Propagation Rate Constants Research Articles

Effect of Hydrogen and External Donor on Propylene Polymerization Kinetics with a 4th‐Generation Ziegler‐Natta Catalyst

AbstractApparent kinetic rate constants and activation energies for catalyst activation, deactivation, and propylene propagation with a 4th‐generation Ziegler‐Natta catalyst are estimated. The rate of polymerization in the presence of donor and hydrogen is higher than in their absence. Catalyst activation and propagation rate constants increase with the addition of hydrogen, likely because hydrogen frees 2–1‐terminated dormant sites. The deactivation term was the highest without electron donor, presumably because the nonspecific sites deactivate faster than the specific sites. Apparent activation energies for catalyst activation, deactivation, and propagation are not significantly influenced by the presence of hydrogen or external donor, indicating that they do not affect these mechanisms substantially. magnified image

Termination of Surface Radicals and Kinetic Modeling of ATRP Grafting from Flat Surfaces by Addition of Deactivator

Surface-initiated controlled radical polymerization has been widely used for grafting polymers from various surfaces. However, how the surface-constrained radicals are terminated remains to be further elucidated. In this work, a simple kinetic model is developed for surface-initiated atom transfer radical polymerization (SI-ATRP) with addition of excess deactivator in solution. The model describes the development of polymer layer thickness, as well as the concentrations of radical, dormant and dead chains. A simple but accurate analytical solution is obtained for the polymer layer thickness as a function of time. The model accounts for the effects of equilibrium constant, activator/deactivator concentration ratio, monomer concentration, grafting density and rate constants of propagation and termination. The model is verified with the experimental data of 2-methacryloyloxythyl phosphorylcholine (MPC), methyl acrylate, acrylamide, and N-isopropylacrylamide under various conditions. In correlating thickness versus time experimental curves at different catalyst concentrations, it is clearly demonstrated that the termination of radicals on surface is facilitated by diffusion of catalyst species in solution. Although radical chains are immobilized, radical centers “migrate” and terminate through activation and deactivation reactions. The termination rate constant is therefore proportional to catalyst concentration. It is also found that the termination is influenced by chain conformation and the rate constant is grafting density dependent.

Copolymerization of ethylene with α‐olefins over supported titanium–magnesium catalysts. II. Comonomer as a chain transfer agent

AbstractThe data on the effect of comonomer (propylene and 1‐hexene) on molecular weight (Mw), molecular weight distribution (MWD), and content of terminal double bonds were obtained for ethylene/α‐olefin copolymers produced over a supported titanium–magnesium catalyst (TMC) upon polymerization in the absence of hydrogen. The experimental data on the effect of comonomer concentration on Mw of polymers were used to calculate the ratios between the effective rate constants of chain transfer with monomer and the propagation rate constant. It was shown that the effective rate constant of chain transfer with monomers increases in the row of monomers: ethylene < 1‐hexene < propylene. Meanwhile, the data on the effect of copolymers on content of terminal double bonds of various types demonstrate that different reactions of chain transfer with comonomer may simultaneously occur during copolymerization. It results in simultaneous formation of terminal vinylidene and trans‐vinylene bonds. Therefore, the calculated rate constants of chain transfer with comonomer are complex values, which include the rate constants of chain transfer with comonomer occurring via different mechanisms. The data on MWD, short chain branching (SCB) and terminal double bonds content of different types were obtained by molecular weight fractionation of copolymers followed by the analysis of narrow fractions. The analysis of the data on MWDs of SCB and terminal double bonds shows that active sites of the TMC are considerably heterogeneous with respect to the rates of different chain transfer reactions with monomers. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Emulsifier-free, organotellurium-mediated living radical emulsion polymerization (emulsion TERP) of methyl methacrylate with dimethyl ditelluride as the catalyst

