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.     

13C NMR spectroscopic study of epoxidized 1,4-polyisoprene and 1,4-polybutadiene

Partly epoxidized cis- and trans-1,4-polyisoprenes and cis-and trans-1,4-polybutadienes were prepared, and their ¹³C NMR spectra examined. All the prominent resonances in the spectra of the epoxidized polymers were assigned by using lanthanide shift reagent and off-resonance decoupling experiments. A ¹³C NMR method of quantitative assessment of the epoxide content was developed following determination of relative spin-lattice relaxation time (T₁) and nuclear Overhauser effect (NOE) parameters of the various carbons in the epoxidized polyisoprenes and polybutadienes.     

Trans-stereospecific polymerization of butadiene and random copolymerization with styrene using borohydrido neodymium/magnesium dialkyl catalysts

The combination of a neodymium borohydride, Nd(BH₄)₃(THF)₃ (1) or Cp^*Nd(BH₄)₂(THF)_x (2), with MgⁿBuEt (BEM), affords an efficient and highly selective (up to 96.7% 1,4-trans) catalyst for butadiene polymerization. In the presence of excesses of Mg co-catalyst, polymer chain transfer takes place between neodymium and magnesium, and significant amounts of 1,2-units are observed. When considered for butadiene-styrene statistical copolymerization, the catalytic system based on 2 showed a good ability to produce poly[(1,4-trans-butadiene)-co-styrene)], with strong impact of the Mg/Nd ratio on the yield and on the copolymer microstructure, including the percentage of inserted styrene (up to 16.9 mol%). Whatever the co-monomers concentration the polybutadiene backbone remained 1,4-trans. The precise microstructure of the polymers and copolymers was thoroughly analyzed by means of high resolution NMR spectroscopy (900 MHz) and MALDI-ToF spectrometry.     

Localization of Failures in Structural Order of <Emphasis Type="Italic">trans</Emphasis>-1,4-Butadiene Units in Interdefective Regions of a Butadiene-Nitrile Elastomer Matrix

The fractional free volume of chains passing and incorporated into the ordered structures of segments in trans-1,4-configuration in the copolymers of butadiene and acrylonitrile at different content of acrylonitrile units is calculated in order to determine the localization of order disturbances of butadiene trans-1,4-units. Amorphization of the structure occurs in the immediate vicinity of structural defects of acrylonitrilebutadiene rubbers formed by alternating acrylonitrile and trans-1,4-units of butadiene as well as cis-1,4-and 1,2-isomers of butadiene.     

Microstructure of 1,2-<Emphasis Type="Italic">trans</Emphasis>-and 1,4-<Emphasis Type="Italic">trans</Emphasis>-poly(penta-1,3-dienes)

Microstructures of 1,2-trans-and 1,4-trans-poly(penta-1,3-dienes) synthesized using different catalysts were determined by high resolution ¹³C NMR spectroscopy. The content of dyad combinations of the 1,4-and 1,2-structures was quantitatively determined in hydrogenated poly(penta-1,3-dienes) from the ratio of intensities of the characteristic signals of the carbon atoms of the methylene groups in the ¹³C NMR spectra. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1113–1118, June, 2007.     

Quantum-chemical study of the effect of butadiene interaction with anionic active sites on the polymer microstructure

The electronic and geometric structure of the models for prereaction complexes of the anionic active sites for polymerization of butadiene have been calculated using a modified CNDO method: C₄H₄Li-cis-C₄H₆(I), C₄H₇Li-trans-C₄H₆(II), (C₄H₇Li)₂-cis-C₄H₆(III) and (C₄H₇Li)₂-trans-C₄H₆. The configuration of complexes I and II resulting from the total energy minimization points to the preferential C₄H₆ attack on the α-C atom of the monomeric active site (AS) leading to 1,4-units in polybutadiene. A more pronounced complexation effect observed with I as compared to II was taken into account when interpreting data on the preferential formation of cis-1,4-structure within macromolecules. The structure of models III and IV and also a decrease in the difference of the energy of interaction with C₄H₆ incorporated in these models, as compared to models I and II, indicate a decrease in the 1,4-cis-units content with increasing initiator concentration. Based on results of the present study, an evaluation was also made of the effect of the interaction between the living macromolecule aggregates and diene on the dissociation processes.     

13C-NMR-spektren stereoisomerer 1,4-dialkylspiro[4.5]decan-derivate

The ¹³C NMR spectra of a series of 7-substituted 1,4-dialkylspiro[4.5]decanes and of suitable reference compounds are assigned and the derived substituent effects upon the chemical shifts are discussed. In particular, consideration of γ and δ-effects allows the differentiation between 1,4-cis- and 1,4-trans-compounds. Furthermore, the (relative) configuration of the chiral spiroatom C-5 is determined.     

