Syndiotactic 1,2-polybutadiene with Co-CS2 catalyst system. III. 1H- and 13C-NMR study of highly syndiotactic 1,2-polybutadiene

The ¹H and ¹³C-NMR spectra of highly crystalline syndiotactic 1,2-polybutadiene (s-PB) are discussed in order to clarify the mechanism of butadiene polymerization with cobalt compound–organoaluminum–CS₂ catalysts. Cis opening of the double bonds in the syndiotactic polymerization is affirmed by the study of the copolymer from perdeuteriobutadiene and cis,cis-1,4-dideuteriobutadiene. S-PB (mp 210°C) has 99.7% 1,2 units, 0.3% isolated cis-1,4 units, and 99.6% syndiotacticity. Polymer ends (2-methyl-3-butenyl group and conjugated diene structure) are also determined. The differences in free energy of activation between 1,2 and cis-1,4 propagation and between syndiotactic and isotactic propagation are 14.0 and 9.6 kcal/mol, respectively, for Co(acac)₃-AlEt₃-AlEt₂Cl-CS₂, and 6.7 and 5.7 kcal/mol, respectively, for the aluminum-free Co(C₄H₆)(C₈H₁₃)CS₂ system. The conformation of s-PB in o-dichlorobenzene at 150°C is described by the sequence (tt)_1.6(gg)(tt).     

Chemicals from Synthesis Gas : By Robert A. Sheldon (Series: Catalysis by Metal Complexes) D. Reidel Publishing Company,Dordrecht, The Netherlands, 1983, xx + 216 pages,Dfl. 110.00, US$ 48.00. ISBN 90-277-1489-4.

Photolysis of cis-HMn(CO)₄PPh₃ in the presence of H₂ and 1-alkene results in catalytic hydrogenation and isomerization of the alkene. The isomerization leads to cis- and trans-2-alkene in the presence or absence of H₂. Catalytic hydrogenation also occurs when cis-CH₃Mn(CO)₄PPh₃ is irradiated in the presence of H₂; use of D₂ leads exclusively to CH₃D. The possible mechanism of the hydrogenation is discussed.     

Studies of crotyl anion reactions and some regularities of copolymerization in hexamethylphosphortriamide

Cis-trans isomerization of butenes in free-anion systems proceeds through the α-butene intermediate. Direct cis- and trans-transformation of crotyl anions does not occur. Reactions of crotyl anion and its ion pairs with proton-donor compounds in hexamethylphosphortriamide (HMPTA) are accompanied by the formation of an equilibrium mixture of butenes containing 97 per cent β-butenes. The addition of these active species to alkylene oxides and sulphides, cyclohexadiene-1,3, styrene and 2,3-dimethylbutadiene occurs with the participation of the terminal carbon atom of the crotyl active centre, resulting in the predominant formation of 1,4-units. It has been shown that the reactivity ratios r₁ and r₂ for the pairs butadiene-styrene, butadiene-2,3-dimethylbutadiene and butadiene-cyclohexadiene-1,3 become very close in HMPTA.Studies of copolymer microstructure reveal an increase of butadiene 1,4-units content paralleling a rise of second monomer content.     

Copolymerization of butadiene and styrene with neodymium naphthenate based catalyst

Copolymerization of butadiene (Bd) and styrene (St) was carried out in toluene at 50 °C by a conventional rare earth catalytic system, Nd(naph)₃-Al(i-Bu)₃-Al(i-Bu)₂Cl. It exhibited a high catalytic activity and high stereospecificity in the copolymerization. The influences of the conditions in polymerization on the yield, composition, microstructure and molecular weight of copolymer were thoroughly studied. According to the 13C-NMR spectrum, the resultant copolymer containing 18% St units, and the diad fraction of St-trans Bd or St-vinyl Bd can hardly be found in its 13C-NMR. The cis-1,4 content of Bd unit of the copolymer decreased little with the increase of St content. The GPC curves indicate the presence of two kinds of active sites in the polymerization.     

Filler‐polymer interactions of styrene and butadiene units in silica‐filled styrene–butadiene rubber compounds

Styrene–butadiene rubber (SBR) is a copolymer of styrene and butadiene, and the butadiene unit is composed of cis‐1,4‐, trans‐1,4‐, and 1,2‐components. Filler‐polymer interactions of each component of SBR in silica‐filled SBR compounds were examined by microstructure analysis of the bound and unbound rubbers. The composition ratio of butadiene and styrene units (butadiene/styrene) of the bound rubber was higher than that of the compounded rubber. Of the butadiene units, the 1,2‐component of the bound rubber was more abundant than the cis‐1,4‐ and trans‐1,4‐components. The filler‐polymer interaction of the butadiene unit with silica was stronger than that of the styrene unit, and the interaction of the 1,2‐component was stronger as compared with the others. The butadiene–styrene ratio of the bound rubber of the compounds containing the silane coupling agent was lower than for the compounds without the silane. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 577–584, 2004     

