Modeling stable free-radical polymerization

A kinetic model has been developed for stable free-radical polymerization (SFRP) processes by using the method of moments. This model predicts monomer conversion, number-average molecular weight, and polydispersity of molecular weight distribution. The effects of the concentrations of initiator, stable radical, and monomer, as well as the rate constants of initiation, propagation, termination, transfer, and the equilibrium constant between active and dormant species, are systematically investigated by using this model. It is shown that the ideal living-radical polymerization having a linear relationship between number-average molecular weight and conversion and a polydispersity close to unity is the result of fast initiation, slow propagation, absence of radical termination, and a high level of dormant species. Increasing stable radical concentration helps to reduce polydispersity but also decreases polymerization rate. Thermal initiation significantly broadens molecular weight distribution. Without the formation of dormant species, the model predicts a conventional free-radical polymerization. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2692–2704, 1999     

Modeling of molecular weight development in atom transfer radical polymerization

A kinetic model has been developed for atom transfer radical polymerization processes using the method of moments. This model predicts monomer conversion, number‐average molecular weight and polydispersity of molecular weight distribution. It takes into account the effects of side reactions including bimolecular radical termination and chain transfers. The determining parameters include the ratios of the initiator, catalyst and monomer concentrations, as well as the ratios of the rate constants of propagation, termination, transfer and the equilibrium constant between radicals and their dormant species. The effects of these parameters on polymer chain properties are systematically simulated. The results show that an ideal living radical polymerization exhibiting a linear relationship between number‐average molecular weight versus conversion and polydispersity approaching unity is only achievable under the limiting condition of slow monomer propagation and free of radical termination and transfers. Improving polymerization rate usually accompanies a loss of this linearity and small polydispersity. For polymerization systems having a slow initiation, the dormant species exercise a retention effect on chain growing and tend to narrow the molecular weight distribution. Increasing catalyst concentration accelerates the initiation rate and thus decreases the polydispersities. It is also shown that for a slow initiation system, delaying monomer addition helps to reduce the polydispersities. Radical termination and transfers not only slow down the monomer conversion rates but also broaden polymer molecular weight distributions. Under the limiting conditions of fast propagation and termination and slow initiation, the model predicts the conventional free radical polymerization behaviors.     

The Role of a Metallocene Additive in Radically Initiated Polymerization when Solving Direct and Inverse Kinetic Problems

Polymerization of methyl methacrylate, initiated by benzoyl peroxide in the presence of titanocene dichloride, is considered from the point of view of formal kinetics. Based on the kinetic scheme of the process (which includes the reactions of classical radical polymerization, the reaction of benzoyl peroxide with titanocene dichloride, the reactions of the controlled radical polymerization of organometallic mediated radical polymerization (OMRP) and atom transfer radical polymerization (ATRP), the reaction of the formation of a coordinating active site and the coordinating chain propagation on a mathematical model of the kinetics of the process is created. This model also makes it possible to calculate the molecular-mass characteristics of poly(methyl methacrylate). As a result of the solution of the inverse kinetic problem at a temperature of 343 K, the values of the reaction rate constants of the kinetic scheme are found under which the discrepancy between the calculated models and experimental data is minimal. Using the developed model of the kinetics of the process, a numerical experiment is performed (i.e., a direct kinetic problem is solved). This problem revealed the following regularities of the process. (1) An increase in the initial concentration of titanocene dichloride at a constant initial concentration of benzoyl peroxide leads to an increase in the rate of consumption of benzoyl peroxide but not to an increase in the initial rate of the process compared to classical radical polymerization. (2) With an increase in the initial concentration of titanocene dichloride, the lifetime of the macroradicals at the initial stage of the process is reduced, and hence the molecular weight of the resulting polymethyl methacrylate is less than that of the polymethyl methacrylate obtained in the absence of titanocene dichloride, and it will increase during the process of approaching the final values. (3) During the polymerization of methyl methacrylate, initiated by benzoyl peroxide in the presence of titanocene dichloride, a smoothing gel effect (as in the case of the polymerization of methyl methacrylate initiated by benzoyl peroxide in the presence of ferrocene) does not occur since titanocene dichloride forms stable complexes with methyl methacrylate and, consequently, it participates in reactions consuming macroradicals to a lesser degree than ferrocene.     

