 resonated at 2.21 ppm in both cis-1,4 and trans-1,4 units, whereas H1 resonated at 2.05, 2.21, and 2.15 ppm. Splitting due to the dyad sequences of 1,4 and 3,4 units was apparent. The methine-proton (H3) in a 3,4 unit showed a broad peak centered around 1.5 ppm in C6D6. The overlapping of this signal with the methyl-proton signals at 1.73 and 1.63 ppm resulted in some uncertainty in the determination of the microstructure of polyisoprene which contained a considerable amount of 3,4 unit.

1H‐NMR study of polyisoprenes

Abstract1H‐NMR spectra of polyisoprene were assigned using polymers of isoprene‐1,1,4,4‐d4, isoprene‐1,1,5,5,5‐d5, and isoprene‐4,4‐d2 polymerized with various catalysts. The methylene‐proton signal at 2.1 ppm in cis‐1,4 ‐ and trans‐1,4‐polyisoprenes was divided into H4‐ and H1‐proton signals; H4 resonated at 2.21 ppm in both cis‐1,4 and trans‐1,4 units whereas H1 resonated at 2.05, 2.21, and 2.15 ppm. Splitting due to the dyad sequences of 1,4 and 3,4 units was apparent. The methine‐proton (H3) in a 3,4 unit showed a broad peak centered around 1.5 ppm in C6D6. The overlapping of this signal with the methyl‐proton signals at 1.73 and 1.63 ppm resulted in some uncertainty in the determination of the microstructure of polyisoprene which contained a considerable amount of 3,4 unit.

Isoprene Polymerization by Catalyst Systems of Rhodium Complex and Allyl Alcohol

Abstract To prepare polyisoprene of controlled microstructure and molecular weight in emulsion systems, a series of polymerizations was carried out with catalyst systems of rhodium complexes and allyl alcohol, with variation of the type of rhodium salts, emulsifiers, and other conditions. A dramatic change in the structure of polyisoprene and also an increase of the polymer yield were brought about by the addition of sodium dodecyl sulfate and allyl alcohol, p-Toluene sulfonic acid and other strong acids exhibited an effect similar to that of the sulfate type emulsifiers on the isoprene polymerization. The degree of polymerization of polyisoprene was increased from 150 to 2000 when the amount of allyl alcohol in the system was decreased from 100 mole % to less than 10 mole % with respect to isoprene monomer. Most samples of polyisoprene prepared with above catalyst systems were found to have almost equi-binary 1,4-cis- and trans- enchainments. The effects of several phosphines and phosphites as additives on the microstructure of polyisoprene prepared with the rhodium complex systems were also examined.

Polymérisation anionique de l'isoprène: Nature de la paire d'ions et stéréospécificité. II. Influence de la nature du solvant

AbstractThe present work reports some new results on the dependence between the stereospecificity of the anionic propagation of isoprene and the nature of the active centers. The stereospecificity of the propagation of macroheterobicylce separated ion pairs does not depend on the nature of the alkali counterions, and the microstructure of the polyisoprenes obtained does not differ from that of those produced by free ions. Variations in the microstructure were observed in the propagation of contact ion pairs, depending on the nature of the cation and the solvent used. These variations are likely to be related to the degree of intimacy of the contact ion pairs. There are two main factors which affect the stereospecificity of the propagations in different ways: the size of the cation and the donating power of the solvent. Finally, the external solvation of the propagation ions pairs was confirmed by the microstructure of polyisoprenes synthesized in mixed (inert + donor) solvents.

Polymerization of Conjugated Dienes at High Pressures. X. Influence of Pressure upon the Microstructure of Anionic Polyisoprene

Abstract The microstructure of polyisoprenes prepared in hydrocarbon solvents with n-butyllithium is very sensitive to pressure, whereas in oxygenated solvents the structure remains unchanged even in the highest pressure range. Thus in n-heptane the content of 1,4-cis units decreases with increasing pressure, leading to polymers with heterogeneous structure. It is assumed that electrostatic as well as steric effects must be taken into account together: pressure acts like a polar solvent; however, in the transition state, steric effects affect the 1,4-cis addition.

Polymérization anionique de l'isoprène par les insertions ternaires graphite–métal alcalin–solvant

AbstractThe examination by HR–NMR of the microstructure of polyisoprenes synthesized in the presence of alkali metal–graphite compounds reveals that the polymerization has an anionic character. The microstructure of the polymers depends on the nature of the active centers, which depends on the solvent used in the preparation of the graphite compounds or as polymerization medium. With powdered alkali metal–graphite compounds, the propagation does not differ from the anionic one occurring in a homogeneous medium. Except for K–graphite, an enhancement of the 1–4 addition mode is observed if grains of Li or Na–graphite are used in mass polymerization. This change in the microstructure could be explained by the competition between the rate of propagation and the rate of diffusion of the growing chains from the grains to the homogeneous phase of the polymerization medium.

Interpretation of Microstructure of Poly-Isoprene and Polybutadiene Obtained in Anionic Polymerization

Abstract A mechanism is proposed for the anionic homopolymerization of the diene monomers isoprene and butadiene. This mechanism qualitatively explains how the various polymer microstructures occur. A quantitative model based on this mechanism is treated by LCAO-MO calculations which include the effect of the counterion on the propagating chain. The results of the calculations are then compared with experimental data on microstructure and relative reactivity in hydrocarbon and tetrahydrofuran solvents. In every instance the comparisons give correct, semiquantitative agreement between calculation and experiment.

Cis-Trans Isomerization of Natural Rubber under the Influence of Organo-Aluminum Compounds and Titanium Tetrachloride

Abstract Ziegler catalysts have become of prime importance for synthesis of polymers with regular structure, particularly cis-polyisoprene. The disruption of the structure of the chain by the formation of trans units was, until now, connected only with the influence of the physical and chemical properties of the catalyst on the nature of the addition of the diene monomers during the polymerization process. In the present work it is shown that destruction of regularity of structure can be caused by isomerization of cis units in complete polymer chains into trans units under the influence of the compounds used for initiating the polymerization process. Attempts to isomerize natural rubber by means of ultraviolet light and iodine did not give the expected effect. Ferri established for the first time that under the influence of zinc chloride and titanium tetrachloride natural rubber undergoes changes accompanied by disappearence of crystallization on stretching as shown by x-ray structure analysis. On the basis of these results the authors presumed that under the influence of these agents isomerization of cis units to trans units occurs in the natural rubber chain. The isomerization of cis-1,4 units to trans units in polybutadiene by means of ultraviolet light in the presence of organic bromine or sulfur compounds was first accomplished and proved by Golub. The formation of trans units in natural rubber under similar conditions was not observed. The study of the isomerizing effect of TiCl4 and organo-aluminum compounds was conducted by us on solutions of milled natural rubber (NK) in benzene in sealed glass ampoules. The benzene used was dried and distilled over sodium. All work was conducted in an atmosphere of dry argon. Unsaturation and microstructure of the chain was determined on each sample. The study of microstructure of polyisoprenes was carried out by means of infrared spectra, the relative content of the different configurations of the polymer chain being determined by the absorption in the 800–1000 cm−1 region. The method previously developed by one of us was used for this purpose.