Polymers from non‐homopolymerizable monomers

There are many inorganic and organic compounds known which are not able to homopolymerize either with well‐known polymerizable monomers or even with other non‐homopolymerizable compounds. The participation of non‐homopolymerizable comonomers with reactivity ratios close to 0 results in copolymers with more or less alternating structure, whereas for a strictly alternating copolymer, both reactivity ratios must be 0. Binary copolymerizations of non‐homopolymerizable and homopolymerizable monomers can give information on the topochemistry, and also on the kinetics of such processes, as in these cases the number of propagating steps is remarkably reduced. Up to now, very little is known on the terpolymerization of three non‐homopolymerizable comonomers. Experimental investigations have shown that only combinations of two monomers with electron donor and one monomer with electron acceptor properties or vice versa yield terpolymers, whereas from three monomers of similar electronic behavior, no terpolymers are obtained. All such terpolymers are of alternating structure where a donor unit is succeeded by an acceptor unit. For copolymerizations of two or three non‐homopolymerizable monomers, two different mechanisms must be considered: the so‐called complex model postulates the incorporation of donor‐acceptor complexes of the monomers into the growing chain, whereas with the terminal or penultimate model the addition of free monomers to growing macroradicals is described. Measurements of the rate of polymerization in combination with determinations of the complex constants of the involved donor and acceptor monomer pairs together with a new kinetic scheme allow us to distinguish between the simultaneous participation of free monomers and complexes in the polymerization process.     

“Charge transfer” polymerization—and the absence thereof!

Mechanisms for “charge‐transfer” spontaneous polymerizations and cycloadditions between electron‐rich olefins and electron‐poor olefins were reviewed. As for propagation, literature proposals involving charge‐transfer complexes were rejected. Instead, alternating copolymerization is ascribed to polar effects in free‐radical reactions. As for spontaneous initiation, literature proposals involving charge‐transfer complexes, with or without proton transfer, were rejected. Instead, the initiating species is postulated to be a tetramethylene zwitterion biradical, which may initiate either ionic homopolymerization or free‐radical copolymerization. A new hypothesis proposes that any interaction that brings vinyl monomers close together may facilitate tetramethylene formation and spontaneous polymerization. These interactions include Coulombic, acid–base, hydrophobic–hydrophilic and templating–tethering interactions. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2069–2077, 2001     

Polymerization behavior of 2,5‐bis(dicyanomethylene)‐ 2,5‐dihydrofuran

2,5‐Bis(dicyanomethylene)‐2,5‐dihydrofuran (TCNF) is not homopolymerizable with any initiators, but copolymerizable with styrene (St) in an alternating fashion. Reactivity of TCNF was compared with that of 2,5‐bis(dicyanomethylene)‐2,5‐dihydrothiophene (TCNT) on the basis of the terpolymerization of the TCNT‐TCNF‐St system and the rates of addition reactions of AIBN with TCNT and with TCNF. TCNF was found to be lower in reactivity than TCNT. The relative reactivity was explained with the energy difference between quinonoid structure and benzenoid one. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1285–1292, 1999     

Copolymerization of vinyl acetate with ethyl α-cyanocinnamate

Trisubstituted ethylene, ethyl α-cyanocinnamate, is readily copolymerized with vinyl acetate by a conventional radical initiator. Terminal, penultimate, and “complex” copolymerization models were applied by using the data of composition of the copolymers obtained in bulk and by copolymerization in benzene, ethyl acetate, and chloroform. The model based on the participation of the monomer complexes describes satisfactorily the deviation from the terminal copolymerization model. The proton NMR analyses of the monomer mixtures indicate that the interaction between the monomers leads to the formation of weak monomer complexes. Kinetic studies of the initial rate dependence on the total monomer concentration and monomer feed composition enabled us to evaluate the degree of participation of the free uncomplexed monomers and the monomer complex in the propagation reactions. The contribution of the complexed monomers in the propagation stages increases with the increase in total monomer concentration. The initial rate of the copolymerization is proportional to the square root of the initiator concentration, thus confirming the bimolecular termination of the macrochains. The rate constants of the addition reactions of the complex and free monomers were evaluated from the kinetic studies. The quantitative kinetic treatment provided information regarding the relative weight of the termination reaction and indicated that the termination in the system occurs predominantly by the cross-termination reaction between two growing polymer radicals with different kinds of monomer units at the ends. Additional information on the termination in this system was obtained from viscosity measurements.     