Quantum Chemistry: A Powerful Tool in Polymer Reaction Engineering

Summary: The potentials of computational techniques based on quantum mechanics, to support and complement the experimental analysis, are examined. Mechanisms and reaction paths involved in the free radical polymerization of widely used monomers are studied through a computational approach based on Density Functional Theory (DFT). First, the attention is focused on the initiation kinetics in order to evaluate the role of the initiators in the polymerization process. Methyl acrylate, methyl methacrylate, acrylonitrile, and styrene homopolymerization using different initiators are studied. Then, propagation kinetics is investigated. In particular, the propagation kinetic rate constants for different kinds of acrylates, methacrylates and acetates are calculated and compared with experimental data reported in the literature. The same computational approach is applied to the study of secondary reactions (backbiting, beta-scission) occurring during free radical polymerizations. Finally, the same methodologies are applied to copolymer systems, with emphasis on the evaluation of the role of penultimate effect. The copolymers vinyl acetate/methyl methacrylate and styrene/methyl methacrylate are investigated as system characterized by weak and strong penultimate effect, respectively.     

Detailed mechanism of radical high polymerization of sterically hindered dialkyl fumarates

The radical polymerization kinetics and mechanism of sterically hindered dialkyl fumarates (DRF) bearing various ester alkyl groups are described comprehensively. The overall polymerization reactivity of DRF, the initiation mechanism and the reactivity of the primary radicals in the polymerizations with azo initiators, the determination of the propagation and termination rate constants by means of electron spin resonance spectroscopy, the propagation mechanism and the microstructure of the polymers, and the chain rigidity of poly(DRF) and bimolecular termination process are discussed.     

General features and specifics of the kinetics of the pseudoliving radical polymerization of 4-vinylpyridine and styrene mediated by TEMPO

The kinetics of the pseudoliving radical polymerization of 4-vinylpyridine mediated by TEMPO is studied for the first time, and quantitative parameters characterizing the pseudoliving mechanism of the reaction, namely, the rate constant of reinitiation and the product of the equilibrium constant and the propagation-rate constant, are estimated. It is shown that the general kinetic features of the TEMPO-mediated polymerizations of 4-vinylpyridine and styrene (the pattern of kinetic curves and the zero reaction order with respect to the concentration of alkoxyamine) and the distinctive features of the polymerization of 4-vinylpyridine (an abnormally low rate and a high steady-state concentration of free TEMPO) are determined by three main factors: the rate of spontaneous initiation, the rate of self-termination of macroradicals, and the constant of equilibrium between active and dormant chains.     

Kinetic scheme and rate constants for methyl methacrylate synthesis occurring via the radical–coordination mechanism

Two kinetic schemes of the bulk radical–coordination polymerization of methyl methacrylate initiated by the benzoyl peroxide–ferrocene system are considered from the standpoint of formal kinetics. The most likely kinetic scheme is the one that includes the reactions characteristic of classical radical polymerization and, additionally, reactions of controlled radical polymerization proceeding via the Organometallic Mediated Radical Polymerization mechanism, a reaction generating a coordination active site, and a chain propagation reaction in the coordination sphere of the metal. The temperature dependences of the rate constants for the reactions of this kinetic scheme at temperatures typical of commercial poly(methyl methacrylate) production (313–353 K) have been determined by solving the inverse kinetic problem.     

Reactions of Initiation and Reinitiation in Polymerization Mediated by Organoborane–Oxygen Systems

The kinetics and mechanism of initiation and reinitiation reactions in the polymerization of methyl methacrylate mediated by the ammonia–tripropylborane–oxygen and 2-isopropyl-2-boraadamantane–oxygen systems are studied by ESR spectroscopy using C-phenyl-N-tert-butylnitrone and 2-methyl-2-nitrosopropane as spin traps. It is shown that alkyl and alkoxyl radicals are the main initiating radicals and the rate of initiation is directly proportional to the concentration of oxygen. Two mechanisms of radical formation are valid in the postpolymerization of methyl methacrylate at room temperature under vacuum. The first one (which is predominant) is the decomposition of poly(methyl methacrylate)–boroxyl macromolecules, i.e., the reinitiation of polymerization; the second one (additional during the first 30 min of the process) is the decomposition of borane peroxide compounds accumulated during the stage of oxidation.     

New (Co)polymers by atom transfer radical polymerization

The fundamentals of atom transfer radical polymerization (ATRP) are presented. This includes the mechanistic considerations including structure of active and dormant species and structural features of the catalyst and reaction conditions as well as the nature of initiation and propagation steps. Extension of homogeneous polymerization to heterogeneous systems including emulsion polymerization is presented. Synthesis of (co)polymers with predefined molecular weights and low polydispersites as well as with controlled compositions, functionalities and architectures is reviewed.     

