Ring-chain Equilibrium Research Articles

New routes to macrocyclic polymers of controlled dimensions

Since the early 1950s a number of theoretical studies have dealt with the prediction of the dimensions and behaviour of macrocyclic polymers. However, owing to the difficulty of synthesis, the comparison of theories with experimental data still remains restricted to a small number of macromolecules. The first cyclic polymers studied were those produced from systems in which ring-chain equilibrium exists, but it is only in the last decade that a method for the preparation of ring-shaped vinylic polymers of controlled dimensions has become available. The method involves the bimolecular end-to-end ring closure of an anionically prepared α,ω-difunctional linear polymer using a coupling agent. Low cyclization yields are generally observed. Besides theoretical considerations the experimental conditions which may lead to these results have been examined. In this paper a new approach to the synthesis of macrocyclic polymers with controlled dimensions is examined. In contrast to previous procedures, it involves a single unimolecular ring-closure process which is much less sensitive to the extremely low concentrations required for cyclizations. It is achieved by the direct end-coupling of an α,ω-heterodifunctional linear polymer precursor after the appropriate activation under high dilute conditions of one of its two end-functions. The advantages of this new approach are discussed both on theoretical and experimental grounds. Its application to the preparation of poly(vinyl ether)s, polystyrenes and poly(vinyl ether- b-styrene) diblock copolymers of controlled dimensions with a macrocyclic architecture is described.

Metathetic ring‐chain equilibrium; synthesis of 1,5,9‐trimethyl‐(1E,5E,9E)‐cyclododecatriene from 1,4‐polyisoprene

Metathetic ring‐chain equilibrium; synthesis of 1,5,9‐trimethyl‐(1<i>E</i>,5<i>E</i>,9<i>E</i>)‐cyclododecatriene from 1,4‐polyisoprene

Dynamics of liquid sulphur around the equilibrium polymerization transition

Liquid sulphur has been known since the beginning of this century to show at a temperature T p = 160°C a liquid-liquid transition assumed to consist of the polymerization of S 8 rings into very long polymeric chains. A first neutron scattering study of the liquid local order enabled us to check this assumption at a microscopic level. Then, the vibrational density of states and local motions were studied by time-of-light (TOF) inelastic neutron scattering as a function of temperature, below and above T p. Both the decrease of the modes characteristic of S 8 rings and the evolution of the quasielastic scattering with temperature are consistent with the picture of a polymer transition at T p followed by a ring-chain equilibrium.

Silicone networks prepared via a living percolation mechanism: postgelation structure for networks with a variety of junction functionalities

Silicone network were prepared by the trifluoromethanesulfonic-acid-catalyzed polymerization of octamethylcyclotetrasiloxane and an oligocyclosiloxane. The polymerization occurred by the random exchange of all siloxane bonds in the sol and the gel. Network where the junctions were tri-, tetra-, penta-, hexa-, hepta-, or octafunctional were prepared. The final structure was modeled by a simple thermodynamic consideration of the ring-chain equilibrium involved. The sol was primarily composed of cyclic oligomers of dimethylsiloxane which were in equilibrium with the network. The composition of the various size rings was analyzed to verify the structure assigned to the network. equilibrium swelling studies were carried out which strongly supported the calculated structures

Thermodynamic Recycling—On Ring-Opening Polymerization and Ring-Closing Depolymerization

Abstract The ring-opening polymerization of various cyclic monomers has been extensively investigated in the past, e.g., the cationic polymerization of tetrahydrofuran, the anionic polymerization of ∊-caprolactone, the metathesis polymerization of cycloalkenes, and the anionic or cationic polymerization of cyclohexamethyltrisiloxane and cyclooctamethyltetrasiloxane. In conjunction with ring–opening polymerization, monomer–polymer equilibrium (ceiling temperature) and/or the formation of a ring–chain equilibrium have also been studied. In the last few years we have studied extensively the anionic ring-opening polymerization and copolymerization of cyclic carbonates, the monomers mostly being provided by depolymerization of the polymers which were obtained by polycondensation polymerization. This fact has led to the development of a new polymer recycling concept by depolymerization and repolymerization, which we call thermodynamic recycling.

