NONIS OCYANATE ROUTES TO POLYURETHANES
There has been development of cross-linked and non-cross-linked chemistries advertised as nonisocyanate where an isocyanate is actually used in the chemistry and simply reacted prior to application. In this case, there is no isocyanate reaction subsequently involved in the final part fabrication. An example might be the reaction of diisocyanate with polyol and the subsequent reaction of a hydroxysilane to make a silane-tipped polyurethane backbone (Fig. 12.3a) [6,7]. The final product in its use is isocyanate-free, but there was at some point an isocyanate in its construction. A similar situation can exist in the formation of acrylate-tipped hybrid polymer building block (Fig. 12.3b) [8, 9]. While these materials may be represented in some venues as "nonisocyanate," this is simply a matter of semantics and is not the intent of this chapter. In this chapter will be presented synthetic routes to making urethane structures without ever employing isocyanate-based building blocks in their construction. The intent of such an exercise should in principle not replace one problematic building block (an isocyanate) with an equally (or more so) problematic substitute. In selective cases, such a result is achieved, while in others the substitution may be just another target for future regulation. Additionally, there is nonisocyanate chemistry based on blocked isocyanates (see Chapters 2 and 10) . These are also isocyanates that are utilized in their urethane form reverting back to isocyanates upon heating. However, their "nonisocyanate" status is perhaps even more suspect.
Reactions of Polycyclic Carbonates with Polyamines
One route to polyurethanes without the benefit of polyisocyanate building blocks is via the reaction of polycyclic carbonates with polyamines [11-13]. As shown in Figure 12.1, research activity on this reaction is growing at an exponential rate. To
FIGURE 12.3 (a) A nomsocyanate containing final product with reactive end groups produced by hydrosilylation. (b) A nomsocyanate containing final product with acrylate end groups.
FIGURE 12.4 (a) Direct transformation of polyoxirane (epoxy) to polycyclic carbonate building block by C02 insertion and (b) by reaction of the hydrolyzed oxirane with phosgene.
some extent, this is due to availability of reagents and the relative ease of their transformation to polyurethane building blocks. Formation of cyclic carbonates is most easily achieved by the catalyzed reaction of an oxirane (epoxy) functionality with C02 (Fig. 12.4a). The transformation can also be performed using phosgene or triphosgene (Fig. 12.4b), as well from halohydrins or halogenated carbonates .
Figure 12.4a is the most prevalent route found in the literature and has been optimized to a great extent. Conversion temperatures are in the range of 25-150 °C, C02 pressures from 1 to 21 bar, and reaction times greater than 3h. Reaction times and extent of conversion are a function of reaction conditions, oxirane substrate, and various cross-influences. Conditions for nearly quantitative selectivity and yield have been presented using heterogeneous catalysis under supercritical C02 conditions and extended reaction times. Conversion of polyoxirane to polycyclic carbonate is accompanied by a significant increase in viscosity that can be used to indirectly track conversion. Conversion of the oxirane to cyclic carbonate is also easily followed directly by FTIR (Chapter 5) with the disappearance of the epoxy ether at 800 cm-1 and the growth of the carbonyl peak at about 1780 cm-1.
Polyurethane formation is achieved by reaction of polycyclic carbonate with polyamines having primary amine functionality (Fig. 12.5). The figure shows the
FIGURE 12.5 Illustrative reaction of a polycyclic carbonate with a polyamine to make a polyurethane. Boxes indicate the urethane group.
potential for ring opening to create either primary hydroxyl or secondary hydroxyl products. The ratio of primary to secondary hydroxyl formation along the backbone is driven by substituent groups on the ring . In the presence of highly electron withdrawing groups, secondary hydroxyls predominate. As drawn in Figure 12.5, the ratio is approximately 1:1. The potential for intrachain hydrogen bonding between the backbone hydroxyls and the urethane carbonyls has been cited to bestow improved hydrolytic stability . Evidence for this improvement is not yet presented in the literature.
