Termination Rate Coefficient Research Articles

Styrene–dimethacrylate copolymerization kinetic modeling

AbstractA model for the copolymerization of styrene with dimethacrylate including the post‐gelation period, diffusion effects and cyclization reactions was developed. The numerical fractionation method, balance of sequences and pseudo‐homopolymerization hypothesis were used for such a purpose. Diffusion effects were taken into account through an exponential correlation between the termination rate coefficient and the monomer conversion. Average rate coefficients of cyclization and crosslinking reactions were fitted. The model was validated through literature data for the copolymerizations of styrene with ethylene glycol dimethacrylate and styrene with tetraethylene glycol dimethacrylate. The predictions were satisfactory in the range 1–20% of dimethacrylate and temperature at 95 °C. © 2015 Society of Chemical Industry

Novel Access to Propagation Rate Coefficients of Radical Polymerization by the SP-PLP-EPR Method

A novel method for kp measurement is presented, in which time-resolved electron paramagnetic resonance detection of radical concentration after applying a single laser pulse (SP−PLP−EPR) is carried out in conjunction with measuring monomer conversion induced by the laser pulse. The method is particularly useful for the study of fully ionized monomers, as will be outlined by experiments on trimethylaminoethyl methacrylate chloride (TMAEMA, 2-(methacryloyloxy)-N,N,N-trimethylethan-1-aminium chloride, 20 wt% in aqueous solution) at 60 °C. In addition to kp, the method provides termination rate coefficient data. The reliability of the so-obtained rate coefficients is demonstrated by chemically induced experiments and by measuring TMAEMA conversion after single-pulse initiation via near-infrared (NIR) spectroscopy.

Termination Kinetics of Surface-Initiated Radical Polymerization Measured by Time-Resolved ESR Spectroscopy after Laser-Pulse Initiation

The single-pulse–pulsed-laser polymerization–electron spin resonance (SP-PLP-ESR) method was used to study the termination reaction in polymerization of n-butyl methacrylate (n-BMA) from silica nanoparticles. The polymerization from the surface was initiated by the surface-attached photoinitiator 2,2-dimethoxy-2-phenyl-1-(2-(2-(trimethoxysilyl)ethyl)phenyl)ethane-1-one (DMPTS). The obtained termination rate coefficient at 335 K of kt = (5.4 ± 0.7) × 106 L mol–1 s–1, which could unambiguously be assigned to the termination reaction between two surface-attached macroradicals, is about 2 orders of magnitude smaller than the value found in homogeneous n-BMA polymerization. Surprisingly, no chain-length dependence of the termination rate coefficient could be observed. This fact and temperature-dependent studies of kt, which yield an activation energy that is in perfect agreement with the activation energy of propagation of n-BMA, indicate that the termination between surface-attached macroradicals proceeds under reaction diffusion control, as their translational diffusion is suppressed.

Chain-Length-Dependent Termination of Styrene Bulk Homopolymerization Studied by SP−PLP−EPR

Single-pulse pulsed-laser polymerization in conjunction with time-resolved quantitative EPR detection of radicals, SP−PLP−EPR, has been carried out in the early period of styrene polymerizations between 73 and 135 °C using dicumyl peroxide as the photoinitiator. To enhance signal-to-noise quality, in addition to styrene-H8, the fully deuterated monomer, Sty-d8, has been investigated. The chain-length dependence of the rate coefficient for termination of two radicals of chain length i, kti,i, is adequately represented by the composite model. The power-law exponents for short-chain and long-chain radicals are obtained as αs = 0.51 ± 0.05 and αl = 0.16 ± 0.05, respectively, with the two ranges being separated by the crossover chain length, ic = 30 ± 10. The termination rate coefficient for two radicals of chain length unity, kt1,1, scales with temperature as does monomer fluidity. From kti,i deduced via SP−PLP−EPR, chain-length-averaged termination rate coefficients, ⟨kt⟩, have been estimated, which are close to the reported experimental values.

