Mechanism and catalysis of urethane formation
There are a large number of molecules and unique structures that catalyze urethane reactions. Examples are shown in Figure 3.4 along with their commercial names. Many others are available as well . The mechanism of their action is a subject of debate. The structures of Figure 3.4 can be conveniently broken into Lewis acid transition metals and tertiary amines. Mechanisms of action of the two classes of catalyst are postulated to proceed by different routes. Basic models usually have the catalyst polarizing either the hydroxyl or the isocyanate moieties via simple polar interactions (Fig. 3.5). One can expect that catalyst removal of electron density from the oxygen or nitrogen of the NCO group will in turn enhance the
Figure 3.4 Common catalysts for urethane and urea formation. Trade names are in parentheses.
electrophilic (electron deficient) character of the isocyanate carbon. This effect will not only increase the rate of urethane formation consistent with the structure of Figure 3.2, but may also influence the myriad side reactions isocyanates undergo, a topic that will be covered at greater length later in this chapter. Alternative mechanisms of catalyst action have also been proposed [15-20]. In many of these studies, the mechanism of action was probed by controlling catalyst concentration or catalyst activity (i.e., by varying acidity for instance). These mechanisms can sometimes be quite elaborate and still be compelling based on presented evidence. A mechanism of Lewis acid metal catalyst activity has been presented by several independent workers (Fig. 3.6) [16, 17]. This mechanism has a unique attribute in
Figure 3.5 Examples of bond polarization mechanisms of catalyst action.
Figure 3.6 Proposed mechanism of Lewis acid catalysis of urethane formation.
that its initial assumption is based on the metal's ability to increase its complexation number and increase reactivity of the associated ligands. It also presents opportunities to probe its validity by varying structures and concentrations of the reactants to see if the expected influence is realized. This deductive approach has had mixed mechanistic success since the kinetics have been observed independent of the ligand acid strength and is as well independent of the presence of total system carboxyl or carboxylate strength. Thus the validity of the mechanism rests
Figure 3.7 Proposed mechanism of bismuth carboxylate urethane catalysis. Coordination of the metal is simplified for illustrative purposes.
on the formation of the alkoxide form of the catalyst. The thermodynamics of this transformation has been evaluated, and NMR data based on significant downfield OH proton shifts and line broadening in the presence of bismuth carboxylate catalysts suggest that the alcohol is both coordinated and undergoing proton exchange in the presence of catalyst. Such data suggest the mechanism illustrate in Figure 3.7 to explain this result .
Due to difficulties associated with (i) making highly controlled measurements, (ii) the catalytic (albeit weak) action of the product urethane groups formed, (iii) the difficulty quantifying the effects of varying associations of reactants and products affecting the rate constants, and (iv) the overall thermal sensitivity of the reactions and the catalysis, it is likely that a defined Lewis acid catalyst mechanism enjoying universal endorsement will remain elusive.
The mechanism of tertiary amine catalyst action is also unsettled and further complicated by the tendency of amines to drive reactions other than urethane formation [21-24]. In fact almost all amines are capable of promoting the formation of urethane, urea (reaction of water or amine with two isocyanates), and numerous crosslinking side reactions of isocyanates with reaction products. The catalytic activity forming urethane bonds is conventionally referred to as the "gel" reaction since in polyurethane chemistry the urethane reaction is most tied to the rapid growth of molecular weight and concomitant increase in viscoelasticity resulting in gelation. The catalyzed reaction of isocyanates with water is referred to as the "blow" reaction due to the formation of carbon dioxide which contributes to the formation of foam or frothed morphologies . As shown in Figure 3.5, the ability of tertiary amines to interact strongly with both hydroxyl and isocyanate groups accounts for their versatility in polyurethane chemistry. Amines can also be designed to preferentially interact with water accounting for the ability of some amines to drive the blow reaction with much higher efficiency than the gel reaction (Fig. 3.8).
Figure 3.8 Energy minimized interaction between water and pentamethylene dipropylenetriamine demonstrating the favorable water interactions that result in good blow catalyst properties (light red surfaces indicate non-bonding electron pairs, red is oxygen, and blue is nitrogen). (See insert for color representation of the Figure .)
It is also easy to visualize how a polyamine catalyst can interact simultaneously with a water (or polyol hydroxyl) and an isocyanate to rapidly promote their reaction with one another; however, it is difficult to predict a priori what the behavior of a given amine will be based on measurables like basicity or even steric hindrance [24, 26, 27]. Once an amine is measured for overall activity and blow/gel reaction ratio, the connection is usually then asserted.
At the same time that amine catalysts are able to promote both blow and gel reactions, they are also capable of promoting numerous side reactions. Most of these side reactions involve the reaction of isocyanates with already formed reaction products, and their efficiency for any given side reaction will depend on isocyanate concentration, co-reactant concentration, temperature, catalyst concentration, etc. Isocyanate side reactions will be covered in greater detail later in this chapter. One isocyanate reaction that has great industrial relevance, and uses entirely different catalysts than amines or heavy metals, is the trimerization of isocyanates to isocy-anurates (Fig. 3.9). Isocyanurates are particularly useful in the manufacture of construction foams because of their contribution to network crosslink formation, their thermal resistance, and their relative inflammability.
While some amines can catalyze isocyanate trimerization, the temperature at which they are efficient can be high enough that many other reactions are also operating. In general, excellent trimerization catalysts are highly basic and nucleophilic [28-30]. One hypothesis is that they operate through a stepwise addition beginning with nucleophilic attack of the catalyst on the carbon of a first isocyanate. This increases the nucle-ophilicity of the neighboring nitrogen to then react with a carbon on a second isocyanate and culminates with the rapid ring closure to the six-member isocyanurate ring.
Figure 3.9 Trimerization of isocyanate to isocyanurate.
Figure 3.10 Proposed mechanism of isocyanurate formation using highly nucleo-philic catalysts.
The final step of the sequence is irreversible and drives the final product formation. Recent work has obtained nearly quantitative conversion at low temperatures using carbenes  and proazaphosphatranes catalysts . The mechanism of Figure 3.10 suggests that the reaction should be first order in catalyst and that has in fact been observed in several studies of trimerization kinetics, though reports of kinetics exhibiting second order in catalyst have also been published. The heat of reaction is measured to be -42 kcal/mol trimer formed with a low activation energy consistent with a third power dependence on isocyanate concentration, a linear dependence on catalyst, and only a weak dependence on temperature. Many industrial processes use carboxylate salts and specific amines due to their availability and cost and compensate for their relative inadequacies through process optimization. Other six-member ring structures can be formed by unintended side reactions of carbodiimides and will be covered later in this chapter.