We have investigated the structure and molecular motions of the side and main chains in the comblike polymer poly(1,4-phenylene-2,5-n-didecyloxy terephthalate) with 10 carbons in the alkyl side chain (PPDOT) with respect to its two polymorphic states by means of C-13 magic-angle spinning (MAS) NMR spectroscopy. Structure and polymorphism of such stiff-chain systems are relevant for many semiconducting polymers. Both modifications, called A and B, exhibit a layered morphology with well-ordered backbones forming pi-pi stacks separated by nanodomains of methylene sequences and are connected by a B-A solid-solid phase transition in the range 70-110 degrees C during heating. At ambient temperature, the polymorph A slowly converts back to the thermodynamically more stable modification B. We use C-13 CP MAS spectra to observe the structural changes during the phase transition and the structural differences in both modifications. In C-13 DIPSHIFT experiments, which inform about motionally averaged H-1-C-13 dipole-dipole couplings, we do not see any indication of a backbone-amorphous phase, and therefore we emphasize that the entire sample exhibits well-ordered pi-pi stacking for T < 180 degrees C. The polymorphs differ in the conformational statistics and dynamics of the side chains as well as in the structural conformation of the backbone. Above 180 degrees C we detect the formation of a liquid-crystalline phase, characterized by the loss of the p-p stacking and molecular motions of the side chains with correlation times tau(c) < 0.01 mu s.