Molecular weight distributions in radiation-induced polymerization. III. γ-Ray-induced polymerization of styrene at low temperatures

The kinetics of the γ-radiation-induced polymerization of styrene was studied at radiation intensities of 8 × 10⁴, 2.4 × 10⁵, 3.1 × 10⁵, and 8.3 × 10⁵ rad/hr over a temperature range of ?10°C to 30°C. The water content of the irradiated samples varied from 1.0 × 10^?3 to 7.5 × 10^?3 mole/l. The power dependence of the rate of polymerization on the dose rate at ?10°C varied from 0.53 to 0.71 as the water content of the sample varied from 7.5 × 10^?3 to 1.0 × 10^?3 mole/l. A value of 3.1 kcal/mole was determined for the overall activation energy. Molecular weight distribution studies by gel-permeation chromatography indicated the presence of two distinct peaks. The contribution of each peak was dependent on specific experimental parameters. Kinetic data and molecular weight distribution data indicate the coexistence of two propagating species. Analysis of the data strongly suggests that a free-radical mechanism and a cationic mechanism are involved.     

The Effect of Active Carbon on the Kinetics of Polymerization of Methyl Methacrylate

Rates of 2–2¹?azobisisobutyronitrile initiated polymerization of methyl methacrylate in benzene were determined at 77.2, 65.0, and 50.0°C. The variation of molecular weight of the polymer with temperature and conversion was also studied. At a fixed conversion of 2.0%, the molecular weight decreased from 2.05 × 10⁵ at 50°C to 1.4 × 10⁵ at 77.2°C. The ratio of the propagation rate coefficient to the square root of the termination rate coefficient was found to be 0.61, 0.397, and 0.374 at 77.2, 65.0, and 50.0°C, respectively, with an uncertainty of ±0.5°C in temperature. The effect of active carbon on the rates of polymerization at 77.2°C was measured. Rates of polymerization decreased in the presence of active carbon. For example, the initial rate of polymerization decreased from 7.8 × 10^?4 mole/(liter min) to 4.6 × 10^?4 mole/(liter min) when the carbon concentration was varied from 0 to 9.65 g/liter. The molecular weight of the polymer increased from an average of 1.4 × 10⁵ in the absence of carbon to 1.5 × 10⁵ when carbon was present.     

Polymerization of Acrylamide with Permanganate/Mercaptosuccinic Acid Initiator System

The aqueous polymerization of acrylamide initiated by the acidified potassium permanganate/mercaptosuccinic acid redox system was studied at 35 ± 0.2°C in nitrogen. In the studied range of activator concentration (2.0 × 10^?3 to 6.25 ± 10^?3 mole/liter) the polymerization rate remains unaffected. The initial rate of polymerization varies linearly with KMnO₄ and acrylamide concentrations in the studied range. The activation energy was found to be 6.61 kcal/mole (27.63 kJ/mole) in the temperature range of 30–50°C. The molecular weight of polyacrylamide was found to be independent of [KMnO₄] but increased with increasing monomer concentration. The effect of DMF on polymerization rate and molecular weight was also investigated.     

Increased rates of initiation of polymerization by 4-4′-dicyano-4-4′-azopentanoic acid in the presence of ferric salts

The initiation of the polymerization of acrylamide by 4-4′-dicyano-4-4′-azopentanoic acid in aqueous solution has been studied kinetically at 25°C. Ferric chloride and ferric sulfate were used to terminate polymerization so that rates of initiation could be calculated from the rates of production of ferrous iron. Velocity coefficients at 25°C. for the initiation reaction were found to be (25.7 ± 2.8) × 10^?7 sec.^?1 for the ferric chloride terminated reaction and (73.6 ± 0.6) × 10^?7 sec.^?1 for the ferric sulfate-terminated polymerization. The value reported for the initiation reaction when acrylamide is polymerized in the absence of metal salts is 1.29 × 10^?7 sec.^?1. Velocity coefficients for the termination reaction have been calculated from the overall rates of polymerization obtained with ferric salts present. In the case of the ferric chloride-terminated reaction, it has been shown that the rate of polymerization is reduced by increasing the total concentration of chloride ions. Termination velocity coefficients at 25°C. for the inner sphere complexes FeCl²⁺·5H₂O and FeSO₄⁺·4H₂O have been calculated to be 18.9 × 10⁴ and 7.98 × 10⁴ l./mole-sec., respectively. The dependence on the concentration of ferric chloride of the molecular weights of the polymers produced has also been considered.     

