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Kinetics of Vinyl Polymerization. 1. Ceric Ion Induced Polymerization of Acrylonitrile in the Presence of Butane1,4-diol
Abstract
The kinetics of acrylonitrile polymerization initiated by butane-l,4-diol-Ce(IV) redox system have been studied in aqueous sulfuric acid in the range 30 to 40°C under nitrogen. There is no experimental evidence for the formation of a complex between diol and oxidant; the kinetics are consistent with a linear mode of termination. The effect of certain neutral salts, acids, water-miscible organic solvents, and temperature on the rate of polymerization and the rate of metal ion disappearance have also been investigated. Various rate and energy parameters have been evaluated.
Several models have been proposed in the literature over the last two decades in order to simulate free-radical homopolymerizations. However, most of these models deal with one specific monomer system, usually under restricted polymerization conditions.
The rate of polymerization of acrylonitrile, using the Ce(IV)–cyclohexanone redox system as an initiator, was studied kinetically, in the presence of 0.015M sodium dodecyl sulfate (SDS), over a temperature range of 25–45°C. The rate of polymerization (RP), percentage of monomer conversion, and rate of Ce(IV) consumption (−RCe) were found to increase with the concentration of SDS, above its CMC. The effect of [AN], [Ce(IV)], [H+], and the ionic strength were also studied. The overall activation energies for the polymerization processes were computed to be 23.14 and 17.64 kcal/mol in the absence and presence of 0.015M SDS. A suitable kinetic mechanistic scheme for the free-radical mechanism was proposed. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2066–2072, 2003
The graft copolymerization of acrylonitrile and hydroxyethyl methacrylate monomers onto poly(vinyl alcohol) backbone using the ceric ion technique was investigated in detail and a high percentage grafting of the monomers onto the backbone was obtained. Another backbone which was investigated was a blend of poly(vinyl alcohol) and polyacrylic acid (PVA-PA). The grafting onto the poly(vinyl alcohol) was found to depend on the concentration of the ceric ion and its acidity and the poly(vinyl alcohol) content. Grafting was observed to proceed very smoothly onto the poly(vinyl alcohol) but was much slower in the poly(vinyl alcohol)/poly(acrylic acid) system. The graft copolymerization was interpreted in terms of an emulsion polymerization in the case of the poly(vinyl alcohol) system. However, grafting did not proceed very readily in the poly(vinyl alcohol)/poly(acrylic acid) blend system, explained by a coagulation which caused the system to react more slowly.
The poly(vinyl alcohol)–acrylonitrile–2-hydroxy ethyl methacrylate (PVA–AN–HEMA) grafted latex membrane was synthesized by grafting AN and HEMA mixture on PVA in aqueous solution. The ceric ammonium nitric in nitric acid was was used as a catalyst. This membrane has well-balanced composition of hydrophilic and hydrophobic components and was proved by transition electron microscopy to process microstructure between continuous and disperse phases. The permeability of alcohol–water solution and separativity of phenol–water solution through this PVA–AN–HEMA membrane were studied. It was found that the permeation rate for aqueous solution as as alcohol–water was greater than that for pure water, and the separativity of phenol–water solution by pervaporation increased as the amount of PHEMA increased.
Virtually all free-radical chain reactions require a separate initiation step in which a radical species is generated in the reaction mixture. Some types of chain reactions are initiated by adding a stable free radical, one that shows little or no tendency for self-combination, directly to the reactants, but a separate initiation step is still involved because these stable radicals are most often inorganic ions or metals.A very effective method of generating free radicals under mild conditions is by one-electron transfer reactions, the most effective of which is redox initiation.This method has found wide application for initiating polymerization reactions and has industrial importance, e.g. in low-temperature emulsion polymerizations.In this review, in addition to the classical examples of redox pairs, recently employed metal-ion–organic-compound redox systems, electrochemical regeneration of reduced metal ions, redox initiation in nonaqueous media and transition metal organic halide initiators and metal chelate initiators are all reviewed.