Mixed micelle of cholinergic anabaseine with sodium dodecyl sulfate: novel determination of the binding constants

Anabaseine (3,4,5,6-tetrahydro-2,3'-bipyridine) when added to aqueous (D 2 O) sodium dodecyl sulfate (SDS) binds to micellar SDS and undergoes a change in the position of its ring-chain hydrolysis equilibrium constant, easily followed by 1 H NMR. Micellized open-chain amino ketone (AK-SDS) is favored over micellized cyclic imine (Im-SDS), both present as their monocations. Binding constants for both are derived in a novel way from changes in the ratio of the two components as a function of the SDS concentration

The anionic ring-opening polymerization and copolymerization of cyclic carbonates

Cyclic carbonates are eligible to ring-opening polymerization using a wide variety of initiators such as carbanionic or alcoholate species as well as initiators known to be effective for the ring-opening polymerization of lactones and for the group transfer polymerization of vinyl monomers. Depending on the catalyst, high molecular weight polymers may be obtained in high yields (kinetically controlled regime) or a ring-chain equilibrium is observed upon end-biting, back-biting and transesterification reactions (thermodynamically controlled regime). The polymerizability of the cyclic carbonates is strongly dependent on their structure. Five-membered cycles generally cannot be polymerized, whereas six-membered cycles can be polymerized and copolymerized in an ideal manner. The polymerizability of higher cyclics, in particular when containing aromatic ring systems, is highly dependent on the substitution pattern of the aromatics. Since the active species in the polymerization of aliphatic cyclic carbonates was disclosed to be of alcoholate type, a copolymerization with ϵ-caprolactone is easily achieved, the reactivity of the cyclic carbonate, however, being by far larger than that of the lactone. On the other hand, the copolymerization with pivalolactone exerts a different behaviour, since the active species of the growing pivalolactone chain after a few steps assumes the character of a carboxylate anion which is unable to promote the ring-opening polymerization of cyclic carbonates. Since carbanionic species may be used as initiators for the ring-opening polymerization of cyclic carbonates, polystyryl, polybutadienyl, and polyisoprenyl anions may be used as initiators to achieve the corresponding block copolymers. To obtain block copolymers with poly(methyl methacrylate) blocks a group transfer polymerization of the respective acrylate has to be performed, followed by the polymerization of the cyclic carbonate. The latter, however, rather proceeds by a metal- free anionic process than by a group transfer process. The ring-opening polymerization and copolymerization of cyclic carbonates allows the preparation of a broad variety of new polymers with remarkable properties.

Photochemistry of 2-amino-2H-benzochromenes

Diabatic photoinduced ring opening of the chromene ring of 2-amino-5,6-benzo-2H-chromenes and 2-amino-6-methyl-7,8-benzo-2H-chromenes leads, in the same way as a thermally induced reaction, to the establishment of ring-chain tautomer equilibrium in the ground electronic state, the position of this equilibrium depending on the polarity of the solvent, the temperature, and structural factors. o-Quinoid tautomers exist as several stable isomeric forms: an acoplanar cis-S-cis-trans-form absorbing in the shortwave region, and S-trans-isomers absorbing in the longwave region of the spectrum. Photoexcitation of the o-quinoid forms in the temperature range 125–190 K initiates mutual conversions of these and the initial 2H-chromene structure. The relative stability of the conformers of the o-quinoid form depends on steric and electronic factors: benzanellation in positions 5,6 leads to stabilization of the cis-S-cis-trans-isomer and in positions 7,8 the S-trans-isomers are more preferred: π-acceptor substituents on the amine component increase the stability of the cis-S-cis-trans isomer and electron-donor substituents stabilize the S-trans-isomer.

Metathesis polymerization — stepwise or chain growth reaction?

The metathesis polymerization of cycloolefins may follow a stepwise or a chain growth mechanism depending on the type of double bond involved (strained or unstrained) and the catalyst (high or low activity). Strained cycloolefins/catalysts of low activity/catalysts with low transfer ability, in the kinetically controlled regime of the polymerization, yield high molecular weight polymers following a chain growth mechanism; in the ideal case, the molecular weight distribution of the polymers is identical with a Poisson distribution. Under suitable conditions, equilibrium may be attained during the course of the reaction, which yields cyclic oligomers corresponding to a ring-chain equilibrium and a most probable distribution of the polymers. On the other hand, with cycloolefins of low ring strain/highly active catalysts, the reaction yields cyclic oligomers in the initial steps of the polymerization, the dimers and trimers being particularly preferred and produced in greater amounts than those corresponding to their equilibrium concentration. In the further course of the reaction they are consumed in a step-wise manner, to form high molecular weight material exhibiting a most probable molecular weight distribution and to result in a homologous series of cyclic oligomers corresponding to a ring-chain equilibrium.

