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1.
A method based on matrix algebra and on the rotational isomeric state scheme to study the configurational- conformational entropy of 1,4-polydienes with geometrical isomerism has been developed. Bernoullian and first-and second-order Markovian statistics for the sequences of cis and trans units along the chains have been considered, and the explicit relation between entropy and mole fraction of cis units has been derived. Calculations performed by using available experimental data for the configurational parameters and the conformational partition functions of 1,4-polybutadiene and 1,4-polyisoprene show that entropy is a monotonic function of the geometrical isomer composition. While the entropy of polybutadiene increases with the content of cis units, the reverse is true for polyisoprene.  相似文献   

2.
An analytical and kinetic study of the thermal reaction of cis- or trans-2-butene has been performed in a static system over the temperature range of 480–550°C and at a low extent of reaction and initial pressures of 10–100 torr. The rate constant of the unimolecular cis–trans isomerization of cis-2-butene, determined under the conditions (2.3 RT in cal/mole) is in good agreement with previous measurements made at lower pressures. A comparison between the formation rates of hydrogen from the thermal reactions of cis- and trans-2 butene around 500°C leads to the rate constant value (2.3 RT in cal/mole) for the unimolecular 1,4? hydrogen elimination from cis? 2? butene.  相似文献   

3.
The kinetics of the gas-phase thermal isomerization between trans- and cis-1,2-bis(trifluoromethyl)-1,2,3,3-tetrafluorocyclopropane as well as their decomposition to trans- and cis-perfluoro-2-butene, respectively, and CF2, was studied in the temperature range of 473–533°K, with an initial pressure of reactant of 1.5 to 7.0 Torr. Some runs were also made with the addition of SF6 as an inert gas up to a total pressure of 100 Torr. The reactions are first order and homogeneous. The rate constants for the geometrical isomerization fit the following Arrhenius relations: and the corresponding equations for the decomposition of the trans and cis-cyclopropane are .  相似文献   

4.
Titanates are versatile in the 1,4 polymerization of isoprene. The (R′O)4Ti/RAlCl2 catalyst gives either cis- or trans-1,4-polyisoprene, depending on the nature of both the titanate and the solvent. Primary titanates give cis-1,4-polyisoprene in both aliphatic and aromatic solvents. Secondary titanates give cis-polyisoprene in aliphatic solvents, and trans-1,4-polyisoprene in aromatic solvents. Tertiary titanates give trans-polyisoprene in both aliphatic and aromatic solvents. A mechanism is postulated which takes into consideration the role of the solvent. ESR studies of the various titanate–RAlCl2 catalysts were made; the paramagnetic structures are related to polymerization mechanisms.  相似文献   

5.
The reversible isomerization of cis-hepta-1,3-diene to cis-2-trans-4-heptadiene via a 1,5 hydrogen shift has been investigated kinetically at nine temperatures in the range of 475° to 531°K. Equilibrium is reached near 94% reaction. Some cis-2-cis-4-heptadiene is also formed, but at a rate some 60 times slower than the cis,trans isomer. A least-squares analysis of the data yielded the Arrhenius equation for the isomerization of the cis-hepta-1,3-diene: Possible errors in the equilibrium constant measurements are discussed, and employing an equilibrium constant calculated by using group additivity estimates together with the values of k1, we obtained for the reverse reaction where .  相似文献   

6.
Stress, strain, and birefringence measurements have been carried out on swollen and unswollen networks of ′cis-1,4-polybutadiene polymers. Neither stress-strain nor birefringence-strain relations of unswollen specimens obey the Gaussian network theory, but both can be fitted by the Mooney-Rivlin equation. On the contrary, data on specimens swollen in tetralin, decalin, benzene, and carbon tetrachloride strictly obey the Gaussian network theory. Existing methods for evaluating the temperature coefficient of the unperturbed dimensions, d In 〈r2〉/dT, from the stress-temperature relation are applied to the present data and discussed in some detail. It is concluded that reliable values of d In 〈r2〉/dT are not obtainable from data on unswollen samples because of the pronounced non-Gaussian effect. The value 7.5 Å3 for the optical anisotropy Å3 (an alternative to the stress-optical coefficient) for unswollen specimens is markedly larger than values (5.8 Å3 on the average) for swollen specimens. This is interpreted as due to the shortrange orientational order among polymer segments. The quantities 〈r2〉, ΔΓ, and their temperature coefficients are calculated for both cis-1,4-polybutadiene and cis-1,4-polyisoprene chains, on the basis of the rotational isomeric state approximation for bond rotations. Values of ΔΓ for cis-1,4-polybutadiene calculated using Clément and Bothorel's set of anisotropic bond polarizabilities are in good agreement with observed values for swollen specimens. Those for cis-1,4-polyisoprene obtained using the same set of anisotropic bond polarizabilities are somewhat smaller than observed values for unswollen specimens. This departure is in the direction expected from the behavior of ΔΓ upon swelling (i.e., a decrease in ΔΓ upon swelling).  相似文献   