Photopolymerization kinetics and molecular interactions in methacrylate-imidazolium based ionic liquid systems

Photopolymerization of poly(ethylene glycol) monomethacrylate (PEGMM) in two imidazo- lium based ionic liquids (ILs) differing with the anion size and H-bond accepting ability, i.e. 1-ethyl- -3-methylimidazolium tetrafluoroborate ((EMIm)(BF4)) and 1-ethyl-3-methylimidazolium trifluoroace- tate ((EMIm)(ATF)) was investigated to find the effect of the IL anion on propagation k p and termination rate k t b coefficients (by DSC) as well as molecular interactions in the system (on the basis of viscosity and FT-IR measurements). Polymerizations in bulk and in tricresyl phosphate (TCP) served as references. Both monomer/IL systems show strong viscosity synergism and viscosity deviations are positive indica- ting that viscosity of associates between the monomer and the ILs is relatively higher than that of pure components. The increase in k p values observed in ILs is higher for polymerization in (EMIm)(BF 4 ), which shows stronger Coulomb cation-anion attractions and lower H-bond accepting ability of the anion. The influence of the ionic liquids on the termination rate coefficient results from the viscosity of the mono- mer-IL system, taking account of viscosity synergism. On the basis of changes of imidazolium ring CH groups vibrations and carbonyl stretching vibrations (both in PEGMM and IL anion) two possible confi- gurations of monomer-IL interactions were proposed: between the monomer carbonyl and C(2)-H hydro- gen of the imidazolium cation in the case of (EMIm)(BF 4 ) and a configuration in which the monomer car- bonyl interacts with the imidazolium cation located on the top of the ring, in the case of (EMIm)(ATF).

(Meth)acrylic monomers with heteroatom-containing ester side chains: a systematic PLP-SEC and polymerization study

The Arrhenius parameters of the propagation rate coefficient for two hetero-atom containing (meth)-acrylates (studied as 1 M solution in N,N-dimethylacetamide (DMAc)) are determined via the pulsed laser polymerization – size-exclusion chromatography (PLP-SEC) method. Absolute molar mass determination is achieved via SEC coupled to on-line multi-angle laser light scattering (MALLS). The data obtained for hydroxypropylcarbamate acrylate (HPCA, A = 3.97 (−1.44 to 1.63) × 106 L mol−1 s−1 and Ea = 14.3 (−1.38 to 5.13) kJ mol−1) are critically compared with the literature known data sets of two structural derivatives, i.e., 2-(phenylcarbamoyloxy)isopropyl acrylate (PhCPA) and 2-(hexylcarbamoyloxy)isopropyl acrylate (HCPA), indicating an increase in the propagation rate coefficient with increasing ester side chain length. Ureidoethyl methacrylate (UMA, A = 2.08 (−0.45 to 0.91) × 106 L mol−1 s−1 and Ea = 19.9 (−0.89 to 0.91) kJ mol−1) represents the first hetero-atom containing methacrylate to be studied via PLP-SEC, evidencing a significantly higher propagation rate coefficient compared to earlier investigated methacrylate-type monomers. Furthermore, the free-radical polymerization behavior of HPCA and UMA is studied via in situ1H-NMR experiments at elevated temperatures allowing for an estimation of average termination rate coefficients (at low conversion) in conjunction with the determined kp data. Furthermore, the polymerization of UMA was successfully controlled by reversible addition–fragmentation chain transfer (RAFT) polymerization as evidenced by the linear evolution of the number-average molar mass, Mn, with conversion (3000 g mol−1 ≤ Mn ≤ 23 000 g mol−1, 1.15 ≤ Đ ≤ 1.3) as well as by nitroxide-mediated polymerization (NMP), as demonstrated by the linear evolution of Mn with conversion (4000 g mol−1 ≤ Mn ≤ 40 000 g mol−1, 1.3 ≤ Đ ≤ 1.4). In addition, HPCA polymerization was successfully controlled by the RAFT process, as evidenced by the linear evolution of Mn with conversion (2000 g mol−1 ≤ Mn ≤ 21 000 g mol−1, 1.2 ≤ Đ ≤ 1.4) and successful chain extension experiments. Finally, the NMP of HPCA exhibited uniform shifts of the molar mass distributions in the range of 5000 g mol−1 ≤ Mn ≤ 70 000 g mol−1 and successful chain extension experiments.

Polymerization of Methyl Methacrylate with Lithium Triflate. A Kinetic and Structural Study

The radical photopolymerization of methyl methacrylate in the presence of lithium triflate is investigated by EPR and NMR spectroscopies in order to assess the effect of the ionic medium on the polymerization kinetics. The EPR spectra show a notorious increase in the concentration of propagating radical as result of a dramatic decrease in the average termination rate coefficient. IR spectroscopy experiments and theoretical studies confirm the formation of a complex between the lithium cation and the oxygen atom at the carbonyl position of the ester in the methyl methacrylate. The electrostatic repulsion between the complexed propagating radicals, specifically at the end of the chains, allows explaining the observed low termination rate. This strong interaction would lead to “quasi-living” polymer chains from the first steps of the polymerization.

