Polymerization of neat 2‐ethylhexyl acrylate induced by a pulsed electron beam

The bulk polymerization of 2‐ethylhexyl acrylate (2‐EHA), induced by a pulsed electron beam, was investigated with pulse radiolysis, gravimetry, and Fourier transform infrared spectroscopy. The roles of the dose rate, pulse frequency, and added acrylic acid (AA) in the polymerization of 2‐EHA were examined at ambient temperature. In the range of 12.6–71.2 Gy/pulse, the polymerization of 2‐EHA was dose‐rate‐dependent: at the same total dose, a lower dose rate yielded a higher conversion. Also, a lower pulse rate gave a higher conversion at the same total dose. The addition of up to 10 wt % AA showed no increase in the conversion of 2‐EHA at a low conversion (8 kGy), but at a higher conversion (16 kGy), a 20 wt % increase in the conversion of 2‐EHA was observed. The estimated values (1.6 ± 0.3) × 10^?3 (dm³ s)^3/2 mol^?1 s^?1/2 for k_p(G/2k_t)^1/2 and 2.6 ± 0.8 dm³ s J^?1 for 2k_tG (where k_p is the rate constant of propagation, k_t is the rate constant of bimolecular termination, and G is the yield of free radicals) were obtained at relatively low conversions. The reaction rate constant of the addition of 2‐EHA^· free radicals to the monomer was measured by pulse radiolysis and found to be 2.8 × 10² mol^?1 dm³ s^?1. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 196–203, 2003     

Free‐radical copolymerization of styrene and m‐isopropenyl‐α,α′‐dimethylbenzyl isocyanate studied by 1H NMR kinetic experiments

The free‐radical copolymerization of m‐isopropenyl‐α,α′‐dimethylbenzyl isocyanate (TMI) and styrene was studied with ¹H NMR kinetic experiments at 70 °C. Monomer conversion vs time data were used to determine the ratio k_p × k_t^?0.5 for various comonomer mixture compositions (where k_p is the propagation rate coefficient and k_t is the termination rate coefficient). The ratio k_p × k_t^?0.5 varied from 25.9 × 10^?3 L^0.5 mol^?0.5 s^?0.5 for pure styrene to 2.03 × 10^?3 L^0.5 mol^?0.5 s^?0.5 for 73 mol % TMI, indicating a significant decrease in the rate of polymerization with increasing TMI content in the reaction mixture. Traces of the individual monomer conversion versus time were used to map out the comonomer mixture composition drift up to overall monomer conversions of 35%. Within this conversion range, a slight but significant depletion of styrene in the monomer feed was observed. This depletion became more pronounced at higher levels of TMI in the initial comonomer mixture. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1064–1074, 2002     

Dark‐cure studies of cationic photopolymerizations of epoxides: Characterization of the active center lifetime and kinetic rate constants

We have characterized the effective rate constants for termination/trapping (k_t/t) and propagation (k_p) for solvent‐free cationic photopolymerizations of phenyl glycidyl ether for conversions up to 50%. We have performed dark‐cure experiments in which active centers are produced photochemically for a specified period of time until the initiating light is shuttered off, and then the polymerization rate is monitored in the dark. This method is especially well suited for characterizing cationic polymerizations because of the long active center lifetimes. Our analysis provides profiles of the instantaneous kinetic rate constants as functions of conversion (or time). For photopolymerizations of phenyl glycidyl ether initiated with iodonium photoinitiators, k_t/t and k_p remain essentially invariant for conversions up to 50%. For the photoinitiator (tolycumyl) iodonium tetrakis (pentafluorophenyl) borate (IPB), the values of k_t/t at 50 and 60 °C are 0.027 and 0.033 min^?1, respectively. The corresponding values of k_t/t for diaryliodonium hexafluoroantimonate (IHA) are 0.041 and 0.068 min^?1. The values of k_p at 50 °C for IPB and IHA are 0.6 and 0.4 L mol^?1 s^?1, respectively. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2064–2072, 2003     

