Effect of SnCl4 on the radical-initiated polymerization of diethyl itaconate: Kinetical and ESR studies

The effect of SnCl₄ on the polymerization of diethyl itaconate ( 1 ) with dimethyl 2,2′-azoisobutyrate ( 2 ) in benzene was investigated kinetically and ESR spectroscopically. The polymerization rate (R_p) at 50°C shows a flat maximum on varying the SnCl₄ concentration. The molecular weight of the resulting polymer decreases with increasing SnCl₄ concentration. The overall activation energy of the polymerization is lowered from 52 to 33 kJ · mol^?1 by the presence of SnCl₄, (0,342 mol · L^?1). An NMR study revealed that 1 and SnCl₄ form 1:1 and 2:1 complexes with a large stability constant in benzene. The propagating polymer radicals in the absence and presence of SnCl₄ are ESR-observable as a five-line spectrum under the actual polymerization conditions. The complexed polymer radicals show further three-line splitting due to two methylene hydrogens of the ethyl ester group. The polymer radical concentration increases with the SnCl₄ concentration. The rate constant (k_p) of propagation was determined using R_p and the polymer radical concentration. k_p (6,3–2,9 L · mol^?1 · s^?1 at 50°C) decreases with increasing SnCl₄ concentration. The presence of SnCl₄ (0,342 mol · L^?1) reduces the activation energy of propagation from 29 to 21 kJ · mol^?1. The rate constant (k_t) of termination was estimated from the decay curve of the polymer radicals, k_t (3,1–1,1 · 10⁵ L · mol^?1 s^?1) also decreases with the SnCl₄ concentration. The activation energies of termination in the absence and presence of SnCl₄ (0,342 mol · L^?1) are 30 and 24 kJ · mol^?1, respectively. Suppression of propagation and termination by SnCl₄ seems to be explicable in terms of an entropy factor.     

Effect of Aromatic Co‐Solvents on the Efficiency of Atom Transfer Radical Coupling Reactions of Fluorinated and Non‐Fluorinated Vinyl Aromatic Polymers

Monobrominated versions of poly(pentafluorostyrene) (PPFSBr), polystyrene (PSBr), and poly(methyl acrylate) (PMABr) are prepared by atom transfer radical polymerization (ATRP) and employed in a variety of atom transfer radical coupling (ATRC)‐type reactions to observe the impact of external aromatic faces on the extent of coupling (X_c). In ATRC reactions assisted with the radical trap 2‐methyl‐2‐nitrosopropane (MNP), X_c is nearly unchanged when the electron‐rich benzene co‐solvent (50% v/v with THF) is replaced with the electron‐poor hexafluorobenzene (HFB) for PSBr and PMABr. In the case of PPFSBr, the addition of benzene to the reaction mixture results in far lower extents of coupling (X_c < 0.2). ¹H NMR spectra of the radical trap MNP in HFB show greater aggregation to the inactive form, compared to the spectra obtained in benzene. To remove the effect of the radical trap interacting with the aromatic co‐solvent and altering the rate of coupling, traditional ATRC reactions are performed with the same co‐solvent systems and, in this case, HFB results in higher X_c values across all polymer types. This is consistent with HFB pushing the position of the K_ATRP further toward the active radical, while benzene increases the reactivity of the MNP radical trap.     

Cationic polymerization of 4-ethyl-1,3-dioxolane

The bulk polymerization of 4-ethyl-1,3-dioxolane, ( 1 ), was investigated at temperatures ranging from ?48 to 0°C with boron trifluoride etherate and the perchloric acid/acetic anhydride binary system as initiators. At temperatures above ? 20°C, liquid oligomers were obtained in poor yield, while at lower temperatures viscous polymer consisting of alternating sequences of oxy-1-ethylethylene and oxymethylene units with number average molecular weights of several thousands was produced. ¹H NMR analysis of the acetal protons of the polymer on the basis of some model compounds disclosed that the bond cleavage of the monomer in the polymerization occurred nearly randomly at the two acetal C–-O bonds. This polymerization involves a monomer-polymer equilibrium. The thermodynamic parameters for the polymerization were determined from the temperature dependence of the equilibrium monomer concentrations: ΔH_1c = ?13,0±0,8 kJ mol^?1 (= ?3,1±0,2 kcal/mol) and ΔS_1c = ?59,4±3,8 J mol^?1 K^?1 (= ?14,2±0,9 cal/(mol K)). 4-Isopropyl-1,3-dioxolane, ( 2 ), was found very reluctant to polymerize above ? 78°C.     