Kinetics of free-radical copolymerization of α-methylstyrene and styrene

The free-radical copolymerization of α-methylstyrene and styrene has been studied in toluene and dimethyl phthalate solutions at 60°C. Gas chromatography was used to monitor the rate of consumption of monomers. For styrene alone, the measured rate of polymerization R_p and M?_n of the polymer coincided with values expected from previous studies by other workers. Solution viscosity η affected R_p and M?_n of styrene homopolymers and copolymers as expected on the basis of an inverse proportionality between η^1/2 and termination rate. The rate of initiation by azobisisobutyronitrile appears to be independent of monomer feed composition in this system. Molecular weights of copolymers can be accounted for by considering combinative termination only. The effects of radical chain transfer are not significant. A theory is proposed in which the rate of termination of copolymer radicals is derived statistically from an ideal free-radical polymerization model. This simple theory accounts quantitatively for R_p and M?_n data reported here and for the results of other workers who have favored more complicated reaction models because of the apparent failure of simple copolymer reactivity ratios to predict polymer composition. This deficiency results from systematic losses of low molecular weight copolymer species in some analyses. Copolymer reactivity ratios derived with the assumption of a simple copolymer model and based on rates of monomer loss can be used to predict R_p values measured in other laboratories without necessity for consideration of depropagation or penultimate unit effects. The 60°C rate constants for propagation and termination in styrene homopolymerization were taken to be 176 and 2.7 × 10⁷ mole/l.-sec, respectively. The corresponding figures for α-methylstyrene are 26 and 8.1 × 10⁸ mole/l.-sec. These constants account for the sluggish copolymerization behavior of the latter monomer and the low molecular weights of its copolymers. The simple reaction scheme proposed here suggests that high molecular weight styrene–α-methylstyrene copolymers can be produced at reasonable rates at 60°C by emulsion polymerization. This is shown to be the case.     

Studies of redox polymerization. I. Aqueous polymerization of acrylamide by an ascorbic acid–peroxydisulfate system

The polymerization of acrylamide initiated by an ascorbic acid–peroxydisulfate redox system was studied in aqueous solution at 35 ± 0.2°C in the presence of air. The concentrations studied were [monomer] = (2.0–15.0) × 10^?2 mole/liter; [peroxydisulfate] = (1.5–10.0) × 10^?3 mole/liter; and [ascorbic acid] = (2.84–28.4) × 10^?4 mole/liter; temperatures were between 25–50°C. Within these ranges the initial rate showed a half-order dependence on peroxydisulfate, a first-order dependence on an initial monomer concentration, and a first-order dependence on a low concentration of ascorbic acid [(2.84–8.54) × 10^?4 mole/liter]. At higher concentrations of ascorbic acid the rate remained constant in the concentration range (8.54–22.72) × 10^?4 mole/liter, then varied as an inverse halfpower at still higher concentrations of ascorbic acid [(22.72–28.4) × 10^?4 mole/liter]. The initial rate increased with an increase in polymerization temperature. The overall energy of activation was 12.203 kcal/mole in a temperature range of 25–50°C. Water-miscible organic solvents depressed the initial rate and the limiting conversion. The viscometric average molecular weight increased with an increase in temperature and initial monomer concentration but decreased with increasing concentration of peroxydisulfate and an additive, dimethyl formamide (DMF).     

