We have investigated phase behaviors of a series of polyphenylacetylene derivatives, poly[di(n- alkyl) ethynylterephthalates] (Pn, n is the number of carbon atoms of n-alkyl group, from 2 to 14), which are largely influenced by the length of n- alkyl tails on 2,5-position of the phenyl group. With short alkyl groups (n <= 6), Pns form columnar liquid crystalline (Col) phases due to the "jacketing effect" of side-chains. As the alkyl tails become longer (n > 8), an isotropic phase, which can be considered as a reentrant one (I-re), is identified between lamellar phase (Lam) at low temperatures and Col phase at high temperatures. This unusual phase behavior is determined by the motional state of the side-chains. Thanks to in-situ variable temperature solid-state NMR experiments, the motion of main- and side-chains in different phases were distinguished, providing strong evidence for the entropy effect of side-chains which drives I-re and then Col phase in Pns (n >= 8) upon heating. (C) 2016 Elsevier Ltd. All rights reserved.