Phase Separation in Poly(N,N‐diethylacrylamide)/D2O Solutions and Physical Gels as Studied by 1H NMR Spectroscopy

The structural‐dynamic changes during temperature‐induced phase separation in poly(N,N‐diethylacrylamide) (PDEAAm)/D₂O solutions and physical gels in a broad range of concentrations (c = 0.5–50 wt.‐%) were studied by conventional high‐resolution and MAS ¹H NMR spectra. In the whole concentration range, irrespective of whether the studied system is a solution (c < 5 wt.‐%) or physical gel (c > 5 wt.‐%), the phase transition is manifested by line broadening (linewidth ≈ 3.6 kHz) of a major part of PDEAAm units, evidently due to the formation of compact globular‐like structures. ¹H MAS NMR spectra have shown that this broadening is not due to near‐static dipolar interactions. The respective motion is effectively isotropic with correlation time ≈ 1 μs, probably corresponding to Brownian tumbling of the whole globular particles. The thermotropic phase transition, as revealed by NMR, is not discontinuous, but ≈  6 K broad (302–308 K). Above the LCST transition, the fraction p* of PDEAAm segments in globular‐like structures is between ≈   0.94–1.0, independent of the polymer concentration. At temperatures below the LCST a certain preaggregation of PDEAAm is indicated by smaller line broadening (linewidths ≈ 100 Hz) in the ¹H NMR spectra.     

Laser flash photolysis of poly(p-vinylbenzophenone) in solution. Intramolecular triplet deactivation processes

Poly(p-vinylbenzophenone) (poly[1-(4-benzoylphenyl)ethylene], PVBP) and p-isopropylbenzophenone (IPB) were irradiated in benzene solution at room temperature with 347, 1 nm laser flashes (duration 25 ns). Triplet-triplet spectra were recorded at the end of the flash. Upon following the decay of the T-T spectra a pronounced polymer effect was found: the decay followed 2nd order kinetics in the case of the polymer and 1st order kinetics in the case of the model compound. The halflife times differ by a factor of about five (τ_1/2(PVBP) < τ_1/2(IPB)). Whereas τ_1/2(IPB) was not influenced, τ_1/2(PVBP) decreased with increasing absorbed dose per flash. From the results it is inferred that at relatively high dose rates PVBP triplets decay essentially via intramolecular T-T annihilation and at lower dose rates via intramolecular selfquenching processes. At comparable conditions IPB triplets were essentially deactivated via interactions with solvent molecules.     

The Influence of Solvent and Temperature Upon the Aggregation of Poly(ethylene glycol)

A viscometric method was used to investigate the influence of solvent and temperature upon the aggregation of poly(ethylene glycol) (PEG) in dilute solution. While no obvious change in the value of the intrinsic viscosity [η] and the Huggins coefficient k_H was observed for PEG in benzene solutions upon increasing the temperature from 15°C to 40°C, for the same increase in temperature, the [η] values in aqueous solutions and aqueous potassium sulfate (0.45 mol/l K₂SO₄) solutions showed a substantial decrease, k_H values showed a substantial increase. Especially for 0.45 mol/l K₂SO₄ solutions, at high temperatures (35°C and 40°C), deviation from the Huggins law and a much higher value of k_H are obtained, which presents the occurrence of polymer aggregation. These facts imply that intersegment attraction (cohesive interaction) gets gradually important relatively to intersegment repulsion, due to the salt out effect and the hydrogen bond breaking action of thermal energy added to the solution. As a consequence, in aqueous K₂SO₄ solutions at higher temperatures, polymer‐polymer contacts are favored. Then above a critical concentration, the phenomenon of aggregation occurs. However, there is no obvious aggregation in PEG/water solution and PEG/benzene solutions in the temperature range by our viscometric measurement, although in PEG/water solutions, the intersegment cohesion causes that the polymer coil shrinks gradually as the temperature increases. The results show that aggregation of polymer is possible to occur only in poor solvent.     

