# Synthesis of Shape-Memory Polyurethanes: Combined Experimental and Simulation Studies
## 2 Materials and Methods
### Computer Simulations: The Model and the Method
To obtain stable results, five independent simulation runs for each synthesis stage were performed, which lasted around 1.6 \(\times\) 10\({}^{4}\), 6 \(\times\) 10\({}^{5}\), 4 \(\times\) 10\({}^{4}\), and 4 \(\times\) 10\({}^{7}\) MCs steps, respectively, for the first to forth stages of the PCUUs synthesis. The statistical error of calculated parameters in all of the simulated synthesis steps did not exceed 5%. A reaction could occur when a pair of objects were located in neighboring lattice sites. The probability of all reactions that occurred in the system studied was assumed to be 0.02. This assumption was based on our previous simulations of macromolecular systems within the frame of the DLL model [47; 48; 49], and each reaction took place with a certain probability. As the result of a reaction, the objects changed their identities, and each step of the polymerization process was assumed to be irreversible. We developed a methodology that allowed us to reflect the most important features of the copolymerization process. It assumed that the reactivity of the functional groups was always the same and, thus, it was independent from the chain length. Furthermore, no termination reactions were assumed in the second synthesis step. It has to be noted that there are elements in the system that cannot react with each other as they are terminated with the same end groups. Figure 2 presents elements (lattice beads), which represent molecules and their fragments in the first stage of the simulated reaction. A possible representation of reactions of the first step of synthesis using these elements is given in Figure 3.  
In the first step of the PCUU synthesis, the reaction between DMC with 1,10-decanediol was simulated. In Figure 2, DMC is illustrated as a bead (1), which, after reaction (depicted as A in Figure 3) with one of -CH\({}_{3}\) groups, changes into a bead (3), while the second -CH\({}_{3}\) group remains unreacted. Other elements, which have been assigned with consecutive numbers, have been described similarly. In this work, the numbers in brackets correspond to the molecules, with the numbers described in the schematic graphs. Concentrations of components in the system were assumed to be: 18:1; 10:1, and 8:1, respectively, indicating a molar ratio of DMC to diol. The element (1) was used with a different molar excess over a bead (2), which is a representation of 1,10-decanediol. The element (2) can react with (1) or (3) beads leading to the creation of (4) bead with one unreacted -OH group. Other reactions between the beads are also possible: A: reaction between -CH\({}_{3}\) and -OH groups of the monomers; B: the reaction between -CH\({}_{3}\) group in DMC and -OH group of the intermediate product; and C: the reaction between -OH group in diol and -CH\({}_{3}\) group of the intermediate product. The simulation was carried out until all -OH groups in the system had reacted, i.e., until elements (2) and (4) were converted. Figure 3: An illustration of the simulation of the synthesis of BMC. The colors and numbers correspond to the objects from Figure 2. The chemical bonds formed in reactions A–C are marked by dotted lines. Figure 2: Schematic representation of base units used in the first stage of the simulation. Subsequently, in the second step of the simulation process, we studied the synthesis of OCDs, in which the elongation of the polymer chains formed in the previous stage takes place. The product obtained in the first reaction reacted here with 1,10-decanediol. The main advantage of a two-step synthesis is that it allows OCD to be obtained with an estimated molar mass and assures that all the OCD molecules are terminated with hydroxyl groups [15]. In this step, a low molar mass transesterification product formation was ignored. In addition, the preset initial reagent ratios were calculated stoichiometrically. Simulations stopped when OCDs of the desired average chain length were built.