F1 is hydrogen and F2 is hydrogen or methyl, the repeating units being the same or different, and (ii) at least one terminal unit which has the formula wherein Cy consists of an n-alkyl moiety containing 18-24 carbon atoms, and b is a bond or a moiety which has a valence of at least 2 and which links the Cy moiety to the polymer backbone; (2) the polymer has a crystalline melting temperature, Tp,