New consideration of the mechanism of chain transfer to monomer in anionic polymerization

A new kinetic scheme for chain transfer to monomer in the anionic polymerization of hydrocarbon monomers is presented. The scheme agrees with generally accepted views on the chemical mechanism of carbanionic reactions better than the one used previously. It is suggested that the most probable path of the transfer reaction is the proton abstraction from the side group of the monomer, the terminal double bond of the monomer molecule remains unchanged, and therefore the intermediate species can participate in succeeding reactions as a macromonomer. The discrepancy between the predictions of the proposed scheme and of the previous one concerning the molar characteristics of polymers are discussed and the ways to establish the true mechanism of transfer in particular systems are suggested.     

Mechanism of anionic (ionic) polymerization of acetylene monomers

Polymerization of phenylacetylene in presence of butyllithium has been studied. It is shown that the process of polymerization is accompanied by the appearance and growth of the ESR signal. The model of anionic polymerization is suggested according to which formation of low-molecular-weight polyphenylacetylene is caused by the destruction of the active centers by means of electron transfer from active center to the conjugated chain. The assumption is made that electron transfer from conjugated chain to an active center may also be a fundamental reaction limiting the chain length in cationic polymerization of acetylene monomers. The kinetic scheme of ionic polymerization of acetylene monomers with consideration of electron transfer is analyzed and molecular weight distribution functions are obtained with good agreement between the calculated parameters and experiment. It is shown that one of the ways of obtaining high-molecular-weight polyacetylenes in ionic polymerization is the formation of donor-acceptor complexes with a polyene chain in the process of the chain growth. The formation of complexes exchanges parameters of electron structure of the polyene chain and decreases the rate of the electron transfer reactions.     

Simulation of Non‐Linear Free‐Radical Polymerization Using a Percolation Kinetic Gelation Model

A new kinetic gelation model that incorporates the kinetics of representative non‐linear free‐radical polymerization is presented. Specifically, free‐radical homopolymerization, polymerization in the presence of a chain‐transfer agent (CTA, CTA‐induced polymerization), and copolymerization of a mixture of the bi‐ and tetrafunctional monomer is used to simulate kinetic effects on polymerization statistics and microstructures. An algorithm for random next‐step selection in a self‐avoiding random walk and efficient mechanisms of a component's mobility are introduced to improve the generality of the predictions by removing commonly occurring deficiencies due to early trapping of radicals. The model has the capability to take into account into several free‐radical polymerization mechanisms such as crosslinking, branching, and transfer reaction, and also to predict the onset of the sol–gel transition, and the effect of chemical composition on the transition point. It is shown that a better understanding of microstructure evolution during polymerization and chemical gelation is attained. Lastly, one important benefit of the model is to simulate very highly packed random chains or microgels within a polymer network.     

Identifying similarities and differences between analogous bisdithiolene and bisdiselenolene complexes: A computational study

Due to ligand non‐innocence and reversible one‐electron‐transfer processes dithiolene complexes have been intensively studied both experimentally and computationally. While the substitution of the ligating sulfur atoms by selenium provides a means to delicately tune the behavior of dithiolene compounds, diselenolene complexes have not been as thoroughly examined. Yet, the search for such ligands has been ongoing since the 1970s. Thus, we have looked at several metal‐bisdiselenolene complexes and have compared key properties of these complexes with their bisdithiolene analogues to determine the effect of substituting the chalcogen atom. The results herein show that substitution of the sulfur atoms by selenium within these complexes only subtly changes the thermodynamics and kinetic reactivity of bisdithiolene complexes while not significantly affecting the geometries of the complexes. The significance being that the relatively minor structural changes that occur upon redox is a key feature of dithiolene complexes. Due to ligand non‐innocence and reversible one‐electron‐transfer processes dithiolene complexes have been intensively studied, however, diselenolene complexes have not. First‐principles calculations show that substitution of the sulfur atoms by selenium within the investigated complexes does offer the ability to subtly tune the thermodynamics and kinetic reactivity of bisdithiolene complexes, while not significantly affecting the geometries of the complexes. © 2015 Wiley Periodicals, Inc.     