Emulsifier-free, organotellurium-mediated living radical emulsion polymerization (emulsion TERP) of methyl methacrylate (MMA) using poly(methacrylic acid) (PMAA)–methyltellanyl (TeMe) (PMAA30-TeMe) (degree of polymerization of PMAA, 30) was carried out at 60 °C with 1000 rpm stirring. The emulsion TERP of MMA did not successfully proceed with maintaining a living manner because of a low chain transfer rate constant relative to propagation rate constant. Moreover, even in the coexistence of (TeBu)2, the emulsion TERP of MMA also did not proceed with living nature. On the other hand, it in the coexistence of (TeMe)2 proceeded with good control/livingness. The difference was caused by the different in the solubility in water between (TeBu)2 and (TeMe)2. Because (TeBu)2 is difficult to dissolve in the aqueous phase, it did not work as a catalyst in the aqueous phase, resulting in a homogeneous nucleation. On the other hand, because (TeMe)2, which is more hydrophilic than (TeBu)2, works as an efficient catalyst in the aqueous phase, resulting in the self-assembly nucleation.

Dibromomaleimide End Functional Polymers by RAFT Polymerization Without the Need of Protecting Groups.

Polymers bearing the dibromomaleimide (DBM) group as a functional chain end have been synthesized by RAFT polymerization. A DBM functional chain transfer agent (CTA) was utilized to afford well-defined PtBA, PMA, and PTEGA, without the requirement of protecting group chemistry. It was found that RAFT polymerization of NIPAM and styrene with this CTA was severely retarded/inhibited which is ascribed to their relatively low propagation rate constants compared to acrylates. This observation is accounted for by a reversible trapping of propagating radicals by the DBM group in RAFT polymerizations using a monomer with low kp. However, further attempts to synthesize DBM-terminated PtBA and PMA by ATRP using an analogous initiator were unsuccessful, and broad PDI were observed. Furthermore, highly efficient postpolymerization functionalization of DBM-terminated PMA produced by RAFT, with the model compound thiophenol was also demonstrated.

Critical Evaluation of the Microwave Effect on Radical (Co)Polymerizations

Critical evaluations of the microwave effect on initiation, propagation, and termination during conventional radical polymerizations (RPs) of methyl methacrylate (MMA) and random copolymerization of styrene (St) with (meth)acrylates are examined by comparing microwave heating (MWH) and conventional heating (CH). Poly(methyl methacrylate) with similar $ \overline M_{\rm n} $, $ \overline M_{\rm w} $/$ \overline M_{\rm n} $, and conversion are obtained under precisely controlled temperature, indicating very small changes of propagation rate constant. Rate enhancement in the absence of precise temperature control is mostly due to the higher reaction temperature of the reaction mixture than the apparent value indicated on display. Rates of initiator decomposition under well-controlled temperature are essentially the same for MWH and CH.

Macromol. Theory Simul. 9/2011

Cover: Using the thermodynamics of the atom transfer radical polymerization (ATRP) equilibrium as the basis of the kinetic modeling of ATRP, a linear correlation between the maximum ATRP equilibrium constant for controlled polymerization and the propagation rate constant is described. This enables the design of well-controlled ATRP systems. Further details can be found in the article by H. Bergenudd,* M. Jonsson, and E. Malmström on page 814.

Can Propagation Reaction in Carbocationic Polymerization and Copolymerization of Styrene be Diffusion Controlled ?

AbstractSummary: The reactivity ratios r1 and r2 in copolymerizations of styrene and parasubstituted styrenes, for which r1 = 1/r2, are in contradiction with diffusion control for their propagation reactions. The cross propagation rate constants k12copol in copolymerization of styrene with p‐chlorostyrene, p‐methylstyrene and p‐methoxystyrene have been shown to increase with their nucleophilicity parameter N. This is also not compatible with diffusion controlled cross propagation and propagation, but agrees with similar rate constants of propagation for these monomers. The capping rate constants k12capp of reactions of poly(p‐methylstyrene)± and poly(p‐methoxystyrene)± with π‐nucleophiles also increase with N, but with a much larger selectivity. This shows that k12copol and k12capp are not identical. The k, from 109 to 6 109 L mol−1 s−1, obtained with p‐chlorostyrene, styrene and p‐methylstyrene by the Diffusion Clock (DC) method are not consistent with those derived from the ionic species concentration (ISC method) for indene, 2,4,6‐trimethylstyrene and p‐methoxystyrene of the order of 104 – 105 L mol−1 s−1, also measured for living polymerization. These last values are in agreement with those measured previously in nonliving systems, and with an approximate compensation between the reactivity of a monomer and that of the corresponding carbocation.

