Diisocyanate coupled polyester chains

Notice that the 67°C endotherm shows no Mw dependency, possibly because the smallest TPU of the series (Mw of 48,000) already consists of soft segments (diisocyanate-coupled polyester chains) long enough to display the maximum temperature for this transition in this system. 

According to O. Bayer, the latter procedure, which is used especially for the preparation of elastomeric polyurethanes, is carried out in two separate stages. First, a carefully dried, relatively low-molecular-weight, aliphatic polyester or polyether with hydroxy end groups is reacted with an excess of diisocyanate. A chain extension reaction occurs in which two to three linear diol molecules are coupled with diisocyanate, so as to yield a linear polymer with some in-chain urethane groups and with isocyanate end groups. 

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