Ring‐Closing Metathesis Reactions: Interpretation of Conversion–Time Data

Conversion–time data were recorded for various ring‐closing metathesis (RCM) reactions that lead to five‐ or six‐membered cyclic olefins by using different precatalysts of the Hoveyda type. Slowly activated precatalysts were found to produce more RCM product than rapidly activated complexes, but this comes at the price of slower product formation. A kinetic model for the analysis of the conversion–time data was derived, which is based on the conversion of the precatalyst (Pcat) into the active species (Acat), with the rate constant k_act, followed by two parallel reactions: 1) the catalytic reaction, which utilizes Acat to convert reactants into products, with the rate k_cat, and 2) the conversion of Acat into the inactive species (Dcat), with the rate k_dec. The calculations employ two experimental parameters: the concentration of the substrate (c(S)) at a given time and the rate of substrate conversion (?dc(S)/dt). This provides a direct measure of the concentration of Acat and enables the calculation of the pseudo‐first‐order rate constants k_act, k_cat, and k_dec and of k_S (for the RCM conversion of the respective substrate by Acat). Most of the RCM reactions studied with different precatalysts are characterized by fast k_cat rates and by the k_dec value being greater than the k_act value, which leads to quasistationarity for Acat. The active species formed during the activation step was shown to be the same, regardless of the nature of different Pcats. The decomposition of Acat occurs along two parallel pathways, a unimolecular (or pseudo‐first‐order) reaction and a bimolecular reaction involving two ruthenium complexes. Electron‐deficient precatalysts display higher rates of catalyst deactivation than their electron‐rich relatives. Slowly initiating Pcats act as a reservoir, by generating small stationary concentrations of Acat. Based on this, it can be understood why the use of different precatalysts results in different substrate conversions in olefin metathesis reactions.     

Initiator efficiency of 2,2′‐azobis(isobutyronitrile) in bulk dodecyl acrylate free‐radical polymerizations over a wide conversion and molecular weight range

An Erratum has been published for this article in J. Polym. Sci. Part A: Polym. Chem. (2004) 42(21) 5559 . The initiator efficiency, f, of 2,2′‐azobis(isobutyronitrile) (AIBN) in dodecyl acrylate (DA) bulk free‐radical polymerizations has been determined over a wide range of monomer conversion in high‐molecular‐weight regimes (M_n ? 10⁶ g mol^?1 [? 4160 units of DA)] with time‐dependent conversion data obtained via online Fourier transform near infrared spectroscopy (FTNIR) at 60 °C. In addition, the required initiator decomposition rate coefficient, k_d, was determined via online UV spectrometry and was found to be 8.4 · 10^?6 s^?1 (±0.5 · 10^?6 s^?1) in dodecane, n‐butyl acetate, and n‐dodecyl acetate at 60 °C. The initiator efficiency at low monomer conversions is relatively low (f = 0.13) and decreases with increasing monomer to polymer conversions. The evolution of f with monomer conversion (in high‐molecular‐weight regimes), x, at 60 °C can be summarized by the following functionality: f^{60 °C} (x) = 0.13–0.22 · x + 0.25 · x² (for x ≤ 0.45). The reported efficiency data are believed to have an error of >50%. The ratio of the initiator efficiency and the average termination rate coefficient, 〈k_t±, (f/〈k_t〉) has been determined at various molecular weights for the generated polydodecyl acrylate (M_n = 1900 g mol^?1 (? 8 units of DA) up to M_n = 36,500 g mol^?1 (? 152 units of DA). The (f/〈k_t〉) data may be indicative of a chain length‐dependent termination rate coefficient decreasing with (average) chain length. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5170–5179, 2004     

Homogeneous free‐radical polymerization of styrene in supercritical CO2

The free‐radical polymerization of styrene has been studied in the homogeneous phase of supercritical (sc) CO₂ at 80°C and pressures between 200 and 1 500 bar. 2,2'‐Azobisisobutyronitrile is used as initiator and CBr₄ as chain‐transfer agent. The polymerization is monitored by means of online FT‐IR/NIR spectroscopy. In the presence of CO₂ a solution polymerization may be carried out up to a considerable degree of monomer conversion. At 500 bar, for example, maximum styrene conversions of 34.4 and 11.9% may be reached in homogeneous phase at CO₂ contents of 16.8 and 44.5 wt.‐%, respectively. Analysis of the measured conversion‐time profiles yields termination rate coefficients, k_t, which are by one order of magnitude larger than k_t for styrene bulk polymerizations at identical temperature and pressure. The enhanced termination rate in fluid CO₂ is assigned to the poor solvent quality of scCO₂ for polystyrene.     

