NMR studies on the growing species of cyclic acetals in their cationic polymerization. 1H-NMR spectra of oxycarbenium ions formed in some model reactions

The ionic species formed in the reaction of 1,3-dioxane with triethyloxonium salts were examined by ¹H-NMR spectroscopy under various conditions. The signal assignable to the protons of the positively charged end-methylene group appeared at δ = 9,9–16,6 ppm, depending on temperature, solvents, and counter ions. The quenching of the ionic species with sodium ethoxide provided 1,3-dioxane, 1,3-diethoxypropane, and 3,5,9-trioxaundecane. The removal of diethyl ether from the reaction mixture containing the ionic species shifted the end-methylene proton signal to a higher magnetic field and favored the formation of 1,3-dioxane in the quenching reaction. Similar phenomena were observed for the ionic species produced in the reaction of 1-chloromethoxy-3-ethoxypropane with antimony pentachloride. These results indicate the oxycarbenium ionic nature of the ionic species formed in the above reactions. On the basis of these model reactions, the growing species of cyclic acetals in the cationic polymerization is discussed.     

Dynamic NMR studies of equilibria involving active species in the polymerization of cyclic acetals. Reaction of the methoxymethyl cation with 5-, 6-, and 7-membered cyclic acetals

The cyclic acetals 1,3-dioxolane ( 4a ), 1,3-dioxane ( 4b ), and 1,3-dioxepane ( 4c ) give in their reactions with the methoxymethyl cation ( 1 ) a mixture of two isomeric tertiary oxonium ions: the simple products of cationation 5a-c and the cationated rings 6 , 7 , 9 , enlarged by two atoms. The equilibria for the studied monomers are shifted to the side of the oxonium ions as shown in the reaction schemes 2, 4, 5, and 6 (SbF^-₆ or SbCIF^-₅ being the corresponding anions). In the case of 4a the equilibrium is displaced almost completely to the enlarged ring 6 , whereas for the other two cyclic acetals 4b and 4c the first cationated products 5b and 5c dominate. Application of the dynamic ¹H NMR method allows the determination of the equilibrium constants of the reaction of 4a with 1 .     

Kinetics and mechanism of the polymerization of cyclic acetals initiated by stable carbenium salts. II. Confirmation of the initiation mechanism for triphenylmethyl hexachloroantimonate in the polymerization of 1,3-dioxolan

The interaction of triphenylmethyl hexachloroantimonate (I) with 1,3-dioxolan (II) gives the corresponding dioxolenium salt IV which decomposes into β-chloroethyl formate (V) and antimon pentachloride (VI) (Eq. (2)) or acts as initiator by reacting with 1,3-dioxolan (Eq. (3)). Triphenylmethyl tetrachloroantimonate does not initiate polymerization of 1,3-dioxolan but forms exclusively β-chloroethyl formate. SbCl₅, formed according to Eq. (2), reacts with 1,3-dioxolan to produce again the dioxolenium salt IV together with HCl and non-initiating by-products: SbCl₃ and (CH₃OCH₂CH₂O)_n SbCl_5hyphen;n, in proportions depending on the reaction conditions. β-Chloroethyl formate has been found to be present in the polymerization mixture at concentrations close to those calculated on the assumption that the reaction between the original initiator (I, IV, VI, triethyloxonium hexachloroantimonate or triphenylmethyl tetrachloroantimonate) and 1,3-dioxolan as well as the further decomposition of IV go to completion. All the initiators (In) give the same polymerization rates which are proportional to [In]₀ in toluene solution and proportional to [In]²₀ in dichloromethane solution. Considering the competitive consumption of the initiator and the formation of the true initiating species these results can be rationalized by the analysis of the kinetics according to the reaction scheme. Polymerization has been found to be of first order in monomer and to proceed without an induction period. Thus, initiation is fast, in agreement with the previously measured rates of hydride transfer to the triphenylmethyl cation and the fast decomposition of the dioxolenium salt in the presence of 1,3-dioxolan.     

