MOLECULAR-DYNAMICS OF MODEL COMPOUNDS OF POLYMERS WITH CHLOROCYCLOHEXYL GROUPS IN THEIR STRUCTURE

The conformational mobility of 2-chlorocyclohexyl isobutyrate (CCHI), a model compound for the repeating unit of vinyl polymers containing chlorocyclohexane residues as side groups, is analyzed employing molecular dynamics (MD) procedures. Close to room temperature (ca. 300 K), the interconversion between axial (i.e., both chlorine atom and eater group in axial positions) and equatorial (both substituents in equatorial orientations) is not observed within the total time of 5 ns allowed to the MD trajectories. The analysis was then performed at temperatures in the range 1000 to 1500 K and the results extrapolated to lower temperatures. These extrapolations give energetic barriers of 5.72 and 8.15 kca/mol, respectively for axial-->equatorial and equatorial-->axial transformations, with life times of tau(ax)approximate to 9.66 and tau 46.3 ns for these two conformations at 300 K. The same procedure applied to unsubstituted cyclohexane gives an energetic barrier of 10.6 kcal/mol for the chair to chair interconversion, in excellent agreement with literature values. Further extrapolation to the temperatures at which the beta subglass relaxation processes take place indicate that this interconversion is practically forbidden and therefore could not be invoked to explain the absorptions exhibited by this kind of polymers. The dipole moment of CCHI is also measured and calculated. Concordance between experimental (2.9+/-0.1 D) and calculated (2.7 D) values is very good. (C) 1995 American Institute of Physics.