Macromolecules

DFT Study on the Propagation Kinetics of Free-Radical Polymerization of α-Substituted Acrylates

I. Değirmenci, V. Aviyente, V. Van Speybroeck, M. Waroquier
Macromolecules
42 (8), 3033–3041
2009
A1

Abstract 

The kinetics of the free-radical propagation of methyl acrylate (MA), methyl methacrylate (MMA), ethyl α-fluoroacrylate (EFA), ethyl α-chloroacrylate (ECA), ethyl α-cyanoacrylate (ECNA), and methyl α-hydroxymethacrylate (MHMA) have been calculated using quantum chemical tools. Various DFT functionals such as BMK, BB1K, MPW1B95, MPW1K, and MPWB1K were used to model the relative propagation kinetics of the monomers. Among the methodologies used, MPWB1K/6-311+G(3df,2p)//B3LYP/6-31+G(d) was found to yield the best qualitative agreement with experiment. We explored chain length effects by examining addition reactions of monomeric, dimeric, trimeric, and tetrameric radicals to the monomers. We have also modeled the tacticity of the widely used monomers MA and MMA by considering all of the alternatives of attack of the radical in the 3D space around the monomer. This study has qualitatively confirmed the experimental syndiotactic/isotactic ratio of 66/3 for MMA. Finally, the kinetics of the initiation to polymerization for MA and MMA is also successfully reproduced.

Density Functional Theory Study of Free-Radical Polymerization of Acrylates and Methacrylates:  Structure−Reactivity Relationship

I. Değirmenci, D. Avci, V. Aviyente, K. Van Cauter, V. Van Speybroeck, G.B. Marin, M. Waroquier
Macromolecules
40 (26), 9590–9602
2007
A1

Abstract 

Radical polymerization processes occur through a complex network of many different reactions. It is well-known that the polymerization rate is directly related to the monomer structure. The experimental polymerizability behavior is expressed as kp/kt1/2, where kp is the rate coefficient of propagation and kt is the rate coefficient of termination. In this study, the reactivity of a series of acrylates and methacrylates is modeled in order to understand the effect of the pendant group size, the polarity of a pendant group, and the nature of the pendant group (linear vs cyclic) on their polymerizability. The geometries and frequencies are calculated with the B3LYP/6-31+G(d) methodology whereas the energetics and kinetics of these monomers have been studied using the two-component BMK/6-311+G(3df,2p)//B3LYP/6-31+G(d) level of theory. For rotations about forming/breaking bonds in the transition state, an uncoupled scheme for internal rotations has been applied with potentials determined at the B3LYP/6-31+G(d) level. Generally the rate constants for propagation kp mimic the qualitative polymerization trend of the monomers modeled and can be used with confidence in predicting the polymerizability behavior of acrylates. However in the case of 2-dimethylaminoethyl acrylate, chain transfer is found to play a major role in inhibiting the polymerization. On the other hand, the disproportionation reaction turns out to be too slow to be taken into consideration as a termination reaction.

Ab Initio Study of Free-Radical Polymerization:  Defect Structures in Poly(vinyl chloride)

K. Van Cauter, B. Van den Bossche, V. Van Speybroeck, M. Waroquier
Macromolecules
40 (4), 1321-1331
2007
A1

Abstract 

The main reaction routes that lead to the formation of structural defects in PVC are studied on a theoretical basis with the BMK/6-311+G(3df,2p)//B3LYP/6-31+G(d) method. All studied reactions can be classified into four classes:  the reactions following a head-to-head addition, intramolecular H-transfer (backbiting), and chain transfer reactions to polymer and to monomer. The head-to-tail propagation is the reference reaction for estimating the probability of the reaction routes leading to defect formation. Variations of chain length of the reacting polymer chain were taken into account in the calculations, leading to more than 100 studied reactions. The ab initio kinetic parameters, combined with typical monomer and polymer concentrations during suspension polymerization, serve as an input for the calculation of the defect concentrations that can be compared to the experimental data. This work supports the overall mechanism of defect formation during vinyl chloride polymerization as established experimentally.