Kinetic Study, by UV–Vis Spectroscopy, on the Strong Effect of LiCl on the Controlled Polymerization of 2-Bromo-3-hexyl-5-iodothiophene and 2-Iodo-3-hexyl-5-bromothiophene: Determination of the Propagation Rate Constants, Application to the Synthesis of High Molecular Weight Polydodecylthiophene

The controlled polymerization of the 2-bromo-3-hexyl-5-iodothiophene (5) and 2-iodo-3-hexyl-5-bromothiophene (2), using Ni(dppp)Cl2 as a catalyst and LiCl as an additive, was followed in real time by UV–vis spectroscopy. It was shown that the propagation rate constant depends strongly on the ratio R = [LiCl]/[M]. For R = 4 the polymerization rate constant for the regular monomer 5 achieves a constant value 20 times higher than the one obtained without LiCl: kp = 44 ± 5 mol–1L s–1. For the reverse monomer 2, the addition of LiCl (R = 4) leads to a polymerization reaction with a similar propagation rate constant. The slow initiation reaction of monomer 2, with and without LiCl, has been analyzed by 31P NMR, UV–vis spectroscopy, SEC. It was shown that the addition of LiCl increases the reactivity of the activated monomers under their Grignard forms but also modifies the active center by exchanging the bromine atom by a chlorine atom. This new process was extended to the synthesis of well-defined poly(dodecylthiophene) of high molecular weight.

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.

Predicting the Limit of Control in the ATRP Process: Results from Kinetic Simulations

AbstractKinetic simulations are reported, where the ATRP equilibrium constant KATRP is varied and the rates and degree of control in different ATRP systems are evaluated. The apparent rate constant kapp increases with increasing KATRP, but a maximum is reached. The limit of control is passed before the maximum, i.e. when KATRP is increased further, apparent first‐order kinetics and well‐controlled molecular weights will no longer be obtained. The equilibrium constant at which the limit of control is reached varies linearly with the propagation rate constant. This enables the design of well controlled ATRP systems. The influence of the conversion and chain length dependence of the termination rate constant on the simulation results is discussed.magnified image

Macroinitiator halide effects in galactoglucomannan‐mediated single electron transfer‐living radical polymerization

AbstractChloro (Cl)‐ and bromo (Br)‐functionalized macroinitiators were successfully prepared from the softwood hemicellulose O‐acetylated galactoglucomannan (AcGGM) and then explored and evaluated with respect to their ability and efficiency of initiating single electron transfer‐living radical polymerization (SET‐LRP). Both halogenated species effectively initiate SET‐LRP of an acrylate and a methacrylate monomer, respectively, yielding brushlike AcGGM graft copolymers, where the molecular weights are accurately controlled via the monomer:macroinitiator ratio and polymerization time over a broad range: from oligomeric to ultrahigh. The nature of the halogen does not influence the kinetics of polymerization strongly, however, for acrylate graft polymerization, AcGGM‐Cl gives a somewhat higher rate constant of propagation, while methacrylate grafting proceeds slightly faster when the initiating species is AcGGM‐Br. For both monomers, the macroinitiator efficiency is superior in the case of AcGGM‐Br. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Kinetic parameters estimation of ring-opening poly(lactic acid) polymerization by modeling and simulation