Ring-chain tautomerism and three- to four-component equilibria in fused-ring tetrahydro-1,3-oxazines

The condensation of cis - (1) and trans -2-hydroxymethyl-4-cyclo-hexenyl-1-amine (2) and 2,3- diexo - (3) and 2,3- diendo -3-hydroxymethylbicyclo[2.2.1]heptylamine (4) with 8 different aromatic aldehydes led to ring-chain tautomeric equilibria between epimeric tetrahydro-1,3-oxazines and open-chain Schiff bases, in all cases the equilibrium mixtures consisted of two epimeric ring forms although the 2-axial epimers of the cyclohexene-fused derivatives had only a minor contribution to the ring-chain tautomeric equilibria. For the norbornane-fused derivatives the ring-chain tautomeric equilibria were taken equal to the ratio of the sum of the ring forms and the amount of the E-isomer of the open form in which case the simple equation, log K X = 0.76σ + + log K X=H derived recently for ring-chain tautomerism in 1,3-oxazines prevailea again for each system studied.

Hydrolysis of cholinergic anabaseine and N-methylanabaseine: Influence of cosolvents on the position of the ring-chain equilibrium—Compensatory changes

The position of the ring-chain equilibrium established by the addition of water to the iminium ring (Im) in the title compounds and subsequent ring cleavage to give the acyclic conjugate acid amino ketones (AK) were investigated by proton NMR using D 2O, dimethyl sulfoxide (DMSO)- d 6, and CD 3OD solvents. A linear free energy relationship expressing the change in the logarithmic value of the equilibrium constant [AK] ([Im][D 2O]) for the mixed solvent with the mole fraction of water added to methanol is found for the substrates, giving the equilibrium constants for the pure solvents. Equilibrium constants for the pure solvents increase in the order water, methanol, DMSO. Although there is less water in the largely nonaqueous solvents, the increase in the magnitude of the equilibrium constants partially compensates for this decrease, thereby resulting in a moderated reduction of the value of the ratio [AK] [Im] . Replacement of the deuteron bonded to the imine nitrogen atom by a methyl group causes much more acyclic keto-ammonium ion to form in D 2O due to steric compression between this methyl group and the adjacent pyridine ring causing an increase in energy of about 1.7 kcal/mol.

The apparent p Ka of the oxidation product of spermine or spermidine: A p Ka masked by a ring-chain tautomeric equilibrium

The previously reported acid titration curve (S. Brandänge, L.-H. Eriksson, and B. Rodriguez, Acta Chem. Scand., 1984, B38, 526) for the aminal oxidation product of the title compounds in D 2O is computer simulated based on a model consisting of a ring-chain tautomeric equilibrium ( K T) for the monodeuteronated bicyclic aminal followed by a second deuteronation of the resultant monocyclic primary amine ( K a1 ) to give a dication. The fractional amount of dicationic acid product is given by the term [D +] ([D +] + K app ) where K app, the apparent dissociation constant, equals 5.47 × 10 −6, the product of K T and K a1 .

Gas-Phase Ring-Chain Tautomerism in 1,3-oxazolidines

Gas-phase ring-chain tautomeric equilibria with fourteen 1,3-oxazolidines derived from norephedrine and norpseudoephedrine were studied by means of mass spectrometry. Using 14 eV ionizing electrons these equilibria were comparable to those in non-polar solvent and obeyed the simple equation: log K x = ρ σ + + c, where ρ = 0.58±0.06 and 0.55+0.03 and c = 0.14+0.05 and 0.30±0.03 for norephedrine and norpseudoephedrine derivatives, respectively. Approximate values of enthalpy differences were also observed.

Ring-chain tautomerism in 1,3-thiazolidines

NMR spectroscopy was applied to study ring-chain tautomeric equilibria in several substituted 1,3-thiazolidines. The system highly prefers the ring form and only in the case of 2-(2 '-hydroxy,5 '-bromophenyl)-l,3-benzothiazo-line ( 2d) 1H and 13C NMR spectra revealed a detectable amount of the openchain tautomer. The relative stability of the 1,3-thiazolidine ring was estimated to be more than 10 4 times higher than that of the 1,3-oxazolidine ring.

Formation of 3,1-Perhydrobenzoxazines and their N-Methyl Derivatives. The Effects of Epimerization and Temperature.