Synthesis of carboxy‐telechelic oligostyrene by dead‐end polymerization: Evaluation of primary radical termination by kinetic study and kinetic simulation model

Styrene was polymerized with azobiscyanovaleric acid (ACVA) to obtain carboxy‐telechelic polystyrene. Our aim was to synthesize an oligostyrene with a molecular weight of about 2000 g/mol. Thus, styrene was polymerized at 90 °C, where t_1/2ACVA is very short, and at very high initiator concentrations. In these conditions a new termination reaction appeared, known as the primary radical termination (PRT). At first, PRT was observed by a deviation from the simple kinetic law. Then, the constant of PRT was determined by several kinetic models to be more important than the constant of bimolecular termination. Finally, the PREDICI simulation program was used to confirm the predominance of PRT. This simulation program allowed the determination of the constant of primary radical deactivation k_rr and the influence of this reaction on the conversion rate. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2740–2750, 2001     

Modeling termination kinetics in free‐radical polymerization using a reduced number of parameters

An approach for modeling chain‐length dependent termination rate coefficients is presented. The method is based on the assumption that free‐radical chain length may be considered as a continuous variable. As compared to discrete numerical methods, in continuous modeling the number of independent dimensionless parameters can be significantly reduced. As a consequence, for a wide variety of monomers the conversion dependence of k_t can be predicted without extensive numerical calculations. The method may also be used to determine polymerization conditions under which simpler models of k_t (which neglect effects arising from the dependence of k_t on chain length) may be applied. Calculations for methyl methacrylate, styrene, and butyl acrylate bulk polymerizations up to high degrees of monomer conversion show that the impact of chain length on termination varies with conversion and strongly depends on the type of monomer.     

Photoinitiated polymerization of methacrylic monomers in a polybutadiene matrix (PB): Kinetic,mechanistic, and structural aspects

The kinetics and mechanism of the photoinitiated polymerization of tetrafunctional and difunctional methacrylic monomers [1,6‐hexanediol dimethacylate (HDDMA) and 2‐ethylhexyl methacrylate (EHMA)] in a polybutadiene matrix (PB) have been studied. The maximum double‐bond conversion, the maximum polymerization rate, the intrinsic reactivity, and the kinetic constants for propagation and termination have been calculated. Unlike the behavior followed by the SBS‐HDDMA and PS‐HDDMA systems, where a reaction‐diffusion mechanism occurs from the start of the polymerization at low monomer concentrations (<30–40%), in the PB‐HDDMA system the reaction diffusion controls the termination process only after approximately 10% conversion is reached, as for the bulk polymerization of polyfunctional methacrylic monomers. Before reaching 10% conversion the behavior observed can be better explained by a combination of segmental diffusion‐controlled (autoaccelerated) and reaction‐diffusion mechanisms. This is probably a consequence of the lower force of attraction between the monomer and the matrix and between the growing macroradical and the matrix than those corresponding to the other systems mentioned. For the PB‐EHMA system, the termination mechanism is principally diffusion‐controlled from the beginning of the polymerization for monomer concentrations below 30–40%, and for higher monomer concentrations, a standard termination mechanism takes place (k_t ≈ 10⁶) at low double‐bond conversions, which is diffusion‐controlled for high conversions (>40%). For PB‐HDDMA and PB‐EHMA systems, crosslinked polymerized products are obtained as a result of the participation of the double bonds of the matrix in the polymerization process. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2444–2453, 2001     

