Showing papers by "Thomas P. Davis published in 2001"

Kinetic Investigations of Reversible Addition Fragmentation Chain Transfer Polymerizations: Cumyl Phenyldithioacetate Mediated Homopolymerizations of Styrene and Methyl Methacrylate

Abstract: A previously published simulation and data fitting procedure for the reversible addition fragmentation chain transfer (RAFT) process using the PREDICI simulation program has been extended to cumyl phenyldithioacetate mediated styrene and methyl methacrylate (MMA) bulk homopolymerizations. The experimentally obtained molecular weight distributions (MWDs) for the styrene system are narrow and unimodal and shift linearly with monomer conversion to higher molecular weights. The MMA system displays a hybrid of conventional chain transfer and living behavior, leading to bimodal MWDs. The styrene system has been subjected to a combined experimental and modeling study at 60 °C, yielding a rate coefficient for the addition reaction of free macroradicals to polymeric RAFT agent, kβ, of approximately 5.6 × 105 L mol-1 s-1 and a decomposition rate coefficient for macroradical RAFT species, k-β, of about 2.7 × 10-1 s-1. The transfer rate coefficient to cumyl phenyldithioacetate is found to be close to 2.2 × 105 L mol-...

Modeling the reversible addition–fragmentation chain transfer process in cumyl dithiobenzoate‐mediated styrene homopolymerizations: Assessing rate coefficients for the addition–fragmentation equilibrium

Abstract: A full kinetic scheme for the free-radical reversible addition-fragmentation chain transfer (RAFT) process is presented and implemented into the program package PREDICI (R). With the cumyl dithiobenzoate-mediated bulk polymerization of styrene at 60 degreesC as an example, the rate coefficients associated with the addition-fragmentation equilibrium are deduced by the careful modeling of the time-dependent evolution of experimental molecular weight distributions. The rate coefficient for the addition reaction of a free macroradical to a polymeric RAFT species (k(beta)) is approximately 5 . 10(5) L mol(-1) s(-1), whereas the fragmentation rate coefficient of the formed macroradical RAFT species is close to 3 . 10(-2) s(-1). These values give an equilibrium constant of K = k(beta)/k(-beta) = 1.6 . 10(7) L mol(-1). Conclusive evidence is given that the equilibrium lies well on the side of the macroradical RAFT species. The high value of k(beta) is comparable in size to the propagation rate coefficients reported for acrylates. The transfer rate coefficient to cumyl dithiobenzoate is close to 3.5 . 10(5) L mol(-1) s(-1). A careful sensitivity analysis was performed, which indicated that the reported rate coefficients are accurate to a factor of 2. (C) 2001 John Wiley & Sons, Inc.

Kinetic investigations of reversible addition fragmentation chain transfer polymerizations: Cumyl phenyldithioacetate mediated homopolymerizations of styrene and methyl methacrylate

Abstract: A previously published simulation and data fitting procedure for the reversible addition fragmentation chain transfer (RAFT) process using the PREDICI simulation program has been extended to cumyl phenyldithioacetate mediated styrene and methyl methacrylate (MMA) bulk homopolymerizations. The experimentally obtained molecular weight distributions (MWDs) for the styrene system are narrow and unimodal and shift linearly with monomer conversion to higher molecular weights. The MMA system displays a hybrid of conventional chain transfer and living behavior, leading to bimodal MWDs. The styrene system has been subjected to a combined experimental and modeling study at 60 °C, yielding a rate coefficient for the addition reaction of free macroradicals to polymeric RAFT agent, kβ, of approximately 5.6 × 105 L mol-1 s-1 and a decomposition rate coefficient for macroradical RAFT species, k-β, of about 2.7 × 10-1 s-1. The transfer rate coefficient to cumyl phenyldithioacetate is found to be close to 2.2 × 105 L mol-1 s-1. The MMA system has been studied over the temperature range 25-60 °C. The hybrid behavior observed in the MMA polymerizations has been exploited (at low monomer conversions) to perform a Mayo analysis allowing the determination of the temperature dependence of the transfer to cumyl phenyldithioacetate reaction. The activation energy of this process is close to 26 kJ mol-1. In contrast to the styrene system, the PREDICI simulation procedure cannot be successfully applied to cumyl phenyldithioacetate mediated MMA polymerizations for the deduction of kβ and k-β. This inability is due to the hybrid nature of the cumyl phenyldithioacetate-MMA system, leading to a significantly reduced sensitivity toward kβ and k-β.