Aqueous polymerization of acrylamide initiated by acidic potassium permanganate–ascorbic acid redox system

The polymerization of acrylamide initiated by the acidic permanganate–ascorbic acid redox pair has been studied in aqueous media at 30 ± 0.2°C in nitrogen atmosphere. The initial rate of polymerization has been found to be proportional to nearly the first power of the catalyst KMnO₄ concentration within the range 6.0 × 10^?3–14.0 × 10^?3 mole/l. The rate is proportional to the first power of the monomer concentration within the range 4.00 × 10^?2–12.0 × 10^?2 mole/l. However, the rate of polymerization is independent of ascorbic acid concentration within the range 3.0 × 10^?3–6.0 × 10^?3 mole/l., but the further increase of the concentration depresses the rate of polymerization as well as maximum conversion. The initial rate increases but the maximum conversion decreases as the temperature is increased within the range 20–35°C. The overall energy of activation has been found to be 9.8 kcal/mole. The optimum amount of sulfuric acid is essential to initiate the polymerization but its presence in excess produces no effect either on the rate of reaction or the maximum conversion. Water-miscible organic solvents and salts, e.g., CH₃OH, C₂H₅OH, (CH₃)₂CHOH, KCl, and Na₂SO₄, depress the rate. Slight amounts of MnSO₄ · H₂O and a complexing agent NaF increase the rate of polymerization. Cationic and anionic detergents have been found to decrease and increase the rate, respectively, while nonionic surfactants have no effect on the rate of polymerization.     

Radiation-induced homogeneous polymerization of ethylene in cyclohexane

The radiation-induced polymerization of ethylene in cyclohexane was carried out in a reactor of 100 ml capacity under a range of temperature of 25–150°C, dose rate of 4.1 × 10⁴–2.9 × 10⁵ rad/hr, pressure of 200 kg/cm², and amount of cyclohexane of 20–90 ml. The polymerization was found to proceed at a steady state from the beginning. The polymerization rate is maximum at ca. 50 ml of cyclohexane. The dose rate exponent of the polymerization rate was 0.6 at every temperature from 25 to 150°C. The polymer molecular weight is in the range of 10³–10⁴, independent of dose rate, and decreases with increasing amount of cyclohexane. The molecular weight distribution is unimodal and narrow. Kinetic analysis of these results indicates that the polymerization proceeds via a simple scheme of homogeneous polymerization and the polymer molecular weight was determined by the chain transfer reaction which takes place mostly with cyclohexane. The unimodal and narrow molecular weight distribution is also consistent with the homogeneous polymerization scheme.     

Molecular weight distribution studies. I. Radiation-induced polymerization of liquid α-methylstyrene

The concentration of water in purified and BaO-dried α-methylstyrene was found to be 1.1 × 10^?4M. The radiation-induced bulk polymerization of the α-methylstyrene thus prepared was studied in the temperature range of ?20°C to 35°C. The polymerization rate varied as the 0.55 power of the dose rate. The theoretical molecular weights and molecular weight distribution were calculated from a proposed kinetic scheme and these values were then compared with those found experimentally. The agreement between these two was reasonably close, and therefore it was concluded that, from the molecular weight distribution point of view, the proposed kinetic scheme for the cationic polymerization of α-methylstyrene is an acceptable one. The rate constant for chain transfer to monomer k_f changed with temperature and was found to be responsible for the decrease in the molecular weight of the polymer with increase in temperature. k_f and k_p at 20°C were found to be 0.95 × 10⁴ l./mole-sec and 0.99 × 10⁶ l./mole-sec, respectively.     

Radiation-induced ionic polymerization of isobutyl vinyl ether in bulk

The radiation-induced ionic polymerization of isobutyl vinyl ether was investigated under conditions where the monomer was dried with molecular sieves. The investigation covered the temperature range from ?16°C to 90°C, and the dose-rate range from 10¹⁵ to 10²⁰ eV/g-sec, using both γ-rays and electrons. A very high overall activation energy of 15.9 kcal/mole was found for the process below 30°C. Above 30°C, however, the value of the overall activation energy dropped to 4.9 kcal/mole, a phenomenon which is ascribed to the solvation of the propagating carbonium ion below 30°C. The dose-rate dependence of the rate of polymerization was found to be 0.58 over the entire dose-rate range investigated. The molecular weight of the polymer was found to be far less sensitive to trace amounts of water than the rate of polymerization. The molecular weight of the polymer depended strongly on the irradiation temperature, reaching a maximum value of about 120,000 at 35°C. It is shown that at temperatures above 20°C regenerative chain transfer processes play an important role in determining the molecular weight of the polymer.     

