AbstractPolyester–urethane block copolymers were prepared by the block polyaddition reaction of diisocyanate with two kinds of linear polyesters, one of which was polyethylene terephthalate. The melting points of polyester–urethane block copolymers containing more than 30 polyethylene terephthalate are independent of the total molar fraction of the two polyester components, so that the melting point–composition curves are step‐shaped. The relation between the second‐order transition point of the block copolymer and its composition, on the other hand, is the same as that for a random copolymer. It is observed that some polyester–urethane block copolymers containing 15–50 of polyethylene terephthalate have elastic properties. For example, the 15/85 polyethylene terephthalate/polyethylene adipate–tetramethylene diisocyanate block copolymer has a tensile strength of 240 kg./cm.2and an elongation of 700. It may be supposed that these elastic properties are due to the structure of the block copolymer chains: the flexible polymer chains in the amorphous region may be joined to the crystallites of polyethylene terephthalate, so that a type of network structure similar to crossli
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