  A nano-second scale in situ probe reveals that a bulk linear polymer
undergoes a sharp phase transition as a function of the degree of conversion,
as it nears the glass transition. The scaling behaviour is in the same
universality class as percolation. The exponents \gamma and \beta are found to
be 1.7 \pm .1 and 0.41\pm 0.01 in agreement with the best percolation results
in three dimensions.