Polymerization and copolymerization of trioxane: Factors influencing molecular weight and end groups

In bulk polymerization and copolymerization of trioxane with ethylene oxide, it has been shown that p-chlorophenyldiazonium hexafluorophosphate is a superior catalyst as compared to boron trifluoride dibutyl etherate (BF₃ · Bu₂O). Polymers and copolymers of significantly higher molecular weight have been obtained. The higher molecular weight has been attributed primarily to less inherent chain transfer during propagation, which in turn can be attributed to the superior gegenion PF₆^?. The polymerization proceeds via a clear period followed by sudden solidification. Faster polymerization and higher molecular weight polymers have been observed for homopolymerization than for copolymerization. The polymer yield obtained after solidification is determined by both rate of polymerization and rate of crystallization of polymers. These rates, in turn, are dependent on the catalyst concentration. The molecular weight is determined both by polymer yield and extent of inherent chain transfer. In the range of monomer to catalyst mole ration [M]/[C] = (0.5–20) × 10⁴ investigated, it has been found that in the higher range, the polymer yield is independent of the catalyst concentration and the extent of inherent chain transfer is inversely proportional to the half power of catalyst concentration: [M]/[C] = (0.5–8) × 10⁴ for homopolymerization and (0.5–3) × 10⁴ for copolymerization with 4.2 mole % ethylene oxide. In the lower range, the yield decreases with catalyst concentration and the extent of inherent chain transfer is inversely proportional to higher power of catalyst concentration. The dependence of molecular weight of polymers on catalyst concentration has been shown to be a complex one. The molecular weight goes through a maximum as the catalyst concentration is decreased. The maximum molecular weights have been obtained at [M]/[C] ≈ 8 × 10⁴ for homopolymerization and ～3 × 10⁴ for copolymerization with 4.2 mole % ethylene oxide. Prior to reaching maximum the molecular weight is inversely proportional to the half power of catalyst concentration indicating it is primarily controlled by inherent chain transfer. Upon further decrease of catalyst, molecular weight decreases as a result of both a decrease in polymer yield and an increase in inherent chain transfer. In copolymerization of trioxane and ethylene oxide, it has been ascertained that methylene chloride exhibits a favorable solvating effect. Although higher inherent chain transfer takes place in copolymerization than in homopolymerization, the extent of chain transfer is independent of ethylene oxide concentration. The difference in polymer yield and molecular weight a t different ethylene oxide concentrations is attributed primarily to the difference in k_p/k_t ratio. It also has been demonstrated that end capping of polymer chains can be accomplished by the use of a chain transfer agent—methylal.     

The thermal polymerization of styrene in diethyladipate at temperatures up to 160°

The thermal polymerization of styrene in diethyladipate has been studied dilatometrically at temperatures from 90 to 160°. The rate was found to be directly proportional to (monomer concentration)² and the molecular weight of the polymer formed was controlled mainly by chain transfer to monomer, particularly at the higher temperatures. A value of 86 ± 2 kJ/mol was obtained for the overall energy of activation for the polymerization, and values of 7.16 × 10^?5. 2.0 × 10^?4 and 5.5 × 10^?4 were found for the transfer constant for diethyladipate at 120. 140 and 160° respectively.     