In this paper, a method is described for the chronoamperometric determination of Ce(IV) in acidic aqueous solutions in order to study the kinetics and the mechanism of polymerization reactions with Ce(IV) as the initiator. The method is based on the FIA principle, in which small samples are injected in a continuous 1.0 mol L(-1) H2SO4 flow, and Ce(IV) is detected chronoamperometrically by reduction at the surface of a gold CDtrode. Such an electrode is made from a commercial CD that has a gold coating acting as a reflecting layer for the laser beam. It was found that a detection limit of 1.5 10(-7) mol L(-1) could be obtained with this method, that high concentrations (order of 10(-2) mol L(-1)) can be detected without IR drop and that the system can be used over several days without renewal or recalibration of the gold CDtrode.
The polymerization of methacrylamide initiated by the redox system K 2 S 2 O 8 /ascorbic acid has been studied at 35 ± 0,2°C under the influence of atmospheric oxygen. The molecular oxygen autocatalyses the polymerization rate. The rate of polymerization is independent of the activator (ascorbic acid) concentration within the range 2,83 · 10 ⁻³ to 11,3 · 10 ⁻³ mol · l ⁻¹ , this does not hold below or above the given concentration range. The rate varies linearly at low monomer concentration (up to 17,76 · 10 ⁻² mol · l ⁻¹ ). The catalyst exponent decreases from nearly unity (0,94) to 0,57 with increasing concentration of the catalyst probably due to participation of primary radicals in the termination of growing chain. The initial rate is not changed by the addition of a strong acid (H 2 SO 4 ) within the range 15 · 10 ⁻⁵ to 30 · 10 ⁻⁵ mol · l ⁻¹ . With the activator (ascorbic acid) alone, an optimum is observed within the pH range 2,9–3,5. The initial rate and the limiting conversion increases with increasing polymerization temperature. The overall energy of activation as calculated from the ARRHENIUS plot has been found to be 16,0 kcal.mol ⁻¹ within the temperature range 30–45°C.
Organic solvents (water miscible only) and small amounts of neutral salts, (KCl, Na 2 SO 4 ) depress the initial rate and the maximum conversion. The addition of small amounts of catalysts like Cu 2⊕ , Mn 2⊕ , Ag oplus; increases the initial rate but no appreciable increase in the limiting conversion is observed.
The cerium(IV) oxidation of thiourea, NN′-dimethyl-,NN′-diethyl-, and ethylene-thioureas and thiosemicarbazide has been examined at [H+]= 0·5M over the range 5–25° using a stopped-flow technique. The rates are consistent with a mechanism involving the formation of free radicals which subsequently dimerise. The thermo-dynamic parameters for the reactions are: thiourea Ea= 7·96 ± 0·4 kcal./mole (ΔS*=–32·7 ± 3 e.u.), dimethyl-Ea= 9·1 ± 0·5 kcal./mole (ΔS*=–29·3 ± 5 e.u.), diethyl-Ea= 8·9 ± 0·5 kcal./mole(ΔS*=–28·5 ± 4 e.u.), ethylene-Ea= 9·6 ± 0·3 kcal/mole (ΔS*=–25·4 ± 3 e.u.), and thiosemicarbazide Ea= 9·8 ± 0·5 kcal./mole (ΔS*=–28·8 ± 3 e.u.). The overall heats of each reaction have been determined calorimetrically. Linear free-energy relationships obtained are discussed.
A kinetic study of the polymerization of acrylonitrile has been made. During polymerization of the pure monomer the polymer is precipitated. The reaction shows kinetic features which are different from those of homogeneous vinyl polymerizations. Among these are the following: (a) the thermal catalyzed and photopolymerizations show an initial increase in rate with time; (b) the photochemical after-effect depends on the duration of irradiation; (c) the molecular weights of polymers prepared in the thermal catalyzed reaction at constant catalyst concentration at different temperatures show a maximum near 60 degrees C; (d) the photopolymer prepared near room temperatures is able at higher temperatures to act as an initiator for the polymerization of acrylonitrile and other vinyl monomers. This initiation can be very rapid, and give rise to a striking 'fast' reaction. It is shown that these and other observations can be interpreted on the assumption that the propagating free radicals become occluded in polymer aggregates during polymerization. This occlusion may affect all the rate coefficients; while the effect of mild degrees of occlusion is chiefly to reduce the termination coefficient, severe occlusion can prevent both termination and propagation. The factors influencing the degree of occlusion are discussed. Similar considerations probably apply to other vinyl polymerizations which produce insoluble polymers; in these reactions it is unlikely that a true stationary state is achieved.