Modeling the Effects of Primary Radicals in Free‐Radical Polymerization

The effects of non‐ideal initiator decomposition, i.e., decomposition into two primary radicals of different reactivity toward the monomer, and of primary radical termination, on the kinetics of steady‐state free‐radical polymerization are considered. Analytical expressions for the exponent n in the power‐law dependence of polymerization rate on initiation rate are derived for these two situations. Theory predicts that n should be below the classical value of 1/2. In the case of non‐ideal initiator decomposition, n decreases with the size of the dimensionless parameter α ≡ (k_tz /k_dz) √r_ink_t, where k_tz is the termination rate coefficient for the reaction of a non‐propagating primary radical with a macroradical, k_dz is the first‐order decomposition rate coefficient of non‐propagating (passive) radicals, r_in is initiation rate, and k_t is the termination rate coefficient of two active radicals. In the case of primary radical termination, n decreases with the size of the dimensionless parameter β ≡ k_t,s r_in^1/2/k_p,s M r_t,l^1/2, where k_t,s is the termination rate coefficients for the reaction of a primary (“short”) radical with a macroradical, k_t,l is the termination rate coefficients of two large radicals, k_p,s is the propagation rate coefficient of primary radicals and M is monomer concentration. As k_t is deduced from coupled parameters such as k_t /k_p, the dependence of k_p on chain length is also briefly discussed. This dependence is particularly pronounced at small chain lengths. Moreover, effects of chain transfer to monomer on n are discussed.     

Free‐Radical Termination Kinetics Studied Using a Novel SP‐PLP‐ESR Technique

Summary: A novel method for measuring termination rate coefficients, k_t, in free‐radical polymerization is presented. A single laser pulse is used to instantaneously produce photoinitiator‐derived radicals. During subsequent polymerization, radical concentration is monitored by time‐resolved electron spin resonance (ESR) spectroscopy. The size of the free radicals, which exhibits a narrow distribution increases linearly with time t, which allows the chain‐length dependence of k_t to be deduced. The method will be illustrated using dodecyl methacrylate polymerization as an example.

Two straight lines provide a very satisfactory representation of the chain‐length dependence of k_t over the entire chain‐length region (c_R = radical concentration).     

Relationship between chain termination rate constant and conversion in radical polymerization

At low and high conversions, the chain termination rate constant for bimolecular termination between polymeric radicals given by k_t = A_tD_s, where A_t is a constant and D_s is the diffusion constant of radical chain end, is completely correct. This termination rate constant does not depend on solution viscosity, but conversion.     

1,3‐Dipolar Cycloaddition of Diazo Compounds to Electron‐Deficient Alkenes: Kinetics and Mechanism of Formation of Dimethyl‐4,5‐dihydro‐1H‐pyrazol‐3,5‐dicarboxylate

1,3‐Dipolar cycloaddition of methyl diazoacetate to methyl acrylate was investigated by kinetic ¹Н NMR spectroscopy. It was established that the mechanism of the process includes parallel formation of trans‐ and cis‐dimethyl‐4,5‐dihydro‐3H‐pyrazol‐3,5‐dicarboxylates as a result of [3 + 2]‐cycloaddition of methyl diazoacetate to methyl acrylate; the corresponding rate constants were denoted k_1t and k_1c. The reaction rate of the isomerization of 3Н‐pyrazolines to 4,5‐dihydro‐1H‐pyrazol‐3,5‐dicarboxylate (3Н → 1Н‐pyrazoline rearrangement) was found to be sensitive to both the methyl acrylate (k_2t, k_2c) and 1Н‐pyrazoline concentrations (k_3t, k_3c). Kinetic analysis showed that the proposed scheme is valid for various reagent concentrations. The numerical solution of the system of differential equations corresponded to the reaction scheme and was used to determine the complete set of reaction rate constants (k (× 10⁵ M^–1·s^–1), 298 K; solvent, benzene‐d₆): k_1t = 2.3 ± 0.3, k_1c = 1.6 ± 0.2, k_2t = 1.1 ± 0.3, k_2c = 1.8 ± 0.5, k_3t = 1.2 ± 0.4, k_3c = 2.2 ± 0.7.     