Kinetics and mechanism of the polymerization of cyclic acetals, 5. Spectroscopic studies of the polymer end-groups in poly(1,3-dioxolane)

Polymerization of 1,3-dioxolane ( 1 ) was initiated in CH₃NO₂ and CH₂Cl₂ solvents with benzoylium hexafluoroantimonate (C₆H₅CO⁺SbF), terminated with sodium ethanolate (C₂H₅ONa), and the endgroups were determined by using UV spectrophotometry and ¹H NMR methods. Polymers with high polymerization degrees DP _n were obtained (M?_n up to 3·10⁵); DP _n calculated for living polymerization conditions (DP _n=([ 1 ]₀?[ 1 ]_e)/[C₆H₅CO⁺SbF]₀; i.e. one molecule of initiator gives one macromolecule) agree well with DP _n measured by osmometry and DP _n found from ¹H NMR and UV methods assuming one respective end- group per macromolecule. ¹H NMR studies were performed on polymers from perdeuterated 1 (1,3-dioxolane-d₆). These polymers, after purification, were shown to bear benzoyloxy and ethoxy end-groups: \documentclass{article}\pagestyle{empty}\begin{document}${\rm C}_6 {\rm H}_5 \hbox{---} {\rm CO\rlap{--} (O} \hbox{---} {\rm CD}_2 {\rm CD}_2 {\rm OCD}_2 {\rm \rlap{--} )}_n {\rm OCH}_2 {\rm CH}_3$\end{document} Similar results were obtained for polymers initiated with triethyloxonium hexafluoroantimonate ((C₂H₅)₃O⁺SbF), and terminated with triphenylphosphine ((C₆H₅)₃P). These results indicate that initiation with stable oxocarbenium or oxonium ions leads to predominantly linear macromolecules, while initiation with perchloric acid (according to the data reported by Plesch) leads to polymers of low molecular weight and claimed to be mostly cyclic after killing with C₂H₅ONa.     

13C NMR studies of the conformation of cyclic paraffins,n-paraffins and polyethylene in solution

High-resolution ¹³C NMR spectra of cyclic paraffins, n-paraffins and polyethylene were measured in 1,2,4-trichlorobenzene solution, and the results were compared with the solid state ¹³C high resolution NMR data measured previously. It is proposed that in cyclic paraffins up to C₃₂, the conformation in the solid state is retained even in solution, all the methylene carbons within the cyclic paraffin exhibiting identical gauche-trans population with fast transition between the conformers in the ring plane, whereas in cyclic paraffins of larger carbon number, the conformation in the solid state is not retained in solution, the gauche-trans populations being different in this case. The conformation of n-paraffins and polyethylene is also discussed.     

13C NMR studies of the polymerization mechanism and the dynamics of poly(alkyl methacrylate) grafted onto silk fibers

¹³C NMR was extensively used to determine the tacticity of poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA), and poly(butyl methacrylate) (PBMA) grafted onto silk fibers using radiation-induced graft copolymerization. All polymers are predominantly syndiotactic and the tacticity is virtually independent of the degree of grafting. The value of PΣ (= P_m/r + P_r/m) was calculated from the triad distribution and was found to be approximately one. This indicates that all the above polymer chains grafted onto silk fibers are syndiotactic and obey Bernoullian statistics. The number-average sequence length of either meso or racemic additions was determined using the triad distribution. The ¹³C NMR relaxation parameters, spin-lattice relaxation time, T₁, and nuclear Overhauser effect, NOE, were observed and then the value of the correlation time for average segmental motion, τ _c, and the width parameter, p, were determined by assuming a log χ² distribution for the correlation time. A relatively small value of p was obtained, indicating that the distribution of the correlation time is large. Taking into account the τ _c values of different poly(alkyl methacrylate) chains grafted onto silk fibers, the following order of segmental motion was furnished: PMMA > PEMA > PBMA.     

