Mechanistic and kinetic studies on the polymerization of styrene initiated by dimethyl sulfonium 2-pyridyl carbonyl methylide

Dimethyl sulfonium 2-pyridyl carbonyl methylide (Y_py-s) initiated radical polymerization of styrene in dimethyl sulfoxide at 85±0.1°C for 6 h under a nitrogen blanket using dilatometric techniques has been studied. The initiator and monomer exponent values were calculated to be 0.5 and 1.2, respectively. The system follows ideal radical kinetics with bimolecular termination. The higher monomer exponent value is ascribed to significant solvent effects on the initiation rate. The overall activation energy and average value of k²_p/k_t are 52.0 kJ mol^?1 and 1.0 × 10^?3 litre mol^?1 s^?1, respectively. The polymerization was retarded in the presence of hydroquinone or benzene; dimethylformamide, however, enhanced the rate of polymerization. Kinetic data and ESR studies indicate that the overall polymerization takes place via triplet carbene formation which acts as a source of free radicals.     

Synthesis and radical polymerization of methacrylic monomers with crown ethers in the ester residue: 1,4,7,10-tetraoxacyclododecan-2-ylmethyl methacrylate

The synthesis and radical polymerization of 1,4,7,10-tetraoxacyclododecan-2-ylmethylmethacrylate (CR4MA) is described. The polymerization reactions of CR4MA were carried out at different temperatures and the kinetic curves of monomer depletion against time were obtained by direct measurements of the instantaneous monomer concentrations by using nuclear magnetic resonance (NMR) spectroscopy. At the same time electron paramagnetic resonance (EPR) spectroscopy was used to determine the actual polymer radical concentration during all the reaction time. The conjunction of both techniques (NMR and EPR) allowed the determination of the polymerization rate parameter (2fk_p/〈k_t〉^1/2) and separately of k_p and 〈k_t〉/f, where f, k_p and 〈k_t〉 are, respectively, the initiator efficiency factor and the overall averages of propagation (k_p is considered to be practically independent of the chain length) and termination rate constants. The values found for this ratio and for k_p were comparatively higher than those recently reported in the literature for its lateral open chain counterpart, the methacrylic monomer with equal number of oxyethylene units in the residue ester (TTEMA). However, the 〈k_t〉 values were similar for the polymerization of both monomers CR4MA and TTEMA. The polymer, PCR4MA, is soluble in water as its open chain homologous, and exhibits a glass transition temperature in the vicinity of the ambient temperature (about 35 °C), much higher than the value found for the homologous polymethacrylate derived from the TTEMA.     

Mechanism of the emulsion polymerization of methyl acrylate: 1. Initiation

The emulsion polymerization of methyl acrylate (MA) has been carried out under two different experimental conditions so as to establish the mode of chain initiation. In the first system, the initiator was Fenton's reagent (FeSO₄ and H₂O₂ redox pair) in acid solution at 20°C in the presence of a cationic detergent, cetyltrimethyl — ammonium bromide (CTAB), while in the second system the initiator was potassium persulphate (K₂S₂O₈) at 50°C in the presence of an anionic detergent, sodium lauryl sulphate (NaLS). The polymerization was followed by the conventional dilatometric and gravimetric methods. Initial rates were determined by keeping the conversion below 10% and by plotting the (yield/time) versus time, and extrapolating the straight line to zero time. It was found that in either case, Initial rate (v) ∝ (Initiator)^0.50 while the viscosity average molecular weight ($\text{M}{}_{\mathrm{v}}$) of the polymers up to 20% conversion was found to be approximately inversely proportional to the square root of the initiator concentration (I), i.e. $\text{(}\text{M}{}_{\mathrm{v}}\text{) ∝ (I)}{}^{\mathrm{?0.50}}$. The rate of polymerization up to 10% conversion was found to be approximately linear with the monomer concentration (from 0.5 to 5.0—, v/v) in the presence of detergents (above CMC) [critical monomer concentration] under the experimental conditions. Since the kinetics of the emulsion polymerization of MA at zero conversion are the same as that of the homogeneous polymerization of MA initiated by the free radicals, it is suggested that the initiation of the emulsion polymerization of MA takes place in the aqueous phase with little or no initiation in the detergent micelles containing monomer. If the polymerization is carried out within the solubility range of MA in the aqueous phase, then the initiation will take place entirely in the aqueous phase. Assuming homogeneous polymerization at zero conversion, the value of the termination rate constant (k_t) has been extracted from the experimental data at different initiator efficiencies.     