Polymerization of butadiene catalyzed by a cobalt-containing catalyst in aliphatic media

The polymerization of butadiene catalyzed by the preformed Co(2-ethylhexanoate)₂-Et₃Al₂Cl₃-isoprene (a molar ratio of 1: 20: 20) catalytic system in toluene, cyclohexane, and hexane has been studied. In toluene, the catalyst demonstrates high activity and stereospecificity and yields a high-molecular-mass polymer. In cyclohexane and hexane, the catalyst has a high activity but affords polymers with lower contents of 1,4-cis-units and reduced intrinsic viscosities. The addition of EtAlCl₂ and/or a decrease in the polymerization temperature make it possible to obtain polybutadiene containing more than 96% 1,4-cis-units and an acceptable intrinsic viscosity (2.0–2.8 dl/g) in the nonaromatic solvents as well.     

Polychloroprene. II. Etude comparée de la microstrusture par RMN 1H et 13C. Thermodynamique de propagation des différentes unités structurales

A microstructure analysis of polychloroprenes by ¹H and ¹³C NMR produced slightly different results which, however, are in good agreement. The main uncertainty concerned the assignment of the 1,4-cis and trans head-to-head and tail-to-tail additions. In the (+ 12, +70°C) region of polymerization temperatures the principal addition mode was 1,4. An increase in the polymerization temperature to 70°C increased the abnormal head-to-head, tail-to-tail additions, and 1,4-cis additions (6–7%), and all vinyl structures that were not higher than 2–3% were equally distributed between 1,2 and 3,4. Under the experimental conditions used in this study no isomerized vinyl structures were found. As in all radical polymerizations the influence of temperature on the propagation mechanisms, namely, the addition modes, was relatively weak, but because of the great number of possible addition modes in microstructure—physicomechanical relationships variations in the microstructure must be taken into account. Only ¹H and ¹³C NMR are capable of determining these variations quantitatively.     

Nature of active centers and stereospecificity of tetravalent titanium benzyl derivatives in polymerization of butadiene

Stereospecificity of tetrabenzyltitanium and its halogeno-derivatives in the polymerization of butadiene has been investigated. The content of 1,2-units decreases while the content of 1,4-cis-units increases in the resulting polybutadiene for the series (C₆H₅CH₂)₄Ti, (C₆H₅CH₂)₃TiCl, (C₆H₅CH₂)₃TiBr, (C₆H₅CH₂)₃Til. Tribenzyltitanium iodide exhibits high stereospecificity for the formation of 1,4-cis-units and their content reaches 94–97%. By determining the number of benzyl groups linked with titanium at different degrees of conversion, it has been shown that the active centre formed from tetrabenzyltitanium contains three benzyl groups and one polymer chain. Two benzyl groups, one iodine atom and one polymer chain are attached to a titanium atom in the active centre for the case of tribenzyltitanium iodide. Electron donors sharply change the stereospecificity of tribenzyltitanium iodide: the content of 1,2-units in the polymer rises to 68%.     

A new insight into the mechanism of 1,3‐dienes cationic polymerization. II. Structure of poly(1,3‐pentadiene) synthesized with tBuCl/TiCl4 initiating system

The microstructure of poly(1,3‐pentadiene) synthesized by cationic polymerization of 1,3‐pentadiene with ^tBuCl/TiCl₄ initiating system is analyzed using one‐dimensional‐ and two‐dimensional‐NMR spectroscopy. It is shown that unsaturated part of chain contains only homo and mixed dyads with trans?1,4‐, trans?1,2‐, and cis?1,2‐structures with regular and inverse (head‐to‐head or tail‐to‐tail) enchainment, whereas cis?1,4‐ and 3,4‐units are totally absent. The new quantitative method for the calculation of content of different structural units in poly(1,3‐pentadiene)s based on the comparison of methyl region of ¹³C NMR spectra of original and hydrogenated polymer is proposed. The signals of tert‐butyl head and chloromethyl end groups are identified in a structure of poly(1,3‐pentadiene) chain and the new approaches for the quantitative calculation of number‐average functionality at the α‐ and ω‐end are proposed. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3297–3307     

Filler-polymer interactions in filled polybutadiene compounds

Bound rubber in a filled rubber compound is formed by physical adsorption and chemisorption between the rubber and filler. Polybutadiene (PB) is composed of three components of 1,2-, cis-1,4-, and trans-1,4-units. Filler-polymer interactions in PB compounds filled with carbon black or silica were studied by analyzing microstructures of the bound rubbers with pyrolysis-gas chromatography. Differences in the filler-polymer interactions of the 1,2-, cis-1,4-, and trans-1,4-units were investigated. The filler-polymer interaction of the 1,2-unit is stronger than those of the cis-1,4- and trans-1,4-units. The interaction of the 1,2-unit with silica is stronger than with carbon black. Bound rubber content is decreased by treatment with ammonia. Change of the bound rubber composition after the ammonia treatment was also studied.     