Molecular weight distributions in radiation-induced polymerization. VII. γ-ray-induced polymerization of “wet” styrene in the solid state

In the present paper kinetic and molecular weight distribution results are reported for the γ-ray-initiated polymerization of styrene in the solid state. “In-source” polymerization over the temperature range ?35°C to ?55°C and post-polymerization at ?35°C have been investigated for “wet” styrene samples (water concentration ≈ 10^?3 mole/l.). An interesting feature of the solid-state polymerization of styrene is the bimodal nature of the molecular weight distribution. On a qualitative basis the results resemble those obtained previously for the polymerization of rigorously dried (“dry”) styrene. However, there are noticeable differences on a quantitative basis resulting from the considerable difference in the water content between wet and dry samples. On the basis of these studies, the kinetic and molecular weight distribution data have been interpreted as being indicative of polymerization occurring simultaneously via free-radical and cationic mechanisms.     

Kinetic study of γ-radiation-induced polymerization of ethylene in the presence of acetylene

The effects of acetylene on the γ-radiation-induced polymerization of ethylene were studied from the viewpoint of kinetics. The experiments were carried out under a pressure of 150–400 kg/cm²; the temperature was 30°C; the dose rates were 2.7 × 10⁴ and 1.1 × 10⁵ rad/hr; the acetylene content was 0–2.21%. Both the polymer yield and the molecular weight increased acceleratively with the reaction pressure in the polymerization containing 0.18% acetylene. The yield increased almost proportionally with the dose rate, and the molecular weight was found to be almost independent of the dose rate in the polymerization containing 2.21% acetylene. The polymerization rate and the molecular weight increased with reaction time, but the increment decreased with increasing acetylene content. The degree of increase in the molecular weight also decreased with increasing time. These results were analyzed by using a graphical evaluation method for kinetics, and the effects of acetylene on each elementary step in the polymerization discussed.     

Molecular weight distribution in radiation-induced polymerization. I. γ-radiation-induced free-radical polymerization of liquid styrene

The kinetics of γ-radiation-induced free-radical polymerization of styrene were studied over the temperature range 0–50°C at radiation intensities of 9.5 × 10⁴, 3.1 × 10⁵, 4.0 × 10⁵, and 1.0 × 10⁶ rad/hr. The overall rate of polymerization was found to be proportional to the 0.44–0.49 power of radiation intensity, and the overall activation energy for the radiation-induced free-radical polymerization of styrene was 6.0–6.3 kcal/mole. Values of the kinetic constants, k_p²/k_t and k_trm/k_p, were calculated from the overall polymerization rates and the number-average molecular weights. Gelpermeation chromatography was used to determine the number-average molecular weight M?_n, the weight-average molecular weight M?_w, and the polydispersity ratio M?_w/M?_n, of the product polystyrene. The polydispersity ratios of the radiation-polymerized polystyrene were found to lie between 1.80 and 2.00. Significant differences were observed in the polydispersity ratios of chemically initiated and radiation-induced polystyrenes. The radiation chemical yield, G(styrene), was calculated to be 0.5–0.8.     

Friedel-crafts copolymerization of thiophene and p-di(chloromethyl)benzene. I. Products,kinetics, and mechanism of the early stages of the reaction

The rate of polymerization of thiophene, at concentrations of catalyst (SnCl₄), and thiophene of the same order as was subsequently used in studying the reaction between thiophene and di(chloromethyl)benzene, is of the order of 10^-2%/hr at 30°C. There is no significant self-condensation of DCMB under the same conditions. Since the reaction between thiophene and DCMB is complete at 30°C in minutes rather than hours, it is assumed that self-condensation of thiophene or DCMB during the reaction between them will be negligible and should not influence the course of the reaction or the structure of the resulting polymer. Reaction at 30°C is much too fast for convenient study. A temperature of 0°C is more appropriate and was used in subsequent kinetic work. The first two products of the condensation of p-di(chloromethyl)benzene (DCMB) with thiophene have been identified by a combination of mass, infrared, and nuclear magnetic resonance spectroscopy as thenylchloromethylbenzene (TCMB) and dithenylbenzene (DTB). DCMB, TCMB, and DTB have been estimated quantitatively during the course of the reaction by gas-liquid chromatography (GLC), and it has been established that the rates of each of the two reaction steps is first-order with respect to the chloro compound (DCMB and TCMB respectively), thiophene, and SnCl₄. Rate constants for these two consecutive reactions were calculated to be k₁ = 2.79 × 10^-4l.²/mole²-sec, k₂ = 6.37 × 10^-3l.²/mole²-sec; the corresponding energies of activation are E₁ = 7.93 kcal/mole, E₂ = 7°67 kcal/mole. These rate constants are appreciably higher than values previously obtained for the corresponding DCMB–benzene reactions.     