The polymerization of acrylonitrile (M) initiated by the Ce(IV)–acetophenone (AP) redox pair has been studied in acetic–sulfuric acid mixtures in a nitrogen atmosphere. The rate of polymerization is proportional to [M]3/2, [AP]1/2 and [Ce(IV)]1/2. The rate of disappearance of ceric ion,–RCe, is proportional to [AP], [M], and [Ce(IV)]. The effect of certain salts, solvent, acid and temperature on both the rates have been investigated. A suitable kinetic scheme has been proposed, and the composite rate constants kp2(k/k/t) and k0/ki are reported.
The polymerization of acrylonitrile (M) initiated by the sorbitol (R)-Ce(IV) redox system has been studied in sulphuric acid in the range 30–40° under nitrogen. At moderately high concentrations of Ce(IV) (0.00015-0.02 M), the rate of polymerization (Rp) is proportional to and and the rate of Ce(IV) disappearance is proportional to [R] and [Ce(IV)]. At lower concentration of Ce(IV) (0.00005–0.00015 M) Rp is proportional to [M], [R]1/2 and [Ce(IV)]1/2 and rate of Ce(IV) disappearance is proportional to [R] and [Ce(IV)]. The effects of certain salts, acid, solvent and temperature on both rates have been investigated. A kinetic scheme involving mutual termination has been proposed and various rate and energy parameters evaluated. At still higher concentration of Ce(IV) (0.02 M), a linear mode of termination seems to operate.
The polymerization of acrylonitrile (M) initiated by the Ce(IV)-propane-1,2-diol (R) redox system has been studied in aqueous sulphuric acid under nitrogen in the temperature range 30 to 40°. The rate of polymerization is proportional to [M]2, [R] and [Ce(IV)]−1 and the rate of ceric ion disappearance is proportional to [R], [Ce(IV)]. The effects of certain salts, acid, solvent and temperature on both rates have been investigated. A kinetic scheme has been proposed, and various rate and energy parameters evaluated.
Cerium(IV) oxidations of a series of cyclic alcohols and glycols were studied in 1.0 M perchloric acid and in mixed sulfuric and perchloric acids. The results are related to the oxidation phase of the cerium(IV) initiation of graft polymerization onto cellulose. Definite evidence was obtained for formation of cerium(IV) complexes with cis- and trans-1,2-cyclohexanediols, trans-2-methoxycyclohexanol, and cyclohexanol in 1.0 M perchloric acid. The magnitudes of the equilibrium constants for complex formation indicated that the 1,2-cyclohexane-diols form chelate complexes. In mixtures of sulfuric and perchloric acids there were no large differences in the over-all oxidation rates of cis- and trans-1,2-cyclohexanediol or of cis- and trans-1,2-cyclopentanediol. The effect of ring size was considerable, however, with the cyclopentanediols reacting much more rapidly (200-1000-fold) than cyclohexanediols. A study of the effect of sulfate ion concentration indicated that CeSO42+ is the most reactive of the cerium(IV)-sulfate complexes present in the sulfuric-perchloric acid systems. The relative reactivities of the cellulose model compounds, trans-1,2-cyclohexanediol, cyclohexanemethanol, and tetrahydropyran-2-methanol, suggested that cerium(IV) oxidation of cellulose will occur mainly, but not exclusively, at the C2-C3 glycol.
The rate of reaction of ceric nitrate with the 1,2-glycol units and the alcohol units of polyvinyl alcohol was determined at 20°C. It was found that the glycol units are oxidized much faster than the alcohol units even though they are outnumbered about one hundred to one in the polymer chain. Under the conditions studied, at least 83 glycol units reacted per 100 hours oxidized. Since the oxidation of the glycol units is accompanied by chain splitting, the free radicals are produced predominantly at the fragments end.