Polymerization of Cyclic Ethers

Abstract

In the present series of studies on the cationic polymerization of cyclic ethers, the reactivities of cyclic ethers were quantified and the effect of the catalyst upon the polymerization kinetics was revealed. These kinetic analyses were successfully performed by means of our “phenoxyl end-capping method”. The change of the reactivity by the ring size of the monomer was interestingly demonstrated. In addition, it is emphasized that the frequency factor as well as the activation energy influence the rate constant of propagation. As to the effect of catalyst upon the polymerization kinetics, the most important conclusion is that the rate constant of propagation changes very little according to the changes of the catalyst components. Variation of the conversion rate by a change of catalyst is due to differences in the rates of the initiation and the termination reactions.     

光调控的活性自由基聚合的研究新进展

从引发和催化两个方面概述了光辐照在活性自由基聚合(LRP)中的应用,从机理上详细地分析了光辐照对氮氧调控自由基聚合(NMP)、原子转移自由基聚合(ATRP)、可逆加成-断裂链转移自由基聚合(RAFT)以及有机钴催化的可控自由基聚合反应(CMRP)的影响。与传统自由基聚合相比,光调控的活性自由基聚合方法可在温和的条件下生成自由基,能够克服传统LRP的一些缺陷,如降低催化反应活化能、提高聚合物末端官能度等。同时,本文对光调控反应的进一步应用以及新方法的产生也进行了展望。     

Initiation Kinetics in Free‐Radical Polymerization: Prediction of Thermodynamic and Kinetic Parameters Based on ab initio Calculations

Initiation kinetics in free radical polymerization is investigated using density functional theory. Thermodynamic and kinetic parameters of the initiation reactions are predicted, and the role of the initiators in the polymerization process is evaluated. Methyl acrylate, methyl methacrylate, acrylonitrile, and styrene homo‐polymerizations with different initiators are studied. Reaction enthalpy and activation energy for each reaction between monomer and the radical fragments arising from the initiators have been determined. The initiation kinetic constants for all of these initiation reactions are evaluated and compared with both computational and experimental propagation kinetic constants of each monomer.

     

Initiation Mechanism of Polymerization of Acryl－amide by Ceric Ion／2－Benzoyl Acetanilide System

ＩｎｉｔｉａｔｉｏｎＭｅｃｈａｎｉｓｍｏｆＰｏｌｙｍｅｒｉｚａｔｉｏｎｏｆＡｃｒｙｌ－ａｍｉｄｅｂｙＣｅｒｉｃＩｏｎ／２－ＢｅｎｚｏｙｌＡｃｅｔａｎｉｌｉｄｅＳｙｓｔｅｍＤＯＮＧＪｉａｎ－ｈｕａ，ＤＥＮＧＣｈａｏ，ＱＩＵＫｕｎ－ｙｕａｎａｎｄＦＥＮＧ...     

Polymerization of alkyl allyl oxalates in the evolution of carbon dioxide at elevated temperatures

Radical polymerization of several alkyl allyl oxalates, including methyl allyl oxalate (MAO), ethyl allyl oxalate, propyl allyl oxalate, butyl allyl oxalate, and octyl allyl oxalate, was conducted in the evolution of carbon dioxide at elevated temperatures, and was compared with the anomalous polymerization behavior of diallyl oxalate (DAO) discussed in our earlier article

1 A. Matsumoto, I. Tamura, M. Yamawaki, and M. Oiwa, J. Polym. Sci. Polym. Chem. Ed., 17 , 1419 (1979).

. The kinetic equations for the polymerization of alkyl allyl oxalate were derived following the kinetic treatment of the DAO polymerization by further consideration of the absence of cyclization of the growing polymer radical and the effective reinitiation by alkyl radical, and were then satisfactorily applied to the polymerization of MAO, as a representative alkyl allyl oxalate. The evolution of carbon dioxide in the polymerization of alkyl allyl oxalates was enhanced with the increase of bulkiness of the alkyl substituent, as a result of steric suppression of the propagation of the growing polymer radical.     

Kinetic model of gas phase polymerization of 1,3-butadiene catalyzed by supported rare earth coordination system

Gas phase polymerization of 1,3-butadiene (Bd) catalyzed by supported rare earth coordination system is studied and a new kinetic model is proposed. Four elementary reactions or processes: the process of exposure and activation of potential active catalytic center, propagation, deactivation and chain transfer reaction to alkyl aluminum, are considered in this model. Some important parameters, such as monomer-consuming rate, are well expressed as the functions of macroscopic polymerization conditions such as pressure, temperature, and duration. The model can simulate the whole polymerization procedure satisfactorily.     