Analysis of nonideal kinetics in the polymerization of methyl methacrylate using some complex initiator systems based on a pyridine–sulfur dioxide charge transfer complex

The kinetic nonideality in the polymerization of methyl methacrylate was studied with the use of pyridine-sulfur dioxide charge transfer complex as the initiator under different conditions. The following systems were studied: (1) aqueous polymerization of methyl methacrylate (MMA) with the use of a pyridine-sulfur dioxide charge transfer complex as initiator, (2) photopolymerization of MMA initiated by the pyridine-sulfur dioxide complex in the presence of carbon tetrachloride, (3) photopolymerization of MMA in bulk and in a pyridine-diluted system with pyridine-sulfur dioxide alone and in combination with benzoyl peroxide as a photoinitiator. Polymerization in all these cases proceeded by radical mechanisms. The kinetic parameter /k_t for the aqueous system was 3.65 L mol⁻¹ s⁻¹, and for nonaqueous systems were 1.27 × 10⁻² to 1.40 × 10⁻² L mol⁻¹ s⁻¹. The monomer exponent and initiator exponent for ideal free radical polymerization systems are 1.0 and 0.5, respectively. In the system studied, the ideal kinetics were followed at specific concentration ranges of both monomer and initiator. At different concentration ranges, the systems behave nonideally. The kinetic nonidealities in monomer exponents, i.e., lower or higher than unity, were explained on the basis of (1) the rate-enhancing effect of different solvents, and (2) a radical generation step by in situ initiator monomer complexation reaction. The kinetic nonidealities in initiator exponent were analyzed and interpreted in terms of (1) primary radical termination, and (2) degradative initiator transfer with little reinitiator. Analysis of kinetic data shows that the degradative initiator transfer effect is more prominent in the present systems. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 585–595, 1998     

Aspects of the kinetics of the radiation-induced polymerization of acrylonitrile in dimethylformamide

The radiation-induced homopolymerization of acrylonitrile (AN) in dimethylformamide (DMF) has been followed in detail over a range of monomer concentrations. In all cases a short induction period was observed which was equivalent to 1.6 × 10⁴ rad. The initial rates of polymerization for solutions of AN in DMF of mole fraction (×) of 0.33, 0.20, and 0.10 are 1.44 × 10^?5, 7.23 × 10^?6, and 2.79 × 10^?6 mole/dm³rad, respectively. Deviations of the polymerization pathway from the standard unity in monomer dependence are examined in terms of radical production ratios to the monomer and the solvent (Φ_M/Φ_S) and the polymer together with the solvent (Φ_p/Φ_s), for various mole fractions of AN in DMF. Thus, an indirect route to G_radical(events) is provided together with the corresponding k/k_t values.     

Equation of consumption rate of free-radical vinyl polymerization. II

The vinyl polymerization reaction is a two-molecule reaction. However, it is more convenient to use a specially defined rate constant than to use the general constant, because a new radical is formed instantaneously in the same radical compound when one monomer combines with an existing radical in a living polymer or an initiator radical. This special rate constant is named the propagation constant and is proportional to the concentration of monomer when the polymer is formed from a unit mole concentration of the initiator radical. The specific propagation constant is related to the concentration of monomers which react in unit time and unit concentration of monomer and radical. Arnett's experiments are discussed in terms of the equation formulated. The value of Δ[M]/([M]₀·t) is found not to be a reaction rate but a value of ln [M]₀/[M] when [M]₀ – [M] is very small. Autoacceleration of the polymerization is found with high concentrations of monomer which yield an increase in the velocity of propagation and also at low concentrations of initiator, which cause prolongation of the propagation stage. When the concentration of initiator is high, this phenomenon does not take place until enough initiator is consumed and the necessary low initiator level is reached. The time required is called the induction period. The larger the polymer molecule is, the higher the viscosity becomes.     