Mechanistic Insights into RNA Transphosphorylation from Kinetic Isotope Effects and Linear Free Energy Relationships of Model Reactions

Phosphoryl transfer reactions are ubiquitous in biology and the understanding of the mechanisms whereby these reactions are catalyzed by protein and RNA enzymes is central to reveal design principles for new therapeutics. Two of the most powerful experimental probes of chemical mechanism involve the analysis of linear free energy relations (LFERs) and the measurement of kinetic isotope effects (KIEs). These experimental data report directly on differences in bonding between the ground state and the rate‐controlling transition state, which is the most critical point along the reaction free energy pathway. However, interpretation of LFER and KIE data in terms of transition‐state structure and bonding optimally requires the use of theoretical models. In this work, we apply density‐functional calculations to determine KIEs for a series of phosphoryl transfer reactions of direct relevance to the 2′‐O‐transphosphorylation that leads to cleavage of the phosphodiester backbone of RNA. We first examine a well‐studied series of phosphate and phosphorothioate mono‐, di‐ and triesters that are useful as mechanistic probes and for which KIEs have been measured. Close agreement is demonstrated between the calculated and measured KIEs, establishing the reliability of our quantum model calculations. Next, we examine a series of RNA transesterification model reactions with a wide range of leaving groups in order to provide a direct connection between observed Brønsted coefficients and KIEs with the structure and bonding in the transition state. These relations can be used for prediction or to aid in the interpretation of experimental data for similar non‐enzymatic and enzymatic reactions. Finally, we apply these relations to RNA phosphoryl transfer catalyzed by ribonuclease A, and demonstrate the reaction coordinate–KIE correlation is reasonably preserved. A prediction of the secondary deuterium KIE in this reaction is also provided. These results demonstrate the utility of building up knowledge of mechanism through the systematic study of model systems to provide insight into more complex biological systems such as phosphoryl transfer enzymes and ribozymes.     

Living Nitroxide‐Mediated Radical Terpolymerization: General Concept and Synthetic Possibilities

The general concept for nitroxide‐mediated radical terpolymerization is advanced. This concept is based on activation‐deactivation equilibria for terminal polymer‐nitroxide adducts. Depending on monomer activity and the stability of terminal nitroxide adducts, terpolymerization can be equilibrium living, quasi‐equilibrium (gradient) living, decaying living, decaying gradient, or non‐living. Expressions for the effective activation‐deactivation equilibrium constant, K_ef, and the rate of terpolymerization are derived from theoretical speculations on equilibrium living and decaying living terpolymerization. For quasi‐equilibrium living terpolymerization, various types of gradient terpolymers are predicted. When activity of the active monomer M₁ is, at least, one order of magnitude different from that of the two other monomers, the effective constant K_ef is shown to approach K₁ of the most active monomer. Experimental kinetic and equilibrium constants agree with the advanced concept for the equilibrium living terpolymerization of styrene with methyl methacrylate, and acrylonitrile in the presence of nitroxide SG1, as well as for decaying living terpolymerization in the same system in the presence of nitroxide TEMPO.

     

Investigation of the radical copolymerization and terpolymerization of maleic anhydride and norbornenes by an in situ 1H NMR analysis of kinetics and by the mercury method: Evidence for the lack of charge‐transfer‐complex propagation

The radical copolymerization of electron‐deficient maleic anhydride (MA) and electron‐rich norbornene (NB) derivatives with 2,2′‐azobis(isobutyronitrile) (AIBN) in dioxane‐d₈ has been monitored in situ by ¹H NMR spectroscopy with free induction decays recorded every 30 min at 60, 70, or 84 °C. The ratios of the monomer pairs were varied in some cases. The NB derivatives employed in this study included bicyclo[2.2.1]hept‐2‐ene (NB), t‐butyl 5‐norbornene‐2‐carboxylate, methyl 5‐norbornene‐2‐methyl‐2‐carboxylate, and ethyl tetracyclo[4.4.0.1^2,5.1^7,10]dodec‐3‐ene‐8‐carboxylate. Decomposition of AIBN, consumption of the monomers, feed ratios, endo/exo ratios, copolymer compositions, and copolymer yields were studied as a function of polymerization time. Furthermore, a homopolymerizable third monomer (t‐butyl methacrylate, methacrylic acid, t‐butyl acrylate, or acrylic acid) was added to the NB/MA 1/1 system, revealing that the methacrylic monomer polymerizes rapidly in the early stage and that the ratio of MA to NB in the terpolymer strongly deviates from 1/1. In contrast, however, the acrylic monomers are more uniformly incorporated into the polymer. Nevertheless, these studies indicate that MA and NB do not always behave as a pair in radical polymerization and disproves the commonly believed charge‐transfer mechanism. Electron‐deficient fumaronitrile was also included in the kinetics study. To further understand the copolymerization mechanism, MA and NB were competitively reacted with a cyclohexyl radical generated by the treatment of cyclohexylmercuric chloride with sodium borohydride (mercury method). A gas chromatographic analysis of the reaction mixtures has revealed that a cyclohexyl radical reacts with MA almost exclusively in competition and that the cyclohexyl adduct of MA essentially accounts for all the products in a mass balance experiment, eliminating a possibility of the formation of an adduct involving the MA–NB charge‐transfer complex. Thus, the participation of a charge‐transfer complex in the copolymerization of MA and NB cannot be important. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3521–3542, 2000     

