Etude par RMN 13C à 62,89 MHz de ramifications longues d'alcanes lourds branchés. Modèles de PEbd et des copolymères éthylène-α oléfines

A series of 12-alkyl tricosanes (four compounds) was examined by use of ¹³C nuclear magnetic resonance at 62,89 MHz as model compounds for isolated short- and long-chain (C₅, C₆, C₇, C₈) branches in low-density polyethylene and ethylene-α olefin copolymers. An array of ¹³C resonances was observed and we note that the ¹³C chemical shifts became insensitive to branch length with octyl and longer branches at this field.     

Caractérisation des branchements courts dans un polyéthylène basse densité par RMN 13C

Carbon-13 NMR analysis of chain branching in low density polyéthylène (LDPE) at 62,89 MHz has revealed the existence of three kinds of complex chain branching besides more than four kinds of well known isolated branches. Well resolved carbon-13 spectra consisting of 26 peaks have been observed for one LDPE sample. For reasonable assignments of all the carbon atoms which play roles in branching signals, model compounds have been used, and three kinds of additivity rules for carbon-13 chemical shifts have been applied. Taking into account the double back-biting scheme in the formation of LDPE, 2-ethyl hexyl, 1,3-diethyl pair and tetrafunctionnal branches have been found besides isolated branches such as ethyl, butyl, pentyl, hexyl and longer chains.     

The occurrence of head-to-head butene units in ethylene-butene copolymers and corresponding ethyl branches in low-density (branched) polyethylenes

A high resolution carbon-13 NMR study of an ethylene-butene copolymer has yielded a spectrum whose resonances could be assigned to isolated ethyl branches, 1,3-diethyl branching as well as 1,2-diethyl branches resulting from head-to-head butene polymerization. All these structures are present in the same sample. An examination of all the published spectra of low-density (branched) polyethylenes has revealed many examples of heretofore unassigned resonances which can be associated with this latter type of diethyl branching in these polymers. Spin-lattice relaxation times have been determined, when feasible, for the carbon atoms in each of the three branched structures.     

Carbon-13 NMR of poly(vinyl acetate) and ethylene–vinyl acetate copolymer

Carbon-13 nuclear magnetic resonance spectra of atactic poly(vinyl acetate) and ethylene–vinyl acetate copolymers are reported and analyzed. Carbon spectra at 22.6 and 62.9 MHz together with use of shift reagents have permitted a more complete assignment than previously reported; in some cases the published assignments are found to be in error.     

Systèmes aromatiques à 10 électrons π dérivés de l'aza-3a pentalène. IX—etude par RMN de l'équilibre azide/tetrazole dans le cas du tetrazolo [5,1-b] benzothiazole

Azido-tetrazole equilibrium is observed in the case of a solution of tetrazolo [5,1-b] benzothiazole in CDCl₃, using proton magnetic resonance at 250 MHz. Analysis of the spectra obtained yields the chemical shifts and the coupling constants of the two tautomeric forms.     

Copolymerisation radicalaire du méthacrylate de méthyle et du chlorure de vinylidène avec ou sans dérive de composition

Two sets of methylmethacrylate-vinylidene chloride copolymers were prepared via radical copolymerization in dimethylformamide. The first set is carried out in batches. Gas-chromatographic analysis of samples allows a kinetic study from which the reactivity ratios r_M - 2.73 and r_C - 0.24 are derived. For the second set a new apparatus, briefly described, has been used. It permits to keep constant the composition of the monomer mixture, through addition of methyl methacrylate, monitored by a chromatographic analysis. The two sets of copolymers are analyzed using nuclear magnetic resonance (60 MHz) leading to the triad distribution, from which the reactivity ratios are derived. Owing to the composition drift, the method cannot be applied to the first set of copolymers; but in that case, it is possible to calculate the triad distribution, the knowledge of the reactivity ratios, and the calculation fits quite well the experimental results.     

Réaction d'imidoylation: Réactivité de composés à méthylène actif avec les O-ethylcarbamates cycliques

Condensation of malononitrile, ethyl cyanacetate and isopropylidene malonate with various O-ethyl cyclic carbamates leads to α-(tetrahydrox- azole-1.3-ylidene-2) and α-(tetrahydroxazine-l,3-ylidene-2)malono- nitriles, ethyl cyanacetates and isopropylidene malonates.     

Carbon-13 NMR of ethylene-1–olefin copolymers: Extension to the short-chain branch distribution in a low-density polyethylene

As model polymers for isolated short-chain branches in low-density polyethylene, a series of ethylene–1-olefin copolymers was examined by use of ¹³C NMR at 25.2 MHz. An array of ¹³C resonances was observed that could be associated independently with methyl through amyl branches. The ¹³C chemical shifts became insensitive to branch length with hexyl and longer branches. Assignments of the various carbon resonances associated with branching were accomplished by using off-resonance decoupling techniques and the behavior of alkane chemical shifts previously observed by other investigators. The ratio of certain backbone and branch resonances could be used to establish the short-chain branch distribution in a low-density polyethylene.     

