Conformation analysis of poly(styrene-alt-maleic anhydride) in solvents

Conformation analyses of polymers lead to special difficulties in molecular modelling. Usually, the size of the structures to be modelled allows only the application of the field force method, in which the electrostatic inter actions are usually only unsatisfactorily considered. Furthermore, a confor mation analysis in vacuum leads to overestimation of intramolecular stabili zations. In the case of styrene-maleic anhydride presented in this work, st rong charge separations occur in the polymer chain. Furthermore, strong pol ymer-solution interactions occur which influence the stability of the confo rmations. To model the solvate shell, an atomistic Monte Carlo process was applied. U sing the EVOCAP (Excluded Volume Constrained Assembly Packing) method the c opolymer molecules were completely covered with solvent molecules. The part ial charge was calculated using the charge equilibration method. This metho d is based on a simple quantum mechanical ab initio approach and calculates the partial charges over an electronegativity balance. The solvate shell a ffects the electronegativity of single atoms within the polymer chain and s o their partial charges change against the vacuum. Beside the conformation energy in vacuum, the stabilization through the sol vate shell is thereby also included. The energy calculation was done in the MMFF94 force field. The stabilization of the polymer conformations in the solvents tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) were each ca lculated from 600 solvatation trials at 300 K for stretched and coiled vacu um conformations. The simulation confirmed the stronger extension of the po lymer chain in DMF as experimentally found through viscosimetric measuremen ts.