Star-polymer synthesis via radical reversible addition-fragmentation chain-transfer polymerization

Abstract: Polystyrene stars were synthesized by reversible addition-fragmentation chain-transfer (RAFT) polymerization using hexakis(thiobenzoylthiomethyl)benzene (I) as a hexafunctional RAFT agent at 80, 100, and 120 degreesC. The polymerizations conformed to pseudo-first-order kinetic behavior. TI-ie molecular weight distributions displayed characteristics consistent with a living radical process. A number of salient features were observed in the molecular weight distributions with the star distribution accompanied by a linear polymer-chain distribution and shoulders on the distributions that can be attributed to radical-radical-termination events. The evidence suggests that high temperatures are required to activate all the RAFT active sites on I, and a hypothesis proposes that there is significant steric hindrance in the initial stages of the RAFT process with I.

Synthesis of Poly(styrene) Star Polymers Grown from Sucrose, Glucose, and Cyclodextrin Cores via Living Radical Polymerization Mediated by a Half-Metallocene Iron Carbonyl Complex

Abstract: Poly(styrene) stars with 5, 8, and 18 arms were synthesized using living radical polymerization from iodinated glucose, sucrose, and cyclodextrin initiator cores, respectively The polymerization system comprised of a half-metallocene iron carbonyl complex coupled with titanium(IV) isopropoxide The reaction kinetics and the molecular weight development were consistent with a living/controlled radical polymerization mechanism Poly(styrene) stars with a very narrow molecular weight distribution were obtained Molecular weight analysis by gel permeation chromatography and NMR confirmed that the star structure was consistent with theoretical predictions The star structure of the polymers was further verified by hydrolysis of the cores to retrieve the polystyrene arms, followed by molecular weight analysis

Free-radical copolymerization of styrene and itaconic acid studied by 1H NMR kinetic experiments

Abstract: The free-radical copolymerization of itaconic acid (IA) and styrene in solutions of dimethylformamide and d(6)-dimethyl sulfoxide (50 wt %) has been studied by H-1 NMR kinetic experiments. Monomer conversion versus time data were used to estimate the ratio k(p).k(t)(-0.5) for various comonomer mixture compositions. The ratio K-p.k(t)(-0.5) varies from 5.2.10(-2) for pure styrene to 2.0.10(-2)mol(0.5)L(-0.5)s(-0.5) for pure IA, indicating a significant decrease in the rate of polymerization. Individual monomer conversion versus time traces were used to map out the comonomer mixture-composition drift up to overall monomer conversions of 60%. Within this conversion range, a slight but significant depletion of styrene in the monomer feed can be observed. This depletion becomes more pronounced at higher levels of IA in the initial comonomer mixture. The kinetic information is supplemented by molecular weight data for IA/ styrene copolymers obtained by variation of the comonomer mixture composition. A significant decrease in molecular weight of a factor of 2 can be observed when increasing the mole fraction of IA in the initial reaction mixture from 0 to 0.5. (C) 2001 John Wiley & Sons, Inc.

Free-radical copolymerization of styrene and itaconic acid studied by1H NMR kinetic experiments

Abstract: The free-radical copolymerization of itaconic acid (IA) and styrene in solutions of dimethylformamide and d6-dimethyl sulfoxide (50 wt %) has been studied by 1H NMR kinetic experiments. Monomer conversion versus time data were used to estimate the ratio kp · kt -0.5 for various comonomer mixture compositions. The ratio kp · kt -0.5 varies from 5.2 ·10-2 for pure styrene to 2.0 · 10-2 mol-0.5 L-0.5 s-0.5 for pure IA, indicating a significant decrease in the rate of polymerization. Individual monomer conversion versus time traces were used to map out the comonomer mixture-composition drift up to overall monomer conversions of 60%. Within this conversion range, a slight but significant depletion of styrene in the monomer feed can be observed. This depletion becomes more pronounced at higher levels of IA in the initial comonomer mixture. The kinetic information is supplemented by molecular weight data for IA/ styrene copolymers obtained by variation of the comonomer mixture composition. A significant decrease in molecular weight of a factor of 2 can be observed when increasing the mole fraction of IA in the initial reaction mixture from 0 to 0.5. © 2001 John Wiley & Sons, Inc.

