Chain Propagation Reactions Research Articles

Radical coordination polymerization

AbstractIn this paper different examples of the effect of coordination‐unsaturated metal compounds on the radical homo‐ and copolymerization of vinyl monomers are considered and analyzed. In all known cases homopolymerization proceeds according to the usual addition chain radical scheme and involve an initiation step with the formation of a primary radical. Complexed radicals or monomers take part in the chain propagation steps. In the case of copolymerization, a complexing agent can considerably influence apparent relative activities of monomers, decreasing or increasing the tendency to alternation. If two monomers are present in the system, one of which forms a complex with a Lewis acid, and the other is a donor with respect to this complex, the formation of a ternary donor‐acceptor complex also occurs in a number of cases. A ternary complex can fulfill the monomer function in the chain propagation reaction, i.e., it can add to the propagating chain as an independent kinetic species. In such cases, strictly alternating copolymers may be obtained. All mechanisms considered agree with the addition‐polymerization scheme. From the kinetic point of view the ternary complex addition is somewhat similar to an elementary step of Butler's inter‐intramolecular cyclopolymerization.

Effect of solvent on chain propagation and termination reaction rates in radical polymerization

The radical polymerizations of acrylamide and methacrylamide and of acrylic, methacrylic and fluoroacrylic acids have been studied in various solvents. The nature of the solvent affects the overall rate due to changes in k p and E p . It has been shown that the effect of solvent also depends on the nature of the monomer. A possible explanation of the observed behaviour is discussed.

The chlorine-atom-sensitized oxidation of chloroform

The chlorine-photosensitized oxidation of chloroform ▪ was studied at 303, 346, 400 and 426 K. At the two higher temperatures the simultaneous oxygen-inhibited photochlorination ▪ was observed. The mechanism and kinetics of these reactions are discussed in detail and several complex rate constants are given concerning chain termination reactions and chain propagation reactions involving Cl ., CCl 3 . and CCl 3O 2 . radicals. The behaviour of this system is very similar to that of the chlorinated ethanes and ethylenes. Evidence for a chain termination reaction ▪ is given.

Polymer reactions—X thermal pyrolysis of poly(isoprene)

Poly(isoprene) (97% cis-1,4 and 3% 3,4) has been pyrolysed in a carrier stream of helium from 315 to 384°. The products were online identified and quantitatively analysed by an interfaced pyrolysis GC peak identification system. The first order rate constant for pyrolysis is 1.1 × 10 −2 sec −1 at 384° with an overall activation energy of 41 kcal mol −1. The main products are isoprene and 1-methyl-4-isopropenyl cyclohexene. The dominant initiation process has been shown to be β chain scission leading to two allylic radicals. The former product is derived from an unzipping reaction, the latter together with 1.5-dimethyl-5-vinylcyclohexene are the main C 10 products from the allylic radicals. All the other minor products can be account for by simple chain propagation reactions with or without intramolecular hydrogen transfers.

Radical cotelomerization of propylene and acrylonitrile with CCl4

1. Cotelomerization of propylene and acrylonitrile with CCl4 was studied during initiation with tert-butyl peroxide with various ratios of the starting reagents. The formation of the propylene homotelomerization products CCl3CH2CHClCH3 and CCl3CH2CH(CH3)CH2CHClCH3 and the two cotelomers CCl3CH2CH(CN)CH2CHClCH3 and CCl3CH2CH(CH3)CH2CH(CN)CH2CHClCH3 was proven. 2. When propylene and acrylonitrile were cotelomerized with CCl4 in stainless-steel autoclaves, the formation of the acrylonitrile-CCl4 homotelomers CCl3(CH2CHCN)nCl with n=1 or 2 was detected together with the mentioned products. 3. The structure of the cotelomers, the correlation of the product yields, and the effect of the initiation method indicate high sensitivity of chain propagation and transfer reactions in the propylene -acrylonitrile-CCl4 system to the polar properties of the components and resulting radicals.

Kinetic method for determining the contribution of donor-acceptor complexes to the reaction of chain propagation during alternating radical copolymerization

This work presents a kinetic method for determining the quantitative contribution made by the addition of free monomers and their donor-acceptor complexes to the propagation reaction during alternating radical copolymerization. The method is based on the determination of positions of the maxima on plots of the copolymerization rate vs. the monomer mixture composition for various molar concentrations of monomers.

The kinetics and mechanism of an anionic polymerization in tetrahydrofuran over various barium compounds

Spectrophotometry has been used to show that dibenzylpolybutadienyl- and polystyryl-barium compounds become isomerized during prolonged storage, and that this process is accompanied by a shift of the absorption maximum to the long wave range. The rate of the butadiene polymerization over various barium compounds is in a dire ct relation with the initiator concentration; this is explained by the preferential progress of the reaction over the undissociated forms of the reactive centres. The rate of styrene polymerization over dibenzyl barium is independent of its concentration. This unusual result is explained by the decisive part of the free ions and the consideration of the equilibrium existing between the free, paired and “triple” ions. When bis-triphenylmethyl barium has been used with styrene, the decisive part is played by paired ions; this results in the chain propagation reaction being of first order with respect to the concentration of reactive centres. The effect of the alkyl group of the initiator on the reactive centre type and the mechanism of the chain propagation reaction in an anionic polymerization has been shown for the first time.