7.
Polychloroprene [CCl?CH? CH2? CH2? ]x of approximately 95% trans-1,4 stereochemical structure was prepared by low-temperature emulsion polymerization. Fractions, obtained by liquid–liquid precipitations were studied in toluene solutions at 30°C by viscometry and osmometry. In addition, force–temperature measurements were carried out on networks of the polymer in the amorphous state. The results obtained on the polymer solutions indicate that the unperturbed dimensions of trans-1,4-polychloroprene are essentially the same as those of trans-1,4-polybutadiene of the same molecular weight. This observation, that substitution of a relatively large Cl atom for one of the methine H atoms in the trans-1,4-polybutadiene repeat unit has little effect on the chain dimensions, suggests that this increase in substituent size is offset by the fact that the length of a C? Cl bond is very much greater than that of a C? H bond. The results obtained on the polymer networks indicate that the unperturbed dimensions of trans-1,4-polychloroprene decrease significantly with increasing temperature, as has also been reported for both trans-1,4-polybutadiene and trans-1,4-polyisoprene.  相似文献   

8.
Supported Organometallic Complexes. VI. Characterization und Reactivity of Polysiloxane-Bound (Ether-phosphane)ruthenium(II) Complexes The ligands PhP(R)CH2D [R = (CH3O)3Si(CH2)3; D = CH2OCH3 ( 1b ); D = tetrahydrofuryl ( 1c ); D = 1,4-dioxanyl ( 1d )] have been used to synthesize (ether-phosphane)ruthenium(II) complexes, which have been copolymerized with Si(OEt)4 to yield polysiloxane-bound complexes. The monomers cis,cis,trans-Cl2Ru(CO)2(P ~ O)2 ( 3b ) and HRuCl(CO)(P ~ O)3 ( 5b ) were treated with NaBH4 to form cis,cis,trans-H2Ru(CO)2(P ~ O)2 ( 4b ) and H2Ru(CO)(P ~ O)3 ( 6b ), respectively (P ~ O = η1-P coordinated; = η2- coordinated). Addition of Si(OEt)4 and water leads to a base catalyzed hydrolysis of the silicon alkoxy-functions and a precipitation of the immobilized counterparts 4b ′, 6b ′. The polysiloxane matrix resulting by this new sol gel route has been described under quantitative aspects by 29Si CP-MAS NMR spectroscopy. 4b ′ reacts with carbon monoxide to form Ru(CO)3(P ~ O)2 ( 7b ′). Chelated polysiloxane-bound complexes Cl2Ru( )2 ( 9c ′, d ′) and Cl2Ru( )(P ~ O)2 ( 10b ′, c ′) have been synthesized by the reaction of 1b–c with Cl2Ru(PPh3)3 ( 8 ) followed by a copolymerization with Si(OEt)4. The polysiloxane-bound complexes 9c ′, d ′ and 10b ′, c ′ react with one equivalent of CO to give Cl2Ru(CO)( )(P ~ O) ( 12b ′– d ′). Excess CO leads to the all-trans-complexes Cl2Ru(CO)2(P ~ O)2 ( 14b ′– d ′), which are thermally isomerized to cis,cis,trans- 3b ′– d ′. The chemical shift anisotropy of 31P in crystalline Cl2Ru( )2 ( 9a , R = Ph, D = CH2OCH3) has been compared with polysiloxane-bound 9d ′ indicating a non-rigid behavior of the complexes in the matrix.  相似文献   

9.
A simple method to determine the chain scission mechanism of the oxidative degradation of rubber vulcanizates is proposed. The method involves the measurement of oxidative stress decay and the change in sol fraction, which allow us to distinguish whether scission occurs randomly along the main chain, near crosslinks or of crosslinks. The applicability of this method was well established using natural rubber vulcanizates as the reference samples. The chain scission of cis-1,4-polyisoprene vulcanizaties was proved to take place randomly along the main chain irrespective of their crosslink structure. On the other hand, the chain scission of dicumyl peroxide cured cis-1,4-polybutadiene takes place selectively near crosslinks. It is suggested that the unusual behavior of cis-1,4-polybutadiene vulcanizates is due to the characteristic structure of the crosslinks.  相似文献   

