Reactivity of diisocyanates

In order to clarify the mechanism of this process, the dibenzyl diisocyanates reactivities were studied by us. For comparison reasons, the reactivity of one of the most common diisocyanates, 4,4-MDI was also investigated. 

A series of intermediates (Fig. 1.12) obtained by us was considered. They were synthesized with two identical functional groups situated in different rings 2,2 -DBDI, 2,4 -DBDI and 4,4 -DBDI as described elsewhere [45]. A study was made by us on the reactivity of 2,2 -, 2,4 -, and 4,4 -DBDI with n-butanol in benzene. 

The molar fraction (-FN) of unreacted diisocyanate was determined as a function of the reduced time t (Fig. 1.13) [44]. Due to the changes in the rate constants during reaction, both the initial and average rate-constant values were calculated. The rate constants of the initial and average stages of the process and the values of the rate-constant ratios were determined. These values corresponded to the reactions of the first and second NCO groups of the symmetrical DBDI isocyanates (2,2 and 4,4 ) and to those of the 4,4 -MDI reaction. Note that the reactivity values of 4,4 -DBDI 

In the case of 2,2 -DBDI, geometric effects were clearly evident. Reactivity was influenced by the steric effects wich lead to significant intramolecular catalytic activity. These effects were responsible for the whole reaction pattern. 

From the reactivity standpoint, 2,4 -DBDI was similar to 2,4 -TDI, although the selectivity of the NCO groups at the ortho and para positions was found to be lower than that of 2,4 -TDI. The former material has an important practical implication since it is a liquid with a low vapour pressure and therefore reduces the risk of inhalation. 

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In the reaction of 2,4-toluene diisocyanate the 4-position isocyanate group reacts first and then the one at the 2-position [12]. Some representative examples of the reactivity of diisocyanates with 2-ethylhexanol are shown in Fig. 2. 

Reactivities of isocyanates depend on their structure. Table 2.6 gives the main isocyanates used for polymer network synthesis. Conjugation with aromatic nuclei makes ArNCO particularly reactive. The reactivity of diisocyanates is well documented in the literature. For symmetric diisocyanates such as diphenylmethane 4,4 -diisocyanate (MDI) or para-phenylene 4,4 -diisocyanate (PPDI), both NCO groups have initially the same reactivity. But as the NCO group itself exhibits an activating effect on isocyanate reactivity, the fact that one NCO group has reacted introduces a substitution effect that usually decreases the reactivity of the second NCO group. 

To throw additional light on the relative reactivity of diisocyanates at higher temperatures Cunningham and Mastin [130] measured rates of reactions with alcohols at 115°C. To provide both primary and secondary hydroxyl groups, 1- and 2-octanol were used (Table 9). At 115°C the... 

The reactivity of diisocyanates with diethylene glycol adipate (0.2M ester, 0.02M isocyanate in monochlorobenzene)... 

The present book is organized into 6 chapters. Chapter 1 describes general aspects on the chemistry of polyurethane elastomers their origins and development, the principles and synthesis mechanisms, as well as general considerations on the main chemical parameters that define such materials, i. e. diisocyanate, macrodiol and chain extender. Selected considerations regarding the reactivity of diisocyanates, the hydrogen bonding and its dynamic and quantum aspects are also discussed in this chapter. 

Polyurethane Formation. The key to the manufacture of polyurethanes is the unique reactivity of the heterocumulene groups in diisocyanates toward nucleophilic additions. The polarization of the isocyanate group enhances the addition across the carbon—nitrogen double bond, which allows rapid formation of addition polymers from diisocyanates and macroglycols. 

Methylene dianiline is normally a very reactive diamine in the presence of diisocyanates. However, a sodium chloride complex that is relatively unreactive at room temperature is commercially available. When the complex is heated to 21°C, it activates to quickly cure the urethane [76]. 

