Kinetic and ESR studies on radical polymerization. Radical polymerization behavior of N-octadecylmaleimide in benzene

The polymerization of N-octadecylmaleimide ( 1 ) initiated with azodiisobutyronitrile ( 2 ) was investigated kinetically in benzene. The overall activation energy of the polymerization was calculated to be 94,2 kJ·mol^?1. The polymerization rate (R_p) at 50°C is expressed by the equation, R_p = k[ 2 ]^0,6[ 1 ]^1,7. The homogeneous polymerization system involves ESR-detectable propagating polymer radicals. Using R_p and the polymer radical concentration determined by ESR, the rate constants of propagation (k_p) and termination (k_t) were evaluated at 50°C. k_p (33 L · mol^?1 · s^?1 on the average) is substantially independent of the monomer concentration. On the other hand, k_t (0,3 · 10⁴ – 1,0 · 10⁴ L · mol^?1 · s^?1) is fairly dependent on the monomer concentration, which is ascribable to a high dependence of k_t on the chain length of rigid poly( 1 ). This is the predominant factor for the high order with respect to the monomer concentration in the rate equation. In the copolymerization of 1 (M₁) and St (M₂) with 2 in benzene at 50°C, the following copolymerization parameters were obtained: r₁ = 0,11, r₂ = 0,09, Q₁ = 2,1, and e₁ = +1,4.     

Propagation and Termination Kinetics of Poly(Ethylene Glycol) Methyl Ether Methacrylate in Aqueous Solution

The propagation rate coefficient, k_p, of poly(ethylene glycol) methyl ether methacrylate (M_n ≈500 g mol^?1) has been measured via pulsed‐laser polymerization (PLP)–size‐exclusion‐chromatography in aqueous solution between 5 wt% monomer and bulk polymerization at temperatures from 22 to 77 °C. k_p increases significantly toward higher water content, as is observed for other water‐soluble monomers. This entropy‐motivated effect enhances the pre‐exponential. The activation energy, E_A(k_p), is more or less identical to the characteristic value of methacrylates. The chain‐length‐dependent rate coefficient, k_t^i,i, for termination of two radicals of chain length i has been investigated at low degrees of monomer conversion via the single‐pulse–PLP–electron paramagnetic resonance technique. k_t^i,i turned out to be adequately represented by the composite model designed by the Russell group. The power‐law exponents for the chain‐length dependence of small and long radicals are close to the numbers reported for other monomers. The rate coefficient for termination of two radicals of chain length unity scales with the fluidity of the reaction mixture. Viscosity measurements prior to polymerization thus enable estimates of termination rate.

     

Kinetic and electron paramagnetic resonance studies on radical polymerization. Radical-initiated polymerization of methyl N-phenylitaconamate in N,N-dimethylformamide

The polymerization of methyl N-phenylitaconamate(methyl 2-methylenesuccinanilate ( 1 )) with dimethyl 2,2′-azodiisobutyrate ( 2 ) was studied in N,N-dimethylformamide (DMF) kinetically and by means of electron paramagnetic resonance (EPR) spectroscopy. The polymerization rate (R_p) at 55°C is given by the equation: R_p = k[ 2 ]^0,58 · [ 1 ]^1,6. The overall activation energy of the polymerization was calculated to be 54,2 kJ/mol. The number-average molecular weight of poly( 1 ) was in the range between 5000 and 17000. From an EPR study, the polymerization system was found to involve the EPR-detectable propagating polymer radical of 1 at practical polymerization conditions. Using the concentration of polymer radicals, the rate constants of propagation (k_p) and termination (k_t) were determined for 55°C. The rate constant of propagation k_p (between 8,4 and 12 L · mol^?1 · s^?1) tends to somehow increase with increasing monomer concentration. On the other hand, k_t (between 1,9. 10^?5 L · mol^?1 · s^?1) increases with decreasing monomer concentration, which results from a considerable dependence of k_t on the polymer-chain length. Such monomer-concentration-dependent k_p and k_t values are responsible for the high dependence of R_p on the monomer concentration. Thermogravimetric results showed that thermal degradation of poly( 1 ) occurs rapidly at temperatures higher than 200°C and the residue at 500°C amounts to 26% of the initial polymer. For the copolymerization of 1 (M₁) with styrene (M₂) at 55°C in DMF the following copolymerization parameters were found: r₁ = 0,52, r₂ = 0,31, and Q, e values Q₁ = 1,09 and e₁ = +0,55.     