Water-insoluble dextrans by grafting, 2. Reaction of dextrans with n-alkyl chloroformates. Chemical and enzymatic hydrolysis

This work deals with the modification reaction of dextran with ethyl and butyl chloroformate using tertiary amines as catalyst/acceptor systems and the DMF/LiCl system as solvent. The structure of the resulting polymers was determined by means of IR, ¹H and ¹³C NMR spectroscopy as well as by chemical analyses. The reaction rate was found to increase in the following order: N,N′-dimethylaniline < pyridine < triethylamine. The presence of cyclic carbonates was observed when triethylamine was used as catalyst. A linear dependence of the reaction rate on polymer and pyridine concentrations and a more complex dependence on the n-alkyl chloroformate concentration were found. Reaction rate and yield decrease with increasing amount of LiCl in the solvent medium and increase with increasing chain length of the n-alkyl chloroformate. The activation energy was found to be 64 kJ/mol (15,3 kcal/mol). The equilibrium water content (EWC) values decreases progressively when either the content of carbonate groups or the side chain length increases. The hydrolysis in the heterogeneous phase showed that the time required for the polymer solubilization is dependent upon the nature of the carbonate groups, the temperature as well as the pH value of the medium. Dextranase was found to be inactive in the hydrolysis of water-insoluble modified dextrans. However, the hydrolysis takes place when water-soluble modified dextrans were used.     

The cAMP-regulated and 293B-inhibited K+ conductance of rat colonic crypt base cells

We have shown previously that secretagogues acting via the second messenger adenosine 3′,5′-cyclic monophosphate (cAMP) activate, besides their marked effect on the luminal Cl⁻ conductance, a K⁺ conductance in the basolateral membrane of colonic crypt cells. This conductance is blocked by the chromanol 293B. This K⁺ conductance is examined here in more detail in cell-attached (c.a.) and cell-excised (c.e.) patch- clamp studies. Addition of forskolin (5 μmol/l) to the bath led to the activation of very small-conductance (probably < 3 pS) K⁺ channels in c.a. patches (n = 54). These channels were reversibly inhibited by the addition of 0.1 mmol/l of 293B to the bath (n = 21). Noise analysis revealed that these channels had fast kinetics and produced a Lorentzian noise component with a corner frequency ( f _c) of 308 ± 10 Hz (n = 30). The current/voltage curves of this noise indicated that the underlying ion channels were K⁺ selective. 293B reduced the power density of the noise (S _o) to 46 ± 8.7% of its control value and shifted f _c from 291 ± 26 to 468 ± 54 Hz (n = 8). In c.e. patches from cells previously stimulated by forskolin, the same type of current persisted in 3 out of 18 experiments when the bath solution was a cytosolic-type solution without adenosine 5′-triphosphate (ATP) (CYT). In 15 experiments the addition of ATP (1 mmol/l) to CYT solution was necessary to induce or augment channel activity. In six experiments excision was performed into CYT + ATP solution and channel activity persisted. 293B exerted a reversible inhibitory effect. The channel activity was reduced by 5 mmol/l Ba²⁺ and was completely absent when K⁺ in the bath was replaced by Na⁺. These data suggest that forskolin activates a K⁺ channel of very small conductance which can be inhibited directly and reversibly by 293B. Received: 1 October 1995/Received after revision: 28 December 1995/Accepted: 28 December 1995     

The initiation of polymerization of unsaturated tertiary amines with carboxylic acids