Abstract The modeling of ring-opening polymerization of lactide to poly(lactic acid) (PLA) has been carried out. Carothers first synthesized PLA in 1932. Since then, hundreds of research papers and patents have appeared in the literature. However, there is a lack of data concerning the rate constants for initiation, propagation and termination steps of PLA polymerization, except some data about the apparent rate constant. This work investigates, theoretically, the individual rate constants using a simple numerical technique. The progress of lactide polymerization can be modeled by assuming a ring opening reaction mechanism comprising chain initiation, chain propagation, and chain termination. The simulator developed, based on the solutionof differential equations corresponding to the above-mentioned kinetic scheme, Generates a detailed molecular weight distribution that can be used to estimate average molecular weights (or average degree of polymerization) vs. polymerization time curves. These simulated curves, on matching with the reported experimental data (for different catalysts), yield the absolute values of rate constants. The values have been determined for zinc lactate. Rate constants could be determined by using the molecular weight and the polydispersity vs. polymerization data. This methodology offers greater opportunity for capturing high, non-equilibrium polymer yield through appropriately timed termination of the polymerization reaction.

Titanium-magnesium catalysts for propylene polymerization: The effect of donors

The chain transfer reaction with hydrogen at propylene polymerization over Ti-Mg catalysts (TMCs) of composition TiCl4/D1/MgCl2-AlEt3/D2 is studied in a wide hydrogen concentration range. A two-step mechanism of this reaction is suggested. This mechanism accounts for the fractional order of the reaction with respect to hydrogen concentration. Constants of chain transfer reaction with hydrogen are determined for TMC with different donors: 1,3-diether or dibutyl phthalate as D1 and tetraethoxysilane or dicyclopentyldimethoxysilane as D2. In propylene polymerization over the TMCs, the length of the polymer chain is mainly determined by the ratio of the propylene and hydrogen concentrations because the propagation and chain transfer rate constants are comparable. The rate constant of chain transfer with hydrogen at ethylene polymerization is significantly (more than one order of magnitude) less, and higher hydrogen concentrations are required for attaining the same degree of polymerization. The results of this study might be helpful in simulation of industrial polymerization processes and in control of the polymer molar mass.

How Fast Can a CRP Be Conducted with Preserved Chain End Functionality?

The preservation of chain end functionality (CEF) is perhaps the most important criterion of the “livingness” for any controlled/living radical polymerization (CRP) system. CEF depends on several parameters but most importantly on concentration of propagating radicals, i.e., rate of polymerization. There are claims in the literature about ultrafast CRP leading to ultrahigh molecular weight with perfectly preserved CEF. However, faster CRP always results in lower CEF. Thus, how fast can a CRP be conducted with preserved CEF? In this paper we evaluate, by kinetic analysis and computation, the dependence of CEF on the rate of polymerization, monomer structure (rate constants of propagation and termination), targeted degree of polymerization, initial monomer concentration, and monomer conversion. Some subtle differences in the definition of CEF are discussed for different types of atom transfer radical polymerization (ATRP) techniques. CEF for polymer chains with and without an α-alkyl group from a (macro)ini...

Polymerization Kinetics of Ethylene Oxide Methacrylates in Ionic Media

AbstractThe radical polymerization of ethylene oxide (PEGylated) methacrylates in ionic media has been studied. Lithium salts interact with the monomer causing a significant increase in the propagation rate constant, kp, and also providing an ionic and highly viscous medium that sharply decreases the termination rate coefficient, kt. Both features make the polymerization reactions with lithium salts faster compared to the bulk monomer. The systems are studied by means of FT‐IR spectroscopy to identify the interactions between the monomer and the lithium salt. In addition, an extensive kinetic study by PLP‐SEC has been performed to study the influence of the lithium salt on kp and kt in the polymerization of these monomers. These results are compared to those obtained when ionic liquids are used as polymerization medium. magnified image