Free radical polymerization in the presence of non-solvents, 1. Styrene

During styrene (STY) polymerization, initiated by radicals formed by thermal or photochemical decomposition of 2,2′-azoisobutyronitrile (AIBN) the overall polymerization rate constant K defined by relation K = R^p/([AIBN]^0,5 [STY] η) and the ratio k_p/(2k_t0) increase with decreasing styrene concentration by hexane or benzene (R_p is the polymerization rate and η_MIX the viscosity of the reaction system). In the thermally initiated polymerization K = k_p (2f k_d/(2k_t0))^0,5 and in the photochemically initiated polymerization K = k_p (2,303 ? I₀? d/(2k_t0))^0,5 where k_d, k_p, and k_t0 are respectively, the rate constants of AIBN decomposition, of propagation, and of termination (for a system of the viscosity 1 mPa·s) reactions, ? is the quantum yield of radicals entering into reaction with the monomer, I₀ the intensity of the incident light, ? the molar absorption coefficient of AIBN, and d the path length of the light. The increase of K and of k_p/(2k_t0) with decreasing monomer concentration is more marked for the system styrene/hexane than for styrene/benzene and this increase is greater at 30°C than at 60°C. For Θ-systems formed by binary mixtures like styrene/hexane, styrene/decane and styrene/C₁ – C₄ alcohols the values of k_p and k_t0 at 30°C range between 57 and 91 dm³·mol^?1·s^?1 and (0,9 to 2,2)·10⁷ dm³·mPa·mol^?1, i.e. they are in principle identical with the tabulated values of these rate constants for styrene bulk polymerization.     

Termination kinetics of free-radical polymerization of styrene over an extended temperature and pressure range

The termination rate coefficient k_t of the free radical bulk polymerization of styrene is determined between 30 and 90°C up to a maximum pressure of 2800 bar. The majority of polymerization experiments has been carried out at monomer conversions up to 20 per cent. In this range a single value of k_t is sufficient to describe termination rate at constant pressure and temperature. Toward higher conversion, significant changes in k_t are observed. The data are measured by a pulsed laser polymerization technique and partly by conventional chemically initiated experiments, both with 2,2′-azoisobutyronitrile (AIBN) as the initiator. Online spectroscopy is applied toward measurement of styrene conversion. The experimental termination rate coefficients up to 20 per cent monomer conversion are adequately represented by the expression: Activation volume and activation energy of k_t are very close to the corresponding activation parameters that characterize the pressure and temperature dependence of the inverse of styrene monomer viscosity. Varying laser pulse repetition rate has been used to investigate a potential chain-length dependence of k_t at low conversion. It turns out that effects of this kind are not sufficiently pronounced to be safely established in view of the experimental precision of ±25 per cent that is reached in the k_t determinations.     