Mechanistic Insights into the UV-Induced Radical Copolymerization of 1,3-Butadiene with Acrylonitrile

An in-depth mechanistic study into the solution based initiator-free UV-induced radical copolymerization of 1,3-butadiene with acrylonitrile is reported. The light induced constant radical flux leads to moderate monomer conversions within 4 to 24 h. The number-average molecular weights of the prepared nitrile butadiene rubber (NBR) range from 2500 to 50 000 g mol–1 (1.7 ≤ PDI ≤ 2.4), while the achievable monomer conversion ranged from close to 7 up to 31% depending on the polymerization temperature, reaction time and UV light intensity. The rate coefficient for the generation of primary radicals, determined as the coupled parameter k1*k3, showed a dependence on the UV light intensity with values between 6.0 s–2 and 34.6 s–2 deduced for the UV light intensity range of 280 to 700 W. The estimated values of the lower limit average termination rate coefficient displayed no dependence on the UV light intensity, with lower limit values between 2.6 × 108 L mol–1 s–1 and 6.3 × 108 L mol–1 s–1 for the UV light int...

Single‐pulse pulsed laser polymerization–electron paramagnetic resonance investigations into the termination kinetics of n‐butyl acrylate macromonomers

AbstractThe termination of model mid‐chain radicals (MCRs), which mimic radicals that occur in acrylate polymerization over a broad range of reaction conditions, has been studied by single‐pulse pulsed laser polymerization (SP‐PLP) in conjunction with electron paramagnetic resonance spectroscopy. The model radicals were generated by initiator‐fragment addition to acrylic macromonomers that were preformed prior to the kinetic experiments, thus enabling separation of termination from the propagation reaction, for these model radicals propagate sparingly, if at all, on the timescale of SP‐PLP experiments. Termination rate coefficients of the MCRs were determined in the temperature range of 0–60°C in acetonitrile and butyl propionate solution as well as in bulk macromonomer over the range of 0–100 °C. Termination rate coefficients slightly below those of the corresponding secondary radicals were deduced, demonstrating the relatively high termination activity of this species, even when undergoing MCR–MCR termination. For chain length of 10, a reduction by a factor of 6 is observed. Unusually high activation energies were found for the termination rate coefficient in these systems, with 35 kJ mol−1 being determined for bulk macromonomer. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Termination and Transfer Kinetics of Sodium Acrylate Polymerization in Aqueous Solution

Rate coefficients of termination and backbiting of secondary chain-end radicals (SPRs) and of propagation of tertiary midchain radicals (MCRs) have been measured for 20 wt % sodium acrylate (NaA), i.e., fully ionized acrylic acid, in aqueous solution at temperatures from 0 to 60 °C. Highly time-resolved SPR and MCR concentration vs time traces, measured after applying an intense laser pulse at t = 0, were analyzed via PREDICI simulation. Cross- and homotermination of MCRs have no major impact on the kinetics. The termination rate coefficient of SPRs in dilute solution of NaA is by more than 1 order of magnitude below the associated value of nonionized AA. The reduction of backbiting rate coefficient upon full ionization is close to the one associated with enhancing the concentration of nonionized AA from 10 to 50 wt %.

Investigations Into Chain‐Length‐Dependent Termination in Bulk Radical Polymerization of 1H, 1H, 2H, 2H‐Tridecafluorooctyl Methacrylate

AbstractThe SP‐PLP‐EPR technique is used to carry out a detailed investigation of the radical termination kinetics of 1H, 1H, 2H, 2H‐tridecafluorooctyl methacrylate (TDFOMA) in bulk at relatively low conversion. Composite‐model behavior for chain‐length‐dependent termination rate coefficients, kti,i, is observed. It is found that for TDFOMA, ic ≈ 60 independent of temperature, and αs ≈ 0.65 and αl ≈ 0.2 at 80 °C and above. However, at lower temperatures the situation is strikingly different, with the significantly higher average values of αs = 0.89 ± 0.15 and αl = 0.32 ± 0.10 being obtained at 50 °C and below. This makes TDFOMA the first monomer to be found that exhibits clearly different exponent values, αs and αl, at lower and higher temperature, and that has both a high αs, like an acrylate, and a high ic, like a methacrylate.