Quantum-chemical study of degradation of molecular analogs of butadiene-styrene copolymer fragments during stretching by end atoms

A comparative study of changes in the electronic structure during stretching of molecular fragments by end atoms is performed by using semiempirical quantum chemical method PM3 with geometry optimization for cis- and trans-hexa-3-ene, 3-methyl-1-pentene and sec-butylbenzene, which model butadiene-styrene copolymer units. In contrast to molecules modeling butadiene-1,4 units, the degradation of molecules modeling butadiene-1,2 and styrene units is found to proceed in two stages. At the first stage, when a maximum in the corresponding stress-strain curves is attained, the molecules with side vinyl and phenyl groups do not break down but experience intramolecular structural transition that is accompanied by the isomerization and displacement of double bonds. At the second stage, the molecules degrade.     

COPOLYMERIZATION OF BUTADIENE AND ISOPRENE CATALYZED BY V(acac)_3-Al(i-Bu)_2Cl-Al_2Et_3Cl_3 CATALYST AND CHARACTERIZATION OF THE PRODUCTS

Copolymerization of butadiene and isoprene catalyzed by the catalyst system V(acac)_3-Al(i-Bu)_2Cl-Al_2Et_3Cl_3 has been studied. Composition, microstructure, crystallinity and melting point of the copolymer obtained were determined by PGC, IR, X-ray diffraction and DSC methods respectively. The results revealed that the product was a copolymer and not a blend. The butadiene units presented in the copolymer were of trans-1,4-configuration, while the isoprene units were of both trans-1,4-and 3,4-forms. The melting point and crystallinity of the copolymer decrcascd with increase of molar ratio of isoprene to hutadiene.     

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.     

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.     

Selective hydrogenation of butadiene-styrene copolymers using a Ziegler-Natta type catalyst 2. Thermal properties

Three types of butadiene-styrene copolymers, diblock, triblock and random, were partially hydrogenated in their elastomeric units in order to determine the influence of hydrogenation extent on their thermal properties. The hydrogenation reactions were carried out using a Ziegler-Natta type catalyst and the extent of hydrogenation was evaluated by FTIR spectroscopic technique. The percentage of crystallinity was determined by differential scanning calorimetry (DSC), considering the low density polyethylene (LDPE) as reference since the saturation of elastomeric units with low content of 1,2-vinyl bonds gives a structure which resembles to LDPE, thus semi-crystalline polymers were obtained. On the other hand, the glass transition temperature (T_g) for the hydrogenated and non-hydrogenated copolymers as well as their heat of fusion, were also determined by DSC. An equation to evaluate the T_g of partially hydrogenated random copolymers is proposed considering both the saturated and unsaturated fractions. The thermo-oxidative behaviour of certain hydrogenated and non-hydrogenated copolymers was evaluated by thermogravimetric analyses (TGA). The results obtained by TGA suggest that a minimum saturation level is necessary in order to improve the thermo-oxidative resistance of the polymers.     

Donor-acceptor complexes in copolymerization. L. Preparation and microstructure of chlorine-containing alternating conjugated diene–acceptor monomer copolymers

Neighboring monomer units cause significant shifts in the infrared absorption peaks attributed to cis- and trans-1,4 units in conjugated diene-acceptor monomer copolymers. Conjugated diene-maleic anhydride alternating copolymers apparently have a predominantly cis-1,4-structure, while alternating diene-SO₂ copolymers have a predominantly trans-1,4 structure. Alternating copolymers of butadiene, isoprene, and pentadiene-1,3 with α-chloroacrylonitrile and methyl α-chloroacrylate, prepared in the presence of Et_1.5AlCl_1.5(EASC), have trans-1,4 unsaturation. Alternating copolymers of chloroprene with acrylonitrile, methyl acrylate, methyl methacrylate, α-chloroacrylonitrile, and methyl α-chloroacrylate prepared in the presence of EASC-VOCl₃ have trans-1,4 configuration. The reaction between chloroprene and acrylonitrile in the presence of AlCl₃ yields the cyclic Diel-Alder adduct in the dark and the alternating copolymer under ultraviolet irradiation. The equimolar, presumably alternating, copolymers of chloroprene with methyl acrylate and methyl methacrylate undergo cyclization at 205°C to a far lesser extent than theoretically calculated, to yield five and seven-membered lactones. The polymerization of chloroprene in the presence of EASC and acetonitrile yields a radical homopolymer with trans-1,4 unsaturation.     

