Solvent effects on free-radical polymerization, 3. Solvent effect on polymerization rate of methyl methacrylate and N-vinyl-2-pyrrolidone

The homopolymerizations of methyl methacrylate (MMA) in N-methyl-2-pyrrolidone (NMP) and of N-vinyl-2-pyrrolidone (NVP) in methyl isobutyrate (MiB) were investigated at 60°C using AIBN as initiator. Reaction rate and dynamic viscosity of the medium were determined as function of monomer dilution. It follows from the estimated monomer exponent n that MMA reacts in NMP faster (n = 0,739) and NVP in MiB slower (n = 1,495) in comparison with reaction rates obtained from the direct proportionality of monomer concentration. The observed deviation was quantitatively separated in partial deviations caused by chain initiation, propagation and termination. It was found that the viscosity effect on chain termination plays a dominant role in the global solvent effect. Chain propagation is influenced by electron donor-acceptor interactions between monomers and solvent.     

Solvent effects on free-radical polymerization, 1. Solvent effect on initiation of methyl methacrylate and N-vinyl-2-pyrrolidone

Initiation constant 2k_d · f, dissociation constant k_d, and efficiency factor f of AIBN were measured at 60°C for two model homopolymerization systems: methyl methacrylate (MMA) in N-methyl-2-pyrrolidone (NMP) and N-vinyl-2-pyrrolidone (NVP) in methyl isobutyrate (MiB) as the solvent. The stable Banfield radical was used as inhibitor. A pronounced solvent effect was observed on the side reaction of the inhibitor with the monomers. Particular solvent sensitivity of monomers in this reaction was identified, whereby opposite solvent properties of NMP and MiB as well as of MMA and NVP were found. The initiation constant 2 k_d · f depends mainly on the dissociation constant of the initiator k_d in regard to its solvent dependence. The efficiency factor f is influenced by monomer concentration, stability (reactivity) of primary radicals and by viscosity of the reaction medium (solvent cage effect).     

Critically evaluated rate coefficients for free-radical polymerization, 1. Propagation rate coefficient for styrene

Pulsed-laser polymerization (PLP) in conjugation with molecular weight distribution (MWD) measurement has emerged as the method of choice for determining the propagation rate coefficient k_p in free-radical polymerizations. Detailed guidelines for using this technique (including essential internal consistency checks) and reporting the results therefrom are given by the authors, members of the IUPAC Working Party on Modeling of kinetics and processes of polymerization. The results for PLP-MWD k_p measurements from many laboratories for bulk free-radical polymerization of styrene at low conversions and ambient pressure are collated, and are in excellent agreement. They are therefore recommended as constituting a benchmark data set, one that is best fitted by (the confidence ellipsoid for the Arrhenius parameters is also given). These benchmark data are also used to evaluate the merits of several other methods for determining k_p; it is found that appropriately calibrated electron paramagnetic resonance spectroscopy appears to yield reliable values of k_p for styrene.     

Solvent effects on radical polymerization of cyclododecyl acrylate, 2. Copolymerization

The radical copolymerization of cyclododecyl acrylate (CDA) with styrene (St) or acrylonitrile (AN) was studied in bulk, benzene, tetrahydrofuran, or dioxane at 60°C, and was compared with that of cyclohexyl acrylate (CHA). In the copolymerization with St, the change in the ester groups or the used solvents influenced the reactivity of both acrylates in the same manner as their homopolymerization rates. Approximately, CDA and CHA behaved like methyl methacrylate in the copolymerization with St. Contrary, in the cases of copolymerization of CDA with AN, peculiar results were obtained and interpreted in terms of characteristic association states of the monomers in the solutions. In benzene solution both the monomer reactivity ratios, r₁ and r₂, are larger than unity (M₁ = AN, M₂ = CDA; r₁ = 1,7, r₂ = 2,0).     

Solvent effects on free-radical polymerization, 2. IR and NMR spectroscopic analysis of monomer mixtures of methyl methacrylate and N-vinyl-2-pyrrolidone in bulk and in model solvents

Binary systems of methyl methacrylate (MMA)/N-vinyl-2-pyrrolidone (NVP) and MMA/N-methyl-2-pyrrolidone (NMP) with NMP as saturated model of NVP and of NVP/methyl isobutyrate (MiB) with MiB as saturated model of MMA were investigated by means of IR and NMR spectroscopy. Investigations were carried out at room temperature or at 60°C in CHCl₃ (IR) or CDCl₃ and C₆H₁₂/C₆D₁₂ (NMR). It can be concluded from IR and NMR spectra that the polarity of MMA increases in the presence of NVP and the polarity of NVP decreases in the presence of MMA. Equilibrium constants K of complex formation were determined to: K = 0,169 ± 0,037 L · mol^?1 for MMA/NVP at 30°C, 0,112 ± 0,024 L · mol^?1 for NVP/MiB at 60°C and 0,125 ± 0,030 L · mol^?1 for MMA/NMP at 60°C.     