Compounds with bridgehead nitrogen. 44—the conformational analysis of perhydropyrido[1,2-c] [1,3]thiazine

The 270 MHz NMR data on trans- and cis-(H-4a, H-7)-7-ethylperhydropyrido[1,2-c][1,3]thiazine show heavy conformational bias to the trans- and S-inside cis-fused conformations, respectively. Comparison of the ¹³C NMR spectra of these anancomeric systems with the ¹³C NMR spectrum of perhydropyrido[1,2-c][1,3]thiazine indicates a trans-?S-inside cis-conformational equilibrium for the latter compound in CDCl₃ at 25°C, containing ca 75% trans-fused conformer. The ¹³C NMR spectrum of perhydropyrido[1,2-c][1,3]-thiazine at ?75°C showed 64% trans-fused conformer and 36% S-inside cis-conformer.     

The structure of chain segments and <Superscript>1</Superscript>H NMR spectra of polychloroprene

The ¹H NMR spectra (working frequencies of 500 and 600 MHz) of polychloroprenes are studied. The spin systems of protons for monomer units of different configurations (1,2, 3,4, 1,4-cis-and 1,4- trans- units), as well as for dyad and triad combinations of 1,4 units, are classified. A high working frequency, the method of double resonance, and the calculation of chemical shifts by empirical increments make it possible to refine the assignment of ¹H NMR signals.     

Isoprene polymerization using a neodymium phenolate pre-catalyst combined to aluminum based co-catalysts

The discrete phenolate Nd(2,6-di-tert-butyl-OC₆H₃)₃ has been assessed as pre-catalyst for the polymerization of isoprene using various aluminum based co-catalysts. In combination with MAO and MMAO in toluene, the reaction is quantitative in 1 h at 60 °C, yielding activities comparable to neodymium versatate and isopropylate in similar conditions. In contrast with the abovementioned systems, the phenolate leads to a single site catalyst species in combination with MAO and MMAO. Increasing amounts of MAO in the reactive medium lead to a decrease of the number–average molecular weight of polyisoprene, highlighting the occurrence of chain transfer between neodymium and aluminum. The polymerization is modestly 1,4-trans selective, in the range 60–75%. Using pentane as a solvent and MMAO, ultra high activities up to 3000 kg PI/mol Nd/h can be reached at room temperature, as well as 85% 1,4-cis selectivity. Ternary systems with co-catalysts based on the chlorinated AlEt₂Cl in combination with AlⁱBu₃ afford finally a 90–98% 1,4-cis stereoselective polymerization depending on the solvent used for the reaction. The SEC traces are in agreement with the presence of several active species for the latter systems.     

Influence of polar modifiers on microstructure of polybutadiene obtained by anionic polymerization. Part 4: Acid-base polar modifiers forming σ-μ complexes: Amine-alkoxide,amine-ether-alkoxide,and ether-alkoxide

The influence of acid-base polar modifiers (σ-μ complex): amine-alkoxide, amine-ether-alkoxide, and ether-alkoxide complexes on microstructure of polybutadiene obtained by anionic polymerization was studied. The content of butadiene isomeric structures was determined based on FT-IR-ATR, ¹H, and ¹³C NMR data analyses. The results obtained indicated a very similar influence of all types of polar modifiers forming σ-μ complex on polymer microstructure leading to high vinyl polybutadiene with nearly equal ratios of cis-1,4 and trans-1,4 butadiene isomeric structures.     

Stereoselective synthesis of tetralins using cationic cyclisations

Tetralins, including the terpene calamenene, were prepared by 6-endo cationic cyclisations, effected by addition of an I(I) reagent to alkenylarenes, followed by reductive deiodination. An activating group on the arene was required for efficient cationic cyclisation. Good diastereoselectivity, relative to a chiral centre in the chain linking the alkene to the arene, was observed, with Z-alkenes giving predominantly 1,4-cis disubstituted tetralins, and E-alkenes giving predominantly 1,4-trans derivatives. Analogous 6-exo cationic cyclisations proved very limited in scope.     