The above prepolymer on treatment with 2 as the chain extender in dry DMF did not proceed at ambient temperature. The mixture had to be heated to 60°C for 3 h before the reaction was complete. After curing at 60°C for 24 h, the yellow, translucent block polyurethane film (BPUR2) again showed the absence of the —NCO peak in the IR spectrum indicating that curing had been complete. The fact that a higher temperature had to be used in the case of 2 as the chain extender compared to 1 is in keeping with the lower order of reactivity of diols with diisocyanates as compared to diamines with diisocyanates. 

The reaction rates of diisocyanates are strongly influenced by their molecular structure. The reactivity of isocyanate groups is enhanced by adjacent electron-withdrawing substituents. Aromatic rings are very effective electron withdrawing groups, and it is for this reason that the majority of commercial diisocyanates are aromatic. Many of the diisocyanates used commercially consist of mixtures of isomers. Some of the more important commercial diisocyanates are illustrated in Fig. 25.6. Diisocyanates must be handled carefully to avoid exposing workers to their hazardous vapors. 

Poly(boronic carbamatejs were prepared by alkoxyboration polymerization of diisocyanates with mesityldimethoxyborane (scheme 33).59 The polymers obtained have boronic carbamate functions in their repeating units and can be expected to be novel reactive polymers. First, alkoxyboration polymerization between mesityldimethoxyborane and 1,6-hexamethylene diisocyanate was examined, and the optimized reaction conditions were bulk reactions at 140°C. Both aliphatic and aromatic diisocyanates gave the corresponding polymers. When aromatic diisocyanates were employed, the... 

A key factor in the preparation of polyurethanes is the reactivity of the isocyanates. Aromatic diisocyanates are more reactive than aliphatic diisocyanates, and primary isocyanates react faster than secondary or tertiary isocyanates. The most important and commercially most readily accessible diisocyanates are aliphatic and colorless hexamethylene-1,6-diisocyanate (HDI), isophorone diisocyanate (IPDI),and aromatic, brownish colored diphenylmethane-4,4 -diiso-cyanate (MDI), 1,5-naphthalenediisocyanate, and a 4 1 mixture of 2,4- and 2,6-toluenediisocyanates (TDI). 

Fig. 2. Reactivity of aromatic diisocyanates 0.02 M with 2-ethyihexanol 0.4 M and diethylene glycol adipate polyester in benzene at 28°C. (A) l-Chloro-2,4-phenylene diisocyanate. (B) m-Phenylene diisocyanate. (C) p-Phenylene diisocyanate. (D) 4,4 -Methylene bis(phenyl isocyanate). (E) 2,4-Tolylene diisocyanate. (F) Tolylene diisocyanate (60%, 2,4-isomer, 40% 2,6-isomer). (G) 2,6-Tolylene diisocyanate. (H) 3,3 -Dimethyl-4,4 -biphenylene diisocyanate (0.002 M) in 0.04 M 2-ethylhexanol. (I) 4,4 -Methylene bis(2-methylphenyl isocyanate). (J) 3,3 -Dimethoxy-4,4 -biphenylene diisocyanate. (K) 2,2,5,5 -Tetramethyl-4,4 -biphenylene diisocyanate. (L) 80% 2,4- and 20% 2,6-isomer of tolylene diisocyanate with diethylene glycol adipate polyester (hydroxyl No. 57, acid No. 1.6, and average molecular weight 1900). Reprinted from M. E. Bailey, V. Kirss, and R. G. Spaunburgh, Ind. Eng. Chem. 48, 794 (1956). (Copyright 1956 by the American Chemical Society. Reprinted by permission of the copyright owner.)...