Solvent effects on radical polymerization of cyclododecyl acrylate, 1. Homopolymerization

The radical homopolymerization of cyclododecyl acrylate at 60°C in benzene, tetrahydrofuran, or dioxane was studied and compared with that of cyclohexyl acrylate. The difference in the polymerization rates of the acrylates could not be interpreted in terms of the polar and steric substituent constants of ester groups in Taft's equation. Remarkable solvent effects on the polymerization rates for both acrylates were also found in the same manner and ascribed to the polar nature of the used solvents.     

Solvent effects on the free-radical copolymerization of butyl acrylate with methyl methacrylate

Free-radical copolymerization of butyl acrylate with methyl methacrylate was carried out at 50°C in a 3 mol/L benzonitrile solution. Differences between the apparent reactivity ratios determined in this work with those previously reported in bulk, toluene and benzene indicate noticeable solvent effects. This fact can be explained through different models, but independent of the chemistry of the system, to obtain the same copolymer composition in bulk or in solution, the monomer feed that should be used is determined by solvent polarity.     

Towards the propagation rate coefficient for the free-radical polymerization of 1,3-butadiene

The propagation rate coefficient k_p for the free-radical polymerization of 1,3-butadiene was determined as a function of temperature in the range of 30–60°C in bulk and as a function of the concentration of 1,3-butadiene in chlorobenzene at 50°C using a combination of a UV flash-lamp induced polymerization and a measurement of the molecular weight distribution of the produced polymer by size exclusion chromamgraphy. Differences of up to around 45% resulted even for the same concentrations and temperatures when these measurements were compared with recent determinations of k_p of 1,3-butadiene in chlorobenzene in which just such an experimental setup was used. This finding was essentially attributed to large differences in the radical termination rates which prevailed in the reaction volumes under the current conditions of the pulse-induced polymerization experiments. Moreover, in the vicinity of bulk concentrations of 1,3-butadiene in chlorobenzene, k_p apparently decreases with concentration.     

Solvent effects on the rate of polymerization of 2-hydroxyethyl methacrylate photoinitiated with aliphatic azo compounds

The rate of polymerization (R_p) of 2-hydroxyethyl methacrylate, at low conversion, using aliphatic azo compounds as photoinitiators is considerably faster in water than in acetonitrile as a solvent. Active radical production in different solvents was analysed in detail. The quantum yield of active radicals showed only a minor dependence on the medium properties. These differences are due to changes in primary radical recombination reactions and they are unrelated to changes of R_p observed in different media. By employing intermittent illumination techniques, it is shown that the increased R_p in aqueous solution is due to a reduced rate of termination. Similar propagation rate constants were obtained in acetonitrile and water, whereas the termination rate constant increases by one order of magnitude when acetonitrile is employed instead of water. These results are discussed in terms of the effect of the cosolvent on the medium viscosity and the macroradical conformation.     

Critically evaluated rate coefficients for free-radical polymerization, 2.. Propagation rate coefficients for methyl methacrylate

Pulsed-laser polymerization (PLP) in conjunction with molar mass distribution (MMD) measurement is the method of choice for determining the propagation rate coefficient k_p in free-radical polymerizations. The authors, members of the IUPAC Working Party on Modeling of kinetics and processes of polymerization, collate results from using PLP-MMD to determine k_p as a function of temperature T for bulk free-radical polymerization of methyl methacrylate at low conversions and ambient pressure. Despite coming from several different laboratories, the values of k_p are in excellent agreement and obey consistency checks. These values are therefore recommended as constituting a benchmark data set, one that is best fitted by The 95% joint confidence interval for these Arrhenius parameters is also given. In so doing, we describe the most appropriate statistical methods for fitting k_p(T) data and then obtaining a joint confidence interval for the fitted Arrhenius parameters. As well, we outline factors which impose slight limitations on the accuracy of the PLP-MMD technique for determining k_p, factors which may apply even when this technique is functioning well. At the same time we discuss how such systematic errors in k_p can be minimized.     

