Carboxylic acid groups were introduced into polystyrene, and the effect both on melt rheology and on mechanical properties of stretched and quenched anisotropic solids below the glass-transition temperature (T-g) was investigated. First, the facile preparation of well-defined copolymers of styrene (S) and 4-vinylbenzoic acid (A) by reversible addition-fragmentation chain-transfer polymerization was demonstrated. The evaluation of monomer reactivity ratios shows that the acid polymerizes faster than styrene but shows that block formation is suppressed under the applied conditions. Addition of acid groups leads to an increase in T-g, and this is documented by the detection of acid dimers in the glassy polymers. Importantly, linear viscoelastic measurements confirm that all the tested samples have a similar number of entanglements per chain (Z), which suggests that the acid groups do not form hydrogen bonds at temperatures above T-g. These are further confirmed by nonlinear extensional rheology of the samples in uniaxial extensional flow. Stretching melts of these polymers at a rate faster than the inverse Rouse time followed by rapid quenching below T-g leads to polymer fibers that remain highly flexible in analogy with nonmodified polystyrene. However, the resulting acid-containing polymer fibers have higher yield stress and ductility at room temperature compared to the nonmodified polystyrene fibers. This approach presents a facile route to improve mechanical properties of anisotropic glassy polystyrene.