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.