Emulsifier-free emulsion polymerization of tetrafluoroethylene by radiation. I. Effects of reaction conditions on the polymerization rate and polymer molecular weight

The radiation-induced emulsifier-free emulsion polymerization of tetrafluoroethylene was carried out at an initial pressure of 2–25 kg/cm², temperature of 30–110°C, and under a dose rate of 0.57 × 10⁴?3.0 × 10⁴ rad/hr. The rate of polymerization was shown to be proportional to 1.0 and 1.3 powers of the dose rate and initial pressure, respectively, and is maximal at about 70°C. The molecular weight of polytetrafluoroethylene (PTFE) lies in the range of 10⁵?10⁶, increases with reaction time in the early stage of polymerization, and is maximal at 70°C but is almost independent of the dose rate. An interesting discovery is that PTFE, a hydrophobic polymer, forms as a stable latex in the absence of emulsifier. When PTFE latex coagulates during polymerization under certain conditions, the polymerization rate decreases, probably because polymerization proceeds mainly on the polymer particle surface. The observed rate acceleration and successive increase in polymer molecular weight may be due to slow termination of propagating radicals in the rigid PTFE particles.     

Radiation-induced polymerization at high dose rate. I. Isobutyl vinyl ether in bulk

Bulk polymerization of isobutyl vinyl ether was studied at 25°C in a wide dose rate range, 8.2-277 rad/sec by γ rays and 8.8 × 10³-2.2 × 10⁵ rad/sec by electron beams. At low dose rate, 8.2-277 rad/sec, only the radical polymerization took place. At high dose rate exceeding 8.8 × 10³ rad/sec, cationic polymerization was found to occur in addition to the radical polymerization. DP _n of the product at high dose rate was 9-10. Further drying of the monomer increased R_p, and molecular weight of the product formed by cationic mechanism also increased.     

Epoxide polymerization—IV. Ethylene oxide polymerization using the diphenylzinc-water system in benzene at 60°

The diphenylzinc-water system was used as catalyst for ethylene oxide polymerization in benzene solution at 60°. The system is greatly influenced by the molar ratio of water to diphenylzinc. H₂O/Ph₂Zn, the maximum catalyst activity being found for a ratio of unity. Ph₂Zn alone and molar ratios of 0.25, 0.5, 1.5, 1.75 and 2.0 gave very low conversion to polymer. For a molar ratio of unity, the yield of polymer and the molecular weight increase with time. The reaction is first order with respect to monomer with k_P = 5.7 × 10^?5 sec^?1 mol^?1 l.     

ESR STUDIES OF CHLOROPHYLL ONE-ELECTRON PHOTOCHEMISTRY: KINETICS AND MECHANISM OF REVERSIBLE HYDROQUINONE OXIDATION IN SOLUTION*

Abstract— ESR studies have been made of the kinetics of semiquinone radical formation and disappearance resulting from the reversible photosensitization by chlorophyll of hydroquinone oxidation in a pyridine-water solvent. The rate of radical decay was found to be second order with respect to the radical concentration, with a rate constant of 6.7 × 10⁵ l./mole sec at -30°C and an activation energy of 6900 cal/mole. The rate of radical formation was recombination-limited and, through the use of β-carotene as a quencher, the rate constant was determined to be 8.81 × 10⁵ l./mole sec at -30°C. The effect of light intensity and hydroquinone concentration on the rate of semiquinone radical formation and on the steady state radical concentration was also investigated and possible mechanisms to explain the results are discussed.