Phenylendgruppen in aromatischen Polycarbonaten sind IR-spektroskopisch nach einer Kompensationsmethode bis zu 0,05% Phenyl erfaßbar. Die phenolischen OH-Endgruppen im Polycarbonat können ebenfalls IR-spektroskopisch und unabhnging davon photometrisch bis zu einer Nachweisgrenze von 0,001% ermittelt werden. Die Endgruppenanalysen gestatten die Berechnung des Zahlenmittels des Molekulargewichtes. Diese Werte stimmen gut mit nach anderen Methoden bestimmten Molekulargewichten überein.Phenyl end-groups in aromatic polycarbonates may be determined IR-spectroscopically by a compensation method. Even 0.05% of phenyl can be detected.The phenolic hydroxyl end-groups in polycarbonates may also be determined, Up to 0.001% of phenolic hydroxyl end-groups can be estimated photometrically.The end-group analyses allow the calculation of the number average of the molecular weight. The values of the molecular weight are in good agreement with those determined by other methods.
The polymerization of acrylamide initiated by ceric nitrate and 3-chloro-1-propanol was studied in aqueous solution at 25°C. At constant hydrogen ion and nitrate ion concentration, the rate of polymerization was found to be independent of the ceric ion concentration. The molecular weight of the polymer was very low. To explain the data, a kinetic scheme is proposed whereby ceric ions are responsible for both initiation and termination. The rate constant for oxidative termination was found to be of the order of 105 l. mole−1 sec.−1 and varied with the hydrogen ion and the nitrate ion concentrations.
Polymerization of the monomers, methyl acrylate (MA) and methyl methacrylate (MMA) was carried out in sulfuric acid medium at 15°C. With the redox initiator system, ceric ammonium sulfate–malonic acid. There was no induction period, and a steady state was attained in a short time. There was found to be no polymerization even after 1 hr. in the absence of the reducing agent R. The initiation was by the radical produced from the Ce4+–malonic acid reaction. The rate of monomer disappearance was proportional to [M]1.5, [R]0.5, and [Ce4+]0.3–0.5, and the rate of ceric disappearance was directly proportional to [R] and [Ce4+]. Chain lengths of the polymers were directly proportional to [M] and inversely to [R]1/2 and [Ce4+]1/2. The experimental results were explained by a kinetic scheme involving the following steps: (a) oxidation of the substrate to give the primary radical which reacts with Ce4+ to give the products, (b) initiation by the primary radical, (c) propagation, and (d) termination of the growing polymer radicals by the mutual type. For the polymerization of acrylonitrile (AN) by the redox system, ceric ammonium sulfate–cyclohexanone (CH), in sulfuric acid at 15°C., the scheme was modified to include linear type of termination by Ce4+, along with the mutual termination to explain the results especially under conditions with [Ce4+] ≥ [CH].
The polymerization in solution of a variety of water-soluble monomers with a variety of water-soluble redox initiators is described and a detailed study is reported on the polymerization, in aqueous solution, of methyl vinyl ketone and of methyl methacrylate under the influence of potassium sulfate-silver nitrate. Keeping the persulfate concentration constant and varying the silver nitrate or, conversely, keeping the silver nitrate constant and varying the persulfate, it is found that, except at very high concentration of catalyst, both the rate of polymerization and the molecular weight of the polymer bear a linear relation to the square root of the catalyst concentration, in harmony with the general theory of catalyzed bulk and emulsion polymerization. The molecular weight of the polymer formed diminishes with increase in the temperature of polymerization. The study of redox polymerization in water, without emulsifier, has been extended to the case of a mixture of a water-soluble and a water-insoluble monomer, and it has been found that copolymerization proceeds at a reasonable rate and frequently forms a stable latex. The copolymerization of styrene and 5 or 10 per cent of its weight of methacrylonitrile in water under the influence of a water-soluble persulfate-bisulfite redox pair has been examined with some closeness. Polymerization is apparently initiated in the aqueous solution, since, contrary to the state of affairs in emulsion polymerization, water-soluble retarders are more effective than oil-soluble ones, when the retarder is added at the outset. If the retarder is added at a later stage of polymerization, oil-soluble reagents are more effective than water-soluble ones. The water-soluble monomer enters into the polymer at a greater rate than the water-insoluble one, but it appears that, even at the earliest stage at which it is practicable to take samples co-polymerization occurs. The influence of catalyst concentration on the rate of polymerization, the molecular weight of the polymer, and the latex particle size was examined.
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