Molecular Dynamics Simulation of C–C Bond Scission in Polyethylene and Linear Alkanes: Effects of the Condensed Phase

The reaction of C–C bond scission in polyethylene chains of various lengths was studied using molecular dynamics under the conditions of vacuum and condensed phase (polymer melt). A method of assigning meaningful rate constant values to condensed-phase bond scission reactions based on a kinetic mechanism accounting for dissociation, reverse recombination, and diffusional separation of fragments was developed. The developed method accounts for such condensed-phase phenomena as cage effects and diffusion of the decay products away from the reaction site. The results of C–C scission simulations indicate that per-bond rate constants decrease by an order of magnitude as the density of the system increases from vacuum to the normal density of a polyethylene melt. Additional calculations were performed to study the dependence of the rate constant on the length of the polymer chain under the conditions of the condensed phase. The calculations demonstrate that the rate constant is independent of the degree of polymerization if polyethylene samples of different lengths are kept at the same pressure. However, if instead molecular systems of different polyethylene chain lengths decompose under the conditions of the same density, shorter chains result in higher pressures and lower rate constants. The observed effect is attributed to a higher degree of molecular crowding (lower fraction of free intermolecular space available for molecular motion) in the case of shorter molecules

Kinetics of the Self Reaction of Cyclopentadienyl Radicals

The kinetics of the self-reaction of cyclopentadienyl radicals (c-C₅H₅) was studied by laser photolysis/photoionization mass spectroscopy. Overall rate constants were obtained in direct real-time experiments in the temperature region 304–600 K and at bath gas densities of (3.00–12.0) × 10¹⁶ molecules cm^–3. The room-temperature value of the rate constant, (3.98 ± 0.41) × 10^–10 cm³ molecule^–1 s^–1, is significantly higher than the rate constants for most hydrocarbon radical–radical reactions and coincides with the estimated collision rate. The observed overall c-C₅H₅ + c-C₅H₅ rate constant demonstrates an unprecedented strong negative temperature dependence: <i>k</i>₁ = 2.9 × 10^–12 exp­(+1489 K/T) cm³ molecule^–1 s^–1, with estimated uncertainty increasing with temperature, from 13% at 304 to 32% at 600 K. Formation of C₁₀H₁₀ as the primary product of cyclopentadienyl self-reaction was observed. In additional experiments performed at the temperature of 800 K, formation of C₁₀H₁₀, C₁₀H₉, and C₁₀H₈ was observed. Final product analysis by gas chromatography/mass spectrometry detected two isomers of C₁₀H₈ at 800 K: naphthalene (major) and azulene (minor)

Initial Stages of the Pyrolysis of Polyethylene

An experimental study of the kinetics of the initial stages of the pyrolysis of high-density polyethylene (PE) was performed. Quantitative yields of gas-phase products (C₁–C₈ alkanes and alkenes) and functional groups within the remaining polyethylene melt (methyl, vinyl, vinylene, vinylidene, and branching sites) were obtained as a function of time (0–20 min) at five temperatures in the 400–440 °C range. Gas chromatography and NMR (¹H and ¹³C) were used to detect the gas- and condensed-phase products, respectively. Modeling of polyethylene pyrolysis was performed, with the primary purpose of determining the rate constants of several critical reaction types important at the initial pyrolysis stages. Detailed chemical mechanisms were created (short and extended mechanisms) and used with both the steady-state approximation and numerical integration of the differential kinetic equations. Rate constants of critical elementary reactions (C–C backbone scission, two kinds of H-atom transfer, radical addition to the double bond, and beta-scission of tertiary alkyl radicals) were adjusted, resulting in an agreement between the model and the experiment. The values of adjusted rate constants are in general agreement with those of cognate reactions of small molecules in the gas phase, with the exception of the rate constants of the backbone C–C scission, which is found to be approximately 1–2 orders of magnitude lower. This observation provides tentative support to the hypothesis that congested PE melt molecular environment impedes the tumbling motions of separating fragments in C–C bond scission, thus resulting in less “loose” transition state and lower rate constant values. Sensitivity of the calculations to selected uncertainties in model properties was studied. Values and estimated uncertainties of four combinations of rate constants are reported as derived from the experimental results via modeling. The dependence of the diffusion-limited rate constant for radical recombination on the changing molecular mass of polyethylene was explicitly quantified and included in the extended kinetic mechanism, which appears critical for the agreement between modeling and experiment, particularly the agreement between the experimental and the calculated activation energies for product formation rates. Calculations were performed to estimate the contribution to the overall rate of radical recombination of the “reaction diffusion” phenomenon, where recombination is driven not by the actual motion of the recombining radical sites but rather by the migration of the radical site through PE melt due to rapid hydrogen transfer; this contribution was shown to be negligible for the conditions of the current work