Kinetic and ESR studies of the radical-initiated polymerization of N-(2,6-dimethylphenyl)itaconimide

The polymerization of N-(2,6-dimethylphenyl)itaconimide (1) with azoisobutyronitrile (2) was studied in tetrahydrofuran (THF) kinetically and spectroscopically with the electron spin resonance (ESR) method. The polymerization rate (R_p) at 50°C is given by the equation: R_p = K [2] ^0,5 · [1] ^2,1. The overall activation energy of the polymerization was calculated to be 91 kJ/mol. The number-average molecular weight of poly (1) was in the range of 3500–6500. From an ESR study, the polymerization system was found to involve ESR-observable propagating polymer radicals of 1 under the actual polymerization conditions. Using the polymer radical concentration, the rate constants of propagation (k_p) and termination (k_t) were determined at 50°C. k_p (24–27 L · mol^?1 · s^?1) is almost independent of monomer concentration. On the other hand, k_t (3,8 · 10⁴–2,0 · 10⁵ L · mol^?1 · s^?1) increases with decreasing monomer concentration, which seems mainly responsible for the high dependence of R_p on monomer concentration. Thermogravimetric results showed that thermal degradation of poly (1) occurs rapidly at temperatures higher than 360°C and the residue at 500°C was 12% of the initial polymer. For the copolymerization of 1 (M₁) with styrene (M₂) at 50°C in THF the following copolymerization parameters were found; r₁ = 0,29, r₂ = 0,08, Q₁ = 2,6, and e₁ = +1,1.     

Kinetics and Modeling of the Radical Polymerization of Trimethylaminoethyl methacrylate chloride in Aqueous Solution

For the radical polymerization of ionized trimethylaminoethyl methacrylate chloride (TMAEMA) in aqueous solution, two strategies to determine the propagation rate coefficient (k_p) are proposed for systems where the pulsed‐laser polymerization–size‐exclusion chromatography (PLP–SEC) method fails. This problem occurs with some fully ionized or sterically highly hindered monomers, where termination may become too slow. As TMAEMA is a borderline case with k_p being accessible by PLP–SEC and from single‐pulse–pulsed‐laser polymerization with electron paramagnetic resonance (SP–PLP–EPR) spectroscopy, studies into this monomer allow for judging the quality of the suggested alternative approaches of k_p measurement and serve for consistency checks of the previously published k_p and termination rate coefficient (k_t) data. Within both approaches, k_p/〈k_t〉 ^0.5 is measured via chemically initiated polymerization, with 〈k_t〉 referring to chain‐length‐averaged termination. The k_p/〈k_t〉 ^0.5 data are combined either with k_p/〈k_t〉 values from highly time‐resolved near‐infrared detection of monomer conversion induced by a single laser pulse (SP–PLP–NIR) or with Predici modeling on the basis of known chain‐length‐dependent termination kinetics. As coupled rate coefficients are measured, the obtained k_p data also provide 〈k_t〉 for a particular chain‐length distribution. The differences between propagation and termination rates of nonionized and fully ionized monomers are discussed.

     

A kinetic study of the emulsion copolymerization of N,N′-methylenebis(acrylamide) and an unsaturated polyester

The emulsion copolymerization of N,N′-methylenebis(acrylamide) (MBA) and an unsaturated polyester (UP) initiated by potassium peroxodisulfate was kinetically investigated at 50°C by conventional gravimetric and dilatometric methods. The rate of polymerization, the size of latex particles and the number of polymer particles were determined as a function of MBA concentration. The rate of MBA polymerization was found to be proportional to the 0,8th (at 0–15% conversion) and 0,9th (at 20–40% conversion) order with respect to [MBA] at 0,0231 mol · dm^?3 of UP. The values of the reaction order on the monomer concentration are discussed in terms of homogeneous and emulsion polymerization and crosslinking effects. The rate of UP polymerization in the emulsion copolymerization of MBA and UP does not depend on the total monomer concentration. The specific rate of MBA (or UP) polymerization increases with increasing monomer concentration and reaches a maximum at a certain concentration of monomer. The size of polymer particles decreases and the number of particles increases with increasing UP fraction. The stability of polymer particles increases with increasing UP fraction in the monomer feed. The ratio (k_p/k_t^0,5)₀ of the relative rate constants for propagation k_p and termination k_t calculated for the MBA or UP polymerizations at zero conversion increases with increasing MBA concentration. The growth of the polymer particles proceeds via polymerization in particles and by interparticle crosslinking reactions.     

