Maciej Bobrowski

Theoretical study of polymerization mechanism of p-xylylene based polymers.

The mechanism of polymerization of p-xylylene and its derivatives is analyzed at the theoretical level. The polymerization reaction takes place in vacuo without any catalyst. The first step is a pyrolytic decomposition of starting material for polymerization, p-cyclophane, a cyclic dimer of p-xylylene, into biradical linear dimer and finally into two quinonoid monomeric molecules of p-xylylene. The quinonoid monomer is diamagnetic; i.e., it has a singlet ground state. The monomers after pyrolysis, when the temperature is lowered, do not re-form cyclic dimers but instead polymerize into long chain molecules. The initiation of polymerization requires dimerization of two monomers leading to formation of a genuine noncoupled biradical dimer. The chain molecules grow through the propagation reaction only one unit at a time, by the attachment of a monomer to a radical chain end. In this work the pyrolysis reaction, the initiation reaction and the first propagation steps of parylene polymerization (up to pentamer) are studied in details using different quantum chemical methods: AM1 and PM6 semiempirical methods and density functional theory (DFT) approach using B3LYP functional with two basis sets of different size (SVP and TZVP).

Towards the Application of Structure-Property Relationship Modeling in Materials Science: Predicting the Seebeck Coefficient for Ionic Liquid/Redox Couple Systems.

This work focuses on determining the influence of both ionic-liquid (IL) type and redox couple concentration on Seebeck coefficient values of such a system. The quantitative structure-property relationship (QSPR) and read-across techniques are proposed as methods to identify structural features of ILs (mixed with LiI/I2 redox couple), which have the most influence on the Seebeck coefficient (Se ) values of the system. ILs consisting of small, symmetric cations and anions with high values of vertical electron binding energy are recognized as those with the highest values of Se . In addition, the QSPR model enables the values of Se to be predicted for each IL that belongs to the applicability domain of the model. The influence of the redox-couple concentration on values of Se is also quantitatively described. Thus, it is possible to calculate how the value of Se will change with changing redox-couple concentration. The presence of the LiI/I2 redox couple in lower concentrations increases the values of Se , as expected.

Predicting the viscosity and electrical conductivity of ionic liquids on the basis of theoretically calculated ionic volumes

Selected physical properties of the ionic liquids might be quantitatively predicted based on the volumes of the ions these systems are composed of. It is demonstrated that the ionic volumes calculated using relatively simple theoretical quantum chemistry methods can be utilised to estimate the viscosities and electrical conductivities of various commonly used ionic liquids. The fitting formulas of the exponential form are offered and their predictive usefulness is verified. The quality of such predictions is discussed on the basis of several ionic liquids involving [Tf2N]‑ and [BF4]‑ anions and 16 various cations. The dependence of the viscosity and electrical conductivity of the ionic liquids on the temperature is also investigated and the temperature-dependent equations are derived and compared to the experimentally measured values.

Molecular dynamics simulations of the growth of poly(chloro-para-xylylene) films.

Parylene C, poly(chloro-para-xylylene) is the most widely used member of the parylene family due to its excellent chemical and physical properties. In this work we analyzed the formation of the parylene C film using molecular mechanics and molecular dynamics methods. A five unit chain is necessary to create a stable hydrophobic cluster and to adhere to a covered surface. Two scenarios were deemed to take place. The obtained results are consistent with a polymer film scaling growth mechanism and contribute to the description of the dynamic growth of the parylene C polymer.