Study on the cationic polymerization mechanism of 1,3-dioxepane in the presence of ethylene glycol

The cationic polymerization of 1,3-dioxepane (DOP) initiated with trifluoromethanesulfonic acid (I) in the presence of ethylene glycol (EG) was investigated. At sufficiently low concentration of the initiator ([I] > 0.01 mol/L vs. [EG] < 0.20 mol/L), the molecular weights of the obtained polyacetal oligodiols are controlled by the mole ratio of consumed DOP to initial EG. Gel-permeation chromatography studies revealed that the concentration of cyclic oligomers in the products are negligible. The mechanism of the polymerization was investigated by means of kinetic studies. The results showed that the polymerization proceeds according to the active chain end mechanism (ACF) in combination with the activated monomer mechanism (AM); thus the cyclic oligomer in the obtained polymer is reduced, and intermolecular chain transfer to EG in ACE is dominant. It was also demonstrated that as [DOP]²[I]/[EG] decreases the contribution of ACE to the polymerization decreases and that of AM increases. In addition, ¹H and ¹³C NMR data illustrated that each macromolecule of polyDOP oligodiols contained one EG unit on average and that no EG end groups exist.     

Quantitative 13C NMR studies of metabolic compartmentation in the adult mammalian brain

We review the information obtained by 13C NMR methods on the metabolic compartmentation of the adult mammalian brain with emphasis on its quantitative aspects. Classical radiotracer evidence and more recent 13C NMR results support the presence in the brain of at least two glutamate pools, small and large, associated with two kinetically different tricarboxylic acid cycles localized in glia and neurons, respectively. Neuronal and glial cycles interact closely, utilizing common substrates like glucose and oxygen and exchanging a variety of metabolites including glutamate, glutamine and GABA. A model for the cerebral metabolism of (1,2-13C2) acetate has made it possible to calculate fluxes through both cycles and evaluate the exchanges of glutamate, glutamine and GABA under different physiopathological conditions. Calculated flux values through the neuronal and glial tricarboxylic acid cycles are 1.0 and 0.4 mumol/min g, respectively. In the adult normoxic brain, the small and large glutamate pools account for approximately 10% and 90% of cerebral glutamate with estimated turnover times of 1.25 and 5.8/min, respectively. Net transfers of neuronal glutamate and GABA to the glial compartment are calculated to be 0.1 and 0.04 mumol/min g while transfer of glial glutamine to the neuronal compartment is estimated as 0.1 mumol/min g. Pyruvate recycling in the adult brain occurs mainly in the synaptic terminals with a calculated flux of 0.3 mumol/min g. These flux values are altered severely in pathological states such as hypothyroidism or ischemia.     

Initiation of the polymerization of 1,3-dioxolane with 2,4,5-trisubstituted 1,3-dioxolan-2-ylium salts, 2. Reaction of the initiator with the polymer. Model studies

2-Mono- and 2,4,5-trisubstituted 1,3-dioxolan-2-ylium salts were reacted with dimethoxymethane representing the model of the polydioxolane chain. The products were identified and analysed by means of ¹H NMR spectroscopy and gas chromatography. The reaction was found to proceed mainly via interchange of the C-2 carbon atom of the salt and the methylene group of dimethoxymethane yielding 1,3-dioxolane or 4,5-disubstituted dioxolane and the unstable dimethoxymethylium derivative, which decomposes into the respective methyl ester and methyl cation. The formation of some by-products follows the main reaction path. The amount and the kind of substituents as well as the configuration influence the reactivity of the salts in the processes investigated.     

