Ring-opening polymerisation of the macro-cyclic oligomer of butylene terephthalate at elevated temperatures was found to produce high molecular weight polybutylene terephthalate that was both highly crystalline and brittle. The impact of reactive and non-reactive additives on the polymerisation, crystallisation and final crystal structure was extensively studied using differential scanning calorimetry and rheological methods and correlated with observed improvements in fracture toughness and tensile properties for both the neat resin and mode I and II fracture toughness of the fibre-reinforced composites. The reactive modifiers used were bi-functional epoxy resins, namely diglycidyl ether of bis phenol A, butanediol diglycidyl ether and bis[(glycidyl ether)phenyl)]-m-xylene, while the non-reactive modifiers were selected beta nucleants and the ductile thermoplastic, polyethylene glycidyl methacrylate.