Autoacceleration in Bulk Free‐Radical Polymerization: Effect of Chain Transfer

The model and methodology for estimating diffusion‐controlled rate coefficients for the methyl methacrylate polymerization system reported in Parts I and II of this series is extended to include the chain transfer reaction to an agent (CTA). Theoretical and experimental results exhibit minima in the rate of polymerization curves at low conversions as it is observed in the absence of CTA. Such minima are proposed to be the onset of the autoacceleration effect where the termination reaction undergoes a transition from chemically controlled to diffusion controlled where the segmental mobility of the chain ends plays a key role. As the CTA concentration increases, the conversion at which the minimum is presented is higher and the autoacceleration is less pronounced. This behavior is explained in terms of the effect of polymer chain entanglements and segmental mobility of the chain ends on the termination rate coefficient k _t.     

Investigation into the Kinetics of n‐Pentyl Methacrylate Radical Polymerization

The precise knowledge of rate coefficients is of key importance for the understanding and the application of radical polymerization processes. Propagation rate coefficients, k_p, of n‐pentyl methacrylate (PnMA) radical polymerization are measured in bulk and partly in toluene solution over an extended temperature range via pulsed laser polymerization (PLP) in conjunction with size‐exclusion chromatography (PLP–SEC). Rate coefficients, k_t^i,i, for termination of two radicals of chain length i are determined as a function of chain length by single‐pulse PLP in conjunction with electron paramagnetic resonance spectroscopy (SP–PLP–EPR). The as‐obtained data that allow for modeling PnMA polymerization kinetics and product properties at moderate degrees of monomer conversion are compared with reported data for several other alkyl methacrylates. A distinct family behavior of this group of monomers is seen.     

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.     

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.

     

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.     

A kinetic study of butyl acrylate free radical polymerization in benzene solution

The free radical polymerization of butyl acrylate has been studied in benzene solutions ranging from 1 to 5 mol·L^–1 at 50°C using 2,2′‐azobisisobutyronitrile as initiator. Under the conditions of our experiments, both the effective rate coefficient for initiation, 2 f k_d , and the coupled parameter, k_p/k_t^1/2, (where k_p and k_t are the constants for propagation and termination reactions, respectively) are dependent on the monomer concentration. The 2 f k_d value shows little increase with monomer concentration. The variation of the k_p/k_t^1/2 parameter has been correlated with the chain length dependence of the termination rate coefficient. This effect is also responsible for the high dependence of the overall polymerization rate, R_p, onthe monomer concentration (1.49).     

The laser flash-initiated polymerization as a tool of evaluating (individual) kinetic constants of free radical polymerization, 1. Outline of method and first results

A method is presented which allows the determination of k_p/k_t-values in free radical polymerization. It is based on measurements of the (average) rate of polymerization under pseudostationary conditions, the polymerization being initiated by laser flashes of short duration. For ρk_t t₀ ? 1 (ρ being the additional polymer radical concentration produced by each laser flash, k_t the bimolecular termination constant between polymer radicals, k_p the rate constant of chain propagation, t₀ the time separating two successive laser flashes) k_p/k_t may be obtained as the slope of a linear plot of the fractional conversion per flash vs. ln t₀. Dividing the intercept by the slope yields ln (pk_t). Thus, if p is accessible, separation of k_p/k_t-data into its individual constituents may be accomplished without making any use of stationary polymerization data. Application of this method to the polymerization of styrene sensitized by benzoin or AIBN at 25°C gives k_p/k_t-values of 1,0 · 10^?6 which are in fair agreement with those obtained by other methods.     

Effects of Chain Transfer Agent and Temperature on Branching and β‐Scission in Radical Polymerization of 2‐Ethylhexyl Acrylate

Poly(2‐ethylhexyl acrylate) is synthesized by conventional radical bulk polymerization both with and without 1‐dodecane thiol as chain transfer agent (CTA) at temperatures from 4 to 140 °C. Electrospray‐ionization mass spectrometry is used to analyze the polymer. This reveals the occurrence of significant β‐scission at high temperature and confirms the presence of CTA‐capped polymers at all temperatures, as well as combination products from 4 to 65 °C. Subsequent ¹³C melt‐state NMR analysis allows quantification of branching and β‐scission. Both are reduced when CTA is present, consistent with a “patching” effect. As expected, the amounts of β‐scission and branching increase with synthesis temperature, although β‐scission dominates at the highest temperature. The backbiting rate coefficient of 2‐ethylhexyl acrylate is determined from NMR results, taking β‐scission into account for the first time. Remarkable agreement with literature k_bb values is obtained, especially for activation energy. This strongly suggests family‐type behavior for acrylate k_bb.     