Photopolymerization of methyl methacrylate with the triethylamine-bromine charge transfer complex as photoinitiator

The triethylamine-bromine (TEA-Br₂) charge transfer complex was employed as photoinitiator in the photopolymerization of methyl methacrylate under light of 440 nm. The initial rate of conversion was 0.418%/min with an induction period of 56 min. The initiator and monomer exponents were 0.5 and 1.0, respectively. The polymerization was inhibited in the presence of hydroquinone but oxygen had a very little inhibitory effect. The value of k/k_t was 5.13 · 10^?2 l/mol · s and the activation energy was 19.18 kJ/mol. The rate constant for the decomposition of the charge transfer complex (k_ε) was 4.61 · 10^?6 1/s. Kinetic data and other evidence indicate that the overall polymerization takes place by a radical mechanism.     

Kinetics and mechanism of radical polymerization of methylmethacrylate using p-acetylbenzylidene triphenylarsonium ylide (p-ABTAY) as an initiator

Solution polymerization of methylmethacrylate (MMA) initiated by p-acetylbenzylidene triphenylarsonium ylide in dioxane was carried out at 60±0.2 °C up to 10 hrs. in a polymerization tube under a nitrogen atmosphere. The values of the initiator and the monomer exponent were computed as 0.46 and 1.03, respectively. The overall activation energy and kp²/k_t were calculated as 53 KJ/mole and 1.19 × 10⁻² L/mol·s, respectively for the polymerization.     

Kinetic study of the free‐radical polymerization of vinyl acetate in the presence of deuterated chloroform by 1H‐NMR spectroscopy

The free‐radical polymerization of vinyl acetate was performed in the presence of deuterated chloroform (CDCl₃) as a chain‐transfer agent (telogen) and 2,2′‐azobisisobutyronitrile as an initiator. The effects of the initiator and solvent concentrations (or equivalent monomer concentration) and the reaction temperature on the reaction kinetics were studied by real‐time ¹H‐NMR spectroscopy. Data obtained from analysis of the ¹H‐NMR spectra were used to calculate some kinetic parameters, such as the initiator decomposition rate constant (k_d), k_p(f/k_t)^1/2 ratio (where k_p is the average rate constant for propagation, f is the initiator efficiency, and k_t is the average rate constant for termination), and transfer constant to CDCl₃ (C). The results show that k_d and k_p(f/k_t)^1/2 changed significantly with the solvent concentration and reaction temperature, whereas they remained almost constant with the initiator concentration. C changed only with the reaction temperature. Attempts were made to explain the dependence of k_p(f/k_t)^1/2 on the solvent concentration. We concluded from the solvent‐independent C values that the solvent did not have any significant effect on the k_p values. As a result, changes in the k_p(f/k_t)^1/2 values with solvent concentration were attributed to the solvent effect on the f and/or k_t values. Individual values of f and k_t were estimated, and we observed that both the f and k_t values were dependent on the solvent (or equivalent monomer) concentration. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Contribution of primary radical termination to radical polymerization of diethyl fumarate estimated from absolute rate constants

Summary Diethyl fumarate was radically polymerized under UV irradiation and concentration of the propagating radical was determined to be of the order of 10^-5 mol/L by scavenge with a stable free radical. The absolute rate constant for propagation (k_p) was evaluated from the overall rate of polymerization at 30°C: K_p =(2.9 ± 0.3) × 10^-2 L/mol · s. The rate constant for mutual termination of the polymer radical (k_t) was calculated from the decreasing rate of the radical concentration in the dark: k_t=8.0 L/mol·s. The k_t value determined is one twentieth of that evaluated previously by a rotating sector method. This discrepancy is accounted for by contribution of much faster primary radical termination.     