The polymerization rate (R_p) of N-methyl-N-phenyl-2-aminoethyl methacrylate (MPAEMA) initiated with 2,2' -azodiisobutyronitrile (AIBN) at 50°C increased considerably after the addition of CCI₃COOH, and distinctly after the addition of CH₃COOH. R_p in a benzene solution of 2 mol. dm^?3 MPAEMA and 5 · 10^?2 mol. dm^?3 CCl₃COOH (without AIBN) was 13% · h^?1. [η] of the obtained polymer corresponded to 64 cm³ · g^?1. The polymerization order of MPAEMA initiated with CCl₃COOH is 0,93 with respect to monomer and 0,51 with respect to CCl₃COOH. The overall activation energy of polymerization of MPAEMA calculated from the temperature dependence of R_p between 20 and 50°C is 43 ± 1,2 kJ · mol^?1. In a benzene solution of 2 mol.dm^?3 MPAEMA, 5 · 10^?2 mol · dm^?3CCl₃COOH and 5 · 10^?3 mol · dm^?3 1,4-benzoquinone at 50°C the polymerization does not proceed for 6 h. In a benzene solution of 2 mol · dm^?3 4-dimethylaminostyrene (4-DMAS) and 2 mol · dm^?3 CH₃COOH (without AIBN), 40% of monomer polymerized within one hour. [η] of the polymer was 4 cm³ · g^?1. The overall activation energy of polymerization of 4-DMAS in the presence of CH₃COOH is ca · 54 kJ · mol^?1. The addition of 5 · 10^?3 1,4-benzoquinone slows down the polymerization rate only slightly. The effect of acids on the elementary polymerization reactions is characterized.     

Estimation of chain molecular dimension of poly(2-methylpent-1-ene sulfone) at critical polymer contents by dielectric measurements

Dielectric measurements were made in the frequency range from 30 Hz to 3 MHz for a series of the solutions of poly(2-methylpent-1-ene sulfone) (PMPS) having two different molecular weights in benzene at 30°C. The unperturbed chain molecular dimension was estimated from dielectric relaxation time at the critical polymer contents, C_cr. This value of the unperturbed dimension was much higher than that from the intrinsic viscosity measurement. The dielectric motion at C_cr involves simultaneous co-operative reorientations of about three spheres. The number of the spheres of reorientation may be related to the flexibility of the molecular chain of the polymer. The ratio of two effective volumes of the polymer obtained from the unperturbed chain molecular dimension and from the dielectric relaxation time, τ_m agreed with that of the two effective volumes obtained from the K value in Onogi's treatment and from the τ_m value.     

Étude cinétique de la polymérisation anionique du styrène en présence d'un contre-ion baryum

The anionic polymerization of styrene in tetrahydrofuran initiated by a bifunctional organo-barium derivative shows a remarkable property: for a given temperature the propagation rate does not depend on carbanion concentration in the range from 3.10^?5 to 5.10^?3 mol/l. This corresponds to a linear relationship between the rate constant of propagation k_p and the reciprocal of active sites concentration. The activation energy of propagation is equal to 4,1 kcal/mol (17,1±1,3 kJ/mol). Our experimental results were interpreted assuming that “living” polymer molecules form rings, their two anionic ends being associated through a divalent cation, and that they constitute aggregates with a “rosace” type structure. The validity of this assumption is supported by spectral and viscosimetric studies of living polymer solutions.     

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.     

Flow properties of concentrated solutions of polyarylate in m-cresol

The flow properties of concentrated solutions of polyarylate from bisphenol A/50:50 isophthalic acid: terephthalic acid in m-cresol are studied. The results are discussed in terms of critical concentration C_c and critical molecular weight M_c for the onset of entanglements. Values of C_c = 8,2 wt.-% and M_c ≈ 3500 are obtained. The dependence of the Newtonian viscosities with temperature allows to obtain different energies of activation of flow for each polyarylate/m-cresol system, ranging from 9 to 13 kcal/mol (= 37,7 to 54,4 kJ/mol). The following free-volume-additivity equation is proposed to fit these data: where E_a, E_ap, and E_ad are the activation energies of the solution, the polymer and the solvent, respectively. V_p and V_d are the polymer volume fraction and the diluent volume fraction, respectively, and α_p and α_d represent the differences between the volume coefficients of expansion above and below the corresponding glass transition temperatures for the polymer and the solvent.     