It has been reported that cure of polyurethanes produced by reaction of cyclic carbonates and amines can occur at room temperature when allowed to proceed for extended times . A similar experiment pursued with the parent poly epoxy and polyamine building blocks is not cured to the same degree, suggesting that electrophilicity of the carbonate linkage for the nucleophilic amine may predominate over ring strain promoting reactivity of epoxy moieties. As observed for conventional urethane synthesis, Lewis acids (such as tin octanoate) and Lewis bases (such as l,8-diazabicycloundec-7-ene (DBU)) are very effective, but do not reportedly result in room temperature cure for normal processing times requiring heating to 70 °C. However, while cyclic carbonates can be more reactive than epoxy functionality, they are by no means as reactive as isocyanates that exhibit diffusion-limited reactivity with amines. An additional influence on reactivity is the volume functional group density. This means that reactivity of building blocks to form polymer will be influenced by the equivalent weight of the reactive components with lower equivalent weight translating into higher reactivity. This is primarily an influence of the exothermic ring-opening reaction and the activation energy of reaction.
A related reaction of aziridines with C02 can lead directly to polyurethanes (Fig. 12.6). This reaction can result in formation of a urethane-amine with the relative ratios affected strongly by polymerization conditions [15-17]. The urethane
FIGURE 12.6 Illustrative reaction of azindine with super critical CO2 to make a polyurethane amine.
FIGURE 12.7 Two potential mechanisms for formation of the polyurethane-amine backbone from azindine polymerization with CO2.
formation reaction occurs via nucleophilic attack of the carbamate anion on the aziridinium cation. The competitive reaction forming the polyamine can occur via ring opening of the aziridinium cation by the primary amine (Fig. 12.7).
Industrial and commercial adoption of cyclic carbonate chemistry for making polyurethane backbones will depend on several factors: (i) the ability to make useful materials, (ii) the ability to make those materials economically, and (iii) the environmental, health, and safety advantage relative to isocyanate-based urethanes [18-21]. The following analysis for polycyclic carbonate-amine reaction would apply equally to each other pathway discussed herein.
Taking each factor into consideration:
1. In principle, cyclic carbonate chemistry can be nearly as flexible as polyurethane. While it would appear that polycyclic carbonate chemistry is less able to form phase separated materials, it may be possible to create hard block/soft block polymer structures as made in polyurethane chemistry. This could be achieved by employing high-molecular-weight soft segment diamines and preparing hard segments from cyclic carbonate reactions with very low equivalent weight chain extenders such as ethylenediamine. The ability to form intrahard segment hydrogen bonding might create sufficient cohesive energy density to result in a durable physical cross-link. With sufficient phase volume, it may be also possible to create a cocontinuous structure within the elastomer and observe traditional polyurethane tensile properties. However, it remains to be seen if the components can produce the required reaction-induced phase separation prior to gross phase separation of the reacting components due to reactivity, viscosity, and solubility differences between the components. A possible solution for this may come by preparation of polycyclic carbonate-polyamine prepolymers. Prepolymer preparation prior to making the hard segment may provide adequate compatibilization to maintain a homogeneous initial state from which microphase separation into hard and soft segments can occur. Additionally, due to the relatively slow reactivity of polycyclic carbonates and incompatibility with blowing agent technology, this technology may not be suitable for flexible or rigid foam purposes.
2. Polyisocyanate prices are generally less expensive than polyoxirane (epoxy resins) due to fundamental costs associated with industrial manufacture. The price difference is as much as 30%. Subsequent addition of C02 to epoxy resin will further increase differential cost depending on the process chosen, the reaction time, the need to purify/separate products, and the volume produced. In addition, the cost of polyamines is highly variable but substantially more expensive than polyether or polyester polyols. The difference can be 100% or more. Thus, it can be anticipated that the cost of a polyurethane elastomer produced by reaction of a polycyclic carbonate with polyamines may be as much as twice more expensive as the same elastomer produced by isocyanate-based chemistry. This may not be the dominant factor if regulatory agencies decide that the risks of polyisocyanates outweigh the benefits.
3. The cost/benefit ratio will require that polyamines employed making nonisocyanate polyurethanes are substantially less problematic than polyisocyanates [22-24]. While the relative toxicity of any particular polyamine relative to a particular polyisocyanate will depend on numerous structure-property relationships, it is unavoidable that amines are strong bases and are therefore dangerous by skin contact and inhalation. It will remain to be seen if the potential benefits of this nonisocyanate route to polyurethanes can withstand a health and safety examination on the same grounds.