Kinetic study of radical polymerization. VII. Investigation into the solution copolymerization of acrylonitrile and itaconic acid by real‐time 1H NMR spectroscopy

Free radical solution copolymerization of acrylonitrile (AN) and itaconic acid (IA) was performed with DMSO‐d₆ as the solvent and 2,2′‐azobisisobutyronitrile (AIBN) as the initiator. Weight ratio of the monomers to solvent and molar ratio of initiator to monomers were constant in all experiments. The initial comonomer composition was the only variable in this study. On‐line ¹H NMR spectroscopy was applied to follow individual monomer conversion. Mole fraction of AN and IA in the reaction mixture (f) and in the copolymer chain (F) were measured with progress of the copolymerization reaction. Overall monomer conversion versus time and also compositions of monomer mixture and copolymer as a function of overall monomer conversion were calculated from the data of individual monomer conversion versus time. Total rate constant for the copolymerization reaction was calculated by using the overall monomer conversion versus time data and then k_p/k_t^0.5 was estimated. The dependency of k_p/k_t^0.5 on IA concentration was studied and it was found that this ratio decreases by increasing the mole fraction of IA in the initial feed. The variation of comonomer and copolymer compositions as a function of overall monomer conversion was calculated theoretically by the terminal model equations and compared with the experimental data. Instantaneous copolymer composition curve showed the formation of alternating copolymer chain during copolymerization reaction. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3253–3260, 2007     

Termination rate coefficients for acrylamide in the aqueous phase at low conversion

The kinetics of acrylamide (AAm) free radical polymerization at low conversion of monomer to polymer in the aqueous phase was investigated at 50 °C using γ-radiolysis relaxation, which is sensitive to radical-loss processes. The values of the termination rate coefficients for AAm ranged from 8×10⁶ to 3×10⁷ M⁻¹ s⁻¹ as the weight fraction of polymer ranged from 0.002 to 0.0035, which is significantly lower than the low-conversion values for monomers such as styrene (2×10⁸ M⁻¹ s⁻¹) and methyl methacrylate (4×10⁷ M⁻¹ s⁻¹) in organic media. These can be quantitatively explained by applying a chain-length-dependent model of free-radical polymerization kinetics [Russell GT, Gilbert RG, Napper DH. Macromolecules 1992;25:2459. [19]] in which termination kinetics are expressed in terms of a diffusion-controlled encounter of radicals which ultimately yields an expression for the chain-length-averaged termination rate coefficient, 〈k_t〉. The lower 〈k_t〉 for AAm arises due to a combination of the high k_p value, promoting rapid formation of slower terminating long chains, and the slow diffusion of short propagating chains, relative to other common monomers. The chain transfer to monomer constant for AAm in water at 50 °C, C_M, was estimated using the chain-length-distribution method with correction for band-broadening [Castro JV, van Berkel KY, Russell GT, Gilbert RG. Aust J Chem 2005;58:178. [21]] and found to be 1.2×10⁻⁴ (±10%). The diffusion characteristics for AAm were adapted from those obtained for a similar aqueous system (hydroxyethyl methacrylate) together with a 0.5 exponent for the power law dependence on penetrant degree of polymerization at zero weight fraction polymer. This provides an adequate fit to the 〈k_t〉 data. This is the first application of the chain-length-dependent model to describe experimental termination rate coefficients for an aqueous system at low conversion to polymer. The result that the experimental termination rate coefficients can be reproduced with an a priori model with physically reasonable parameters supports the physical assumptions underlying that model.     

