Polyurethane formation from diisocyanates

One of the first kinetic investigations of the reaction of diisocyanates with difunctional hydroxyl compounds was that of Bailey et al. [153]. Several diisocyanates were studied, using hydroxyl-terminated diethylene glycol adipate of 1800 molecular weight. Data obtained using 2,4-tolylene 

The decrease in rate after 50% reaction is quite apparent in the reaction of tolylene diisocyanate with the polyester at 29°C (Fig. 11). This change in rate illustrates the reduced reactivity of the 2-position isocyanate group, having steric hindrance from the ortho methyl substituent, as well as the lesser activating influence of a meta urethane substituent compared to a meta isocyanate substituent. An increase in reaction temperature favours the slow reaction more than the fast, as would be expected if differences in activation energy accounted for at least part of the difference in rates. Thus at 100°C there was little decrease in rate of reaction with TDI. In the case of 4,4 -diphenylmethane diisocyanate (Fig. 12), there was little change in rate after 50% reaction at any of the temperatures studied. 

The effect of several catalysts on the reaction between 80 20-TDI and a ten-molar excess of diethylene glycol adipate was also reported by Bailey et al. [153]. o-Chlorobenzoyl chloride was a slight retarder tertiary amines and cobalt naphthenate were catalytic. 

The reaction between TDI and polyols has been simulated in a computer study [154], The objective was to prepare NCO-terminated prepolymers containing a minimum of monomeric TDI. Twelve simultaneous and/or consecutive reactions were treated mathematically. The calculated predictions were in good agreement with experimental results. 

Some data on the effect of structure of the hydroxyl compound on reactivity with p-phenylene diisocyanate were given by Cooper et al. [155]. The system was not specified except that the temperature was 

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Stepwise polymerization is characteristic, for example, of the polyurethane formation from diisocyanates and polyols. Nylon 66 is formed by condensation polymerization from hexamethylenediamine and adipic acid. 

Step growth polymerization can also take place without splitting out a small molecule. Ring-opening polymerization, such as caprolactam polymerization to nylon 6, is an example. Polyurethane formation from a diol and a diisocyanate is another step growth polymerization in which no small molecule is eliminated. 

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. 

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. 

Let us now analyze the application of this technique to the formation of a polyurethane synthesized from toluene diisocyanate (TDI, a mixture of 80% toluene 2,4- diisocyanate and 20% toluene 2,6-diisocyanate, as shown in Fig. 5.15) and a stoichiometric amount of a polyfunctional polyol based on sorbitol, using triethylamine (TEA) as a catalyst (Aranguren and Williams, 1986). 

It should be stressed that the viscosity changes during formation of polyurethanes even from bifiinctional compounds can be correlated with gelation most likely they are connected with the formation of a physical network then crosslinks arise from sufficiently strong specific interactions like hydrogen bonds [50]. An example of such a process is the reaction of macro (diisocyanate) with 3,3 -dichloro-4,4 -diaminodiphenylmethane [43]. 

Example Soft polyurethane foams formed from diisocyanates n6glycols are used for insulation and for furniture stuffing. The formation of a polyurethane is shown below ... 

The formation of polyurethane nanoparticles from inverse nano-emulsions (W/O) has also been achieved. Interfacial polyaddition in inverse nano-emulsion is of special interest since this allows the encapsulation of hydrophilic active materials such as proteins or nucleic acids. Thus, taking advantage of the high reactivity of tolylene 2,4-diisocyanate with water molecules, polyurea lipid nanocapsules with aqueous cores obtained from W/O nano-emulsions and prepared by PIT method were designed. Polymer synthesis occurs by in situ interfacial polymerization after nano-emulsion formation. Volatile oils employed as the continuous phase were removed by evaporation and the nanocapsules were redispersed in water. These nanocapsules could be potentially used for encapsulation of both hydrophilic and lipophilic molecules simultaneously. 

Reactive groups on the surface of molds may react with suitable groups in the processed material. For example, isocyanate groups readily react with hydroxyl groups found on the surfaces of many metals. It is also possible that the metal surface will have a catalytic effect on reactions on the surface and will make modification of surface properties of materials. Reflection spectroscopy in the IR region showed that the chemical properties of the metal walls of molds used to form polyurethane articles have an effect on the reaction of diisocyanates contacting the walls. In the manufacture of polyurethane moldings from polyols and diisocyanates, polyurea formation in the boundary layer or in directly adjacent materials represents a concurrent reaction to the polyurethane formative reaction. Urea formation depends on the metal Cu promoted the reaction, which, however, proceeded slowly on polished 1r surfaces. The metal-specific chemical reactions... 

Step-growth polymerization is characterized by the fact that chains always maintain their terminal reactivity and continue to react together to form longer chains as the reaction proceeds, ie, a -mer + -mer — (a + )-mer. Because there are reactions that foUow this mechanism but do not produce a molecule of condensation, eg, the formation of polyurethanes from diols and diisocyanates (eq. 6), the terms step-growth and polycondensation are not exactly synonymous (6,18,19). 

Water-borne polyurethane coatings are formulated by incorporating ionic groups into the polymer backbone. These ionomers are dispersed in water through neutrali2ation. The experimental 1,12-dodecane diisocyanate (C12DI Du Pont) is especially well suited for the formation of water-borne polyurethanes because of its hydrophobicity (39). Cationomers are formed from IPDI, /V-methyIdiethan olamine, and poly(tetramethylene adipate diol)... 

Polyurethanes are important synthetic macromolecules. They are manufactured, for example, in the form of foams. Such a foam is obtained during the formation and solidification of the polyurethane when a gas escapes from the reaction mixture, expanding the material. An elegant possibility for generating such a gas uniformly distributed everywhere in the reaction medium is as follows besides the diol, one adds a small amount of H20 to the diisocyanate. H20 also adds to the C=N double bond of the diisocyanate. According to the uncatalyzed addition mechanism of Figure 8.12, this produces an A-arylated free carbamic acid Ar—NH— C(=0)—OH. However, such a compound decomposes easily in perfect analogy to the decom-... 

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