The influence of methallyl sulphonate and itaconic acid on the copolymerization of acrylonitrile and methyl acrylate in an aqueous solution of sodium thiocyanate

The influence of methallyl sulphonate and itaconic acid on the kinetics of the copolymerization of acrylonitrile and methyl acrylate has been investigated in an aqueous solution of sodium thiocyanate, using DAA and 4,4′-azo-bis (4-cyano-pentanol-1 as initiators). Methallyl sulphonate and itaconic acid participate in the chain propagation reaction and inhibit the copolymerization of acrylonitrile and methyl acrylate. The orders of the copolymerization reaction in respect to initiators have been calculated, the values being determined by the influence of the methallyl sulphonate and itaconic acid. The action of methallyl sulphonate and itaconic acid lead to a reduction in the rate constant for the breakdown of 4,4′-azo-bis (4-cyano-pentanol-1). The orders of the copolymerization reaction have been calculated, as well as the rates of initiation relative to the total concentration of monomers. The effective rates of inhibition have been determined for the copolymerization of acrylonitrile and methyl acrylate with methallyl sulphonate and itaconic acid as the inhibitors. The effective activation energies have been calculated for the studied reactions.

The autoignition of hydrocarbon fuels at high temperatures and pressures—Fitting of a mathematical model

A systematic study has been carried out of the autoignition of a number of hydrocarbon fuels under engine-like conditions in a rapid compression machine. The fuels were selected on the basis of their octane quality and sensitivity as measured in standard octane tests, and included a number of primary and toluene reference fuels and the olefin 2-methylhex-2-ene. The results confirm that there is a strong correlation between the performance of these fuels in engines and their autoignition properties measured in a rapid compression machine. The importance of charge temperature and density in “de-rating” sensitive fuels under “severe” engine conditions is demonstrated. The mathematical model of hydrocarbon auto-ignition described previously has been fitted to the experimental results for each of the fuels studied. This model is based on a relatively simple chemical mechanism, but is capable of giving a good simulation of all the experimentally observed trends. It is not possible to obtain a unique fit and thereby to determine the Arrhenius parameters for individual rate coefficients or groups of coefficients within the chemical model, but some qualitative pointers towards the nature of the chemical mechanism are possible using this approach. In particular the results suggest that degenerate-branching agent is formed by a radical reaction whose kinetic competition with the alternative main chain propagation reaction exhibits first-order dependence on oxygen concentration rather than on fuel concentration.

Polymerization of acrylonitrile in an aqueous solution of sodium thiocyanate

A kinetic study was made of the polymerization of acrylonitrile initiated by DAA and 4,4′-azo-bis-(4-cyanopentanole-1) in an aqueous solution of sodium thiocyanate. The rate of AN polymerization in aqueous sodium thiocyanate solution is 8 times higher at 60° than in DMF solution. The order of the polymerization reaction in respect to monomer and initiator concentrations is 1·45 and 0·7 respectively. The rate of initiation is a function of monomer concentration. An aqueous solution of sodium thiocyanate, used as solvent, has a marked effect on all stages of the radical polymerization of acrylonitrile. The rate constants for decomposition of the initiators were calculated, as was the ratio of rate constants for chain propagation and termination reactions.

The mechanism of oxidative polymerization in films

The kinetics of oxidative polymerization of oligoester-acrylates in films have been studied by the circulating volumetric method. It was found that the process is autocatalytic and that the kinetics are dependent on film thickness, the partial pressure of oxygen and the degree of reaction. It is shown that during the course of polymerization a transition takes place from kinetic to diffusion control of the reaction rate. Ideas about the effect of the consistency of the medium on the chain propagation and termination reactions proved to be inadequate for interpretation of the relationships found. For this reason the model of layer-by-layer progress of oxidative polymerization in the film, taking into account the concentration gradient of oxygen through the thickness of the latter, is examined in detail.

Cationic polymerization of glycolide in the presence of antimony trifluoride

The polymerization of glycolide in a melt containing SbF 3 proceeds to 100% conversion in the temperature interval 160–175° and the number of active centres increases with time. The polymerization process features reactions of chain propagation and acyl ion binding by carbonyl groups of the monomer and polymer molecules, as normally occurs with lactones, but an additional feature of this process is the dissociation of ion pairs to free ions. The initial stages of polymerization involve relatively slow initiation with the participation of two initiator molecules. In the light of data on the viscosity of the polyglycolide in the melt the “living” character of the growing chain ends is postulated. A formula is derived which relates the viscosity of polyglycolide in the melt to number-average molecular mass.

Termination of propagation reaction in steric networks

The theory of branching processes is used to describe the gel formation during a steric polycondensation and also takes into account the termination of the chain propagation reaction for the steric network.

