K. Van Cauter

Ab Initio Study of Poly(vinyl chloride) Propagation Kinetics: Head-to-Head versus Head-to-Tail Additions

K. Van Cauter, V. Van Speybroeck, M. Waroquier
ChemPhysChem
8, 541-552
2007
A1

Abstract 

The relative importance of head-to-head versus head-to-tail addi-tions during the free-radical polymerization of vinyl chloride isdetermined by ab initio methods for different chain lengths ofthe polymer. First, a level of theory study is performed to deter-mine cost-effective methods for the ab initio description of thepropagation kinetics of vinyl chloride. The study includes the fol-lowing DFT-based methods: B3LYP, B3PW91, BHandH, BHandH-LYP, BLYP, BP86, MPW1K and MPW1PW91, in combination withdouble or triple zeta basis sets 6-31G(d) and 6-311GACHTUNGTRENNUNG(d,p). Also,the more recently developed BMK and MPW1K functionals are in-cluded. The influence of diffuse functions is tested by comparisonwith the basis sets 6-31+G(d) and 6-311++GACHTUNGTRENNUNG(3df,2p). The best-performing methods are B3LYP, B3PW91 and MPW1K combinedwith the 6-31+G(d) basis set. The converged probability of head-to-head propagation (2 per 1000 monomer units) is put into rela-tion with the experimental concentrations of defect structures. Acomparison is made with the head-to-head (HH) content of fluo-rine-substituted polymers and poly(vinyl acetate). The ab initiocalculations correctly predict the relative sequence of HH contentamong the various polymers.

Open Access version available at UGent repository

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.

Ab Initio Study of Free-Radical Polymerization: Polyethylene Propagation Kinetics

K. Van Cauter, V. Van Speybroeck, P. Vansteenkiste, M-F. Reyniers, M. Waroquier
ChemPhysChem
7 (1), 131-140
2006
A1

Abstract 

The chain-length dependence of the propagation rate coefficient for the free-radical polymerization of ethylene was investigated on an ab initio basis. Polyethylene was chosen as a test system because of its structural simplicity. Ab initio density functional theory at the B3LYP/6-31g(d) level was applied to study the kinetics of a set of addition reactions of a systematically growing radical alkyl chain to ethylene. These reactions are propagation steps in the free-radical polymerization of ethylene. Special attention was paid to low normal modes corresponding to internal rotations (IR), since the latter are important for an accurate description of the partition functions. The effect of coupling of the IR modes is also discussed. A comparison is made with the propagation rate constant derived from experiment. The results indicate that the propagation rate coefficient has largely converged by the hexyl radical stage, though a weaker chain-length dependence of kpfor longer chains was detected.

Comparative study of kinetics and reactivity indices of free radical polymerization reactions

K. Van Cauter, K. Hemelsoet, V. Van Speybroeck, M. Waroquier
International Journal of Quantum Chemistry
102 (4), 454-460
2005
A1

Abstract 

Density functional theory calculations are used to determine the kinetics and reactivity indices of the first propagation steps of the polyethylene and poly(vinyl chloride) polymerization. Transition state theory is applied to evaluate the rate coefficient from the microscopically determined energies and partition functions. A comparison with the experimental Arrhenius plots validates the level of theory. The ability of reactivity indices to predict certain aspects of the studied propagation reactions is tested. Global softnesses of the reactants give an indication of the relative energy barriers of subsequent monomer additions. The correlation between energy and hardness profiles along the reaction path confirm the principle of maximum hardness. Local indices predict the regioselectivity of the attack of the growing radical to vinyl chloride. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Ab Initio Study of Free-Radical Polymerizations: Cost-Effective Methods to Determine the Reaction Rates

V. Van Speybroeck, K. Van Cauter, B. Coussens, M. Waroquier
ChemPhysChem
6 (1), 180-189
2005
A1

Abstract 

The addition of carbon-centered radicals to ethene, which are important in free-radical polymerization processes, are studied from a theoretical point of view. Experimental data for the rate constants are only available for the addition of methyl, ethyl, propyl and butyl radicals. The latter reactions are taken as model systems to derive a cost-effective method for the addition of alkyl radicals to ethene. The proposed model must be accurate and computationally feasible for additions in which larger radicals are involved. Accuracy is validated by direct comparison of theoretical and experimental rate constants in the temperature range from 300 to 600 K. A variety of electronic-structure methods were tested ranging from Hartree–Fock and post-Hartree–Fock methods to pure and hybrid density functional theory methods. Molecular partition functions were refined by treating large amplitude vibrations beyond the harmonic oscillator approximation.

Pagina's

Abonneren op RSS - K. Van Cauter