Kinetics of free radical solution polymerization of methyl methacrylate over an extended conversion range

The low and intermediate conversion solution polymerization kinetics of methyl methacrylate in toluene and in 2-butanone are investigated. For this purpose, repetitively applied laser pulses are employed to stimulate initiation, and quantitative near infrared spectroscopy is used to determine methyl methacrylate concentrations. The monomer concentration change per pulse is measured; it is shown how this information may be converted into coupled values of the rate coefficient for propagation k_p, the rate coefficient for termination k_t, and the total efficiency of (photo)initiation. In the present case k_p is already known from independent investigations, which enables our pseudostationary state methods to yield k_t directly. The major thrust of this paper, therefore, concerns the effect of solvent on k_t: it is found that at both low and intermediate conversions, k_t increases as the solvent concentration increases; possible causes of this are discussed. The effect of initial solvent fraction on the variation of k_t with conversion is also reported. Although all our experimental measurements are carried out at an elevated pressure (1000 bar) and a single temperature (30°C), there is no reason for suspecting that our findings regarding solution polymerization termination should not be representative for polymerizations of methyl methacrylate (and other methacrylate monomers) in general.     

Evaluation of the kinetic parameters for styrene polymerization and their chain length dependence by kinetic simulation and pulsed laser photolysis

Pulsed laser photolysis (PLP) has been employed to determine propagation rate constants k_p for styrene polymerization in benzene over a wider temperature range (20?80°C) than previously converd. It is proposed that a small chain length dependence of k_p (overall) may, in part, be a consequence of a marked chain length dependence of k_p for the first few propagation steps [i.e. k_p(1) > k_p(2) < k_p(3) ≥ k_p(≥4)]. The propagation rate constant for styrene polymerization is given by the expression: In k_p = 16,0₉ ? 289₅₀/(RT) (overall) or In k_p = 16,4₇ ? 300₈₄/(RT) (chain length ≥ 4). Kinetic simulation has been applied both as an aid in data analysis and to demonstrate the reliability of the PLP technique for evaluation of propagation rate constants (k_p) in radical polymerization. This has been achieved by examining the sensitivity of the molecular weight distribution of polymers formed in PLP experiments to the values of the kinetic parameters associated with polymerization and their chain length dependence. The termination rate constants (k_t = k_c + k_d) and the ratio of combination to disproportionation (k_c: k_d) markedly affect the molecular weight distribution of polymer formed in PLP experiments. The prospects for evaluating the values of k_t, its chain length dependence and k_c : k_d by direct analysis of the molecular weight distribution are discussed in the light of these results.     

Functionalized monomers, 2. Kinetics of radical polymerization of oxirane- and oxetane-derivatives of styrene and copolymerization of 4-vinylphenyloxirane with styrene

The kinetics of radical polymerization of 4-vinylphenyloxirane ( 1 ), 4-vinylbenzyloxirane ( 2 ), and 2-(4-vinylphenyl)oxetane ( 3 ) initiated by 2,2′-azoisobutyronitrile (AIBN) was studied. In the case of 1 the initial rate of polymerization was found to depend on the initiator and monomer concentrations as (r_p)_total ∝? [AIBN]^0,5 · [M₁]^1,36. The higher order of the polymerization rate with respect to 1 was interpreted as due to a concurrent thermally initiated polymerization; the rate of the latter was found to depend on the monomer concentration squared. The value of the ratio propagation rate constant over square root of termination rate constant k_p/k_t^1/2 = 2,8 · 10^?2 dm^3/2 · mol^?1/2 · s^?1/2 was determined from the measured dependences (r_p)_total = f[AIBN] and (r_p)_total = f([M₁]), corrected for the rate of thermally initiated polymerization of 1 . On the other hand, the kinetics of radical polymerization of 2 and 3 did not deviate from the standard scheme valid for radical polymerization; in both cases the observed reaction order with respect to initiator and monomer was 0,5 and 1, respectively. Radical copolymerization of 4-vinylphenyloxirane (M₁) with styrene (M₂) was characterized by monomer reactivity ratios r₁ = 1,06 and r₂ = 0,78, respectively, corresponding to the Q, e-scheme values Q = 0,9 and e = ?0,36 for monomer 1 .     

