Low molecular weight triblock copolymers (TBCs) with poly(styrene-co-acrylonitrile) (SAN) end-blocks and poly(ethylene oxide) (PEO), poly(propylene oxide) (PPO) or polycaprolactone (PCL) mid-blocks were synthesized using atom transfer radical polymerization (ATRP). The influence of molecular weight, composition (mid-block mote fraction), and interaction parameter on the crystallinity and on the formation of an ordered nanoscale phase-separated structure was investigated using thermal analysis, X-ray scattering, and electron microscopy. The TBCs with PEO mole fractions of over 0.5 exhibited PEO crystallinities of around 40% (compared to 72% for the PEO homopolymer) and lamellar nanoscale periodicities of around 176 angstrom (compared to 143 angstrom for the PEO homopolymer). The TBCs with PEO, PCL or PPO mole fractions of less than 0.5 exhibited relatively low crystallinities and did not exhibit ordered structures. These results emphasize the importance of the mid-block mole fraction in determining the ability to form an ordered nanoscale structure through mid-block crystallization. The ordered structure disappeared on heating the TBCs above the mid-block melting point, but below the SAN glass transition temperature. The crystallinity was reduced significantly in TBCs that were annealed or cast from a solvent. (c) 2006 Elsevier Ltd. All rights reserved.