The cyclization reactions of p-nitrobenzaldehyde with cis- and trans-2-hydroxymethylcyclohexylamine and of their N-methyl derivatives have been studied by 1 H NMR spectroscopy in CDCl 3 solution. Time-dependent spectra confirmed that the reactions with both cyclohexylamines proceeded via open-chain intermediates whereas those with the N-methyl derivatives showed no signs of such intermediates. The effects of temperature and the presence of the minor epimer on the ring-chain tautomeric equilibria are also discussed

Quantitative determination of the ring-chain hydrolysis equilibrium constant for anabaseine and related tobacco alkaloids

L'anabaseine (tetrahydro-3,4,5,6 bipyridyle-2,3p), une neurotoxine, s'hydrolyse rapidement en solution aqueuse en aminocetone a chaine ouverte.

Medium effects on formation of cyclic enolates from (2E,4Z)-3-formyl-2,4-hexadienedioates, a novel ring-chain equilibrium: importance of hydrogen bonding, solvent dipolarity/polarizability, and ion-pairing effects. Stereoisomerism of 3-formyl-2,4-hexadienedioates

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTMedium effects on formation of cyclic enolates from (2E,4Z)-3-formyl-2,4-hexadienedioates, a novel ring-chain equilibrium: importance of hydrogen bonding, solvent dipolarity/polarizability, and ion-pairing effects. Stereoisomerism of 3-formyl-2,4-hexadienedioatesJerzy W. Jaroszewski and Martin G. EttlingerCite this: J. Org. Chem. 1989, 54, 7, 1506–1518Publication Date (Print):March 1, 1989Publication History Published online1 May 2002Published inissue 1 March 1989https://doi.org/10.1021/jo00268a007RIGHTS & PERMISSIONSArticle Views404Altmetric-Citations9LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alerts Get e-Alerts

Tautomerism of 1-methyl-5-amino-2,3-dihydro-1,2,4-triazolium-1-alkylidene(or arylidene)-2-methylaminoguanidinium iodide

1H and 13C NMR spectroscopy has been used to examine the structures of the products of the condensation of aldehydes and ketones with 1-methyl-1-aminoguanidinium iodide, which in some instances in solution are involved in a ring-chain tautomeric equilibrium between 1-methyl-5-amino-2,3-dihydro-1,2,4-triazolium iodide and 1-alkylidene(or arylidene)-2-methylaminoguanidinium iodide.

ChemInform Abstract: Reactions of Carbonyl Compounds in Basic Solutions. Part 10. Methoxide‐Catalysed Cyclisation of Methyl‐2‐Acylbenzoates and 8‐Acyl‐1‐naphthoates

The detailed mechanism of the methoxide-catalysed rearrangement of both normal and pseudo methyl o-acylbenzoates and 8-acyl-1-naphthoates to form indane-1,3-diones and phenalene-1,3-diones, respectively, has been studied. Rate–acidity function correlations for the reactions in methanolic dimethyl sulphoxide give linear relations. The kinetic isotope effects have been observed with kH/kD equal to 0.7–0.9 and 5–7 for methyl o-acetylbenzoate and normal methyl 8-acetyl-1-naphthoate, respectively. The effect of substituents and ring–chain tautomerism on the rates has been considered. For the o-acylbenzoates the initial state is the ring-chain tautomeric equilibrium mixture and the rate-determining step is the intramolecular attack of the anion of the normal ester. For normal 8-acyl-1-naphthoates the rate-determining step is the ionisation of the normal esters; for the corresponding pseudo esters, the rate-determining step is the isomerisation of the psuedo to the normal ester.

Anionic ring‐opening polymerization of 2,2‐dimethyltrimethylene carbonate

AbstractThe anionic ring‐opening polymerization of 2,2‐dimethyltrimethylene carbonate in solution with sec‐butyllithium as initiator was found to result under suitable conditions in a bimodal molecular weight distribution, vic. cyclic oligomers and a high‐molecular‐weight polymer. The concentration of cyclic oligomers approaches that of a ring chain equilibrium. In the kinetically controlled regime high polymer yields were obtained. Optimum reaction conditions for a polymer yield of 85 – 95% are a temperature of −10°C, an initial monomer concentration of 10 weight‐% and toluene as solvent. In THF the ring chain equilibrium is achieved readily. The ceiling temperature was determined to be ≈ 30°C. The homopolymer was characterized by spectroscopic, thermoanalytical and viscosity measurements. A high degree of crystallinity was observed.