Copolymers of acrylates with sulfur dioxide: Synthesis and structural studies by 13C NMR spectroscopy

The copolymerization of sulfur dioxide with methyl, ethyl, butyl or 2-ethylhexyl acrylate was studied. The influence of the composition of monomer feed and reaction temperature on the composition of the copolymers was investigated. The maximum content of sulfonyl units in the copolymers, obtained at ?78°C, was found to be ca. 30 mol-% and to decrease with increasing temperature. The sequence distribution in the copolymers was determined by ¹³C NMR spectroscopy. It was proved that acrylates do not undergo alternating propagation with SO₂, and the minimum number of acrylate monomeric units in a homosequence is 2. The results obtained were compared with those obtained from studies on the copolymerization of SO₂, with other monomers, and are discussed in terms of the alternating copolymerization theory and the penultimate model.     

13C NMR studies on the relative reactivity of isocyanate groups of isophorone diisocyanate isomers

Model reactions of isophorone diisocyanate (IPDI ( 1 )) with ethanol were examined using ¹³C NMR technique to locate the chemical shift of different urethane and isocyanate groups in the products. The reactivity of the secondary and primary isocyanate group of the IPDI isomers is catalyst-dependent. In the presence of Lewis acids (e.g., dibutyltin dilaurate) the secondary NCO group is more reactive, while in the presence of Lewis bases (e.g., triethylamine) it is the primary one. This effect allowed the chemical shift assignment of urethane- and isocyanate carbonyl C atoms in adducts formed by reaction of IPDI with ethanol in mole ratio NCO:OH = 2:1. In ethyl carbamates from the main isomer the chemical shift δ = 155,93 ppm is assigned to the carbonyl C atom of the urethane group at the secondary C atom, the signal at δ = 157,07 ppm to the primary urethane group. Furthermore, it was possible to assign the signal at δ = 122,12 ppm to the primary NCO group and the signal at δ = 122,98 ppm to the secondary NCO group of isophorone diisocyanate.     

Initiation of the polymerization of 1,3-dioxolane by 2,4,5-trisubstituted 1,3-dioxolan-2-ylium salts, 1. Reaction of the initiator with the monomer. Model studies

The reaction of 2,4,5-trisubstituted 1,3-dioxolan-2-ylium salts with 1,3-dioxolane results in the formation of the more reactive 1,3-dioxolan-2-ylium salts monosubstituted in 2-position and 4,5-dimethyl-1,3-dioxolane. Values of the rate constants of this reversible reaction, which is supposed to precede the actual initiation process in the polymerization of 1,3-dioxolane by trisubstituted dioxolenium salts, depend on the structure of the dioxolenium cations. Steric hindrances and positive charge delocalization effects due to the substituents in dioxolenium cations were found to reduce the rate of that reversible reaction. The trans-isomers are less reactive than the respective cis-isomers, while aromatic derivatives are less reactive than those with aliphatic substituents.     

Toward dynamic isotopomer analysis in the rat brain in vivo: automatic quantitation of 13C NMR spectra using LCModel

The LCModel method was adapted to analyze localized in vivo (13)C NMR spectra obtained from the rat brain in vivo at 9.4 T. Prior knowledge of chemical-shifts, J-coupling constants and J-evolution was included in the analysis. Up to 50 different isotopomer signals corresponding to 10 metabolites were quantified simultaneously in 400 microl volumes in the rat brain in vivo during infusion of [1,6-(13)C(2)]glucose. The analysis remained accurate even at low signal-to-noise ratio of the order of 3:1. The relative distribution of isotopomers in glutamate, glutamine and aspartate determined in vivo in 22 min was in excellent agreement with that measured in brain extracts. Quantitation of time series of (13)C spectra yielded time courses of total (13)C label incorporation into up to 16 carbon positions, as well as time courses of individual isotopomer signals, with a temporal resolution as low as 5 min (dynamic isotopomer analysis). The possibility of measuring in vivo a wealth of information that was hitherto accessible only in extracts is likely to expand the scope of metabolic studies in the intact brain.     