Modelling the molecular weight distribution in terpolymerization systems with donor–acceptor complexes

In this work we present, for the first time, a mathematical development of the moments of the molecular weight distribution in terpolymerization systems where donor–acceptor complexes are formed and the propagating reactions are carried out according to the complex participation model. The resulting set of equations is applied to the system formed by vinyl chloride (VC), vinyl acetate (VAc) and maleic anhydride (MA), in order to show the use of the equations and the type of information that might be obtained from them.     

Complex-radical terpolymerization of phenanthrene,maleic anhydride,and trans-stilbene

The radical terpolymerization of the donor-acceptor-donor monomer system, phenanthrene (P)—maleic anhydride (M)—trans-stilbene (S), was studied. These monomers are known to be nonhomopolymerizable. The terpolymerization was carried out in p-dioxane and/or toluene at 70°C in the presence of benzoyl peroxide used as the initiator. P and S were found to form charge transfer complexes (CTC) with M in p-dioxane at 35°C. The results obtained are discussed in terms of the free monomer and complex propagation models. It is shown that terpolymerization is carried out at a stage close to binary copolymerization of two complexomers. The reactivity ratio of P … M and S … M complexes was estimated by the Kelen-Tüdös method. Absorbance ratios at 1770 cm^?1 (v_C=0 of anhydride group), 764 cm^?1 (δ_CH in monosubstituted benzene of S), and 820 cm^?1 (δ_CH in disubstituted benzene of P) as a function of terpolymer composition were established. P—M—S terpolymers are shown to have high thermal stabilities. © 1995 John Wiley & Sons, Inc.     

Complexed copolymerization of vinyl compounds with alkylaluminum halides

The monomer reactivity in the complexed copolymerization of vinyl compounds with alkylaluminum halides has been extensively surveyed. Equimolar copolymers were obtained in various combinations of monomers which are classified into two monomer groups, A and B. The group B monomers are conjugated vinyl compounds having nitrile or carbonyl groups in the conjugated position and form complexes with alkylaluminum halides. The group A monomers are donor monomers having low values, such as olefins, haloolefins, dienes, and unsaturated esters. These A monomers belong to the same group of monomers which give alternating copolymers in conventional radical copolymerization with maleic anhydride, SO₂, and so on. In addition the complexed copolymerization has the same specific characteristics as the conventional alternating copolymerization, i.e., high reactivities of allyl-resonance monomers and inner olefins and no transfer of halogen atom to the copolymers in CCl₄. These features suggest little or no participation of the A monomer radical. The Q-e scheme is also discussed in terms of the monomer reactivity. More than two monomers selected from groups A and B give multicomponent copolymers in which alternating sequential structures hold with respect to A and B. Anomalous mutual reactivities between two B monomers in the terpolymerization were observed and indicate that the nature of radical in the complexed copolymerization may be different from that expected by the Lewis-Mayo equation. The complexed radical mechanism previously proposed is discussed in connection with the specific behavior mentioned above.     

Initiation of cationic polymerization by photoinduced electron transfer

The objective of this paper is to discuss: (i) the general approaches to the initiation of cationic polymerization by photinduced electron transfer reactions (ii) the use of photoinduced electron transfer reactions for block copolymer synthesis. For the first, it is concluded that three general methods are currently available which involve reduction of onium salts by (a) photogenerated radicals, (b) photoexcited sensitizers or (c) electron donor compounds in charge transfer complexes. According to this view, a variety of initiating systems are discussed. For the second, recent developments on the application of photoinduced electron transfer reactions to the synthesis of block copolymer of monomers polymerizable with different mechanisms are presented.     