Compartmentalization Effects on Bimolecular Termination in Atom Transfer Radical Polymerization in Nanoreactors

AbstractThe factors that determine the magnitude of the segregation effect in atom transfer radical polymerization (ATRP) in a dispersed system (miniemulsion, emulsion, etc.) have been elucidated by means of simulations based on two‐dimensional Smith–Ewart equations. The segregation effect is not a function of merely the average number of propagating radicals per particle, but also depends on the values of the deactivation and termination rate coefficients. This occurs as a result of these rate coefficients exerting an influence on the distribution of propagating radicals between the particles, which in turn affects the termination rate. The results provide further information as to how one can potentially exploit compartmentalization to improve livingness (end‐functionality) in ATRP in dispersed systems. magnified image

Chain‐Length‐Dependent Termination in Radical Polymerization of Acrylates

AbstractThe technique of SPPLP EPR, which is single‐pulse pulsed‐laser polymerization (SPPLP) in conjunction with electron paramagnetic resonance (EPR) spectroscopy, is used to carry out a detailed investigation of secondary (chain‐end) radical termination of acrylates. Measurements are performed on methyl acrylate, n‐butyl acrylate, and dodecyl acrylate in bulk and in toluene solution at −40 °C. The reason for the low temperature is to avoid formation of mid‐chain radicals (MCRs), a complicating factor that has imparted ambiguity to the results of previous studies of this nature. Consistent with these previous studies, composite‐model behavior for chain‐length‐dependent termination (CLDT) rate coefficients, $k_{{\rm t}}^{i,i} $, is found in this work. However, lower and more reasonable values of αs, the exponent for variation of $k_{{\rm t}}^{i,i} $ at short chain lengths, are found in the present study. Most likely this is because of the absence of MCRs, thereby validating the methodology of this work. Family‐type termination behavior is observed, with the following average parameter values adequately describing all results, regardless of acrylate or the presence of toluene: αs = 0.79, αl = 0.21 (long chains) and ic ≈ 30 (crossover chain length). All indications are that these values carry over to termination of acrylate chain‐end radicals at higher, more practical temperatures. Further, these values largely make sense in terms of what is understood about the physical meaning of the parameters. Variation of the rate coefficient for termination between monomeric radicals, $k_{{\rm t}}^{{\rm 1,1}} $, is found to be well described by the simple Smoluchowski and Stokes–Einstein equations. This allows easy prediction of $k_{{\rm t}}^{{\rm 1,1}} $ for different alkyl acrylates, solvent, and temperature. Through all this the unrivalled power of SPPLP EPR for measuring and understanding (chain‐length‐dependent) termination rate coefficients shines through. magnified image

Termination Kinetics of 1‐Vinylpyrrolidin‐2‐one Radical Polymerization in Aqueous Solution

AbstractTermination kinetics of 1‐vinylpyrrolidin‐2‐one radical polymerization in aqueous solution has been studied at 40 °C between 20 and 100 wt.‐% VP. The <kt>/kp values from laser single‐pulse experiments with microsecond time‐resolved NIR detection of monomer conversion, in conjunction with kp from literature, yield chain‐length‐averaged termination rate coefficients, <kt>. Because of better signal‐to‐noise quality, experiments were carried out at 2 000 bar, but also at 1 500, 1 000, and 500 bar, thus allowing for estimates of <kt> at ambient pressure. The dependence of <kt> on monomer conversion indicates initial control by segmental diffusion followed by translational diffusion and finally reaction diffusion control. To assist the kinetic studies, viscosities of VP–water mixtures at ambient pressure have been determined. magnified image

EPR Measurement of Fragmentation Kinetics in Dithiobenzoate-Mediated RAFT Polymerization

The kinetics of reversible addition−fragmentation chain transfer (RAFT) polymerization of butyl acrylate with ethyl S-thiobenzoyl-2-thiopropionate (ETTP) as the RAFT agent has been studied. The concentrations of propagating (P•) and intermediate radicals (INT•) were measured via highly time-resolved EPR spectroscopy after initiation by a laser single pulse. Predici simulation of experimental data results in rate coefficients for −40 °C of kad = (1.4 ± 0.4) × 106 L mol−1 s−1, kβ = (4.7 ± 1.5) s−1, and ktcross = 0.25 × kt, where kt is the rate coefficient for termination of two P• species. Fast fragmentation of the intermediate radical is thus observed at this low temperature. Measuring the ratio of INT• to P• concentrations during stationary polymerization at −40 and 70 °C yields Keq = kad/kβ values of (2.3 ± 0.6) × 105 and 75 ± 15 L mol−1, respectively, which correspond to an apparent activation energy difference, EA(kad/kβ), of −49.5 kJ mol−1.