A new approach to controlled/living radical polymerization by DPE method

Controlled free radical polymerizations of methyl methacrylate and styrene in bulk by 1,1-diphenylethene (DPE) were demonstrated in a two-step process, preheating treatment of initiators followed by a living polymerization of monomers. Over the course of polymerization, continuous growing of polymers with unimodal molecular weight distribution and a relatively small polydispersity index (around 1.5 even in the range of Mn ∼ 10⁵ g/mol) on GPC diagrams was observed. In our previous study, the DPE controlled radical polymerization with constant molecular weight throughout the polymerization was caused by the intrinsically low reactivation rate constant (k₂) of DPE capped dormant chains. To raise the reaction temperature in order to increase k₂, a continuous molecular weight growing but broader or bimodal molecular weight distribution was obtained if the living polymerization was conducted in a one-step process. In this work, a two-step polymerization process was proposed. In the first step, the initiator 2,2′-azobisisobutyronitrile (AIBN), control agent DPE, and small amount of monomer were mixed and heated for a specific time period. Then a living polymerization of monomers was conducted in the second step of polymerization. This two-step new approach had minimized the imperfections of the DPE system; thus the polymerization showed better living characters and revealed its enhanced control abilities.     

Viscosity of bulk free radical polymerizing systems under near-isothermal and non-isothermal conditions

A viscometer-reactor assembly is used to generate data on the viscosity, η(t), of an example polymerizing system exhibiting the Trommsdorff effect, namely, the bulk free radical polymerization of methyl methacrylate (MMA), at different temperature conditions [near-isothermal and non-isothermal (near-step increase and near-step decrease in temperature)] and at two different initiator, 2,2′-azoisobutyronitrile (AIBN), concentrations. Two types of cup and bob assemblies, viz., the Haake^® SV-2 and the Haake^® HV-DIN, have been used to measure η(t) of the reaction mass, until reasonably high values of viscosity, well into the gel effect region. Only three sets of experimental data on x_m(t), M_w(t) and η(t) under near-isothermal conditions, are used to develop general correlations for the viscosity. These tuned correlations predict the values of the viscosity for a whole variety of other experimental conditions, including non-isothermal cases, reflecting that the physics of the system is well represented by them. Hence, these correlations can be used for other systems after tuning their parameters. The feasibility of on-line soft sensing is demonstrated for a few cases.     

Kinetics of butylacrylate polymerization in a starved feed reactor

A starved feed reactor (SFR) is a semibatch polymerization reactor where initiator and monomer are fed slowly into a fixed amount of solvent. The polymerization is carried out isothermally at elevated temperatures. The added initiator decomposes instantaneously and the added monomer polymerizes immediately. The molecular weight of the product polymer can be effectively controlled by the feed ratio of monomer to initiator. This article presents a study on the kinetics of butylacrylate polymerization in an SFR. The model parameters are regressed with experimental data. Although the solids fraction in the SFR is high (>50%), viscosity is not high and the “gel effect” is weak because of the low molecular weight of the products. It is found that the termination rate constant is a power function of molecular weight, and the lumped rate constant k_p/(k)^1/2 can be modeled through an Arrhenius equation. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1519–1525, 2004     

Free-radical propagation and termination kinetics of the butyl acrylate dimer studied by pulsed laser polymerization techniques

The propagation and termination rate coefficients for bulk polymerization of the butyl acrylate dimer (BA dimer) are determined by pulsed laser techniques. The rate coefficient for propagation, k_p, is deduced for temperatures from 20 to 90 °C via the pulsed laser polymerization-size exclusion chromatography (PLP-SEC) method at pulse repetition rates between 1 and 10 Hz. The Arrhenius parameters were found to be: E_A(k_p) = (34.2 ± 1.0) kJ mol⁻¹ and A(k_p)/L mol⁻¹ s⁻¹ = (1.08 ± 0.49) × 10⁷ L mol⁻¹ s⁻¹. The termination rate coefficient, k_t, has been measured via SP-PLP-ESR, single pulse-pulsed laser polymerization in conjunction with time-resolved electron spin resonance detection of radical concentration. The resulting Arrhenius parameters as deduced from the temperature range −15 to +30 °C are: E_A(〈k_t〉) = (22.8 ± 3.7) kJ mol⁻¹ and log(A/L mol⁻¹ s⁻¹) = 10.6 ± 1. The chain-length dependence of k_t was studied at 30 °C. For short chains a significant dependence was found which may be represented by an exponent α = 0.79 in the power-law expression k_t(i) = k_t⁰i^−α.     