Polymerization of chloro-p-xylylenes, quantum-chemical study

The p-xylylene monomers of parylene N, C and D have similar high polymerization reactivity. For effective copolymerization processes this fact is basically a drawback and for instance the copolymerization with styrene doesn’t go at all (Corley et al. J Pol Sc 13(68):137–156, [15]). Substitution of terminal hydrogen atoms by chlorine atoms reduces reactivity dramatically. 7,7,8,8-tetrachloro-p-xylylene and 2,5,7,7,8,8-hexachloro-p-xylylene can be isolated as yellow crystals. These crystals can be kept without any change in temperature below 0 ∘C, but they polymerize slowly at room temperature. Perchloro-p-xylylene is stable even at elevated temperatures and does not polymerize under any conditions. Both 7,7,8,8-tetrachloro-p-xylylene and 2,5,7,7,8,8-hexachloro-p-xylylene copolymerize with various vinyl monomers, such as styrene and others. In this work the polymerization reactions of different chloro-derivatives of p-xylylene were modeled by means of the DFT method with hybrid correlation functionals (B3LYP and PBE0) and, for comparison, by means of the Hartree Fock methods. We inquired both initiation as well as elongation polymeric reactions for each of the reactants. We survied their reactivity analytically examining energetics and configurations in Szwarc-like process. The quantitative influence of chlorine atoms on the reactivity in polymerization steps, their location in the reactants’ structure (aromatic and/or aliphatic) as well as their number, were reviewed. The polymerizations of p-xylylenes with chlorine atoms as terminal aliphatic substituents yet revealed one more access path for parylenes’ in situ functionalization.

Molecular dynamics simulation of polymerization of p-xylylene

Poly-para-xylylene based polymers, also known as parylenes, are polymers that can be deposited in thin film form at room temperature by chemical vapor deposition (CVD). In the CVD process the p-xylylene monomers obtained by pyrolysis of paracyclophane polymerize into long chain molecules. The initiation of polymerization requires dimerization of two monomers leading to formation of a biradical dimer. The chain molecules grow through the propagation reaction only one unit at a time, by the attachment of a monomer to a radical chain end. In this work the polymerization of parylene is studied using all-atom molecular dynamics (MD) simulations. MD is extended with Monte Carlo step allowing for bond formation between two monomers or between an active radical chain end and a monomer. Simulations are performed for deposition of para-xylylene onto three different substrates: water, octane and silicon surface.

Ab initio study of the mechanism of singlet-dioxygen addition to hydroxyaromatic compounds: Negative evidence for the involvement of peroxa and endoperoxide intermediates

In this article we report our study of two possible mechanisms of photooxidation of hydroxyaromatic compounds, involving the intermediacy of zwitterionic peroxa intermediates or 1,4‐endoperoxides. To study the pathway of the first of them, as yet unexplored by theoretical methods, a simpler system composed of 1,3‐butadiene‐1‐ol and singlet (1Δg) dioxygen was considered first, for which calculations were carried out at the CASSCF/MCQDPT2 ab initio level, mostly with the 6‐31G* basis set. The cumulative activation barrier to this reaction was found to be 20 kcal/mol and corresponded to a proton transfer (from the hydroxy oxygen atom to the attached oxygen molecule) in the cyclic zwitterionic peroxacyclopenta‐3‐ene‐2‐ol intermediate. This intermediate and the proton‐transfer transition state were found to have a closed‐shell character, which enabled us to estimate the corresponding activation barrier for the phenol‐dioxygen system by carrying out optimization at the RHF level and single‐point calculations at the MP2, CASSCF, and MCQDPT2 levels of theory. The energy barrier to the reaction was estimated to at least about 40 kcal/mol, rendering this mechanism for the phenol‐oxygen system unlikely for nonpolar solvents. Similarly, calculations of the barrier to proton transfer from the 1,4‐endoperoxide of phenol to its hydroperoxide were found to exceed 60 kcal/mol, eliminating such a mechanism too, which leaves only the earlier postulated mechanisms involving an initial charge or hydrogen‐atom transfer to dioxygen as probable.

Reactions of parylenes with double bonds: An ab initio study

The reactions of the hydrogen atom transfer from various compounds (containing the same ethylene fragment in their framework) to the neutral di-para-xylylene dimer were examined at the B3LYP/6-31G level to estimate the corresponding kinetic barriers. Also, an alternative path (involving the π-bond cleavage) for such reactions were considered. It was found that the certain derivatives should easily react with the dimer at room temperature since the overall reactions are thermodynamically favorable and kinetically possible.