Radically initiated polymerization of a methacryloylamido-terminated saccharide, 1. Monomer synthesis,homopolymerization and characterizations

A methacrylamido-terminated saccharide, 6-deoxy-6-methacryloylamido-D -glucopyranose (MAG), was synthesized using a new and easy pathway. Kinetics of the radically initiated solution homopolymerization of this monomer was investigated at 50°C using 4,4′-azobis(4-cyanopentanoic acid); the obtained k_p/k_t^1/2 value around 1.0 mol^−1/2 ċ L^1/2 ċ s^−1/2 is indicative of the strong ability of MAG to polymerize (k_p and k_t being the rate constants of propagation and termination, respectively). The chemical structure of the homopolymer, as analyzed by ¹³C NMR spectroscopy shows a tacticity which is in favor of a syndiotactic configuration resulting from the bulkiness of the side group. The dimensional characteristics of the polymers were studied by light scattering as an on-line detection in size exclusion chromatography (SEC). As expected from the k_p/k_t^1/2 value, high weight-average molecular weights were obtained. The small average radii of gyration were found to reflect a relatively compact structure in water due to inter- and intramolecular interactions; viscosity studies in various solvents as well as values of the second virial coefficient corroborate such a behavior which reflects that water is a poor solvent for poly(MAG).     

Vinyl polymerization. 290. Solvent effects on the decomposition of 1.4-dimethoxycarbonyl-1.4-diphenyl-2-tetrazene

The rate of thermal decomposition of 1.4-dimethoxycarbonyl-1.4-diphenyl-2-tetrazene (DDT) has been measured in various solvents by UV spectroscopy. The rate measurements were carried out over a temperature interval from 65 to 85°C. The activation parameters have been calculated from the dependence of the rate constants on the temperature. On the basis of the kinetic data, the influence of the solvent on the decomposition of DDT was discussed. In this connection the solvent effect on the rate of polymerization of styrene initiated by DDT was also examined.     

Variation of the propagation rate coefficient with pressure and temperature in the free-radical bulk polymerization of styrene

The propagation rate coefficient K_p of the free-radical bulk polymerization of styrene is determined between 30 and 90°C up to a maximum pressure of 2800 bar. The data from pulsedlaser polymerizations and product analyses by gel-permeation chromatography (PLP-GPC) are adequately represented by the expression: The conseaquences of deducing activation volumes and activation energies from k_p/(L · mol^?1 · s^?1) or fromk*_p/(kg · mol^-1 ·. S^?1) are outlined.     

The laser-flash-initiated polymerization as a tool of evaluating (individual) kinetic constants of free-radical polymerization, 2. The direct determination of the rate of constant of chain propagation

When a polymerizable system is subjected to periodic light flashes, which induce the formation of primary radicals, a pseudostationary state is established which is characterized by a periodic profile of the (polymer) radical concentration. Within such a period of length t₀ the radical concentration will decay according to a second-order rate law. At the end of this period the radicals, which have escaped termination up to this moment, have propagated up to a chain length L₀ = t₀·k_p·c_M, k_p representing the propagation rate constant and c_M the monomer amount concentration. When the next flash arrives these radicals are opposed to a strongly increased overall concentration of radicals which leads to an enhanced probability for their termination. As a consequence the formation of dead polymer molecules with a chain length close to L₀ is favoured. The chain-length distribution of polystyrene prepared under such pseudostationary conditions, which was evaluated by gel permeation chromatography, in fact exhibits such a peak. The analysis of the theoretical distribution curves, derived in this communication, reveals that it is easily possible to correlate this peak to L₀, independently of the mode of termination (disproportionation or combination). Thus, a method of evaluating k_p is derived without any reference to the termination rate constant k_t and largely independent of all features which usually cause problems in the evaluation of k_p and k_t (such as primary radical termination etc.). The experimental results agree fairly well with the data reported in literature, especially with those obtained from the number of particles and the rate of polymerization in emulsion systems.     

Rate coefficients of the free-radical polymerization of 1,3-butadiene

Rate coefficients of termination and transfer in the free-radical polymerization of 1,3-butadiene in chlorobenzene were determined in the temperature range 318 K < T < 333 K. On the basis of an earlier published temperature dependence of the rate coefficient of propagation, for the termination reaction the Arrhenius equation K_t = 1,13 · 10¹⁰ · exp(? 711 K/T) L · mol^?1 · s^?1 was obtained. For the transfer to monomer the experiments yielded the Arrhenius equation K_tr,M = 4,22 · 10⁶ · exp(? 5140 K/T) L · mol^?1 · S^?1 and for the transfer to the solvent K_tr,S = 2,25 · 10⁸ · exp(? 7050 K/T) L · mol^?1 · S^?1.     

