A new type polyoxymethylene (POM) having a long chain alkyl group as an end group was synthesized. This is a block copolymer comprising polar POM and non-polar polyethylene (PE) blocks. Since both the block chains have crystallizability, this copolymer has another aspect of being a crystalline block copolymer. To discuss the effects of the PE block chain length on the crystallizability, differential scanning calorimetric (DSC) measurements were conducted on the POM-PE blocks having different alkyl chain lengths. Small-angle X-ray scattering (SAXS) measurements were also conducted to confirm the microphase separation in the molten state (at 190 degrees C). It was found that the microphase separation even took place at 190 degrees C in a diblock sample having considerably low composition as the fraction of PE moieties is similar to 0.05. (C) 2000 Elsevier Science Ltd. All rights reserved.