Influence of high pressure on the reaction of radical substitution

1. The volume effect of activation in radical reactions of chain transfer through CCl4 depends little on. the structure of the radical and is close to the corresponding effect in the reaction of chain propagation. 2. Substantial changes in the volume effect of activation of the reaction of stripping of a hydrogen atom by a free radical are associated chiefly with the difference in the lengthening of the C-H bond in the formation of the transition state. Calculation showed that the stretching of the C-H bonds may reach a value of the order of 80% of its normal length.

The type of reaction centres for polymerization of cyclic acetals

The conclusion that carboxonium ions are the reactive centres for polymerization of cyclic acetals hasbeen reached on the basis of our own and literature data. A modified mechanism of the chain propagation reaction, S N 1 is suggested and is based on a synchronous carboxonium ion formation as the main characteristic of the transition state.

Kinetics of the adiabatic polymerization of methyl methacrylate

The kinetics of the adiabatic polymerization of methyl methacrylate have been investigated. The main equation derived on the basis of theoretical analysis correlate rates of change in temperature, in the induction period, and in the maximum rate with time and with the initial conditions of the polymerization process. The rate constants and the activation energies were calculated for the reactions of initiation and chain propagation.

Polymerization kinetics of butadiene in the presence of secondary butyl-lithium

The polymerization kinetics of butadiene initiated by sec.butyl-lithium have been studied in detail by dilatometry in heptane, cyclohexane, and toluene in the temperature range 20–60°C, a vacuum method being used and the walls of the apparatus employed being rinsed with a solution of the organo-lithium catalyst. The ways in which the polymerization rate and the molecular weights of the polymers depend on the concentration of sec.C 4H 9Li at 20 and 60°C have been found for the three solvents with active site concentrations in the range 1 × 10 −2−1 × 10 −5 mole/l.; the apparent activation energies for chain propagation have been determined. It has been shown that the order of the chain propagation reaction with respect to the catalyst depends not only on the polymerization temperature and the sec.C 4H 9Li concentration, but also on the type of hydrocarbon solvent being used. It is suggested that, during polymerization in aliphatic solvents as distinct from aromatic solvents, the degree of association of the active sites does not correspond to the observed order of the reaction with respect to the catalyst since, in this case, associated forms of the organo-lithium compounds also participate in chain propagation acts in addition to the monomeric forms.

Pre‐exponential factors of rate constants of chain propagation reactions in free radical polymerization

AbstractA method is described for calculating the pre‐exponential factor of the chain propagation rate constants in free radical vinyl polymerizations taking place in the liquid phase. The pre‐exponential factors calculated for some typical monomers are in satisfactory agreement with the experimental values.

Studies in ring‐opening polymerization. II. Cyclohexane spiro‐5‐1,3,2‐dioxathiolan‐4‐one 2‐oxide

AbstractThe effect of spiro cyclohexane substitution on the polymerizability of the 1,3,2‐dioxathiolan‐4‐one‐2‐oxide ring in various solvents has been examined. The steric hindrance of the cyclohexane ring inhibits the bimolecular chain propagation reaction which involves direct attack by a terminal hydroxyl group on the ring and which has been shown to occur in simpler dioxathiolan systems. The conjoined cyclohexane ring does not, however, markedly affect the “thermal” polymerization which occurs in nonhydroxylic solvents and in which chain propagation is thought to involve a reactive α‐lactone intermediate. The rate‐determining step in the sequence of reaction leading to polymer formation is a ring‐scission process in which sulfur dioxide is evolved and the α‐lactone intermediate formed. The values of the activation energy (25–30 kcal/mole) and frequency factor (1011–1013sec−1) associated with this reaction are, therefore, those which govern the the overall polymerization, since the subsequent steps are sensibly instantaneous. In the presence of adventitious traces of water the resultant polymer, poly(1‐hydroxycyclohexanecarboxylic acid) has one carboxyl and one hydroxyl endgroup per chain. Polymers having M̄n ∼ 15,000 are readily obtained; these are amorphous materials, in contrast to the analogous poly‐β‐ester and dialkyl‐substituted poly‐α‐esters which are crystalline. At temperatures in excess of 120°C a competitive first‐order fragmentation reaction leading to the formation of cyclohexanone, carbon monoxide, and sulfur dioxide was observed. Kinetic studies demonstrated that this reaction, which is characterized by an activation energy of ∼40 kcal/mole is unimportant, in the sense that it does not interfere with polymer formation at temperatures below 100°C.

Mechanism of the reaction of chain propagation in processes of liquid-phase oxidation in the presence of copper stearate

1. The rates of formation and consumption of alcohols, ketones, and acids in the oxidation of n-decane catalyzed by copper stearate were measured. A comparison of the rates of formation of the oxidation products with the rate of consumption of hydroperoxide indicated that the bulk of the alcohols, ketones, and acids are formed directly from the peroxide radical, bypassing the state of hydroperoxide formation. 2. In the catalysis of CuSt2, part of the peroxide radicals are converted to oxidation products with cleavage of the C-C bond before the stage of acid formation.