Propagation kinetics in free-radical polymerizations

The combination of pulsed laser initiated polymerizations (PLP) with analysis of the generated polymer by size-exclusion chromatography (SEC) yields reliable individual rate coefficients for polymerizations of a large number of monomers in bulk and in solution. PLP-SEC experiments carried out in the presence of scCO₂ as a solvent show no unambiguous trend: while a significant reduction of k_p is seen for some monomers, e.g. acrylates, k_p for monomers such as vinyl acetate and styrene is not affected. It is suggested that the influence of CO₂ on acrylate k_p is not a true kinetic effect and that the experimental findings may be understood in terms of the occurrence of local monomer concentrations in the vicinity of the propagating radical. It is discussed that such local monomer concentrations may also contribute to a better understanding of why k_p increases with ester size within the acrylate or within the methacrylate family, and why k_p frequently is influenced by the initiating laser pulse repetition rate.     

Case studies with mathematical modeling of free-radical multi-component bulk/solution polymerizations: Part 2

In Part 2 of this series of two extensive overviews of multi-component polymerization case studies, we again present mathematical modeling results with experimental confirmations. Part 2 represents a refinement and expansion of the detailed and extensive mathematical model presented in Part 1 for free-radical, bulk and/or solution multi-component polymerizations. The expansion is mainly with respect to depropagation, thus making the model more fluent at elevated polymerization temperatures and, in parallel, with additional features as backbiting (with systems involving butyl acrylate). The model considers up to six monomers (unique in the literature), for either batch or semi-batch reactor modes. As the simulator database contains several monomers, initiators, solvents, chain transfer agents and inhibitors, all tested over a wide range of polymerization conditions, from data in both academic and industrial laboratories, several hundred combinations of ingredients can be modeled. The many outputs generated by the model include conversion, molecular weight, polymer composition, branching indicators, sequence length, as well as many other polymerization characteristics related to both production rate and polymer quality. Although the only literature data found to date contain a maximum of four monomers, model predictions for homo-, co-, ter- and tetra-polymerizations show reasonable agreement against the data at both regular and elevated temperatures. With these expansions, this model is directed towards becoming a complete free-radical polymerization tool for training and educational uses both in industry and academia.     

Applications of limiting conversion free-radical polymerizations. II. Azobisisobutyronitrile-initiated polymerization of methyl methacrylate in benzene

Data for the azobisisobutyronitrile-initiated polymerization of methyl methacrylate in dilute benzene solution is presented at 80, 100, and 126°C. in terms of Tobolsky's limiting conversion kinetics. Number-average molecular weights were obtained viscometrically. Attempts to evaluate the relative amounts of combination and disproportionation in the chain-terminating reaction were foiled by the discovery that under the conditions reported the initiator efficiency is temperature-dependent decreasing markedly with increasing temperature.     

Applications of limiting conversion free-radical polymerizations. I. tert-butyl hydroperoxide-initiated polymerization of styrene in benzene

Data for the tert-butyl hydroperoxide-initiated polymerization of styrene in dilute benzene solution at 126°C. are presented. These data are discussed in terms of Tobolsky's limiting conversion kinetics. Several rate laws are considered. Simple unimolecular decomposition of the peroxide appears to be unimportant under these conditions, as does solvent interaction.     

Ab initio study of free-radical polymerizations: cost-effective methods to determine the reaction rates.

The addition of carbon-centered radicals to ethene, which are important in free-radical polymerization processes, are studied from a theoretical point of view. Experimental data for the rate constants are only available for the addition of methyl, ethyl, propyl and butyl radicals. The latter reactions are taken as model systems to derive a cost-effective method for the addition of alkyl radicals to ethene. The proposed model must be accurate and computationally feasible for additions in which larger radicals are involved. Accuracy is validated by direct comparison of theoretical and experimental rate constants in the temperature range from 300 to 600 K. A variety of electronic-structure methods were tested ranging from Hartree-Fock and post-Hartree-Fock methods to pure and hybrid density functional theory methods. Molecular partition functions were refined by treating large amplitude vibrations beyond the harmonic oscillator approximation.     

Kinetic studies of free-radical polymerizations of I-vinylimidazole initiated by benzoyl peroxide and azoisobutyronitrile

Kinetic features of radical polymerizations of 1-vinylimidazole initiated by azoisobutyronitrile and by benzoyl peroxide have been investigated in several solvents and at several temperatures. Polymerizations initiated both by the azo compound and by the peroxide have abnormally low monomer exponents and abnormally high initiator exponents, features first noted by Bamford and Schofield and attributed by them to degradative addition of monomer. Polymerizations initiated by benzoyl peroxide appear to be further complicated by induced decomposition of the initiator; conversion vs time curves, especially for polymerizations in dimethylformamide and particularly at 80°C, show pronounced curvature. These curves can be approximately interpreted in terms of the Tobolsky theory of “dead-end” radical polymerization.     