Pulsed‐laser polymerization‐gel permeation chromatographic determination of the propagation‐rate coefficient for the methyl acrylate dimer: A sterically hindered monomer

Abstract: The methyl acrylate dimer (MAD) is a sterically hindered macromonomer, and the propagating radical can fragment to an unsaturated end group. The propagation-rate coefficient (k(p)) for MAD was obtained by pulsed-laser polymerization (PLP). The Mark-Houwink-Sakaruda parameters required for the analysis of the molecular weight distributions (MWDs) were obtained by multiple-detector gel permeation chromatography (GPC) with on-line viscometry. The small radical created by the fragmentation results in a short-chain polymer that means the MWD may no longer be given by that expected for "ideal" PLP conditions; simulations suggest that the degree of polymerization required for "ideal" PLP conditions can be obtained from the primary point of inflection provided the GPC traces also show a clear secondary inflection point (radicals terminated by the second, rather than the first, pulse subsequent to initiation). Over the temperature range of 40-75 degreesC, the data can be best fitted by k(p)/dm(3) mol(-1) s(-1) = 10(6.1) exp(-29.5 kJ mol (1)), with a moderately large joint confidence interval for the Arrhenius parameters. The data are consistent with an increased activation energy and reduced frequency factor as compared with acrylate or methacrylate; both of these changes can be ascribed to hindrance. (C) 2001 John Wiley & Sons, Inc.

Copolymerization Behavior of 7-Methylene-2-methyl-1,5-dithiacyclooctane: Reversible Cross-Propagation

Abstract: The copolymerization behavior of 7-methylene-2-methyl-1,5-dithiacyclooctane (MDTO) with a range of common comonomers has been investigated and found to be highly unusual. The resulting copolymers contained blocks of homo-poly(MDTO), even at low MDTO feed ratios. Changes in temperature and monomer concentration have a significant effect on the copolymer composition of methyl methacrylate-MDTO and styrene-MDTO mixtures and therefore on the apparent reactivity ratios. The magnitude of the concentration effect is independent of the solvent. A new copolymerization mechanism is proposed in which the addition of an MDTO radical- to the comonomer is reversible while all other propagation reactions are effectively irreversible. To our knowledge, this is the first observation of such a mechanism. Parameters for MDTO-M-2 (M-2 = STY,MMA) according to this model are reported as (r(MDTO), r(2), k(-12)/k(22)) = (0.06, 7, 17; M-2 = STY) and (0.38, 3.2, 2.5; M-2 = MMA) at 60 degreesC.

Copolymerization Propagation Kinetics of Dimethyl Itaconate and Styrene: Strong Entropic Contributions to the Penultimate Unit Effect

Abstract: In this work, pulsed-laser polymerization has been utilized to measure average propagation rate coefficients, (k(p)), in the bulk copolymerization reaction of dimethyl itaconate and styrene at 20 degreesC. The measured (k(p)) values were substantially higher (up to a factor of 3) than those predicted by the terminal model. This unusual result is attributed to strong entropic penultimate unit effects on the DMI-terminal radicals. We discuss the significance of this result particularly for other copolymerization systems where there are large size differences between the monomers and for systems where the comonomer substituents are similar (for instance, acrylate-methacrylate pairs), where the enthalpic penultimate unit effect is diminished and the entropic effect may become observable.