A systematic investigation of the ring size effects on the free radical ring-opening polymerization (rROP) of cyclic ketene acetal (CKA) using both experimental and theoretical approach

Radical ring-opening polymerization (rROP) reaction of cyclic ketene acetals (CKA) is an interesting route to biodegradable polymers. Contrary to their tremendous potential, fundamental understanding of their reaction kinetics and thermodynamics is still limited. We present experimental and theoretical investigations for rROP reactions of CKA to systematically elucidate the effects of monomer ring sizes on the homopolymerization. We aim to provide insights on the structural-reactivity relationship of CKA by studying the thermodynamics and kinetics of the forward ring-opening propagation reactions and key side reactions, namely ring-retained propagation and radical back-biting reaction leading to branching. Experimental results show that for the CKA with smaller ring sizes, significant amount of ring-retained side products are formed when up to 90% of the monomers are converted. However, for the larger ring sizes (7 and 8 membered), almost complete ring-opening polymerization with <1% of ring-retained products are formed. Density functional theory (DFT) calculations show that kinetic effects from the collision frequency dominate in differentiating between ring-opening propagation, ring-retained propagation, and backbiting. The results corroborate well with experiments and reports in the literature. Our systematic study from the first principle and experimental validation provide insights into CKA rROP to apply radical polymerization to generate biodegradable polymers.     

Controlled Radical Polymerization: from Oxygen Inhibition and Tolerance to Oxygen Initiation

Molecular oxygen is a radical scavenger in both conventional and controlled radical polymerization(CRP), resulting in many time-consuming methods for physically removing oxygen before the polymerization. Different approaches have been developed to have oxygen tolerance by chemically consuming or converting molecular oxygen into non-initiating species to address this issue. Recently, we propose another approach called oxygen initiation that directly transforms molecular oxygen into the initiating carbon radical in CRP. This feature article summarizes our recent developments in this direction. Oxygen-initiated reversible addition-fragmentation transfer(RAFT) polymerization has been successfully conducted using oxygen and trialkylborane as co-initiators under the ambient conditions and atmosphere without any prior degassed procedures. This gas-triggered initiation provides the opportunity for spatiotemporal control of the polymerization by molecular oxygen or air. Rationally synthesized alkylborane compounds could derive the predesigned structure of the initiating alkyl radical to minimize the side reactions and free polymer chains, achieving the synthesis of ultra-high molecular weight polymers. The challenges and perspectives are also discussed in the end.     

Comments on M.Yu. Zaremski, Chen Xin, A.P. Orlova, I.V. Blagodatskikh, and V.B. Golubev’s Paper “General Features and Specifics of the Kinetics of the Pseudoliving Radical Polymerization of 4-Vinylpyridine and Styrene Mediated by TEMPO”

It is shown through a numerical experiment that, in the presence of extra (spontaneous) initiation, the equilibrium constant and the ratio between the rates of quadratic and reversible terminations cannot affect the rate of pseudoliving radical polymerization. The kinetics of the process in the steady-state region is determined only by the ratio between the rates of additional initiation and quadratic chain termination. Discrepancies between experimental data and theory must be explained with allowance for side reactions that can occur in these complex systems rather than via re-examination of the reliably verified mechanism of pseudo-living radical polymerization.     

Kinetics and mechanism of cyclohexane oxidation with molecular oxygen in the presence of propionic aldehyde

The kinetics and mechanism of the liquid-phase oxidation of cyclohexane with molecular oxygen in the presence of the additives of propionic aldehyde are studied at 303.0, 322.5, and 341.5 K by measuring the rates of oxygen and propionic aldehyde consumption and the yields of the main reaction products (cyclohexanol (COL), cyclohexanone (CON), cyclohexyl hydroperoxide, and propionic acid and peracid). A kinetic scheme is proposed and rate constants of elementary reactions are estimated based on the analysis of their rates and the yields of the main cyclohexane products. The key reactions of the main steps (including chain initiation, propagation, and termination) are determined. An increase in the rate of cyclohexane oxidation and the yield of the target products (cyclohexanol, cyclohexanone, and cyclohexyl hydroperoxide) in the presence of propionic aldehyde suggests that highly active acylperoxy radicals participate in chain propagation. The [CON]/[COL] ratio indicates that these products are mainly formed in chain propagation. The strong effect of the Baeyer-Villiger rearrangement on both the rate of oxygen consumption and the yield of the target products at the initial stages of the process and at high propionic aldehyde concentrations is explained.