Random copolymer of butadiene and propylene prepared with TiCl4–Et3Al–phosgene catalyst

The copolymerization of butadiene and propylene was investigated. It was found that the catalyst system of TiCl₄–Et₃Al–COCl₂ yields a random copolymer of high molecular weight with a small amount of gel polymer above room temperature. Tetrachloroethylene was a good solvent for the production of high polymer containing a high proportion of propylene units in high yield. The fractionation and the analysis of degradation experiments of copolymer indicate that the copolymer is of random distribution of propylene units in the copolymer. However, the monomer reactivity ratios, r_BD = 6.36 and r_Pr = 0.42, suggest some degree of blocked character. The properties of the copolymer were superior to those of cis-1,4–polybutadiene, especially in resistance to thermal aging.     

Alternating copolymer of butadiene and propylene. III

Alternating copolymerizations of butadiene with propylene and other olefins were investigated by using VO(acac)₂–Et₃Al–Et₂AlCl system as catalyst. Butadiene–propylene copolymer with high degree of alternation was prepared with a monomer feed ratio (propylene/butadiene) of 4. Alternating copolymers of butadiene and other terminal olefins such as butene-1, pentene-1, dodecene-1, and octadiene-1,7 were also obtained. However, the butadiene–butene-2 copolymerization did not yield an alternating copolymer but a trans-1,4-polybutadiene.     

Electronic spectra of α,β-unsaturated carbonyl compounds—II : An evaluation of increments characteristic of structural features of β-oxy-α,β-unsaturated ketones

By comparing UV spectra of β-alkoxy-α,β-unsaturated ketones of established steric structure, spectral constants characteristic of the cis/trans configuration change and s-cis/s-trans and O-s-cis/O-s-trans conformation changes have been evaluated. These are: Δλ^cis_trans = 0, Δλ^s-cis_s-trans = 8 nm and Δλ^O-s-trans_O-s-cis = 6 nm. A comparison of cis-s-cis enol ethers with the parent enols yielded the increment for the intramolecular (“chelating”) H-bond, Δγ_chel = 24 nm. The methanol-induced bathochromic shift has been found to depend strongly on s-cis/s-trans isomerism. The substituent increments have been shown to be dependent on the degree of substitution in the reference molecule. The results obtained have been summarized in a set of spectral increments complementing the basic system of Woodward and the Fiesers.     

Mechanochemical-induced cis-to-trans isomerization of poly(2-ethynyl-3-n-octylthiophene) prepared with a Rh complex catalyst

Highly stereoregular polymerization of 2-ethynyl-3-n-octylthiophene was successfully performed with a [Rh(norbornadiene)Cl]₂ catalyst to produce the corresponding polymers in fairly high yields by using triethylamine or a mixture of it with other solvents as the polymerization solvent. We found that the obtained polymer using CHCl₃ was a mixture of cis-transoid form, ca. 68% and trans-transoid form, ca. 32% unlike our previous conjecture. Further, we found that the cis-to-trans isomerization can be also induced when the pristine predominant cis polymer was subjected to mechanochemical grinding (MCG) treatment at 77 K using a mortar filled with liquid nitrogen to decrease the cis content from ca. 68% to ca. 7%. The polymers obtained before and after the MCG treatment were characterized in detail using ¹H NMR, laser Raman, solution UV-vis, diffuse reflective UV-vis, and ESR methods in order to determine the geometry of the main-chain CC bonds in the polymer. The data showed that the polymer obtained by the treatment has a fairly distorted trans conjugation length, i.e., bent trans structure in which less mobile unpaired electrons generated by the rotational scission of the original cis CC bonds are stabilized.     

Filler–polymer interactions in both silica and carbon black‐filled styrene–butadiene rubber compounds

Bound rubber in a filled rubber compound is formed by physical adsorption and chemisorption between the rubber and the filler. Styrene–butadiene rubber (SBR) is composed of four components of styrene, cis‐1,4‐, trans‐1,4‐, and 1,2‐units. Filler–polymer interactions in both silica and carbon black‐filled SBR compounds were studied by analyzing microstructures of the bound rubbers with pyrolysis‐gas chromatography. Differences in the filler–polymer interactions of the styrene, cis‐1,4‐, trans‐1,4‐, and 1,2‐units were investigated. The filler–polymer interactions of the butadiene units were found to be stronger than that of the styrene unit. The interactions of the cis‐1,4‐ and trans‐1,4‐units were stronger with carbon black than with silica, whereas the 1,2‐unit interacted more strongly with silica than with carbon black. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 439–445, 2001     

Anionic Polymerization and Copolymerization of Hydrocarbon Monomers Catalyzed by Organobarium Initiators

The barium salt of the dimeric dianion of 1,1-diphenylethylene (Ba-DPhE) initiates polymerization and copolymerization of monomers capable of anionic polymerization (butadiene, isoprene, styrene) in ethereal and hydrocarbon solvents. Ba-DPhE is more stereospecific in butadiene polymerization (up to 70% of cis-1, 4-units in hydrocarbon medium) than initiators based on other metals of Groups I and II. The relative reactivity of monomers in copolymerization processes in THF decreases in an order typical for anionic polymerization: styrene > butadiene > isoprene. The most interesting feature of organobarium initiators is their ability to form random butadiene-styrene copolymers with high cis-1,4-butadiene unit content when copolymerization proceeds in a hydrocarbon medium.