10.
Polymerization of butadiene catalysed first with V(acac)_3-Al(i-Bu)_2Cl, then with Co(acac)_3-H_2O-Al(i-Bu)_2Cl has been studied. The polymer obtained was identified to be a new variety of cis-1,4-polybutadiene which contained a fraction of trans-1,4-polybutadiene chemically bonded to the cis-1,4-polybutadiene chains. Its molecular weight and trans-1,4 content can be regulated by varying the catalyst composition and concentration as well as other polymerization conditions. The trans-1,4 fraction, although it presents only in 9—16%, forms a crystalline phase in the matrix at room temperature and facilitates the crystallization of the polymer.  相似文献   

11.
Cyclopropyl cyanide isomerizes in the gas phase at 660°–760°K and 2–89 torr to give mainly cis- and trans-crotonitrile and allyl cyanide, with traces of methacrylonitrile. The reactions are first order, homogeneous, and unaffected by the presence of radical-chain inhibitors. The rate constants are given by Overall: cis-Crotonitrile: trans-Crotonitrile: Allyl cyanide: where the error limits are standard deviations. On the basis of a biradical mechanism, it is deduced that the ? CH? CN radical center is resonance stabilized by ca. 30 kJ mole?1. Approximate equilibrium data are given for interconversion of the 1- and 3-cyanopropenes.  相似文献   

12.
Polybutadienes (PBs) are found to form inclusion complexes with cyclodextrins (CDs) stereoselectively to give crystalline compounds in bulk. These complexes have been isolated and characterized by means of 1H NMR and 13C CP/MAS NMR spectroscopy, and X‐ray diffraction. Although α‐CD did not form inclusion complexes with any kinds of PBs in aqueous solutions, α‐CD did form inclusion complexes with PBs having 1,4‐cis‐ and 1,4‐trans‐butadiene units in bulk by heating at 100 °C. On the other hand, PB having 79% of a 1,2‐structure did not form inclusion complexes with α‐CD. The yield of the inclusion complexes increases with an increase in the content of the 1,4‐cis‐structure of PB and decreases with the molecular weights of the PBs.

  相似文献   


13.
A new trithiocarbonate functionalized cis-1,4-polyisoprene was obtained from oxidative degradation of natural rubber followed by reductive amination and amidation. The structure of the resulting functionalized cis-1,4-polyisoprene was confirmed by a combination of 1H NMR spectroscopy, 13C NMR spectroscopy, MALDI-TOF mass spectrometry and FTIR spectroscopy. 1H NMR spectroscopy showed that the trithiocarbonate functionality was equal to one. The well-defined trithiocarbonyl-end functionalized cis-1,4-polyisoprene was used as a macromolecular chain transfer agent (macroCTA) to mediate the RAFT polymerization of t-BA using AIBN as the initiator ([t-BA]0/[macroCTA]0/[AIBN]0 = 250/1/0.2) in toluene at 60 °C. The resulting PI-b-P(t-BA) diblock copolymer presents an unimodal SEC trace shifted toward higher molecular weight in comparison with the SEC trace of the macroCTA, indicating that the polymerization of the second block is effective. The characteristics of the copolymer were determined by SEC = 26,000 g mol−1, PDI = 1.76) and 1H NMR spectroscopy ( (PI) = 62 and (P(t-BA)) = 87).  相似文献   

14.
A study was made of the microstructural changes that occur in ultraviolet irradiation under vacuum of thin films of 1,2-poly(cis-1,4-hexadiene) (CHD), 1,2-poly(trans-1,4-hexadiene) (THD), 1,2-poly(trans-1,3-pentadiene) (TPD), equibinary (1,2,-1,4) polybutadiene (EB), and equibinary (3,4-1,4) polyisoprene (EI). These polymers—all containing pendant double bonds—undergo important photoinduced loss of unsaturation, presumably through cyclization of the double bonds, by analogy to the previously reported photocyclization of 1,2-polybutadiene (VB) and 3,4-polyisoprene (VI)films. For the equibinary polymers, which contain internal as well as external (or pendant) double bonds, the loss of unsaturation is considered to involve photocyclization of 1,2-1,4 and 1,2-1,2 dyads in EB and of 3,4-1,4 and 3,4-3,4 dyads in EI. Accompanying thecyclization process in CHD, THD, and TPD is a direct photochemical cis-trans isomerization of ? CH?CH? double bonds analogous to that originally noted for 1,4-polybutadiene. The photorearrangements in the above polymers with pendant double bonds were compared to the corresponding thermally induced rearrangements reported previoulsy;for VB and VI, in particular, the thermal, photo-and radiation-induced cycli-zations were found to be very similar, possibly having a common nonradical, nonionic mechanism involving excited double bonds.  相似文献   

