  A simple theory for glassy polymeric mechanical response which accounts for
large scale chain relaxation is presented. It captures the crossover from
perfect-plastic response to strong strain hardening as the degree of
polymerization $N$ increases, without invoking entanglements. By relating
hardening to interactions on the scale of monomers and chain segments, we
correctly predict its magnitude. Strain activated relaxation arising from the
need to maintain constant chain contour length reduces the $N$ dependence of
the characteristic relaxation time by a factor $\sim \dot\epsilon N$ during
active deformation at strain rate $\dot\epsilon$. This prediction is consistent
with results from recent experiments and simulations, and we suggest how it may
be further tested experimentally.
