  A simple Rouse-type model, generalised to incorporate the effects of chemical
crosslinks, is used to obtain a theoretical prediction for the critical
behaviour of the normal-stress coefficients $\Psi_{1}$ and $\Psi_{2}$ at the
gelation transition. While the exact calculation shows $\Psi_{2}\equiv 0$, a
typical result for these types of models, an additional scaling ansatz is used
to demonstrate that $\Psi_{1}$ diverges with a critical exponent $\ell = k+z$.
Here, $k$ denotes the critical exponent of the shear viscosity and $z$ the
exponent governing the divergence of the time scale in the Kohlrausch decay of
the shear-stress relaxation function. For crosslinks distributed according to
mean-field percolation, this scaling relation yields $\ell =3$, in a accordance
with an exact expression for the first normal-stress coefficient based on a
replica calculation. Alternatively, using three-dimensional percolation for the
crosslink ensemble we find the value $\ell \approx 4.9$. Results on
time-dependent normal-stress response are also presented.