Oligomérisation anionique de l'ethylène. III. Application à la synthèse d'alcools à longue chaine

Living oligomers of ethylene obtained by n-BuLi complexed with TMEDA have been deactivated by ethylene oxide. The nuclear magnetic resonance study of the product obtained allowed us to follow the influence of TMEDA toward the functionalization. Three products have been characterized: By increasing the ratio [TMEDA]/[n-BuLi] one obtains a decrease of the functionalization reaction.     

Sels d'aziridinium stables C-fonctionnalisés III: Synthèse et caractérisation d'aziridines protonées épimères à l'azote

Aziridinium ions were prepared from aziridines bearing a functionality on a ring carbon and as isolated crystalline fluorosulfate salts either by protonation in fluorosulfuric acid solution or by alkylation with methyl fluorosulfate. The structural assignement of these ions is based on nmr spectra and on quenching experiments with sodium carbonate. The configurations of the epimeric ions have been assigned by the nuclear Qverhauser effect and the stereoselectivity in protonation is disucussed.     

聚(苯乙烯-丁二烯-苯乙烯)三嵌段共聚物链结构的研究

<正> 以丁基锂引发二步加料法制备聚(苯乙烯-丁二烯-苯乙烯)三嵌段共聚物(简称SBS),往往在共聚物的中间链段聚丁二烯段中含有少量苯乙烯,即中间链段末端含有部份无规丁苯,得不到很纯的三嵌段共聚物,本文用超导核磁共振仪(H~1-NMR)、傅立叶红外光谱仪(FT-IR)、粘弹谱仪研究了上述SBS链结构,发现非嵌段苯乙烯芳环质子共振峰由二     

Alternating copolymers of propylene and alkyl acrylates

High-molecular-weight alternating and acrylate-rich copolymers of propylene and ethyl acrylate were prepared by using boron trifluoride to complex the acrylate ester. The polymerizations were run at room temperature and autogeneous pressures with free-radical initiation. The polymers were characterized by their nuclear magnetic resonance (NMR) and infrared (IR) spectra, differential scanning calorimetry, and gel permeation chromatography. The proton and ¹³C NMR spectra show that the equimolar copolymers are alternating to a high degree.     

Pyrolysis–gas chromatographic analysis of chain branching in polyethylene

A system with flash pyrolysis and gas chromatography (PGC) which includes in-line hydrogenation and packed columns is described. Under optimal conditions of temperature programming and flow rate, a high degree of resolution was obtained over a broad range of fragment lengths. The system has been applied to the analysis of side groups and long branches in polyethylene. It was found that, for low concentration of branches, yield and distribution of the isoalkanes is very sensitive to the type of polymerization used to form the polyethylene. A great number of the single-branched isoalkanes have been identified by comparing their retention time with the scale of respective boiling points and by pyrolyzing well characterized ethylene-propylene copolymers. The pyrograms of the ethylene-propylene copolymers demonstrated that a C? C bond in an α- or β-position to a tertiary carbon cleaves about two times more readily than a C? C bond in a linear chain. It was further found that the branch length of the side chain is indicated by the identity of the fragments produced, and the probability of formation of these fragments is directly related to the possibility of a cyclic decomposition mechanism. From the high yields of 3- and 5-methylalkanes it was possible to detect about 20 ethyl and 10 n-butyl branches per thousand in a low-density polyethylene. Thus, pyrolysisgas chromatography has to be considered seriously as a method for the determination of branches, even when in present in low concentration (Ziegler-type polyethylene).     

Etude en Résonance Magnétique Nucléaire et Stéréochimie d'un Composé Cyclopropanique d'Intérět Thérapeutique. Résonance du Proton à 250 MHz avec Triple Irradiation

1-Pivaloyl-2-hydroxymethylcyclopropane is studied with nuclear magnetic resonance. The C-1? C-2 configuration is determined from the 250 MHz n.m.r. spectrum (triple irradiation experiments have been performed for this purpose). Rotational isomerism around the ring-carbonyl bond is studied from the ASIS effect. Rotational isomerism around the ring-hydroxymethyl bond is studied from vicinal coupling constants over a temperature range of ?20 to +125°C. From the J(HOCH) coupling constant (in CCl₄) rotamer populations of the hydroxyl group are examined and the overall conformational distribution can be established.     