Kinetics of Bulk Free‐Radical Polymerization of Butyl Methacrylate Isomers Studied by Reaction Calorimetry

The kinetics of bulk free‐radical polymerizations of n‐butyl methacrylate (n‐BMA), iso‐butyl methacrylate (i‐BMA), and tert‐butyl methacrylate (t‐BMA) are studied by differential scanning calorimetry and with the aid of a mathematical model previously reported by the authors. In all the cases, the rate of polymerization (R_p) evolution curve exhibits a minimum at low conversions and the characteristic maximum of the autoacceleration effect. It is found that the monomer conversion x_min at which the minimum is observed, follows the order n‐BMA > i‐BMA > t‐BMA and that for monomer conversions (x) smaller than x_min, the termination rate coefficient (k_t) shows a plateau. According to the model results it is obtained that for x > x_min, the termination reaction is chemically controlled whereas for x > x_min, it is diffusion‐controlled and that the x_min values are related to the value of the termination rate coefficient of the chemical step (k_t0) of every isomer, which is highly influenced by the steric hindrance of the alkyl substituent group.     

Influence of the stereochemical configuration on the radical polymerization of methacrylic monomers: cis‐ and trans‐(2‐cyclohexyl‐1,3‐dioxan‐5‐yl) methacrylate

The synthesis of two new isomeric monomers, cis‐(2‐cyclohexyl‐1,3‐dioxan‐5‐yl) methacrylate (CCDM) and trans‐(2‐cyclohexyl‐1,3‐dioxan‐5‐yl) methacrylate (TCDM), starting from the reaction of glycerol and cyclohexanecarbaldehyde, is reported. The process involved the preparation of different alcohol acetals and esterification with methacryloyl chloride of the corresponding cis and trans 5‐hydroxy compounds of 2‐cyclohexyl‐1,3‐dioxane. The radical polymerization reactions of both monomers, under the same conditions of temperature, solvent, monomer, and initiator concentrations, were studied to investigate the influence of the monomer configuration on the values of the propagation and termination rate constants (k_p and k_t ).The values of the ratio k_p /k_t ^1/2 were determined by UV spectroscopy by the measurement of the changes of absorbance with time at several wavelengths in the range 275–285 nm, where an appropriate change in absorbance was observed. Reliable values of the kinetics constants were determined by UV spectroscopy, showing a very good reproducibility of the kinetic experiments. The values of k_p /k_t ^1/2, in the temperature interval 45–65 °C, lay in the range 0.40–0.50 L^1/2/mol^1/2s^1/2 and 0.20–0.30 L^1/2/mol^1/2s^1/2 for CCDM and TCDM, respectively. Measurements of both the radical concentrations and the absolute rate constants k_p and k_t were also carried out with electron paramagnetic resonance techniques. The values of k_p at 60 °C were nearly identical for both the trans and cis monomers, but the termination rate constant of the trans monomer was about three times that of the cis monomer at the same temperature. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3883–3891, 2000     

Kinetic Study of Radical Polymerization VIII. A Comprehensive Study of Solution Copolymerization of Vinyl Acetate and Methyl Acrylate by 1H‐NMR Spectroscopy

Radical copolymerization reaction of vinyl acetate (VA) and methyl acrylate (MA) was performed in a solution of benzene‐d₆ using benzoyl peroxide (BPO) as the initiator at 60°C. Kinetic studies of this copolymerization reaction were investigated by on‐line ¹H‐NMR spectroscopy. Individual monomer conversions vs. reaction time, which was followed by this technique, were used to calculate the overall monomer conversion, as well as the monomer mixture and the copolymer compositions as a function of time. Monomer reactivity ratios were calculated by various linear and nonlinear terminal models and also by simplified penultimate model with r _2(VA)=0 at low and medium/high conversions. Overall rate coefficient of copolymerization was calculated from the overall monomer conversion vs. time data and k _p  . k _t ^?0.5 was then estimated. It was observed that k _p  . k _t ^?0.5 increases with increasing the mole fraction of MA in the initial feed, indicating the increase in the polymerization rate with increasing MA concentration in the initial monomer mixture. The effect of mole fraction of MA in the initial monomer mixture on the drifts in the monomer mixture and copolymer compositions with reaction progress was also evaluated experimentally and theoretically.     

