A quantum mechanical study on polymer flexibility: Extended model from monomer to tetramer of 2- and 4-bromostyrenes

DFT Quantum chemical descriptors as dipole and quadrupole momenta as well as molecular volume have been calculated for the monomer, dimer and tetramer of 2- and 4-bromostyrene in order to study the effect of small changes in their molecular structure on the glass transition temperature of the corresponding polymers. In addition, correlation with the chain stiffness has been pursued by analysing torsional barriers both for monomer and oligomers of the title compounds obtained at ab initio and DFT levels of theory. First of all, the performance of the theoretical methods has been checked by optimizing the molecular geometry of the monomer and comparing with experimental data. The internal rotation barrier of the vinyl moiety has been also calculated and a conformational analysis has been afforded in detail through different energy factorization schemes, i.e., the total kinetic and potential scheme and that provided by the NBO theory. In addition, the topological analysis of the electron density provided by the Atom-in-Molecules, AIM, theory has allowed us to rationalize the stable conformers on the basis of bond critical points and ring critical points featuring intramolecular contacts. The most stable conformations of the dimer and tetramer have been determined as models for their respective polymers. The values of the central dihedral angles for the most populated dimer and tetramer of both 2- bromo and 4-bromostyrene indicate that the position of the halogen hardly affects the backbone chain arrangement. Predictive performance of the internal rotation barriers as well as the molecular volume and dipole and quadrupole momenta have been assessed through comparison with the experimental Tg value for the title compounds. As a conclusion, although the internal rotation barrier does not provide conclusive results for the higher chain flexibility in the case of poly-2-bromostyrene, the dipole and quadrupole momenta, as well as the molecular volume calculated for the monomer, dimer and tetramer follow the same trend as the measured Tg for poly-2-bromostyrene and poly-4-bromostyrene demonstrating this way the capability of those descriptors for predicting small variations in glass transition temperatures.