Relationship between the autoacceleration of polymerization rate and conversion in radical polymerization

By using a simple treatment for the kinetics of radical polymerization with primary radical termination, the ratio k_ty/k_tx of chain termination rate constant k_ty at conversion y to that k_tx at conversion x and the ratio k_tiy/k_tix of the primary radical termination rate constant k_tiy at conversion y to k_tix at conversion x were calculated for the polymerizations of methyl methacrylate and ethyl acrylate in the conversion range 0 to 0.4. k_ty/k_tx and k_tiy/k_tix were treated by using the following equations based on the variation of conversion: where g(T,y) is the average fractional free volume of radical chain end at conversion y and absolute temperature and β(T) is a function depending on T, and where g_i(T,y) is the average fractional free volume of primary radical at conversion y and T and β_i(T) is a function depending on T. The autoacceleration for the above monomers was successfully interpreted by the above treatment.     

Chromocene catalysts for ethylene polymerization: Scope of the polymerization

Chromocene deposited on silica supports of high surface area forms a highly active catalyst for polymerization of ethylene. Polymerization is believed to occur by a coordinated anionic mechanism previously outlined. The catalyst formation step liberates cyclopentadiene and leads to a new divalent chromium species containing a cyclopentadienyl ligand. The catalyst has a very high chain-transfer response to hydrogen which permits facile preparation of a full range of molecular weights. Catalyst activity increases with an increase in silica dehydration temperature, chromium content on silica, and ethylene reaction pressure. The temperature-activity profile is characterized by a maximum near 60°C, presumably caused by a deactivation mechanism involving silica hydroxyl groups. A value of 72 was estimated for the ethylene–propylene reactivity ratio (r₁). Linear, highly saturated polymers are normally prepared below 100°C. By contrast with other commercial polyethylenes, the chromocene catalyst produces polyethylenes of relatively narrow molecular weight distribution. Above 100°C, unsaturated, branched polymers or oligomers are formed by a simultaneous polymerization–isomerization process.     

Auto-acceleration of radical polymerization rate in polymerization of styrene under the condition of predominant transfer

Styrene was polymerized at 20°C for 252 days in the presence of 2,2′-azobis(isobutyronitrite) azobis(isobutyronitrile) (AIBN). When [AIBN]_σ = 0.268 mmol l⁻¹, the instantaneous weight-average degree of polymerization (140,000) of the polymer formed is independent of conversion x between 0.05 and 0.60. This independence shows predominant transfer to monomer. Auto-acceleration of polymerization starts at about x = 0.02. In order to explain this auto-acceleration, an equation derived previously for the relationship between termination rate, free volume and volume fraction of polymer is applied to the kinetic data obtained under the condition of predominant transfer to monomer. It is concluded that polymer molecules may move by reptation and the mobility of segments decreases with decreasing free volume.     

Stereocontrol in the cationic polymerization of N-vinylcarbazole

The dependence of the steric microstructure of cationically polymerized poly(N-vinylcarbazole) (PVK) upon catalyst, polymerization temperature, and polymerization solvent has been investigated. The effect of polymerization temperature variation was found to be small, whereas the choice of catalyst and polymerization solvent was found to have a strong influence upon the PVK steric microstructure. A correlation was found between the syndiotacticities X_s and the π^* solvent polarities of the polymerization solvents for a given catalyst. A decrease in X_s with increasing π^* solvent polarity was observed using BF₃OEt₂ and AlEt₂Cl catalysts and has been interpreted in terms of propagation via contact ion-pair ring structures reversibly formed between the active end group and a preceding repeating unit. The increase in X_s with increasing π^* solvent polarity observed with several of the catalysts investigated has been interpreted in terms of chain ion pairs whose separation increases with increasing π^* solvent polarity. The influence of the various Lewis acid catalysts upon the steric microstructures of cationically polymerized PVK allowed the following order of nucleophilicity to be established:      

p-Quinones in the radical polymerization of styrene

Russian Chemical Bulletin - The polymerization of styrene in the presence of p-quinones of various structures was studied. The polymerization kinetics and molecular weight characteristics of...     