Liquid crystalline block copolymers by sequential cationic or promoted cationic and free-radical polymerizations

The scope for the study of the synthesis and properties of liquid crystalline (LC) block copolymers is briefly outlined. While there are many approaches to the synthesis of LC block copolymers, the use of azo macroinitiators is very versatile and allows one to produce diverse block copolymer architectures. Azo macroinitiators are prepared by cationic or promoted cationic polymerization of tetrahydrofuran (1) or cyclohexene oxide (2), and are then used to initiate the free-radical polymerization of various methacrylates 3,4 or acrylates 5–9 containing mesogenic azobenzene or biphenyl units thereby yielding block copolymers. The AB or ABA block copolymers are microphase-separated and form smectic and/or nematic mesophases similar to the respective LC homopolymers.     

Thermal gas-phase decomposition of chloroethylenes. II. Vinyl chloride

The thermal gas-phase decomposition of vinyl chloride has been studied behind shock waves over the temperature range of 1350-1900°K and the density range of 7 × 10^?7-1.5 × 10^?3 mol/cm³ (at 1600°K) in mixtures of C₂H₃Cl highly diluted with argon. The ultraviolet absorption of C₂H₃C was recorded at 230 nm as a function of time. The decomposition proceeds via molecular elimination of HCl. The unimolecular dissociation rate is pressure dependent at all but the highest pressures applied. Application of modified HKRR theory results in the rate expression for the limiting high pressure rate constant, and in a collision efficiency of derived from the limiting low-pressure rate constant.     

Mesomeric Vinyl Cations. Part III. Stereochemistry of cations derived from 2-bromo-1, 3-dienes

In contrast to their acyclic analogues, 2-bromocyclohexa-1, 3-diene ( 11a ) and 3-bromo-1, 5-dimethyl-cyclohexa-1, 3-diene ( 11b ) do not react with silver ion, nor do they undergo solvolysis in 80% ethanol at temperatures up to 180°. The inertness to ionization of these cyclic bromodienes is attributed to steric inhibition of mesomerism involving the π electrons of the second double bond in the planar vinyl cation 12 and to the bent structure of this hypothetical intermediate.     

Vinyl polymerization by metal complexes. III. Vinyl polymerization initiated by oligoamide and cupric ion system

Polymerization of methyl methacrylate and other vinyl monomers was studied in the presence of oligoamide (?-aminocaproic acid, its dimer, trimer, tetramer, and pentamer) and cupric ion in aqueous media. The polymerization was found to be of free-radical character and selective for the type of vinyl monomer. Carbon tetrachloride can accelerate the polymerization. The initiation mechanism of the polymerization is discussed. Spectroscopic measurements were indicative of formation of 1:1 complex between oligoamides and cupric ion in aqueous NaClO₄ solution. Some chemical and physical properties of the resulting polymers were measured.     

Radiochemical studies of free-radical vinyl polymerizations. VI. Polymerization of methyl methacrylate in the presence of organotin PVC stabilizers

¹⁴C-Azoisobutyronitrile was used to initiate polymerizations of methyl methacrylate in the presence of the organotin compounds: tetrabutyltin, dibutyltin diacetate, dibutyltin dilaurate, dibutyltin di(ethyl mercaptide), dibutyltin di(dodecyl mercaptide), and dibutyltin dichloride. Only dibutyltin dichloride affected rates of polymerization significantly, and this was ascribed to an increase in the velocity constant k_p for the propogation reaction. No evidence was obtained for radical displacement reactions of the polymer radicals with bonds between tin and carbon, oxygen, sulfur, or chlorine. Transfer activity exhibited by the mercaptides was ascribed to traces of thiol impurity, possibly formed during storage. The relevance of these results to the mechanism of stabilization of poly(vinyl chloride) is briefly discussed.     

Vinyl chloride polymers and their use in polymer blends

A short introduction to polymer-polymer miscibility and to the prediction of the miscibility of polymers is given. The four main types of polymer-modified poly(vinyl chloride) (plastification, impact modification, processing aids and heat deflection temperature modification) are explained by examples. The thermal stability of poly(vinyl chloride) in such blends is discussed; the effectivity of tin-stabilizers may be higher in such blends than in pure poly(vinyl chloride).     

