Poly (tetrarnethylene terephthalate)-poly (tetramethylene ether) block copolymers with various contents of hard segments and with soft segments of various molecular weights were prepared. It was shown
that contents of hard segments and molecular weights of soft segments affect the. block copolymerization reaction
especially
the formation of homopolymer. The short hard segments can crystallize even with only 2-4 formal units. The polyester-polyether multiblock copolymer with 20 wt% hard segments is melt-spun to give elastic fibers with good elastic recovery.