**2. Raw materials for shape memory polyurethane**

PU is an important subclass of polymers and it contains multi-disperse blocks of soft and hard segments in an alternative fashion. The urethane linkage (─NHCOO─) is produced by a rearrangement reaction between a diisocyanate and a polyol. The common raw materials used for PU synthesis are categorized into macroglyol, diisocyanate, and chain extender. The brief descriptions of these components are presented in this section.
