**2.2. Macroglycol**

In the synthesis of SMPU, the aliphatic diols or polyols with a molecular weight of 400–5000 g mol−1 are mainly used as macroglycols. The long-chain macroglycol containing low functionality provides elastomeric soft SMPU, whereas the short-chain macroglycol with high functionality forms rigid SMPU. Various types of macroglycol such as polyester, polyether, polycarbonate, hydrocarbon, and so on are used in the synthesis of SMPU. However, the polyester and polyether macroglycols are mostly used among them. Polyester macroglycol provides high flexibility, which is very useful in SMPU. However, the ester groups are easily hydrolyzed by alkali, thereby weakening the performance of SMPU in special biological application under alkaline condition [18]. A few of polyester polyol macroglycols such as polycaprolactone, polycarbonate, and so on demonstrate high hydrolytic stability in alkaline condition too. On the other hand, polyether macroglycol contributes some specific properties such as metal chelation, hydrophilicity, crystallinity, surface activity, and so on. In addition to that, different dendritic or hyperbranched polyols are also used to prepare SMPU. Among all, the crystalline polycaprolactone diol (PCL) and polyethylene glycol (PEG) are widely used for SMPU as macroglycol.

### **2.3. Chain extender**

of the components during polymerization process. A large number of versatile components with different structures and properties are available for this polymerization [12]. Thus, PU is versatile with respect to its structure and properties. Further, it is available in various forms, thermoplastic, foam, elastomer, and fiber [13, 14]. Thus, among the different types of SMP, PU is a good candidate as SMP for various advanced applications. In this chapter, we focus on the raw materials required for the synthesis of shape memory polyurethane (SMPU), the principle of shape memory function, the design of protocol of SMPU, the recent research progress

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

It is a fundamental precursor for the synthesis of PU. Although both types of diisocyanate viz. aromatic and aliphatic are used, aromatic diisocyanates are most commonly used for SMPU. The properties of SMPU greatly depend on the structure of diisocyanate component. Thus, appropriate selection of diisocyanate component envisages the properties of SMPU. The most commonly used diisocyanates are toluene diisocyanate (TDI), 4,4-diphenyl methane diisocyanate (MDI), polymeric methylene diphenyl diisocyanate (PMDI), 3,3-dimethyl diphenyl methane diisocyanate (DDI), naphthalene diisocyanate (NDI), hexamethylene diisocyanate

Among them, TDI and MDI are widely used in the preparation of SMPU [15, 16]. In most of the cases, TDI is used as a mixture of the 2,4- and 2,6-isomers in 80:20 mol ratio. Similarly, MDI also has three isomers namely 4,4-, 2,4-, and 2,2-diphenyl methane diisocyanates. However, 4,4-isomer is used in most of the SMPUs [15]. Though aromatic diisocyanates have higher reactivity than aliphatic ones and obtained SMPU exhibited good thermal and mechanical properties, the former SMPU also suffers from a few inadequacies such as lower oxidation and weaker ultraviolet stabilization [17]. Therefore, the use of aliphatic diisocyanates is

In the synthesis of SMPU, the aliphatic diols or polyols with a molecular weight of 400–5000 g mol−1 are mainly used as macroglycols. The long-chain macroglycol containing low functionality provides elastomeric soft SMPU, whereas the short-chain macroglycol with high functionality forms rigid SMPU. Various types of macroglycol such as polyester, polyether, polycarbonate, hydrocarbon, and so on are used in the synthesis of SMPU. However, the

of SMPU, and their potential applications.

**2.1. Diisocyanate**

54 Aspects of Polyurethanes

**2.2. Macroglycol**

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

descriptions of these components are presented in this section.

(HDI), isophorone diisocyanate (IPDI), and lysine diisocyanate (LDI).

required, where color and transparency are important for end application.

Several low-molecular-weight (generally below 500 g/mol) diols, diamines, and amino alcohols are used as chain extenders for the synthesis of SMPU [18–20]. The most commonly used chain extenders are ethylene glycol, 1,4-butanediol, 1,6-hexanediol, diethylene triamine, and diethanol amine. The chain extender is generally used to increase the chain length and the molecular weight of SMPU. The amine chain extender reacts rapidly and increases the crosslinking density of SMPU by bridging with biuret linkages in case of SMPU thermoset, whereas diol chain extenders sometimes required organometallic catalysts to obtain SMPU. On the other hand, multifunctional and highly branched chain extenders like trimethylol propane, glycerol, triglyceride ricinolate, hyperbranched polyol, and so on are used to prepare hyperbranched structure in SMPU.