Radiation-induced polymerization at high dose rate. IV. Chloroprene in bulk

Bulk polymerization of chloroprene was studied at 25°C in a wide does rate range. Variations of the rate of polymerization (R_p) and molecular weight as a function of does rate were essentially the same as those in several monomers that are capab;e of radical and cationic polymerizations. The polymerization proceeds with radical mechanism at low dose rate ans with radical and cationic mechanism concurrently at high dose rate. The number-average molecular weight of the high-dose-rate was ca. 2400. Microstructure of the polymers was mainly of trans-1,4 unit with small fraction of cis-1,4 and 3,4-vinyl unit. Fractions of the vinyl unit and the inverted unit in trans-1,4 sequence which increased at high does rate inflected the change of dominant mechanism of polymerization.     

Determination of astaxanthin and astaxanthin esters in the microalgae <Emphasis Type="Italic">Haematococcus pluvialis</Emphasis> by LC-(APCI)MS and characterization of predominant carotenoid isomers by NMR spectroscopy

The oily product ZANTHIN® consists of natural astaxanthin, which is manufactured from the microalgae Haematococcus pluvialis by supercritical CO₂ extraction. An HPLC method was developed to separate all of the components of the complex astaxanthin extract using a C₃₀ column. The separation resulted in different isomers of astaxanthin accompanied by two other carotenoids. The main component consisted of astaxanthin singly esterified with several different fatty acids. C18:3, C18:2, C18:1 and C16:0 were identified as the most commonly occurring fatty acids. Doubly esterified astaxanthin was also found, although in lower concentrations compared to singly esterified astaxanthin. After performing a detailed fatty acid analysis by GC-MS, the peaks from the extract were assigned via HPLC-MS. A trans to cis transmutation of the all-trans compound was performed by thermal treatment in order to obtain an enrichment of cis isomers as the basis for unambiguous identification via NMR experiments. The all-trans as well as the 9- and 13-cis isomers of astaxanthin were characterized in detail by UV/Vis, ¹H, and ¹H,¹H COSY NMR spectroscopy.     

Electrostatic forces that determine O,O-trans versus O,S-cis conformers in the aminated porphyrin complexes of Cd(N-NHCO-2-C4H3O-tpp)(OAc) and Cd(N-NHCO-2-C4H3S-tpp)(OAc)

Two new diamagnetic, mononuclear and aminated porphyrin complexes of O,O-trans-Cd (3-trans) and O,S-cis-Cd (4-cis) have been synthesized and characterized by ¹H, ¹³C NMR spectroscopy. The crystal structures of (acetato)(N-2-furancarboxamido-meso-tetraphenylporphyrinato)cadmium(II) [Cd(N-NHCO-2-C₄H₃O-tpp)(OAc); 3-trans] and (acetato)(N-2-thiophenecarboxamido-meso-tetraphenylporphyrinato)cadmium(II) [Cd(N-NHCO-2-C₄H₃S-tpp)(OAc); 4-cis] were determined. The coordination sphere around Cd²⁺ is a distorted square-based pyramid in which the apical site is occupied by a bidentate chelating OAc⁻ group for 3-trans and 4-cis. The plane of three pyrrole nitrogen atoms [i.e., N(1), N(2), N(4) for 3-trans and N(1), N(2), N(3) for 4-cis] strongly bonded to Cd²⁺ is adopted as a reference plane 3N. The N(3) and N(4) pyrrole rings bearing the 2-furancarboxamido (Fr) and 2-thiophenecarboxamido groups in 3-trans and 4-cis, respectively, deviate mostly from the 3N plane, thus orienting separately with a dihedral angle of 33.4° and of 31.0°. In 3-trans, Cd²⁺ and N(5) are located on different sides at 1.06 and −1.49 Å from its 3N plane, while in 4-cis, Cd²⁺ and N(5) are also located on different sides at 1.04 and −1.53 Å from its 3N plane. An attractive electrostatic interaction between the Cd²⁺ and O(4)⁻ atoms in furan stabilizes the O,O-trans conformer of 3. A repulsive electrostatic interaction between Cd²⁺ and S(1)⁺ destabilizes the O,S-trans conformer of 4. Both of these repulsive and the mutually attractive interactions between S(1)⁺ and O(3)⁻ atoms favor the O,S-cis rotamer of 4 both in the vapor phase and in low polarity solvents. NOE difference spectroscopy, HMQC and HMBC were employed for the unambiguous assignment of the ¹H and ¹³C NMR resonances of 3-trans and 4-cis in CDCl₃ at 20 and −50 °C.