SP-PLP-EPR Study into the Termination Kinetics of Methacrylic Acid Radical Polymerization in Aqueous Solution

The termination kinetics of methacrylic acid (MAA) radical polymerization in aqueous solution containing 10 wt % MAA was measured via the single pulse−pulsed laser polymerization−electron paramagnetic resonance (SP-PLP-EPR) method from 0 to 50 °C. The termination rate coefficient, kt, was determined as a function of radical chain length. Addition of high-molecular-mass poly(methacrylic acid), which mimicks monomer conversion, leads to a reduction of kt, which is however far below the associated decrease in fluidity, η−1.

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

Online Low-Field 1H NMR Spectroscopy: Monitoring of Emulsion Polymerization of Butyl Acrylate

The emulsion polymerization of butyl acrylate was monitored by online nuclear magnetic resonance spectroscopy at 20 MHz 1H frequency. The reaction progress could be followed, monitoring the conversion of the reactant time-resolved without any need of sample preparation. Experimental data were analyzed with kinetic models for free-radical polymerization. The data comprised the polymerization rate in seeded batch emulsion polymerizations of butyl acrylate with doubly deionized water and D2O as solvent. The polymerization rate versus conversion curve behaves compatible with the three rate intervals model, typically observed in emulsion polymerizations. Zero−one kinetics explains the experimental results appropriately, leading to the determination of entry and termination rate coefficients.

Termination and Transfer Kinetics of Butyl Acrylate Radical Polymerization Studied via SP-PLP-EPR

Butyl acrylate (BA) solution polymerization (1.5 M in toluene) was investigated via single-pulse pulsed laser polymerization in conjunction with electron paramagnetic resonance spectroscopy (SP-PLP-EPR) with emphasis on the termination and transfer kinetics of the system in which two distinctly different types of radicals, secondary chain-end radicals (SPRs) and midchain radicals (MCRs), are present. MCRs are produced by intramolecular hydrogen transfer (backbiting). They may react back to SPRs by monomer addition. The evolution of SPR and MCR concentrations after photoinitiation with an intense laser pulse was measured via highly time-resolved EPR at temperatures between −40 and +60 °C. At very low temperatures the MCR concentration is negligible, enabling the chain-length-dependent rate coefficient of SPR termination, ktss(i,i), to be directly determined. At higher temperatures it was necessary to use PREDICI simulation of the radical concentration vs time traces, a process which yields the chain-length-dependent rate coefficient of SPR termination for monomeric radicals, ktss(1,1), as well as the rate coefficients for backbiting, kbb, for monomer addition to an MCR, kpt, and for SPR-MCR cross-termination, ktst. The composite model adequately represents ktss(i,i), with the power-law exponents αs = 0.85 ± 0.09 and αl = 0.16 ± 0.07 for short-chain and long-chain radicals, respectively, and a crossover chain length between short-chain and long-chain behavior at around ic = 30. The activation energy for both ktss(1,1) and ktst(1,1) is found to be as one would expect for translational diffusion of small molecules.

Termination Rate Coefficients for Radical Homopolymerization of Methyl Methacrylate and Styrene at Low Conversion

AbstractA comprehensive and systematic study of overall termination rate coefficients, kt, in low‐conversion radical (homo)polymerization of methyl methacrylate and styrene is presented. Values of kt were determined by gravimetric analysis of steady‐state experiments, employing 2,2′‐azoisobutyronitrile as initiator. The values delivered by this simple method were found to be in qualitative and quantitative agreement with those from more modern and sophisticated techniques for measuring kt. Accordingly, correlations for bulk, low‐conversion kt as a function of temperature are given for each monomer. The effects of initiator concentration, cI, and temperature on bulk kt were studied in a controlled way for both monomers. Additionally, ethyl benzene was used as solvent in order to investigate rigorously the effect of monomer concentration, cM, on styrene kt. The trends found by these systematic studies were considered in the light of what is known about the chain‐length dependence of termination. Styrene's behavior was always found to be qualitatively in accord with expectation, although the variations of kt with cI and cM were not as strong as should be the case. However its activation energy, 15 kJ · mol−1, is shown to be almost perfectly in agreement with theory. Methyl methacrylate, on the other hand, is recalcitrant in that its overall kt does not make manifest the chain‐length dependent termination that has been directly measured by other techniques. Possible reasons for these discrepancies are discussed, as are reasons for the difference in values between kt for the two monomers. On the latter topic it is concluded likely that the chain‐length dependence of termination at short chain lengths is primarily responsible for styrene having kt that is higher by a factor of about 3, with there also being a contribution that arises from styrene's slower propagation.magnified image