Acyclic diene metathesis (ADMET) polymerization was used as a new synthetic route to a class of processable, hydrophobic, and crystalline polymers which contain only aromatic and aliphatic hydrocarbon units in their backbone. Poly(p-phenylene octylene) (PPPO), as a first example, was prepared by the ADMET polymerization of 1,4-bis(pent-4-ene)benzene, followed by the catalytic reduction of the resulting unsaturated polymer. The weight-average molecular weights of the polymers obtained with this method were in the range of 13 000-25 000. Poly(p-phenylene octylene) was found to be highly soluble in common solvents and to have a melting temperature of around 90 degrees C, which enabled melt processing into fibers and films without decomposition. Wide-angle X-ray diffraction and annealing experiments on PPPO revealed two different crystal modifications. The polymer, as precipitated from the reaction mixture, predominantly forms the kinetically preferred modification alpha, while the annealing of pristine PPPO almost exclusively leads to the thermodynamically more stable modification beta.