The mechanism of propagation in free‐radical copolymerization

Abstract: A brief overview of free-radical copolymerization kinetics is presented. Recent developments have highlighted the shortcomings of the terminal (or Mayo–Lewis) model. However, for many practical reasons, approximate models are useful for assisting our understanding of any given copolymerization reaction and allowing comparisons with the vast database on copolymerization reactions. Terminal model reactivity ratios have limited meaning, so it may be imprudent to use these values as a quantitative measurement of radical reactivity. Presently, experimental limitations restrict the amount of information that can be obtained by models being fitted to composition or propagation rate data; therefore, the direct measurement of radical reactivity is required together with high-level quantum calculations. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 597–603, 2001

Investigation of the penultimate unit effect in halogen atom chain transfer in free radical copolymerization

Abstract: Chain transfer constants to carbon tetrachloride (CCl4) and carbon tetrabromide (CBr4) were measured for the STY-MMA copolymerization system at 40, 50, and 60 degreesC, and Arrhenius parameters were determined across this range of temperatures. A value for Cs of CBr4 in STY (2.8 x 10(2)) was determined using Bamford's moderated copolymerization method. The chain transfer constants obtained were fitted to a terminal model and a penultimate unit effect (PUE) model, of which only the PUTE model provided a realistic fit to the data. The chain transfer reaction of STY-STY-terminated radicals to CCl4 at 60 degreesC was found to be 3.3 times faster than that of MM-A-STY-terminated radicals. With CBr4 at the same temperature, STY-STY-terminated radicals reacted 9.4 times faster than MMa-STY-terminated radicals. Subsequently, the effect of temperature on the PUE was quantified. Additional experiments on CCl4 were performed using dimethylformamide (DMF) as solvent. These experiments showed an increase in the magnitude of both the chain transfer constants and the PUE in DMF compared to the bulk experiments (from a factor of 3.3-7.2 times faster), suggesting the involvement of a polar transition state in transfer to CCl4 and a polar contribution to the penultimate unit effect. We also discuss the effect that polar stabilization may have in atom transfer radical polymerization.

Using Kinetics and Thermodynamics in the Controlled Synthesis of Low Molecular Weight Polymers in Free-Radical Polymerization

Abstract: A simple way of controlling molecular weights in the free-radical copolymerization of styrene and alpha -methylstyrene (AMS) is presented and investigated ly simulation via the program package PREDICI(R). It is shown that the molecular weight of the product copolymers may be varied in a wide range (from (M) over bar (w) = 500 to 2.10(6)) by variation of the comonomer feed ratio and the reaction temperature. The reasons for this simple molecular weight control are associated with the AMS comonomer and are threefold: (i) AMS has a low propagation rate coefficient, due to the increased steric requirements of the monomer. (ii) AMS has a high transfer to monomer constant(C-M) in comparison with styrene and (iii) AMS has a low ceiling temperature, so that the effective propagation rate; coefficient decreases with increasing temperature. In addition to the styrene/AMS system, other comonomer ems showing similar kinetic and thermodynamic features (e.g. the styrene/methyl ethacrylate (MEA) system) may also be used to generate a wide range of molecular weights. The possibilities for controlling molecular weight and end group functionalities by replacing the slowly propagating monomer by a functional monomer are discussed.

A novel method for the measurement of chain transfer to monomer constants in styrene homopolymerizations: The pulsed laser rotating reactor assembly

Abstract: The chain transfer to monomer constants (C-m) in styrene bulk polymerizations have been determined in the temperature range between 25 and 90 degreesC using both thermal polymerization and the novel pulsed laser polymerization (PLP) with extended dark times. This novel technique determines transfer to monomer rate coefficients by combining a pulsed laser setup with a rotating reactor/cuvette assembly. The resulting transfer controlled molecular weight distributions (MWD) were analyzed by the well-known Mayo and chain length distribution (CLD) methods. The results of both experimental methods agree well, which indicates that PLP with extended dark times is a reliable technique to determine transfer to monomer constants. For this reason, we report average values for the activation parameters determined from both thermal and extended dark times PLP experiments: E-a = 21.6 kJ mol(-1) and A = 0.22 L mol(-1) s(-1). These numbers are in excellent agreement with those reported in the literature.