Characterization of creatine guanidinium proton exchange by water‐exchange (WEX) spectroscopy for absolute‐pH CEST imaging in vitro

Chemical exchange saturation transfer (CEST) enables indirect detection of small metabolites in tissue by MR imaging. To optimize and interpret creatine‐CEST imaging we characterized the dependence of the exchange‐rate constant k_sw of creatine guanidinium protons in aqueous creatine solutions as a function of pH and temperature T in vitro. Model solutions in the low pH range (pH = 5–6.4) were measured by means of water‐exchange (WEX)‐filtered ¹H NMR spectroscopy on a 3 T whole‐body MR tomograph. An extension of the Arrhenius equation with effective base‐catalyzed Arrhenius parameters yielded a general expression for k_sw(pH, T). The defining parameters were identified as the effective base‐catalyzed rate constant k_b,eff(298.15 K) = (3.009 ± 0.16) × 10⁹ Hz l/mol and the effective activation energy E_A,b,eff = (32.27 ± 7.43) kJ/mol at a buffer concentration of c_buffer = (1/15) M. As expected, a strong dependence of k_sw on temperature was observed. The extrapolation of the exchange‐rate constant to in vivo conditions (pH = 7.1, T = 37 °C) led to the value of the exchange‐rate constant k_sw = 1499 Hz. With the explicit function k_sw(pH, T) available, absolute‐pH CEST imaging could be realized and experimentally verified in vitro. By means of our calibration method it is possible to adjust the guanidinium proton exchange‐rate constant k_sw to any desired value by preparing creatine model solutions with a specific pH and temperature. Copyright © 2014 John Wiley & Sons, Ltd.     

Polymerization of Vinyl Acetate Initiated with Di‐tert‐butyl Perfumarate

The polymerization behavior of vinyl acetate ( 2 ) was studied in benzene using di‐tert‐butyl perfumarate ( 1 ) as an initiator. Low molecular weight polymer (M̄_n ≈ 3 000) is formed in the early stage of polymerization where 1 is substantially consumed by thermal decomposition, copolymerization with 2 , and chain transfer reactions through an addition‐substitution mechanism. As a result, the low molecular weight polymer formed in the early stage of polymerization contains five peroxy ester groups per polymer molecule. Then, the polymerization of 2 initiated with the low molecular weight polymer further proceeds to yield high molecular weight poly( 2 ) (M̄_n = 2.5–23×10⁴). Decomposition of the peroxy ester group of 1 in benzene was studied in the absence and in the presence of methyl methacrylate (MMA) or 2 . The activation energy of decomposition of the peroxy ester group of 1 is 118 kJ/mol in the absence of the monomers. The decomposition of the peroxy ester group of 1 is highly accelerated in the presence of MMA. The peroxy ester groups derived from 1 decompose in two stages in the presence of 2 . In the first stage, some of them are rapidly consumed mainly by the chain transfer reaction. In the second stage, the peroxy ester groups of copolymer from 1 and 2 decompose slowly.     

The role of water in the electrodeposition and doping of polythiophene and two of its derivatives