Etude du mécanisme de perte d'ethylène (propène) des N-ethyl (propyl) bromo-1,2,4-triazoles

C-bromo-1,2,4-trizole is generated in three different tautomeric forms by ethylene elimination from the N-ethyl compounds and these toutomes are shown to retain their structure prior to further fragmentation. The analysis of mass analysed ion kinetic energy and collision incuded dissociation spectra confrrms that ethylene loss proceeds by a tw-step mechanism with a five- (or four-) centred hydrogen transfer. The results show also that the 3- and 5-bromotriazole structures only are responsible for the mass spectrum of the parent heterocycle. Similar data are dicussed for the loss of propene from N-propylbromotriazoles.     

Radical cations in irradiated solid <Emphasis Type="Italic">n</Emphasis>-alkanes and polyethylene

The method of time-resolved magnetic field effect (TR MFE) in recombination fluorescence of spin-correlated radical ion pairs has been used to detect and identify radical cations generated at early stages after the pulse irradiation of polyethylene and tricosane. From analysis of TR MFE curves, the widths of unresolved EPR spectra of radical cations have been determined, and their g factors have been evaluated. The results have demonstrated that, as distinct from n-alkanes, primary radical cations in polyethylene are localized on chemical defects of the polymer chain in a time of about 1 ns or less.     

Structural study of the influence of partially crystalline poly(ethylene butene) random copolymers on paraffin crystallization in dilute solutions

Random crystalline–amorphous copolymers containing ethylene and butene segments, yielded from dilute-solution, and self-assembled to one-dimensional, needle-shaped aggregates, can modify wax crystal structures through the cocrystallization of the copolymer and wax molecules into hairy platelets. These copolymers show selectivity in their wax crystal modification capacities that depends on the ethylene content of the backbone. Thus, it has been qualitatively established that a copolymer containing larger crystallizable polyethylene sections [poly(ethylene butene) with 7.5 ethyl branches per 100 backbone carbons (PEB-7.5)] is very efficient for longer wax molecules (C₃₆), whereas for shorter waxes (C₂₄), its efficacy diminishes. Here we present a quantitative evaluation of the small-angle neutron scattering results obtained in a complex study of the self-assembling behavior of PEB-7.5 and paraffin waxes (C₂₄ and C₃₆) in decane and of cocrystallization for different polymer–paraffin combinations and solution conditions. The richness of the morphologies was evaluated with a contrast variation technique and the application of structural models. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3113–3132, 2004     

Molecular design of multicomponent polymer systems. XIV. Control of the mechanical properties of polyethylene–polystyrene blends by block copolymers

The present investigation deals with the tensile mechanical properties of the melt-blended polyethylene/polystyrene pair as controlled by poly(hydrogenated butadiene-b-styrene) copolymers. It is clearly demonstrated that moderate amounts of these copolymers (2–10%) significantly increase both the ultimate strength and elongation at break of blends of polystyrene with various types of polyethylene (low-density, high-density, linear low-density, and hydrogenated polybutadiene) and synergistic effects may result. The mechanical performance is strikingly dependent on the molecular characteristics of the copolymers. Over a broad range of molecular weights (60,000–270,000), diblocks are more effective than graft, triblock, or star-shaped copolymers. It is also demonstrated that using polymeric emulsifiers under usual processing conditions in the melt state is powerful technique for preparing valuable polymer alloys.     

The effects of overall composition and monomer sequence distribution on the infrared carbonyl stretching frequencies of ethylene–vinylacetate and styrene–vinylacetate copolymers

The vinyl acetate centered triad fractions of some free radically prepared ethylene–vinyl acetate and styrene–vinyl acetate copolymers have been determined from the patterns of vinyl acetate methine carbon peaks in their ¹³C nuclear magnetic resonance (NMR) spectra. The positions and shapes of the carbonyl bands in the infrared (IR) absorption spectra of the copolymers recorded in chloroform are shown to depend on the compositions of the copolymers and on the proportions of the various vinyl acetate centered triads. Infrared absorption measurements may thus be used in part to characterize the monomer sequence distributions of these copolymers.     

Polymérisation anionique des diènes. VI. Microstructure des polybutadiène et polyisoprène par résonance magnétique protonique à 250 MHz et mécanismes de propagation

Analysis by 250-MHz proton magnetic resonance (PMR) allows more precise examination of the microstructure of anionic polyisoprenes and polybutadienes obtained in the presence of organo-alkali metals. Besides vinyl addition, the propagation of contact ion pairs in dioxane solvent gives a mixture of cis and trans products with isoprene, whereas only trans product is obtained with butadiene. The presence of 1,4-trans-polyisoprene appears to be inconsistent with the cis configuration of the polyisoprenyllithium living end in dioxane or tetrahydrofurane media. In taking into account the existence of complexation phenomena, it was suggested that propagation mechanisms should involve a transition state which should begin with the trans configuration before isomerization into the more stable cis configuration. For the free ions, the living end of the carbanion should be entirely trans.