Freely standing polystyrene films show an anomalous drop of the glass tempe
rature T-g when the molecular weight is high and the thickness smaller than
the coil size R-0. We present here a tentative explanation for these featu
res, where two types of motions compete: a) standard motions, controlled by
the free volume, and independent of chain length! b) collective motions al
ong the chain, which require a weaker free volume (except for the end group
s). For bulk systems, the standard motion always wins because of the end gr
oup hindrance. But for films thinner than the coil size, the dominant proce
ss may be the collective motion of a "loop" which does not involve the chai
n ends. What matters then is not the overall polymerisation index (N), but
the length g of a typical loop starting from the surface (which is a more f
luid region) and reaching deep into the film. With these ingredients, some
surprising aspects of T-g(h, N) may possibly be understood.