The galvanostatic electropolymerization of thiophene ( 1 ), 3-methylthiophene ( 2 ) and 2,2′-bithiophene ( 3 ) in acetonitrile, containing 0,002 to 20 mol/L water, was studied in detail. The current efficiency γ for the polymer film formation decreased rapidly for 1 , but much slower for 2 and 3 with increasing water concentration c. For c = 0,1 mol/L, γ was found to be 10, 82 and 95% for 1 – 3 . It was possible to describe the experimental γ vs. c plots quantitatively by a model where the electrogenerated radical cations (rc) of the thiophene (derivative) are consumed by two competing second-order follow up reactions, the nucleophilic attack of the rc by water (i) and the rate-determining dimerization of two rc's (ii). Their rates are given by: (i) v₁ = k₁ c_rc c and (ii) v₂ = k₂ c²_rc. The side reaction (i) leads to soluble products, while reaction (ii) contributes to the polymer film growth. The ratio k₁/k₂ was found to be 3 · 10^?7, 2 · 10^?8 and 4 · 10^?9 for 1, 2 and 3 , respectively, and this shows that the electropolymerization of 2,2′-bithiophene is by a factor of about 100 less sensitive toward the present water than thiophene. The steady state concentration c_rc is in the order of 10^?9 mol/L in all three cases. The relative insensitivity of doping/undoping cycles with respect to the presence of water is discussed in terms of a lessened reactivity of the polarons and a low solubility of water in the heteroaromatic polymer.     

The polymerization of isobutyl vinyl ether by trifluoroacetic acid

The polymerization of isobutyl vinyl ether (IBVE) initiated by trifluoroacetic acid (TFA) in 1,2-dichloroethane and in 1,2-dichloroethane/carbon tetrachloride mixtures has been studied over the temperature range ?2,5°C to 35°C. Provided that the ratio [IBVE]/[TFA] did not exceed 88 and that 2·10^?4 mol·1^?1 < [TFA] < 4·10^?3 mol·1^?1 polymerization was the only detectable reaction, and the initial rate of reaction R₀ was equal to k[IBVE]²[TFA]. The activation energy of the rate = (34 ± 2) kJ · mol^?1 and the activation energy of the molecular weight = ? (11,9 ± 0,5) kJ · mol^?1. When [TFA] < 1·10^?4 mol/l, no reaction was detectable. The polymers produced had molecular weights less than 4400; the only transfer is by monomer and this is the main chain-breaking step. The reaction was unaffected by the addition of water for [H₂O] < [TFA]. The rate depended on the solvent dielectric constant, but in a manner suggestive of a dipole-dipole reaction. A mechanism is proposed for the reaction in which the reactive intermediates are trifluoroacetate esters solvated by monomer.     

Differential scanning calorimetry studies on water restrained in hydroxyethylcellulose and hydrophobically modified hydroxyethylcellulose

Differential scanning calorimetric (DSC) studies were carried out for hydrophobically modified hydroxyethylcellulose (HMHEC)- and hydroxyethylcellulose (HEC)-water systems to examine the binding of water with polymer molecules and the formation of a structure. The amount of freezing-bound water in HMHEC was found to be much less than that in HEC. The above fact suggests that the water-polymer interaction is hindered by the presence of the hydrophobes. The formation of a structure above the melting point of ice was detected by DSC for both systems HMHEC- and HEC-water. Polarizing light micrographs of these two aqueous polymer systems showed the formation of a liquid crystalline state in the same range of temperature. These phenomena were observed in a wider range of water content for HMHEC (W_c <4.0) than for HEC (W_c <2.5).     

Polymerization of vinyl ethers by 3d-transition metal oxides

Studies on the relative efficiencies of 3d-transition metal oxides as catalysts for the polymerization of vinyl ethers revealed that V₂O₅ is the most efficient catalyst. Some kinetic studies with the system isobutyl vinyl ether/V₂O₅/benzene were carried out at ambient temperature. It was found that the rate is second order in isobutyl vinyl ether (IBVE) concentration (0,25 to 2,0mol.1^?1) and the inhibition of the polymerization by pyridine suggests a cationic mechanism. R_p increases by the use of benzaldehyde along with V₂O₅, whereas water (3.10^?3mol.1^?1) seems to have no effect on R_p. Over the temperature range of 15 to 60°C, R_p as well as [η] attain a maximum value at about 32°C. [η] is unaffected by varying the amounts of oxides, whereas it increases with increasing IBVE concentration. No stereoregularity in the product polymer was observed by using V₂O₅ alone or in combination with modifiers such as benzaldehyde or carbon disulfide. The kinetics are interpreted in terms of the Hinshelwood-Langmuire mechanism assuming a single point adsorption.