Solvent effect on standard electrode potential: Part III. Silver-silver chloride electrode in methanol-water mixtures

From measurements of the electromotive force of the Pt, H₂ (gas, 1 atm); HCl (m), X% methanol, Y% water; AgCl, Ag cells at nine temperatures from 15 to 55°C at 5° intervals, the standard potential of the silver-silver chloride electrode has been determined over a broad range of methanol concentrations (0–90 wt. % methanol). The standard molal potential in the various solvent mixtures has been expressed as a function of temperature. The primary medium effects of various media on hydrochloric acid, and the standard thermodynamic quantities accompanying the transfer of HCl from water to the respective solvent media have been computed. The results have been discussed both in terms of the acid-base behaviour of the solvent mixtures and also their structural effects on the transfer process.     

Modulation effects in free-radical spin-echo signals

Calculations are presented for some cases typical of free radicals for the modulation of the decay of the echo signal on account of anisotropic hyperfine interactions.     

Nucleophilicity and Basicity in the Quaternization of 4-Substituted Quinuclidines. Polar effects. Part III

The second order rate constants (k) for quaternization of thirty 4-substituted quinuclidines ( 1 ) by methyl iodide in methanol have been measured. The effects of the substituents on rate are relatively small and indicate an early transition state with little charge separation. They also indicate less involvement of the nitrogen atom, i.e. a looser transition state, for quaternization by allyl halides than for the less reactive propyl halides. The log k values of 4-substituted quinuclidines correlate moderately well with their pK_a values, the main reason being the imperfect correlation between nucleophilicity and basicity. The assumption that substituents exert equal effects on rates and equilibria is therefore only partly justified. Certain irregularities in the correlation can be attributed to a reduction of the nucleophilicity of nitrogen in 1 due to hydrogen bonding to the solvent methanol. Furthermore, quaternization is faster and responds more strongly to substituent effects in the aprotic solvent acetonitrile.     

Pulsed initiation polymerization as a means of obtaining propagation rate coefficients in free-radical polymerizations. II Review up to 2000

The application of the pulsed initiation polymerization technique (PIP) to determine propagation rate coefficients has evolved into a valuable tool. The number of monomers studied has rapidly increased since 1987. Besides homopropagation rate coefficients also copolymerizations are studied extensively. The calibration of the size exclusion chromatography is still a limiting factor in studying delicate differences in reactivities, for example in families of monomers like that of the methacrylates. In studying solvent effects in propagation, PIP is capable of disclosing even small solvent effects, typically in the order of magnitude of plus or minus 20%. For some families of monomers PIP does not result in reliable parameters. The reasons for the breakdown of the PIP technique with the acrylates are discussed.     

Semicontinuous Emulsion Polymerization of Vinyl Acetate. Part I. Homopolymerization with Poly-(Vinyl Alcohol) and Nonionic Coemulsifier

The semicontinuous emulsion polymerization of vinyl acetate has been studied. Poly(vinyl alcohol) as a protective colloid and ethoxylated cetyl alcohol as a coemulsifier were used. The conversion and particle diameter were affected by the stirring speed and the coemulsifier distribution between initial reactor charge and continuously introduced monomer. The amount of unreacted monomer oscillates with time.     

Phosphorus Chemistry. III Preparation of Vinyl Deuterated Trisubstituted Alkenes

A facile procedure for the preparation of vinyl deuterated trisubstituted alkenes via reduction of the corresponding (2-oxoalkyl)triphenyl-phosphonium halides with sodium borodeuteride is described.     

Magnesium chloride supported high-mileage catalysts for olefin polymerizations. XVIII. Effect of hydrogen and lewis bases

Hydrogen has been found earlier to increase the initial rate of polymerization by MgCl₂/EB/PC/AlEt₃/TiCl₄-AlEt₃/MPT, CW-catalyst (+B_i, +B_e) (EB, ethyl benzoate; PC, p-cresol; MPT, methyl-p-toluate), but decays more rapidly as compared to polymerizations in the absence of H₂. In this study the effect of H₂ was studied when either the internal Lewis base, EB B_i, or the external Lewis base, MPT B_e, or both are deleted from the CW-catalyst. H₂ does not affect the stereospecificity of all the catalysts, but causes a slight increase of polymer yield, whereas the yield is virtually unchanged by H₂ for the catalysts activated with B_e. Unlike the catalyst (+B_i, +B_e) where H₂ increases active site concentrations [Ti^*] about threefold, it affects [Ti^*] negligibly when B_e is absent. The rate constants of propagation is about the same with or without H₂ for the CW-catalyst (+B_i, –B_e) or (–B_i, –B_e); the same statement can be said about the rate constant of chain transfer with AlEt₃ or with H₂. Hydrogen increases the rate of catalyst site deactivation for the various catalysts in the order of(+B_i, +B_e) > (–B_i, –B_e) > (+B_i, –B_e).