Studies on functional micelles, 5. Solvent and salt effects on the micellar condensation of thioalanine S-alkyl esters

The condensation reaction of thioalanine S-dodecyl ester ( 1c ) was carried out in several organic solvents, or in their mixtures with water. In acetone, benzene, chloroform, or hexane, no cyclic dimer of alanine, 3,6-dimethyl-2,5-piperazinedione ( 2 ) and in the water/organic solvent mixtures only low yields of 2 were obtained, contrary to the good yields which resulted in water alone. In the condensation reaction of thioalanine S-octyl ester ( 1b ) obvious salt effects were observed, especially with FeCl₃.     

Solvent effect on the propagation rate constant in the radical polymerization of bis(2-ethylhexyl) itaconate

Polymerization of bis(2-ethylhexyl) itaconate ( 1 ) with dimethyl azobis(isobutyrate) ( 2 ) was carried out at 50°C in various solvents. Polar solvents caused a significant decrease in the polymerization rate (R_p) and the molecular weight of resulting poly( 1 ). The propagating poly( 1 ) radical could be observed as a five-line ESR spectrum in the actual polymerization systems used. The stationary concentration of poly( 1 ) radical was determined by ESR to be 4,2–6,4 · 10^?6 mol · L^?1 at 50°C when the concentrations of 1 and 2 were 1,03 and 3,00 · 10^?2 mol · L^?1. Using R_p the monomer concentration and the polymer radical concentration, the propagation rate constant (K_p) was estimated to be 1,4–6,8 L · mol^?1 · s^?1, depending on the solvents used. The k_p value was smaller in more polar solvents. The solvent effect is explained in terms of the solvent affinity for the propagating polymer chain.     

Solvent effects on stereoregularity of poly(vinyl trifluoroacetate) obtained by radical polymerization

Vinyl trifluoroacetate was polymerized by free radicals in various kinds of solvents at temperatures between +60 and -40°C. The syndiotactic and isotactic diad contents of the polymer were determined via the triad content, which was obtained by their NMR-spectra. The differences of the activation enthalpies and those of the activation entropies between isotactic and syndiotactic arrangements were calculated from the temperature dependence of the ratio of isotactic to syndiotactic diads formed in the different solvents. A compensation effect exists between the two differences. The syndiotacticities of the polymers synthesized in alkanes in which the polymers were insoluble were higher than those in esters and ketones in which the polymers were soluble. The syndiotactic diad contents of the polymer synthesized in alkanes and ketones at ?40°C amount to 60～63% and 52～56% respectively.     

Initiation mechanism of free-radical polymerization of methyl methacrylate by p-substituted N,N-dimethylanilines

In order to investigate the initiation mechanism of the polymerization of MMA by N, N-dimethylaniline (DMA), p-substituted DMA's were prepared and used for a kinetic study. Electron donating groups increase and electron withdrawing substituents decrease the over-all rate of polymerization. Moreover, different kinds of amines were examined as initiators. The results were related with the Hammett equation and the ionization potential of the amines. The solvent effect and the composition of the initiator complex were investigated. From these results, it is concluded that the initiating species is a 1:1 electron transfer complex of DMA with the monomer methyl methacrylate.     

Stereospecific polymerization of methylthiirane in homogeneous phase, 4. Solvent effect on stereospecificity and enantioasymmetry

The living stereospecific polymerization of methylthiirane initiated by a chiral cadmium compound was run in different solvents. The general features of the mechanism of stereoregulation are the same as in bulk. In heptane and in heptane/propylene carbonate mixtures, the stereospecificity of the reaction does not depend on monomer concentration and on the dielectric constant ε of the medium. This independence of stereospecificity is in agreement with the absence of a variation of the complexation constant between a model cadmium derivative and the monomer with the variation of ε in a large range. The enantioasymmetric process depends on the molecular weight of the chain bearing the active sites. A change from diastereoisomeric to enantiomeric forms of active sites occurs for molecular weights around 50000. The enantioasymmetric constant depends on the nature of the medium and we observed a Born law for the dependence of the enantioasymmetric constant versus reciprocal dielectric constant: log k_R ∝ 1/ε. This effect can be explained by a variation of the reactivities of the two types of active sites and occurs without a change in their relative amounts.