Introduction to Polyurethane chemistry
In principle, this chapter should be a simple matter of discussing the means of achieving the urethane structure (Fig. 3.1); however, given the reactivity of polyurethane chemical building blocks and the kinetics of the urethane forming reaction, many other structures form, and these can have significant effects on the final materials. In some cases, the experimental approach is to control variables to a level that optimizes the final product and its structure. In other cases, the reaction is allowed to follow its course, find equilibrium, with the object to simply achieve a desired level of reproducibility. This chapter will cover the mechanism and catalysis to polyurethane structure, the side reactions that can take place, and the results these variances can have on the final product. Also covered are the means of minimizing some routes, maximizing others, and the properties achieved in such control. Only isocyanate-based reactions will be covered in this chapter while other routes to urethane will be covered in Chapter 12.
Although isocyanates are reactive species, at room temperature isocyanate reacts with hydroxyl relatively slowly. To some extent, the slow speed is a reflection of phase incompatibility of the relatively nonpolar and denser isocyanate phase, and the relatively polar and less-dense polyol phase (hydroxyl components). However, even when phase compatabilized with a surfactant, the reaction is slow at room temperature. In general, when simply allowed to sit in each other's presence, a hydroxyl unit and an isocyanate will slowly form urethane or urea (in the case of
Figure 3.1 Urethane structure.
Figure 3.2 Generally accepted transition state for uncatalyzed formation of urethane.
reaction with water) at the interface, forming a crystalline barrier, which slows the observed reaction rate further. For this reason, it is commonplace and industrially essential to utilize catalysts and provide some method of phase mixing to enhance the reaction rate between hydroxyls and isocyanates. Even in the presence of catalysts, isocyanate reactions can show sharp temperature threshold behavior. Observations such as these have created numerous theories about the action of catalysts in these systems. In the absence of catalyst, it is believed that urethane formation proceeds through a six-member ring transition state [1, 2] (Fig. 3.2). The simplest transition state would be a four-member ring, unmediated by an additional third body (i.e., second alcohol) [3, 4]. Several points argue against this. The first is that Fourier transform infrared spectroscopy (FTIR) analysis shows that polyols are predominantly in a hydrogen bonded state suggesting that the availability of lone OH groups may be a limiting factor. Figure 3.3 shows FTIR spectra for a 1000 equivalent weight commercial polyol. The spectra focus on the relevant OH stretch region. The breadth of the peak centered at 3500 cm-1 is indicative of the extensive hydrogen bonding these molecules engage in. An OH group that was uninvolved in intermolecular interactions would reveal itself at about 3650 cm-1  and would increase its intensity as a function of temperature. The absence or very slight population indicated by the peak intensity and the temperature independence of the response (25-85 °C) indicate very little free OH under these conditions. Additionally, quantum mechanical calculations predict that the expected rate of reaction in the absence of bond polarization (as shown in Fig. 3.2) is a much slower reaction than observed. However, when mediating influences such as shown in Figure 3.2 are introduced, the observed and calculated reaction rates are in good agreement .
Along with self associations, there are a large number of structural and experimental factors that can influence the rate of reaction. One of the main factors is isocyanate structure since it was first shown that phenyl isocyanate is nearly a factor
Figure 3.3 Fourier transform infrared spectroscopy of a polyether polyol focusing on the OH stretch region. The nested spectra are the evolution of the signal from 25 (top curve) to 85°C (bottom curve) in 10°C increments.
of 50 more reactive with an alcohol than ethyl isocyanate with the same alcohol [7-9]. This has inevitable consequences for the application of aliphatic isocyanates in weatherable formulations. In addition, it has been proposed that the reaction rate might depend on the dielectric strength of a solvent environment. The rate of reaction can decrease with increasing solvent dielectric strength [10-12] and also can be strongly affected by the solvent's ability to associate with hydroxyls through polar or hydrogen bonding interactions[13, 14], thereby decreasing the availability of hydroxyl functionality to react with the isocyanate.