Initially, water can cause the hydrolysis of the anhydride or the isocyanate, Scheme 28 (reaction 1 and 2), although the isocyanate hydrolysis has been reported to occur much more rapidly [99]. The hydrolyzed isocyanate (car-bamic acid) may then react further with another isocyanate to yield a urea derivative, see Scheme 28 (reaction 3). Either hydrolysis product, carbamic acid or diacid, can then react with isocyanate to form a mixed carbamic carboxylic anhydride, see Scheme 28 (reactions 4 and 5, respectively). The mixed anhydride is believed to represent the major reaction intermediate in addition to the seven-mem bered cyclic intermediate, which upon heating lose C02 to form the desired imide. The formation of the urea derivative, Scheme 28 (reaction 3), does not constitute a molecular weight limiting side-reaction, since it too has been reported to react with anhydride to form imide [100], These reactions, as a whole, would explain the reported reactivity of isocyanates with diesters of tetracarboxylic acids and with mixtures of anhydride as well as tetracarboxylic acid and tetracarboxylic acid diesters [101, 102]. In these cases, tertiary amines are also utilized to catalyze the reaction. Based on these reports, the overall reaction schematic of diisocyanates with tetracarboxylic acid derivatives can thus be illustrated in an idealized fashion as shown in Scheme 29. 

Kennedy, A.L., Wilson, T.R., Stock, M.F., Alarie, Y. Brown, W.E. (1994) Distribution and reactivity of inhaled C-labeled toluene diisocyanate (TDI) in rats. Arch. Toxicol., 68,434-443... 

Asymmetric diisocyanates such as 2,4-TDI are more complex because the initial reactivity of the two isocyanate groups is not equivalent and the substitution effect amplifies the difference. The 4-NCO is about 10-20 times more reactive than the 2-NCO, but the reactivity ratio also depends on temperature (see Chapter 5). This difference also explains why the TDI dimer can be prepared quantitatively (Eq. 2.28). 

In addition, Eq. (5.1) should not be applied in systems that exhibit different initial reactivities of functional groups, such as in the copolymerization of double bonds of a particular unsaturated polyester with styrene or in the formation of a polyurethane starting from 2,4-toluene diisocyanate. 

In this case of three-monomer polyurethane synthesis, there is no thermodynamic driving force for phase separation. The formation of clusters is fully controlled by the initial composition of the system, the reactivity of functional groups, and the network formation history (one or two stages, macrodiol or triol reacted with diisocyanate first, etc.). 

The reactivity of unsaturations with ozone has been applied to produce structures which allow subsequent degradation of materials by ozonolysis. In this way, Peters et al. [105] prepared polyurethanes using novel unsaturated diisocyanates which can be degraded by oxidative cleavage of the double bonds. 

The reactivity of aliphatic diisocyanates is low in comparison to aromatic isocyanates. It is a problem in the manufacturing stage when using the prepolymer route. Quasiprepolymers and one-shot reactions require the correct choice of curative and catalyst for the system to work. 

The unstable prepolymers are typified by the Bayer Vulkalon polyurethane. Vulkalon is generally made by reacting naphthalene diisocyanate (NDI) and hydroxyl terminated polyesters to form a prepolymer. The reactivity of the... 

The differential reactivity of the sterically hindered and unhindered isocyanate groups of tolylene-2,4-diisocyanate facilitates the stepwise conjugation of hapten (R) and protein (P) amino groups (Fig. 3, Rn 7). jd.jj -Difluoro-m,m -dinitrobenzene (DFDNB) reacts with numerous functionalities including primary and secondary amines, imidazoles, and phenols to yield mixtures of conjugated materials (Fig. 3, Rn 8). This reaction is apparently harder to control than the diisocyanate reactions, but it is much more versatile. 

Such diisocyanates can lead to polyurethanes. The first fluorinated polyurethane was patented in 1958 [74], An interesting survey [75] details the comparison of the reactivity of fluorinated diols about that of non halogenated ones for the preparation of such polymers. The first one which contained fluorine was synthesized by reaction of hexafluoropentanediol and hexamethylene diisocyanate [76] ... 

Ono, H.-K., Jones, F. N., and Pappas, S. P., Relative Reactivity of Isocyanate Groups of Isophorone Diisocyanate. Unexpected High Reactivity of the Secondary Isocyanate Group, J. Polym. Sci., Polym. Lett. Ed., 23, 509-515 (1985). 

Introduction of carboxylic acid groups enhances surface active properties (17) of lignins as well as their reactivity with propylene oxide (50). (A more highly propoxylated lignin has superior solubility and reactivity with diisocyanates.) Carboxylated lignins have, however, not been the target of network-forming reactions. 

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