Absolute rate constants for radical-monomer reactions

Summary A method is described for determining the absolute rate constants for the first few propagation steps in radical polymerization. The procedure involves a product analysis of the oligomeric alkoxyamines formed when an initiator is decomposed in monomer containing a very low concentration of a nitroxide radical scavenger. The method is illustrated with analysis of data for methyl acrylate. The rate constants for the first two propagation steps for polymerization of this monomer,k _p(1) andk _p(2), are at least an order of magnitude greater thank _p(average). Values of the absolute rate constants for reactions of phenyl and primary alkyl radicals with methyl acrylate are also estimated.     

Radical polymerization behavior of dimethyl vinylphosphonate: Homopolymerization and copolymerization with trimethoxyvinylsilane

Dialkyl vinylphosphonates such as dimethyl vinylphosphonate (DMVP) and diethyl vinylphosphonate were quantitatively polymerized with dicumyl peroxide (DCPO) at 130°C in bulk. The polymerization of DMVP with DCPO was kinetically studied in bulk by fourier transform near‐infrared spectroscopy (FTNIR) and electron spin resonance (ESR) spectroscopy. The initial polymerization rate (R_p) was given by R_p = k[DCPO]^0.5[DMVP]^1.0 at 110°C, being the same as that of the conventional radical polymerization involving bimolecular termination. The overall activation energy of the polymerization was estimated to be 26.2 kcal/mol. The polymerization system involved ESR‐observable propagating polymer radicals under the practical polymerization conditions. ESR‐determined rate constants of propagation (k_p) and termination (k_t) were k_p = 19 L/mol s and k_t = 5.8 × 10³ L/mol s at 110°C, respectively. The molecular weight of the resultant poly(DMVP)s was low (M_n = 3.4 ? 3.5 × 10³), because of the high chain transfer constant (C_m = 3.9 × 10^?2 at 110°C) to the monomer. DMVP (M₁) showed a considerably high reactivity in the radical copolymerization with trimethoxyvinylsilane (TMVS) (M₂) at 110°C in bulk, giving an inorganic component‐containing functional copolymer with potential flame‐retardant properties; r₁ = 1.6 and r₂ = 0. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effect of pressure on the precipitation and sol-phase aqueous polymerization of methyl methacrylate

Effect of pressure (atmospheric to 120 kg/cm²) on the K₂S₂O₈–Na₂S₂O₄-initiated aqueous polymerization of methyl methacrylate has been studied at 25°C. When the concentrations of the redox initiator are so adjusted as to obtain the separating polymer phase as a coarse coagulum, the conversion, rate, and molecular weight of polymerization tend to rise initially with increase of pressure up to a certain value and fall subsequently to a limiting value. However, these parameters fall monotonously with an increase in pressure when the polymer phase separates out as a fine colloid at a lower concentration of the initiator. The initial rise in rate is consistent with an increase in k_p and or a decrease in k_t under high pressure; the ultimate fall in rate may be due to a decrease in the diffusion of monomer from the aqueous phase to the growing polymer radical site. The fall in the molecular weight with pressure is explained on the basis of enhanced monomer transfer. In the colloidal range the pressure dependence trend is related to the stability of the colloidal phase. The rate is proportional to the square root of the product of K₂S₂O₈–Na₂S₂O₄ and varies linearly as the first power of the monomer concentration as also observed under normal pressure conditions. The MWD values of the polymers are ca. 2.5 and do not change with applied pressure.