13C NMR studies of ethene-norbornene copolymers: assignment of sequence distributions using 13C-enriched monomers and determination of the copolymerization parameters

Ethene and norbornene were copolymerized using metallocene catalysts that produce copolymers having isolated norbornene units or microblocks with a maximum of two norbornene units. The resonances of the norbornene C_5/6 and the ethene carbon atoms, which overlap extensively in the ¹³C NMR spectrum, were differentiated and assigned by comparing the ¹³C NMR spectra of the copolymers obtained from monomers having ¹³C at natural abundance with those prepared from feedstocks containing ¹³C₁-enriched ethene or ¹³C_5/6-enriched norbornene. The NMR analysis revealed that the chemical shifts of the norbornene C_5/6 carbon atoms are triad sensitive and those of the ethene carbon atoms are pentad sensitive. ¹³C NMR analysis of copolymers containing isolated norbornene units in various proportions allowed the resonances of the norbornene C_5/6 and the ethene carbon atoms to be assigned to the respective triads and pentads. The complete triad distributions of these copolymers determined in this way were used to calculate the copolymerization parameters for a representative metallocene catalyst.     

Exchange reactions between covalent and carbocationic species in polymerization of vinyl ethers in the presence of lewis acids: dynamic NMR studies

The ionization of 1-chloro-1-isobutoxyethane (IBVCI) in the presence of BCI₃ in CD₂Cl₂ and CD₂Cl₂/CDCl₃ mixtures at variable temperatures was studied by ¹H NMR spectroscopy. The presence of only one NMR signal for the methine proton in the intermediate stage of ionization indicated a fast exchange between covalent and cationic species. The dependence of its chemical shift on the [BCI₃]₀/[IBVCI]₀ ratio was used to calculate the equilibrium constant of ionization at various temperatures, assuming only the ionization-recombination process. The dependence of the calculate the equilibrium constant (K^app) on the [BCI₃]₀/[IBVCI]₀ ratio indicated the involvement of additional side equilibria, including the formation of oxonium ions by the reaction between the carbocation and the ether functionality, and the complexation of Lewis acid by the ether group. The width at half-height of the methine proton signal was used to calculate the apparent rate constant of the recombination of the counterions in the ion pairs (K). It was found that K is larger in CDCI₃/CD₂CI₂ mixtures than in CD₂CI_2. When BCI₃ is present in excess, the calculated rate constants for ionization and recombination are higher than the reported rate constant of propagation for the cationic polymerization of isobutyl vinyl ether (IBVE). Attempts to use this finding in order to prepare well-defined polymers were unsuccessful because of the consumption of BCI₃ through the addition reaction to the double bond of the monomer.     

13C NMR study on the mechanism of stereoselective polymerization of methyloxirane catalyzed by a model compound for enantiomorphic catalysts

The reaction of [Zn(OCH₃)₂·(C₂H₅ZnOCH₃)₆] ( 1 ) with methyloxirane was investigated by ¹³C NMR spectroscopy. Complex 1 was assumed to be a model compound for enantiomorphic catalyst sites in the stereoselective polymerization of methyloxirane by organozinc catalyst. Complex 1 was found to retain its structural feature, determined by X-ray diffraction, even after the propagation reaction had proceeded. Based on ¹³C NMR studies of the polymerization reaction in terms of spin-lattice relaxation and signal intensity analysis, the mechanism of the stereoselective polymerization of methyloxirane by the chiral catalyst sites was discussed at molecular level in reference to the enantiomorphic structure of complex 1 .     

13C NMR characterization of branched structures in polyesters resulting from the reaction of maleic and phthalic anhydride with 1,2-propanediol

Several model compounds were synthesized in order to study the side reactions taking place during the synthesis of polyesters, especially chain branching due to the addition of hydroxyl groups to maleate or fumarate double bonds. Diesters or polyesters from 1,2-propanediol with fumaric acid and various dialkyl alkoxysuccinates were used as models. Their synthesis and characterization are described. The interpretation of their ¹³C NMR spectra is reported. These spectra are compared with those of the diesters and polyesters of phthalic acid previously described and with the spectrum of a commercial unsaturated polyester. It was possible to identify and to assign the various end groups and the structures resulting from chain branching.