Synthesis of zwitterionic polymer by SET-LRP at room temperature in aqueous

The Cu0-mediated single electron transfer-living radical polymerization of acrylamide and N,N-dimethyl-N-methacryloyloxyethyl-N-sulfobutyl ammonium in aqueous at 25 °C using 2-chloropropionamide as initiator with Cu0 powder/tris-(2-dimethylamino ethyl)amine (Me6-TREN) as catalyst system is studied. The results showed the characteristic of the “living” polymerization that were the Mn of polymers increased linearly with monomer conversion and the ln([M]0/[M]) increased linearly with time too, meanwhile the narrow molecular of weight distributions were found at most cases. Because of the high rate constant of propagation and bimolecular termination of the acrylamide, the external addition of CuCl2 is required to mediate deactivation the early stage of polymerization. In addition, the disproportionation constant of CuIX/L in H2O is higher than in other solvents and the coordination of amino group and CuII takes place easily, so the isopropanol or N,N-dimethylformamide is added to control the polymerization. High conversions were achieved within short time and the polymers prepared showed good antipolyelectrolyte properties in inorganic salts solutions. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Irreversible Anionic Polymerization Kinetics Revisited

Irreversible anionic polymerization is revisited with an emphasis on determining the dependence of the MW distribution on initial conditions. It is shown that only two parameters govern the distribution: the ratio of initiation to propagation rate constants and the ratio of initial monomer to initiator concentrations. New and simple criteria are developed that predict the breadth of the MW distribution based on these parameters. Even when initiation is slower than propagation, it is still possible under certain conditions to obtain a narrow, Poisson-like MW distribution. Since it impacts self-assembly of block copolymers into microphases, the compositional variation of block copolymers produced by living polymerization techniques is also analyzed. A somewhat unexpected result is the composition variability in block copolymers can be significant (10−15%) even when the blocks have dispersity indices less than 1.01.

Dramatic acceleration of SET‐LRP of methyl acrylate during catalysis with activated Cu(0) wire

AbstractA simple method for the activation of the Cu(0) wire used as catalyst in single‐electron transfer living radical polymerization (SET‐LRP) is reported. The surface of Cu(0) stored in air is coated with a layer of Cu2O. It is well established that Cu2O is a less reactive catalyst for SET‐LRP than Cu(0). We report here the activation of the Cu(0) wire under nitrogen by the reduction of Cu2O from its surface to Cu(0) by treatment with hydrazine hydrate. The kinetics of SET‐LRP of methyl acrylate (MA) catalyzed with activated Cu(0) wire in dimethyl sulfoxide (DMSO) at 25 °C demonstrated a dramatic acceleration of the polymerization and the absence of the induction period observed during SET‐LRP catalyzed with nonactivated Cu(0) in several laboratories. Exposure of the activated Cu(0) wire to air results in a lower apparent rate constant of propagation because of gradual oxidation of Cu(0) to Cu2O. This dramatic acceleration of SET‐LRP is similar to that observed with commercial Cu(0) nanopowder except that the polymerization provides excellent molecular weight evolution, very narrow molecular weight distribution and high polymer chain‐end functionality. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010

Stopped-Flow NMR: Determining the Kinetics of [rac-(C2H4(1-indenyl)2)ZrMe][MeB(C6F5)3]-Catalyzed Polymerization of 1-Hexene by Direct Observation

Stopped-flow NMR measurements suitable for determination of reaction kinetics on time scales of 100 ms or longer have been achieved by adaptation of a commercial NMR flow probe with a high-efficiency mixer and drive system. Studies of metallocene-catalyzed alkene polymerization at room temperature have been complicated by high rates, imprecise knowledge of the distribution of different catalyst species with time, and the high sensitivity of the catalysts to low concentrations of impurities. Application of the stopped-flow NMR method to the study of the kinetics of 1-hexene polymerization in the presence of (EBI)ZrMe[MeB(C(6)F(5))(3)] demonstrates that NMR spectroscopy provides an efficient method for direct and simultaneous measurement of substrate consumption and catalyst speciation as a function of time. Kinetic modeling of the catalyst and substrate concentration time courses reveal efficient determination of initiation, propagation, and termination rate constants. As first suggested by Collins and co-workers (Polyhedron 2005, 24, 1234-1249), a kinetic model in which Zr-HB(C(6)F(5))(3) forms rapidly upon beta-hydride elimination but reacts relatively slowly with alkene to reinitiate chain growth is supported by these data.