15.
The issue of connectivity in polymer networks is discussed by employing graph theory. Several topics are addressed, including the theory of rubberlike elasticity and computer simulations of network formation. The formulation of the partition function is reviewed and new applications are presented on the irradiation cure of polybutadiene and the sulfur vulcanization of cis(1,4)-polyisoprene and cis(1,4)-polybutadiene.  相似文献   

16.
The thermal unimolecular reactions of cis- and trans-penta-1,3-diene (c-PTD and t-PTD) have been studied over the temperature range of 1002–1235 K using the technique of very low-pressure pyrolysis (VLPP). c-PTD decomposes via 1,4-hydrogen elimination analogous to that previously reported for cis-but-2-ene. RRKM calculations incorporating a six-center transition state show that the experimental rate constants are consistent with the following high-pressure rate expression at 1100 K: where θ = 2.303RT kcal/mol, and the A factor was assumed to be the same as that for cis-but-2-ene. The activation energy is in excellent agreement with that obtained for cis-but-2-ene. t-PTD also undergoes decomposition by H2 elimination presumably via the prior rapid isomerization to c-PTD the results are in exact agreement with those for c-PTD.  相似文献   

17.
The cis-trans photoisomerization reaction of 1,4-polybutadiene was carried out below the melting points on films of polymers containing high trans-1,4 contents. Under the proper conditions of temperature and polymer composition, the reaction was observed to undergo an anti-equilibrium behavior, which was attributed to an irreversible crystallization of repeating units after isomerization from cis to trans structure. As a result, the trans composition passed through a minimum with reaction time while crystallinity increased throughout the reaction, and unexpectedly the β crystalline form was observed well below the α–β transition temperature. The composition–time behavior observed was rationalized on the basis of incorporation of trans units into crystalline regions on the lamellar fold surfaces and discussed within the framework of the proposed requirements for crystallization-induced reactions of copolymers.  相似文献   

18.
The ~(13)C-NMR spectra of alternating copolymers of conjugated dienes, butadiene (BD), isoprene(IP) and chloroprene (CP), with methyl acrylate (MA) were studied. It is proved that they are allalternating copolymers. The BD units in Poly (BD-alt-MA) are joined to MA mainly in the formof trans 1,4-structure. The contents of trans 1,4-, cis 1,4-and 1,2-structure are 88, 7 and 5%, res-pectively. The IP and CP units in Poly(IP-alt-MA) and Poly(CP-alt-MA) exist essentially as trans1,4-configuration and connect with MA units in "head to head" arrangement predominantly, whileCP-CP units present in Poly(CP-alt-MA) in a small quantity.  相似文献   

19.
Summary: The performances of readily available Ln(allyl)2Cl(MgCl2)2 · (THF)4 precursors (Ln = Nd, 1 ; Y, 2 ; La, 3 ), in combination with alkyl aluminum activators [MAO, AlMe3, AlEt3, Al(iBu)3], have been studied in isoprene polymerization. The catalyst combination 1 /MAO (1:30) shows a high activity (average TOFs up to ca. 5 × 104 mol (Ip) · mol (Nd)−1 · h−1 at 20 °C) and produces polyisoprene in a controlled fashion with up to 98.5% cis content, number‐average molecular weights in reasonable agreement with calculated values, and relatively narrow polydispersities index ( = 1.20–1.70). The yttrium precursor 2 affords systems with much lower activity and degree of control, but enables the formation of either 1,4‐cis‐enriched (75%) or 1,4‐trans‐enriched (91%) polyisoprenes, simply replacing the MAO activator by AlEt3 or Al(iBu)3, respectively.

Formation of 1,4‐cis‐ or 1,4‐trans‐enriched polyisoprenes upon activation with MAO.  相似文献   


20.
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 Et1.5AlCl1.5(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.  相似文献   

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