Radical polymerization of phenyl acrylate as studied by ESR spectroscopy: concurrence of propagating and mid‐chain radicals

The electron spin resonance (ESR) spectrum of the propagating radical of phenyl acrylate (PhA) was successfully recorded in benzene as a non‐polar solvent. The hyperfine coupling constants for the α and β‐protons were evaluated on the basis of the spectra of the propagating radicals of PhA and phenyl acrylate‐α‐d. The simulated spectrum satisfactorily fits those observed during polymerization, and the spectrum of the poly(PhA) radical obtained at low conversion (<15%) was assigned to the propagating radical. The spectra observed at higher conversions (>15%) indicated the presence of two types of radical species, a propagating radical and a mid‐chain radical produced by abstraction of the α‐hydrogen of the monomeric unit. The content of branching in the polymers as a result of the formation of a mid‐chain radical was found to be 1–3% by ¹³C NMR spectroscopy. The absolute rate constants for propagation (k_p) and termination (k_t) of PhA at low conversions were determined based on the quantification of the propagating radical by ESR spectroscopy at 60°C: k_p = 3 580 dm³·mol^–1·s^–1 and k_t = 6,8×10⁶ dm³·mol^–1·s^–1. However, these seemed to be apparent values because the propagating radical is expected to be converted to the mid‐chain radical by intra‐ and intermolecular hydrogen abstraction before the loss of its activity by bimolecular termination. Conversion from the propagating radical to the mid‐chain radical followed by reinitiation was estimated to occur more than twenty times during the lifetime of each polymer chain.     

Untersuchung der teilschritte der ethylen-hochdruck-polymerisation, 1. Druckabhängigkeit der einzelnen geschwindigkeitskonstanten

In order to calculate the average molecular weight, the molecular weight distribution or the branching of low density polyethylene, the individual values of the rate constants of the different reactions involved in the free radical polymerization are required. The method of rotating sector was applied to determine the rate constants of chain propagation and chain termination in a wide range of temperatures and pressures. This first part of the work reports the influence of the pressure. It was found that the rate constant k_p of the chain propagation increases with rising pressure whereas the rate constant k_t of chain termination decreases. The effect of the pressure on k_p can be explained in terms of the activation volume. The decrease of k_t with pressure is caused by a diffusion controlled termination reaction.     

Termination Kinetics of 1‐Vinylpyrrolidin‐2‐one Radical Polymerization in Aqueous Solution

Termination kinetics of 1‐vinylpyrrolidin‐2‐one radical polymerization in aqueous solution has been studied at 40 °C between 20 and 100 wt.‐% VP. The <k_t>/k_p values from laser single‐pulse experiments with microsecond time‐resolved NIR detection of monomer conversion, in conjunction with k_p from literature, yield chain‐length‐averaged termination rate coefficients, <k_t>. Because of better signal‐to‐noise quality, experiments were carried out at 2 000 bar, but also at 1 500, 1 000, and 500 bar, thus allowing for estimates of <k_t> at ambient pressure. The dependence of <k_t> on monomer conversion indicates initial control by segmental diffusion followed by translational diffusion and finally reaction diffusion control. To assist the kinetic studies, viscosities of VP–water mixtures at ambient pressure have been determined.

     

Pulsed Laser Polymerization of Alkyl Acrylates: Potential Effects of the Oxygen Presence and High Laser Power

Summary: Unexpected difficulties are encountered in the determination of propagation rate coefficients (k_p) in free radical polymerization of alkyl acrylates by pulsed laser polymerization (PLP), mainly due to intramolecular transfer to polymer. 1 This article is focused on the role played by the high laser power in these difficulties and the possible reactions of mid‐chain radical with residual oxygen. Removing the oxygen by simple bubbling of nitrogen is sufficient to avoid alteration of the polymerization kinetics of acrylates by residual oxygen under PLP conditions. Moreover, no degradation of polymer (or solvent) has been detected after irradiation with the high laser power typically used in PLP experiments. However, it has been shown that this high laser power completely prevents from having a temporally and spatially homogeneous radical concentration in the PLP cell. A model is proposed here to simulate the pulsed laser polymerization taking initiator consumption and laser energy absorption into account. According to our simulation results, this non‐negligible initiator consumption and laser power absorption can indeed have a positive influence, i.e., it favors the obtainment of a bimodal molar mass distribution fulfilling the IUPAC consistency criteria. This observation may contradict the idea that PLP‐SEC is not suitable to determine accurate k_p values for acrylates above 20–30 °C.

Instantaneous MMDs formed after N pulses. Simulation taking initiator consumption and laser absorption into account.     