Termination Kinetics of Methyl Acrylate and Dodecyl Acrylate Free‐Radical Homopolymerizations up to High Pressure

The single pulse (SP) – pulsed laser polymerization (PLP) technique has been applied to measure k_t /k_p, the ratio of termination to propagation rate coefficients, for free‐radical bulk homopolymerizations of methyl acrylate (MA) and dodecyl acrylate (DA) between 10 and 50°C at pressures from 10 to 2 500 bar. k_t /k_p is obtained from experimental monomer concentration vs. time traces that are determined via time‐resolved (μs) near infrared monitoring of monomer conversion induced by single excimer laser pulses of about 20 ns width. With k_p being known from PLP–SEC experiments, chain‐length averaged k_t is immediately obtained from k_t /k_p. For MA, k_t remains constant up to about 15% monomer conversion and clearly decreases upon further polymerization. For DA at pressures of 100 bar and above, a plateau value of constant k_t is observed up to about 60% monomer conversion whereas at lower pressure, e. g. at 10 bar, k_t slightly increases in the very initial conversion region, but also exhibits a plateau k_t value at moderate and high conversions. The occurrence of such plateau k_t values and their pressure and temperature dependence are consistent with the view that plateau regions of k_t are best understood in terms of diffusion control via segmental mobility.     

Critically Evaluated Termination Rate Coefficients for Free‐Radical Polymerization, 1. The Current Situation

This is the first publication of an IUPAC‐sponsored Task Group on “Critically evaluated termination rate coefficients for free‐radical polymerization.” The paper summarizes the current situation with regard to the reliability of values of termination rate coefficients k_t. It begins by illustrating the stark reality that there is large and unacceptable scatter in literature values of k_t, and it is pointed out that some reasons for this are relatively easily remedied. However, the major reason for this situation is the inherent complexity of the phenomenon of termination in free‐radical polymerization. It is our impression that this complexity is only incompletely grasped by many workers in the field, and a consequence of this tendency to oversimplify is that misunderstanding of and disagreement about termination are rampant. Therefore this paper presents a full discussion of the intricacies of k_t: sections deal with diffusion control, conversion dependence, chain‐length dependence, steady state and non‐steady state measurements, activation energies and activation volumes, combination and disproportionation, and theories. All the presented concepts are developed from first principles, and only rigorous, fully‐documented experimental results and theoretical investigations are cited as evidence. For this reason it can be said that this paper summarizes all that we, as a cross‐section of workers in the field, agree on about termination in free‐radical polymerization. Our discussion naturally leads to a series of recommendations regarding measurement of k_t and reaching a more satisfactory understanding of this very important rate coefficient.

Variation of termination rate coefficient k_t with inverse absolute temperature T^?1 for bulk polymerization of methyl methacrylate at ambient pressure.^[6] The plot contains all tabulated values^[6] (including those categorized as “recalculated”) except ones from polymerizations noted as involving solvent or above‐ambient pressures.     

Polymerization Kinetics of Ethylene Oxide Methacrylates in Ionic Media

The radical polymerization of ethylene oxide (PEGylated) methacrylates in ionic media has been studied. Lithium salts interact with the monomer causing a significant increase in the propagation rate constant, k_p, and also providing an ionic and highly viscous medium that sharply decreases the termination rate coefficient, k_t. Both features make the polymerization reactions with lithium salts faster compared to the bulk monomer. The systems are studied by means of FT‐IR spectroscopy to identify the interactions between the monomer and the lithium salt. In addition, an extensive kinetic study by PLP‐SEC has been performed to study the influence of the lithium salt on k_p and k_t in the polymerization of these monomers. These results are compared to those obtained when ionic liquids are used as polymerization medium.