A novel method for the measurement of chain transfer to monomer constants in styrene homopolymerizations: The pulsed laser rotating reactor assembly

Abstract: The chain transfer to monomer constants (Cm) in styrene bulk polymerizations have been determined in the temperature range between 25 and 90 °C using both thermal polymerization and the novel pulsed laser polymerization (PLP) with extended dark times. This novel technique determines transfer to monomer rate coefficients by combining a pulsed laser setup with a rotating reactor/cuvette assembly. The resulting transfer controlled molecular weight distributions (MWD) were analyzed by the well-known Mayo and chain length distribution (CLD) methods. The results of both experimental methods agree well, which indicates that PLP with extended dark times is a reliable technique to determine transfer to monomer constants. For this reason, we report average values for the activation parameters determined from both thermal and extended dark times PLP experiments: Ea = 21.6 kJ mol-1 and A = 0.22 L mol-1 s-1. These numbers are in excellent agreement with those reported in the literature.

Catalytic chain transfer isomerization reactions involving 2-phenylallyl alcohol

Abstract: A catalytic chain transfer agent, bis[(difluoroboryl)dimethylglyoximato]cobalt(II) (COBF), was utilized in free radical polymerizations involving 2-phenylallyl alcohol (PAA). In homopolymerization only oligomers were formed, and the predominant product was 2-phenylpropanal consistent with a radical-induced isomerization reaction. In copolymerizations with methyl methacrylate (PAA included at 5 and 10 mol %) the chain transfer constants increased significantly (compared to those obtained for methyl methacrylate homopolymerization). NMR analyses of the co-oligomer chains confirmed PAA incorporation into the copolymers and provided evidence for the presence of an aldehyde group resulting from chain transfer isomerization. Further analyses by matrix-assisted laser desorption ionization (MALDI) mass spectrometry indicated conclusively that a PAA monomer unit was incorporated into copolymer chains. The thermal degradation behavior of the oligomers was measured using thermogravimetric analysis (TGA). The thermograms clearly indicate that the PAA is situated at the chain ends of the majority of the polymer chains even at low concentrations of PAA in the initial monomer feed.

Complications in the 355 nm pulsed-laser polymerization of N-vinylcarbazole

Abstract: The propagation kinetics of N-vinylcarbazole (NVC) were carefully investigated via the IUPAC-recommended pulsed-laser polymerization/size-exclusion chromatography technique (PLP-SEC) in the temperature range between -20 and 20 degreesC using 355 nm pulsed irradiation and the photo initiator 2.2-dimethoxy-2-phenylacetophenone (DMPA) as a source of primary radicals. Using this experimental approach, propagation rate coefficients, kp, were not accessible for temperatures exceeding 20 degreesC. There is strong evidence that the monomer itself is excited by pulsed-laser light of 355 nm, thus contributing to the polymerization process via the formation of free radicals. In addition, UV light-induced cationic polymerization processes can not be ignored as a possible side reaction. NVC polymer also absorbs strongly at 355 nm and we speculate that this may lead to bond scission and branch network formation in the PLP process. Laser-controlled molecular weight distributions are only obtained for reaction temperatures below 20 degreesC. The apparent Arrhenius parameters, E-A and A, are 22.8 kJ . mol(-1) and 3.6 x 10(7) L . mol(-1) . s(-1), respectively. These results are divergent from recent literature data.

Complications in the 355 nm pulsed-laser polymerization of N-vinylcarbazole

Abstract: The propagation kinetics of N-vinylcarbazole (NVC) were carefully investigated via the IUPAC-recommended pulsed-laser polymerization/size-exclusion chromatography technique (PLP-SEC) in the temperature range between -20 and 20 °C using 355 nm pulsed irradiation and the photo initiator 2.2-dimethoxy-2-phenylacetophenone (DMPA) as a source of primary radicals. Using this experimental approach, propagation rate coefficients, kp, were not accessible for temperatures exceeding 20 °C. There is strong evidence that the monomer itself is excited by pulsed-laser light of 355 nm, thus contributing to the polymerization process via the formation of free radicals. In addition, UV light-induced cationic polymerization processes can not be ignored as a possible side reaction. NVC polymer also absorbs strongly at 355 nm and we speculate that this may lead to bond scission and branch network formation in the PLP process. Laser-controlled molecular weight distributions are only obtained for reaction temperatures below 20 °C. The apparent Arrhenius parameters, EA, and A, are 22.8 kJ · mol-1 and 3.6 x 107 L · mol-1 · s-1, respectively. These results are divergent from recent literature data.