Kinetics of inhibited or retarded polymerization. Investigations on the cerium(IV)/toluene/acrylonitrile system

The kinetics and mechanism of the inhibiting action of toluene on the Ce(IV) initiated polymerization of acrylonitrile were studied together with the effects of [M], [Ce(IV)], [Toluene], [HClO₄], [NaClO₄], and [Acetic acid] on the rate of polymerization. The values of composite rate constant kk_t/k_pk_ok_i for toluene (and substituted toluenes) was calculated by plotting [M]/R_p versus [M]^?1. A very significant observation in the present study was that the plot of [M]/R_p versus [M]^?1 gave a negative intercept, which seems to be a general observation for all inhibiting substrates. On the basis of the experimental data, it could be concluded that the benzyl radical obtained by hydrogen abstraction, and not a chain transfer, is responsible for the inhibition.     

Radical polymerization of a trimer of methyl acrylate as polymerizable α-substituted acrylate

The radical polymerization of a trimer of methyl acrylate was investigated in relation to the steric hindrance-assisted polymerization of an α-(substituted methyl)acrylic ester. The trimer can also be regarded as a model of the unsaturated end group formed by the addition-fragmentation chain transfer of methyl α-(bromomethyl)acrylate during methyl acrylate polymerization. The trimer polymerizes slowly to a low-molecular-weight polymer at 30–60°C, and electron spin resonance (ESR) quantification of the propagating radical of the trimer allowed the determination of the absolute rate constants of propagation (k_p) and termination (k_t). The k_p and k_t values for the trimer indicate slow propagation and slow termination of polymerizable acrylates bearing a bulky α-substituent. In conformity with a higher reactivity of the trimer of methyl acrylate than the corresponding trimer of methyl methacrylate, poly(methyl acrylate) bearing an unsaturated end group, which is produced by the polymerization of methyl acrylate in the presence of methyl α-(bromomethyl)acrylate, was confirmed to copolymerize with methyl acrylate to yield a branched homopolymer.     

Radical Polymerization and Copolymerization of 12‐[(N‐Methacryloyl)carbamoyloxy]octadecanoic Acid

Radical polymerization of 12‐[(N‐methacryloyl)carbamoyloxy] octadecanoic acid ( 1 ) was kinetically and ESR spectroscopically investigated in acetone, using dimethyl 2,2′‐azobisisobutyrate ( 2 ) as initiator. The polymerization rate (R_p) is given by R_p = k [2]^0.7[1]^1.4 at 50°C. Propagating poly( 1 ) radical was observed as a 13‐lines spectrum by ESR under the actual polymerization conditions. The ESR‐determined k_p values (1.8–7.9 L/mol·s) are much lower than those of usual methacrylate monomers. The rate constant (k_t) of termination was determined to be k_t = 1.0–2.7·10⁴ L/mol·s from decay curve of the propagating radical. The Arrhenius plots of k_p and k_t gave the activation energies of propagation (63 kJ/mol) and termination (24 kJ/mol). A significant solvent effect was observed on the radical polymerization of 1 . The copolymerizations of 1 with styrene(St) and acrylonitrile were examined at 50°C. Copolymerization parameters obtained for the 1  (M₁)/St (M₂) system are as follows; r₁ = 0.73, r₂ = 0.57, Q₁ = 0.83, and e₁ = 0.13.     

Termination Kinetics of tert‐Butyl Methacrylate and of n‐Butyl Methacrylate Free‐Radical Bulk Homopolymerizations

Summary: Termination kinetics in tert‐butyl methacrylate (tert‐BMA) and n‐butyl methacrylate (n‐BMA) bulk homopolymerizations has been studied via the single pulse‐pulsed laser polymerization‐near infrared (SP‐PLP‐NIR) method between 40 and 80 °C at pressures from 500 to 2 250 bar. Toward increasing monomer conversion, the chain‐length averaged termination rate coefficient, 〈k_t〉, for both monomers exhibits the methacrylate‐specific sequence of an initial plateau region, assigned to control by segmental diffusion, followed by a steep decrease of 〈k_t〉 at intermediate conversion, which is assigned to translational diffusion control, and a weaker decrease of 〈k_t〉, associated with reaction‐diffusion control, at still higher degrees of monomer conversion. Despite this similarity, the two isomeric monomers clearly differ in absolute size of 〈k_t〉 and in the monomer concentration ranges where the transitions between the different types of diffusion control occur. The differences are assigned to effects of chain mobility which is hindered to a larger extent in tert‐BMA than in n‐BMA. As a consequence, the 〈k_t〉 behavior of tert‐BMA at 80 °C is close to the one of n‐BMA at 40 °C. Investigations into the chain‐length dependence of k_t, in particular into k_t(i,i), the rate coefficient for termination of two radicals of identical size, support the evidence on the different types of diffusion control that operate as a function of monomer conversion. In the initial conversion range, the power‐law exponent which characterizes the chain‐length dependence of larger (entangled) radicals, is found for both monomers to be close to the theoretical value of α = 0.16.