     

ESR determination of rate constants for the radical polymerization of methyl phenethyl itaconate

Phenethyl 2-(methoxycarbonylmethyl)acrylate (methyl penethyl itaconate) ( 1 ) was prepared and its polymerization with dimethyl 2,2′-azoisobutyrate ( 2 ) was investigated kinetically in benzene. The polymerization rate (R_p) was found to be expressed by R_p = k·[ 2 ]^0,5·[ 1 ]^1,8 (at 50°C). Further, a higher dependence of R_p (2nd order) on the concentration of 1 was observed at 61°C. The overall activation energy of the polymerization was calculated to be a low value of 50,3 kJ/mol. The initiator efficiency (f) of 2 was determined to be 0,48 to 0,22 at 50°Cand 0,50 to 0,28 at 61°C. f decreases with increasing monomer concentration due to the high viscosity of 1 . The poly( 1 ) radical is stable enough to be observable by ESR at high temperatures (50–60°C). Rate constants of propagation (k_p) and termination (k_t) were estimated using the poly( 1 ) radical concentration determined by ESR. k_p [6,0 to 121/(mol·s) at 50°Cand 7,1 to 15 1/(mol·s) at 61°C] shows a trend to increase with the concentration of 1 . On the other hand, k_t [2,9·10⁴ to 17·10⁴1/(mol·s) at 50°Cand 6,9·10⁴ to 45·10⁴1/(mol. s) at 61°C] decreases with increasing MPI concentration. This behavior is responsible for the high order with respect to monomer concentration. Copolymerization of 1 (M₁) with styrene (M₂) at 50°Cgave the following results: r₁ = 0,36, r₂ = 0,40, Q₁ = 0,82 and e₁ = + 0,59. Using the above results, the rate constants of the cross-propagations were estimated to be k₁₂ = 22 1/(mol·s) and k₂₁ = 308 1/(mol·s) at 50°C.     

Evaluation of individual rate constants from the chain-length distribution of polymer samples prepared by intermittent (rotating sector) photopolymerization, 3. Theoretical foundations for termination by disproportionation

The chain-length distribution and its moments were calculated for a periodically interrupted photopolymerization with termination by disproportionation and negligible chain transfer. For a wide range of experimental conditions, which are readily accessible in the overwhelming majority of cases, the position of the first point of inflection in the chain-length distribution corresponds exactly to the quantity k_p [M]t₀, k_p representing propagation rate constant, M monomer and t₀ the period length. Thus k_p can be directly evaluated from the chain-length distribution of the polymer. Together with the ratio k/k_t, k_t being the termination rate constant, calculated from the rate and weight-average degree of polymerization according to a universal relationship—the validity of which is also verified — resolution into the individual rate constants is easily accomplished. The dependence on experimental conditions of the ratio weight- to number-average degrees of polymerization P_w /P_n , which is a measure of the width of the molecular weight distribution of the polymer formed, is discussed.     

Kinetics of the early stages of high‐energy radiation initiated polymerization

We describe our method based on pulse radiolysis with optical detection developed for the examination of the kinetics and mechanism of the first steps of high‐energy radiation initiated polymerization. The absorption spectra of the intermediates were obtained in cyclohexane solution of hexanediol diacrylate (HDDA) of different concentrations. In dilute solution (10 mmol·dm^–3) and short time (10 μs) after the pulse, the spectrum of the monomer radicals was observed. On increasing the monomer concentration, the maximum of the spectrum was shifted to longer wavelength indicating the start of the oligomerization reaction. The increase in the time of observation resulted in a similar shift in dilute solution. From the kinetic curves the rate coefficients of termination for the monomer radicals (2·k_t,m) and average rate coefficients of termination for the oligomer radicals of different chain length (2·k_t) were determined. The average rate coefficient of termination was found to decrease in time (that is with increasing chain‐length).     

