The conformational transition behavior around the glass transition temperat
ure has been studied by means of molecular dynamics through a model system,
atactic polypropylene in bulk. Various criteria of the conformational tran
sition were examined. A domain at barrier site is defined as the transition
state of the conformers and the other conformational states are labeled in
code. The transition is accepted when the code is changed. Variation of th
e width of the domain at barrier distinguishes the deep jumps of the torsio
n rotations from the shallow jumps that do not reach the well bottom within
+/- 20 degrees from the minimum. The behavior of the deep jumps obeys the
rate equation of the transition-state theory, being independent of T-g. The
shallow jumps were found significant in the characterization of the motion
of polymer chains through T-g. This study thus proves that the changes of
properties of polymers resulting from the glass transition are predominatel
y mastered by the shallow jumps. (C) 2000 American Institute of Physics. [S
0021-9606(00)50204-9].