Stereospecific post-metallocene polymerization catalysts: the example of isospecific styrene polymerization

In the context of developing single-site stereoselective post-metallocene catalysts, the case for isospecific styrene polymerization catalysts based on methylaluminoxane-activated group 4 metal bis(phenolato) complexes is summarized. Ligands derived from the 1,4-dithiabutanediyl-linked bis(phenol)s have been found to induce stereochemical rigidity by the presence of the hemi-labile sulfide donor functions. Isospecific styrene polymerization was achieved using easily accessible catalyst precursors of the type [MX₂(OC₆H₂-^tBu₂-4,6)₂{S(CH₂)₂S}] (M = Ti, Zr, Hf; X = Cl, OⁱPr, CH₂Ph). Activating the dibenzyl titanium complex [Ti(CH₂Ph)₂(OC₆H₂-^tBu₂-4,6)₂{S(CH₂)₂S}] with B(C₆F₅)₃ and AlⁱBu₃, controlled isotactic polymerization became possible at lower temperatures. A remarkable dependence of both the activity and stereoselectivity on the ligand substitution pattern was observed. Analogous precursors with the 1,5-dithiapentanediyl-linked bis(phenolato) ligand gave syndiotactic polystyrene with lower activity.     

Equilibrium polymerization behavior in the cationic single ring‐opening polymerization of bicyclo orthoesters

The synthesis and cationic polymerization of the following bicyclo orthoesters were examined: 4‐ethyl‐2,6,7‐trioxabicyclo[2.2.2]octane, 1,4‐diethyl‐2,6,7‐trioxabicyclo[2.2.2]octane, 4‐ethyl‐1‐phenyl‐2,6,7‐trioxabicyclo[2.2.2]octane, 4‐ethyl‐1‐(4‐methoxyphenyl)‐2,6,7‐trioxabicyclo[2.2.2]octane, and 4‐ethyl‐1‐(4‐nitrophenyl)‐2,6,7‐ trioxabicyclo[2.2.2]octane. All the monomers underwent equilibrium polymerization, which was confirmed by the relationships between the polymerization temperature and monomer conversion. The obtained polymers afforded the original monomers via an acid‐catalyst treatment with a low reagent concentration in CH₂Cl₂ at 20 °C. The equilibrium monomer concentration was constant, regardless of the initial reagent concentration, in both polymerization and depolymerization. The bicyclo orthoesters with a bulky and electron‐withdrawing substituent showed a larger equilibrium monomer concentration. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3159–3167, 2001     

茂钛对乙烯基单体的催化聚合 VIII.二氯二茂钛催化乙烯基单体聚合机理的证明

本文以UV和'HNMR跟踪方法研究了二氯二茂钛(Cp2TiCl2)在二甲亚砜中的光化学反应,用自旋捕获技术来捕获Cp2TiCl2催化丙烯腈单体聚合体系中所生成的自由基。UV和'HNMR跟踪结果表明,Cp2TiCl2逐渐分解并生成环戊二烯,ESR测定表明,用自旋捕获剂PBN和BNO都捕获到了环戊二烯自由基,进一步证明了Cp2TiCl2催化乙烯基单体聚合反应是一种自由基机理。     

Kinetics of radical polymerization—XXXVII. Investigation of molecular-inhibitors in the radical polymerization of acrylonitrile

The effect of molecular inhibitors with different reactivities was studied for the homogeneous (solution) and heterogeneous bulk-polymerization of AN. p-Nitro-acetophenone, which acts as a weak retarder in solution, strongly decreases the accelerating character of the bulk polymerization. Aromatic nitroso-compounds are strong inhibitors in both homogeneous and heterogeneous polymerizations. The length of the inhibition period depends linearly on the inhibitor concentration. The character and kinetics of the polymerization after the inhibition period are not affected by the nitroso compounds. A novel method has been introduced to determine the length of inhibition period for accelerating heterogeneous polymerization. In every studied system, a considerable stoichiometric anomaly was observed, and attributed to the hot radical effect.     