Termination kinetics of styrene free‐radical polymerization studied by time‐resolved pulsed laser experiments

The single pulse (SP)‐pulsed‐laser polymerization (PLP) technique has been applied to measure k_t/k_p, the ratio of termination to propagation rate coefficients, for the free‐radical bulk polymerization of styrene at temperatures from 60 to 100°C and pressures from 1800 to 2 650 bar. k_t/k_p is obtained by fitting monomer concentration vs. time traces that are determined via time‐resolved (μs) near infrared monitoring of monomer conversion induced by single excimer laser pulses of about 20 ns width. Styrene is a difficult candidate for this kind of measurements as conversion per pulse is small for this low k_p and high k_t monomer. Thus between 160 to 300 SP signals were co‐added to yield a concentration vs. time trace of sufficient quality for deducing k_t/k_p with an accuracy of better than ± 20 per cent. With k_p being known from PLP–SEC experiments, chain‐length averaged k_t values are immediately obtained from k_t/k_p. At given pressure and temperature, k_t is independent of the degree of overall monomer conversion, which, within the present study, has been as high as 20%percnt;. The k_t value, however, is found to slightly increase with the amount of free radicals produced by a single pulse in laser‐induced decomposition of the photoinitiator DMPA (2,2‐dimethoxy‐2‐phenyl acetophenone). This remarkable observation is explained by DMPA decomposition resulting in the formation of two free radicals which significantly differ in reactivity. Extrapolation of SP–PLP k_t data from experiments at rather different DMPA levels and laser pulse energies toward low primary free‐radical concentration, yields very satisfactory agreement of the extrapolated k_t values with recent literature data from chemically and photochemically induced styrene polymerizations.     

On the kinetics of the polymerization of some ammonium methacrylates with different alkyl chain lengths in aqueous solution

Methacryloyloxyalkyltrimethylammonium chlorides ( 1a–c ) with different alkyl chain lengths were synthesized and polymerized radically with 4,4′-azobis(4-cyanovalerianic acid) and K₂S₂O₈ as initiators. With K₂S₂O₈ as initiator, reaction orders of 0,5 and 1 with respect to initiator and monomer, were found. For the ionic monomers with longer alkyl chains the ratio of rate constants k_p(2fk_d/k_t)^0,5 was determined over a wide concentration range. It was found that with decreasing monomer concentration the ratio of rate constants increases, which is caused by a diminished termination rate because of an increased electrostatic repulsion of the ionic polymer radicals in dilute solution (increase of dissociation). Methacryloyloxyethyltrimethylammonium chloride ( 1a ) doesn't show this effect. This can be explained by the diminished effective charge of the ammonium group in the preferred gauche conformation of the molecule.     

Chain-length dependent termination in pulsed-laser polymerization, 7. The evaluation of the power-law exponent b from the chain-length distribution in the low frequency (single-pulse) limit for the reference systems styrene and methyl methacrylate in bulk at 25°C

The chain-length distribution (CLD) was examined for polymers prepared by “low frequency pulsed laser polymerization” (LF-PLP), i. e. for very long pulse separations in the so-called “low frequency” or “single pulse limit”. The data were fitted to the theoretical CLD which could be derived in a closed form for a chain-length dependent rate coefficient k_t and the parameter b that characterizes this chain-length dependence was determined by this fitting procedure. b values close to 0.2 were obtained for styrene as well as for MMA, indicating a moderate chain-length dependence of k_t at low conversions. This result, which is in good agreement with data evaluated by other methods in our laboratory, points to the fact that under these conditions end-segment diffusion is the rate-determining step in bimolecular termination. Factors like moderate chain transfer to monomer and uncertainties with respect to the mechanism of termination (combination or disproportionation) appear to have very little influence on this result.