  An evolution operator L_n with n arbitrary, typical of several models, is
analyzed. When n= 1, the operator characterizes the standard linear solid of
viscoelasticity, whose properties are already established in previous papers.
The fundamental solution {\epsilon}n of L_n is explicitly obtained and it's
estimated in terms of the fundamental solution {\epsilon}1 of L_1. So, whatever
n may be, asymptotic properties and maximum theorems are achieved. These
results are applied to the Rouse model and reptation model, which describe
different aspects of polymer chains.