Kinetic analysis of styrene atom transfer radical polymerization: Extraction of radical‐radical termination rate coefficients

Kinetic studies of the atom transfer radical polymerization (ATRP) of styrene are reported, with the particular aim of determining radical‐radical termination rate coefficients (<k_t>). The reactions are analyzed using the persistent radical effect (PRE) model. Using this model, average radical‐radical termination rate coefficients are evaluated. Under appropriate ATRP catalyst concentrations, <k_t> values of approximately 2 × 10⁸ L mol^?1 s^?1 at 110 °C in 50 vol % anisole were determined. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5548–5558, 2004     

Free‐radical copolymerization of styrene and itaconic acid studied by 1H NMR kinetic experiments

The free‐radical copolymerization of itaconic acid (IA) and styrene in solutions of dimethylformamide and d₆‐dimethyl sulfoxide (50 wt %) has been studied by ¹H NMR kinetic experiments. Monomer conversion versus time data were used to estimate the ratio k_p · k_t^−0.5 for various comonomer mixture compositions. The ratio k_p · k_t^−0.5 varies from 5.2 · 10⁻² for pure styrene to 2.0 · 10⁻² mol^0.5 L^−0.5 s^−0.5 for pure IA, indicating a significant decrease in the rate of polymerization. Individual monomer conversion versus time traces were used to map out the comonomer mixture–composition drift up to overall monomer conversions of 60%. Within this conversion range, a slight but significant depletion of styrene in the monomer feed can be observed. This depletion becomes more pronounced at higher levels of IA in the initial comonomer mixture. The kinetic information is supplemented by molecular weight data for IA/styrene copolymers obtained by variation of the comonomer mixture composition. A significant decrease in molecular weight of a factor of 2 can be observed when increasing the mole fraction of IA in the initial reaction mixture from 0 to 0.5. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 656–664, 2001     

Radical‐initiated polymerization of β‐methyl‐α‐methylene‐γ‐butyrolactone

β‐Methyl‐α‐methylene‐γ‐butyrolactone (MMBL) was synthesized and then was polymerized in an N,N‐dimethylformamide (DMF) solution with 2,2‐azobisisobutyronitrile (AIBN) initiation. The homopolymer of MMBL was soluble in DMF and acetonitrile. MMBL was homopolymerized without competing depolymerization from 50 to 70 °C. The rate of polymerization (R_p) for MMBL followed the kinetic expression R_p = [AIBN]^0.54[MMBL]^1.04. The overall activation energy was calculated to be 86.9 kJ/mol, k_p/k_t^1/2 was equal to 0.050 (where k_p is the rate constant for propagation and k_t is the rate constant for termination), and the rate of initiation was 2.17 × 10^?8 mol L^?1 s^?1. The free energy of activation, the activation enthalpy, and the activation entropy were 106.0, 84.1, and 0.0658 kJ mol^?1, respectively, for homopolymerization. The initiation efficiency was approximately 1. Styrene and MMBL were copolymerized in DMF solutions at 60 °C with AIBN as the initiator. The reactivity ratios (r₁ = 0.22 and r₂ = 0.73) for this copolymerization were calculated with the Kelen–Tudos method. The general reactivity parameter Q and the polarity parameter e for MMBL were calculated to be 1.54 and 0.55, respectively. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1759–1777, 2003     

Easy Access to Chain‐Length‐Dependent Termination Rate Coefficients Using RAFT Polymerization

Chain‐length‐dependent termination rate coefficients of the bulk free‐radical polymerization of styrene at 80 °C are determined by combining online polymerization rate measurements (DSC) with living RAFT polymerizations. Full k_t versus chain‐length plots were obtained indicating a high k_t value for short chains (2 × 10⁹ L · mol⁻¹ · s⁻¹) and a weak chain‐length dependence between 10 and 100 monomer units, quantified by an exponent of −0.14 in the corresponding power law 〈k_t^i,i〉 = k_t⁰ · P^−b.

Double logarithmic plots of 〈k_t^i,i〉 versus P, evaluated from experimental time‐resolved R_p data according to the procedure described in the text, for different CPDA and AIBN concentrations. The best linear fit for (10 < P < 100) is indicated as full line.