Depolymerization of Free‐Radical Polymers with Spin Migrations

The mechanism of depolymerization is one of the most essential issues in chemical engineering and materials science. In this work, we investigate the depolymerization reactions of three typical free‐radical poly(alpha‐methylstyrene) tetramers by using first‐principles density functional theory. The calculated results show that these reactions all need to overcome the energy barriers in the range of 0.58 to 0.77 eV, and that breaking the C?C bond at the chain end leads to the dissociation of alpha‐methylstyrene monomers from the polymers. Electronic‐structure analysis indicates that the reactions occur easily at the CR₃ unsaturated end, and that the frontier molecular orbitals that participate in the reactions are mainly localized at the unsaturated ends. Meanwhile, spin population analysis presents the unique net spin‐transfer process in free‐radical depolymerization reactions. We hope the current findings can contribute to understanding the free‐radical depolymerization mechanism and help guide future experiments.     

Time‐dependent Density Functional Theory Study on the Hydrogen‐bonded Dimers Formed by Gauche‐1PA and Trans‐1PA

Three hydrogen bonding complexes of the gauche‐1PA dimer (GG), trans‐1PA dimer (TT) and mixed dimer (GT) have been calculated for the geometry conformations and excited‐state energies. The electron distribution at the site of C‐O of H‐donor moiety in HOMO transfers to the direction of O‐H of H‐acceptor moiety in LUMO. The hydrogen bond between two 1PAs is the bridge of the intermolecular charge transfer. By the Zhao and Han's excited‐state hydrogen bonding dynamics rule, the first excited‐state hydrogen bonding change has been discussed without optimizing the excited‐state geometry conformations. According to the distinct difference between GT and GG (TT), we concluded that two gauche‐1PA monomers of one dimer are transformed at the same time to two trans‐1PA monomers.     

Modeling CO2 electrolysis in solid oxide electrolysis cell

A modified Butler–Volmer equation for the reduction of CO₂ by considering multi-step single-electron transfer reactions is presented. Exchange current density formulations free from arbitrary order dependency on the partial pressures of reactants and products are proposed for Ni and Pt surfaces. Button cell simulations are performed for Ni-YSZ/YSZ/LSM, Pt-YSZ/YSZ/Pt, and Pt/YSZ/Pt systems using two different electrochemical models, and simulation results are compared against experimental observations. The first electrochemical model considers charge transfer reactions occurring at the interface between the electrode and dense electrolyte, and the second model considers the charge transfer reactions occurring throughout the thickness of the cermet electrode. Single-channel simulations are further performed to asses the O₂ production capacity of CO₂ electrolysis system.     

Biospecific Properties of Random Copolymers, 1

The kinetic study of copolymer synthesis is important to understand how a material is built and how it can get some particular properties. The radical copolymerization of one, two or more monomers can be simulated with classical analytic models, but can also be simulated by a Monte Carlo model that allows a bigger flexibility and is a lot easier to compute and use. We have shown that this Monte Carlo simulation gives the same results of the kinetic study of the terpolymerization as the analytic model, and it can create easily a bank of virtual copolymers for all the compositions needed to analyze the structure of the macromolecular chains in terms of sequences of monomers occurring as a function of the global composition. The search for the sequences is very simple to compute as it consists in a simple reading of the virtual chains previously simulated. This result can thus be applied to make appear a simple correlation between the distribution of functional groups and some specific observed biological activities of biospecific copolymers.

Simplified schema of the numerical virtual terpolymerization.     

Comprehensive method based on model free method and IKP method for evaluating kinetic parameters of solid state reactions

This article presents, firstly, a short review of methods for evaluating kinetic parameters of solid state reactions and a critical analysis of the isoconversional principle of model free methods. It shows theoretically that the activation energy for complex reactions is not only a function of the reaction degree but also of heating programs, and points out that any method that attempts to extract the dependences of activation energy on conversion degree without considering the dependences of heating programs is problematic. Then an analysis is given of the invariant kinetic parameters (IKP) method and recommends an incremental version of it. Based on the incremental IKP method and model free method, a comprehensive method is proposed that predicts the degree of the dependences of activation energy on heating programs, selects reliable values of activation energy and extracts the values of variable pre‐exponential factor. This comprehensive method is tested using both simulation data and experimental data, the results of which show it can not only give reliable values of kinetic parameters but also be helpful in explaining inconsistencies of kinetic results in solid state reactions. © 2012 Wiley Periodicals, Inc.