Dependence of log(〈k_t〉/k_p) on monomer conversion, X, for n‐BMA and tert‐BMA bulk homopolymerizations at 2 000 bar and 70 °C. Circles and triangles represent independent data sets obtained from separate experiments.     

High‐Temperature Propagation and Termination Kinetics of Styrene to High Conversion Investigated by Electron Paramagnetic Resonance Spectroscopy

Summary: The free radical bulk polymerization of styrene at 120 °C has been investigated over almost the entire conversion range using electron paramagnetic resonance spectroscopy, Fourier‐transform near‐infrared spectroscopy and gel permeation chromatography. The free radical concentration went through a sharp maximum that coincided with the peak in the rate of polymerization during the gel effect, and shifted to higher conversion with increasing initiator concentration. The termination rate coefficient (k_t), decoupled from the initiator efficiency (f) by use of the instantaneous degree of polymerization, remained close to constant up to as high as approximately 80% conversion, at which a dramatic decrease occurred. Both the propagation rate coefficient (k_p) and f fell orders of magnitude near 80% conversion in spite of the temperature being above the glass transition temperature of the system. The value of k_p increased with the initiator concentration at a given conversion in the highest (diffusion‐controlled) conversion range.

Termination rate coefficient (k_t) versus conversion for bulk free radical polymerization of St initiated by TBP at 120 °C. [TBP] = 0.15 (○), 0.10 (?) and 0.05 M ().     

Termination Kinetics of Dibutyl Itaconate Free‐Radical Polymerization Studied via the SP–PLP–ESR Technique

Summary: The termination kinetics of dibutyl itaconate (DBI) bulk polymerization was studied via SP–PLP–ESR single pulse–pulsed laser polymerization with time‐resolved detection of free‐radical concentration by electron‐spin resonance, at temperatures between 0 and 60 °C. As is characteristic of PLP experiments, termination rate coefficients, k_t(i,i), are measured for radicals of (almost) identical chain length (CL) i. CL‐averaged 〈k_t〉, for chain lengths up to 200 monomer units, and also kequation/tex2gif-stack-1.gif referring to termination of very small‐size radicals are directly deduced from measured DBI radical concentration vs time traces. At 45 °C, 〈k_t〉 is (3.4 ± 0.6) · 10⁵ L · mol^?1 · s^?1 and kequation/tex2gif-stack-2.gif is (7.2 ± 1.0) · 10⁵ L · mol^?1 · s^?1. Both rate coefficients are independent of monomer conversion up to the highest experimental conversion of 18%. The associated activation energies are E_A(〈k_t〉) = 23.0 ± 3.2 kJ · mol^?1 and E_A(kequation/tex2gif-stack-3.gif) = 27.6 ± 2.8 kJ · mol^?1, respectively. “Model‐dependent” and “model‐free” analyses of radical concentration vs time profiles indicate a pronounced CL dependence of k_t(i,i) for DBI radicals of moderate size, 5 < i < 50. The lowering of k_t(i,i) with CL corresponds to an exponent α close to 0.5 in a power‐law expression k_t(i,i) = kequation/tex2gif-stack-4.gif · i^?a. At higher chain lengths, the variation of k_t(i,i) with CL becomes weaker and may be represented by an α value of 0.16 or even below. These results are consistent with models according to which α varies to a larger extent at low CL and to a smaller extent at high CL with the crossover region between the two ranges being located somewhere around i = 100.

Conversion‐dependence of 〈k_t〉 and kequation/tex2gif-stack-5.gif from laser‐induced photopolymerization of DBI.     

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.     

Polymerization‐Induced Thermal Self‐Assembly of Functional and Thermo‐Responsive Diblock Copolymer Nano‐Objects via RAFT Aqueous Polymerization

The polymerization‐induced self‐assembly (PISA) of amphiphilic diblock copolymer nano‐objects, which are synthesized via reversible addition‐fragmentation chain‐transfer (RAFT) aqueous polymerization, is discussed. First, the effectiveness of the (S)‐2‐(ethyl propionate)‐(O‐ethyl xanthate) as a RAFT chain transfer agent for the polymerization of N,N‐dimethylacrylamide (DMAm) yielding a water‐soluble macro RAFT agent is investigated. In a second step, poly(DMAm) macro‐CTA is chain‐extended with acrylate monomers in water inducing a PISA process. Besides the use of pentafluorophenyl acrylate (PFPA) as core‐forming block, the thermo‐responsive nature of N‐isopropylacrylamide (NIPAm) and the reactive character of pentafluorophenyl acrylate (PFPA) for a subsequent post‐modification is exploited. Monomer conversion and reaction kinetics are determined via nuclear magnetic resonance spectroscopy, while gel permeation chromatography is used to evaluate the molecular weight distribution of the polymers. The obtained spherical nanostructures are analyzed via dynamic light scattering, scanning electron microscopy, transmission electron microscopy and atomic force microscopy.