Radiation-induced solid-state polymerization in binary systems. IV. Solid-state polymerization in the glassy phase

The radiation-induced polymerization of glass-forming systems containing monomers has been investigated. It was found that irradiation below the second-order transition temperature T_g of the systems causes no in-source polymerization but causes a rapid postpolymerization on warming above the T_g after initial irradiation below the T_g. The post-polymerization was followed by differential thermal analysis and ESR spectra. It is caused above the T_g by the release of peroxy radicals trapped below the T_g, and its rate is proportional to the irradiation dose to some extent, often is explosively high, and brings about a remarkably large temperature rise by accumulation of polymerization heat. Irradiation above the T_g causes rapid in-source polymerization which is accelerated by the high viscosity of the monomeric system between T_g and T_s (WLF temperature) compared to crystal or ordinary solution polymerization. The temperature dependence of the in-source polymerization of glassy systems shows a peak between the T_g and T_s which may be the result of competing effects of the rate increase by the decreased termination near T_s and the rate decrease by the decreased propagation caused by the diffusion prevented near the T_g. The degree of polymerization was also investigated. The temperature dependence of the degree of polymerization of the polymers obtained by in-source polymerization shows a peak similar to that of the temperature dependence of conversion. Unusually large values of the Huggins constant k' are noted between T_g and T_s. The degree of polymerization of the polymer obtained by post-polymerized increases with the increase of irradiation dose and the polymerization rate; this may be the result of decreased chain transfer to nonpolymerizable components.     

Salt effect in the base-catalyzed polymerization of unsaturated amide compounds. III. Nature of the polymerization system

Coordination complexes of lithium chloride with polar solvents and monomers were isolated, and their physical properties were studied. The parallel between stabilities of isolated complexes and coordination function in the polymerization system is discussed. The increase of the Q and e values of p-vinylbenzamide (VBA) supports the mechanism of vinyl-type polymerization of VBA in the presence of the salt. The specific solvent effect of dimethyl sulfoxide was determined by measurement of electrical conductivity of a model system.     

Kinetics of radical polymerization—XL. Investigation of the effect of molecule inhibitors in polymerization in solution

The effect of monomer concentration on the stoichiometric coefficients of nitroso-type molecule-inhibitors was studied for solution polymerizations of AN, MA and St (the solvents were DMF and Bz). The anomalies found in the dependence on the monomer concentration was interpreted; the μ = f(x_mon) relationship was quantitatively described with the inhibition parameters determined from our experimental results.     

Advancing from unimechanism polymerization to multimechanism polymerization: binary polymerization

Polymerizations with multiple mechanisms performed simultaneously are promising but very challenging. As the key limitation,the complicated mutual influence between different mechanisms can be hardly defined and measured. Herein we establish a universal framework for the assessment of mutual influence between different mechanisms using binary polymerization for demonstration. The kinetics and thermodynamics of polymerization with two mechanisms are compared with the corresponding homopolymerization and the difference is expressed by a hybrid function. The hybrid function is composed of a hybrid parameter that describes the extent of mutual influence and a function that describes necessary conditions for mutual influence to occur. The extent of mutual influence can be calculated using kinetic and thermodynamic data without details of reaction mechanisms, for the first time providing a straightforward method to assess the mutual influence between different polymerization mechanisms.We envision that the method has potential in more complex systems with multiple mechanisms/monomers with mutual influence.     

Single-electron and two-electron transfer in the anionic polymerization of vinyl monomers and the ring-opening polymerization of lactones*

Single-electron-transfer (SET) and two-electron-transfer reactions and their mechanisms were examined in the anionic polymerization of vinyl monomers and in the ring-opening polymerization of lactones. SET resulted in the formation of radical anions or enolates at the initiation step of styrene or lactone polymerization with naphthalene sodium as a catalyst. However, alkali-metal supramolecular complexes such as M⁺crown–M⁻ (M = Na or K) were able to transfer two electrons to both these monomers to form carbanions as reactive intermediates. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2158–2165, 2002