  Semiflexible polymer glasses (SPGs), including those formed by the recently
synthesized semiflexible conjugated polymers (SCPs), are expected to be brittle
because classical formulas for their craze extension ratio $\lambda_{\rm
craze}$ and fracture stretch $\lambda_{\rm frac}$ predict that systems with
$N_e = C_\infty$ have $\lambda_{\rm craze} = \lambda_{\rm frac} = 1$ and hence
cannot be deformed to large strains. Using molecular dynamics simulations, we
show that in fact such glasses can form stable crazes with $\lambda_{\rm craze}
\simeq N_e^{1/4} \simeq C_\infty^{1/4}$, and that they fracture at
$\lambda_{\rm frac} = (3N_e^{1/2} - 2)^{1/2} \simeq (3C_\infty^{1/2} -
2)^{1/2}$. We argue that the classical formulas for $\lambda_{\rm craze}$ and
$\lambda_{\rm frac}$ fail to describe SPGs' mechanical response because they do
not account for Kuhn segments' ability to stretch during deformation.