Solvent Effects on Free Radical Polymerization Reactions: The Influence of Water on the Propagation Rate of Acrylamide and Methacrylamide

B. De Sterck, R. Vaneerdeweg, F. Du Prez, M. Waroquier, V. Van Speybroeck
Macromolecules
43 (2), 827–836
2010
A1

Abstract 

The polymerization of acrylamide (AA) and methacrylamide (MAA) was studied by an extensive set of computational methods with a particular focus on the possible influence of water molecules on the propagation reaction. An extensive set of electronic structure methods was tested, consisting of B3LYP, BMK, MPWB1K, MP2, and B2-PLYP of which some include dispersion effects. The effect of water on the transition state is modeled in two different ways. Explicit water molecules are added to the system, showing that replacing the hydrogen bond that dominates the transition state structure by a water-mediated hydrogen bond, results in more stable, more feasible transition states. This effect is the largest for AA polymerization, a monomer that is known to experience a larger solvent effect than MAA. Additionally, a conductor-like polarizable continuum model (C-PCM) is applied on both the transition states in gas phase and the ones bearing explicit water molecules. This model has a dramatic effect on all the propagation rates, raising them by about 3 orders of magnitude. The inclusion of explicit water molecules gives insight into the role of water molecules and the formation of prereactive complexes. The relative rate of polymerization of AA with regard to MAA is well reproduced for a trimeric propagating radical with inclusion of explicit water molecules or by using an implicit solvation model at the BMK and MPWB1K level of theory.

Modeling the Solvent Effect on the Tacticity in the Free Radical Polymerization of Methyl Methacrylate

I. Değirmenci, S. Eren, V. Aviyente, B. De Sterck, K. Hemelsoet, V. Van Speybroeck, M. Waroquier
Macromolecules
43 (13), 5602–5610
2010
A1

Abstract 

The control of stereochemistry in the free radical polymerization of methyl methacrylate (MMA) is important because the physical properties of PMMA are often significantly affected by the main-chain tacticity. In this study, the role of the solvent on the tacticity of MMA polymerization has been investigated by considering the propagation rate constants for the syndiotactic and isotactic free radical polymerization of MMA in vacuum, in methanol (CH3OH), and in 1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)propan-2-ol ((CF3)3COH). All geometry optimizations have been carried out with the B3LYP/6-31+G(d) methodology. The kinetics of the propagating dimer have been evaluated with the B3LYP/6-31+G(d), B3LYP/6-311+G(3df,2p), MPWB1K/6-311+G(3df,2p), and B2PLYP/6-31+G(d) methodologies. The role of the solvent has been investigated by using explicit solvent molecules and also by introducing a polarizable continuum model (IEF-PCM) with a dielectric constant specific to the solvent. Experimentally, the free radical polymerization of MMA in (CF3)3COH is found to be highly syndiotactic (rr = 75% at 20 °C): the stereoeffects of fluoroalcohols are claimed to be due to the hydrogen-bonding interaction of the alcohols with the monomers and growing species. This modeling study has revealed the fact that the solvents CH3OH and (CF3)3COH, which are H-bonded with the carbonyl oxygens located on the same side of the backbone hinder the formation of the isotactic PMMA to some extent. Methanol is less effective in reducing the isotacticity because of its small size and also because of the relatively loose hydrogen bonds (1.9 Å) with the carbonyl oxygens. The methodologies used in this study reproduce the solvent effect on the free radical polymerization kinetics of MMA in a satisfactory way.

Kinetic and Mechanistic Study on p-Quinodimethane Formation in the Sulfinyl Precursor Route for the Polymerization of Poly(p-phenylenevinylene) (PPV)

L. Hermosilla, S. Catak, V. Van Speybroeck, M. Waroquier, J. Vandenbergh, F. Motmans, P. Adriaensens, L. Lutsen, T. Cleij, D. Vanderzande
Macromolecules
43 (18), 7424–7433
2010
A1

Abstract 

The kinetics of p-quinodimethane formation in the sulfinyl precursor route for the poly(p-phenylenevinylene) (PPV) polymerization was studied using stop-flow UV−vis spectroscopy and theoretical first principle calculations. Different sulfinyl monomers were studied by means of quantitative kinetic experiments regarding the p-quinodimethane formation in 2-butanol. The influence of the solvent, the nature of the aromatic moiety, and the substituents on the phenyl core was analyzed by means of qualitative experiments. Quantitative measurements, using pseudo-first-order reaction conditions, were performed in order to assess the effect of the polarizer and the leaving group on the reaction rates. To obtain additional fundamental insight into the pathway leading to p-quinodimethane formation, density functional theory calculations were performed and subsequent reaction rate coefficients were determined from a theoretical point of view, enabling a profound comparison with experiment. From all these data, an E2 mechanism is proposed for the p-quinodimethane formation in the sulfinyl precursor route.

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