A new phenomenon in anionic polymerization, the dependence of diene units structure on copolymer composition, was observed. Thus an increase of styrene content in butadienestyrene copolymer leads to conversion of cis-1,4-butadiene units into trans-1,4-units (in benzene) or to conversion of 1,4-units to 1,2-units (in THF). Similarly, an increase of butadiene content in its copolymer with isoprene (in benzene) leads to conversion of cis-1,4-isoprene units into trans-1,4-units.

Spectrophotometric, conductometric, and viscometric methods were used to study organobarium active centers. Certain anomalies connected with the formation of specific aggregates due to coupling of bifunctional hydrocarbon chains with bivalent counterions were observed.     

An AB initio study of the cis- and trans -conformers of 1,3-butadiene,acrolein and glyoxal: evidence for a stabilizing interaction in cis-acrolein

The cis- and trans-conformers of 1,3-butadiene, acrolein and glyoxal have been studied using the 4-31G and (7, 3) basis sets with full geometry optimization. The changes in geometry in going from the cis- to the trans-structures indicate that the terminal carbon and oxygen atoms in cis-acrolein are closer together than would otherwise be expected from the butadiene and glyoxal data, and that the hydrogens of the CH₂ group are also affected. These results, together with a comparison of the changes in force constants for key structural elements, suggest that some sort of attractive interaction is present in cis-acrolein. ΔE_T, ΔE_K, ΔV_ee, ΔV_nn and ΔV_en are reported for the cis-trans isomerizations, the special geometrical features of the cis-conformer being reflected in ΔV_nn for the acrolein reaction. The positive ΔE_T values for the disproportionation reactions of both conformers2 acrolein → 1,3-butadiene + glyoxal show that the acrolein framework provides the most stable combination of the H₂C, CH- and O structural elements, while the bigger value for the cis-conformers affords additional evidence that the interaction in cis-acrolein is stabilizing in nature. However, in going from the trans- to the cis-structure neither the change in f_C-H,C-H for the C-H that would be involved in an H- bond, nor the change in charge on the atoms in the C-H ? O unit. based on Mulliken population analyses, are in accord with what would be expected for a hydrogen-bonding type of interaction. Instead the stabilization may be a manifestation of the different pattern of charge distribution along the polyene chain of acrolein, i.e. with the ends oppositely charged, compared to butadiene and glyoxal, in which the ends carry the same charge.     

Hydridocyanoalkyl complexes of platinum(II). Insertion of olefins into the PtH bond

The preparation and spectroscopic properties are described of some platinum(II) complexes having a hydride ligand cis or trans to an sp³ carbon, viz. trans-PtH(YCN)(PPh₃)₂ and cis-PtH(YCN)(LL) with YCN = C₂H₄CN, n-C₃H₆CN, o-CH₂C₆H₄CN and LL = bis(diphenylphosphino)-ethene or -ethane. The complexes trans-PtH(YCN)(PPh₃)₂ can add a fifth ligand in solution; the resulting five-coordinate complex was observed by ³¹P NMR in the case of PtH(C₃H₆CN)(PPh₃)₃. Insertion of olefin (ethen, 1-cyanoethene, norbornadiene, allen) into the PtH bond of the trans-hydrido complexes occurs to give cis-dialkyl complexes, but the cis-hydrido complexes are unreactive. The mechanism of insertion is discussed in terms of the kinetics and the geometries of reactants and products.     

Etude du mecanisme de photolyse des complexes cis- et trans-[PtCl2(C2H4)(4-CH3C5H4N)]. Influence de la concentration et de la longueur d'onde

Photodimerization, photoisomerization and photosubstitution quantum yields are measured for cis- and trans-[PtCl₂(C₂H₄)(4-CH₃C₅H₄N)], at various concentrations and wavelengths. Dissociation of the platinumethylene bond o?curs with a quantum yield nearly unity when the cis-compléx is irradiated in the charge transfer bands 5d → π^*(C₂H₄). Dissociation is also observed, but with a lower efficiency, at longer wavelengths. A cis → trans-photoisomerization reaction, probably via a low energy dd excited state is observed at 313,366 and 405 nm, with a constant quantum yield.