Urethane-based compounds

Determination of die process parameters that ensure a permissible temperature profile and optimal solidification path is based on the general principles of the theory of batch reactors formulated in Section 2.7. Let us illustrate this approach with the example of solidification of a urethane-based compound for use as a coating.176... 

Stability study was run on model compound urethanes and the data are shown in Fig. 11. It is apparent from the study that urethanes based on primary hydroxyls should have better thermal stability than those based on secondary or tertiary alcohols. 

Figure 25 shows model reactions of some urethane/unsaturated compound-based hybrid-resin formation. 

The dissociation temperature of a urethane model compound based on p-tolylisocyanate and 2-propanol was studied also. One gram of the urethane model compound was dissolved in Af,ALdimethylformamide containing 250 mg of anhydrous LiBr. An aliquot of this solution was transferred to a DSC sample pan and dried at 75°C in a forced air draft oven. The thermogram was then measured. 

Ojelund K, Loontjens T, Steeman P, Palmans A, Maurer F (2003) Synthesis, structure and properties of melamine-based pTHF-urethane supramolecular compounds. Macromol Chem Phys 204(l) 52-60... 

The hydrolytic stability of certain urethane potting compounds was not believed to be a problem until it resulted in the failure of many potted electronic devices during the 1960s military action in Vietnam. The reversion rate also depends on the amount of catalyst used in the formulations. Best hydrolytic properties are obtained when the proper stoichiometric ratio of base material to catalyst is used. Reversion is usually much faster in flexible materials because water permeates it more easily. 

Recently, a different catalytic system has been reported for the synthesis of urethanes, based on orthometallated ruthenium complexes of the kind [RuL(CO)2C1]2 (best ligand LH = 2-phenylpyridine) in the presence of a base (best NaOMe) [186], The initial rate is first order in catalyst concentration and second order in CO pressure, but is independent on nitrobenzene concentration. If aniline was used as a substrate in place of nitrobenzene, no carbonylation was observed and this was said to represent an evidence that aniline is not an intermediate in the catalytic cycle. However this experiment is inconclusive, since carbonylation of aniline may require the previous oxidation of the starting complex by nitrobenzene, analogously to what found for the Ru(C0)3(DPPE) and Ru3(CO)i2 based systems (vide supra). No experiment was attempted using a nitro compound in the presence of a fferently substituted aniline, analogous to the ones describes for the aforementioned catalytic systems. 

Silane polyurethane hybrids are urethane-based polymers which have been end-capped with reactive silane groups. Urethane-based and silicone-based sealants are two major, single component sealant technologies useful in many applications. For instance, they are used for sealing and bonding cement-containing compounds, metals, plastics, and glass. 

Urethane-based sealants improve rheological and mechanical properties, and adhere well to a variety of substrates as do silicone-based sealants. However, urethane-based sealants tend not to accumulate dirt and dust and are easier to compound than silicone-based sealants. For this reason, hybrid sealants based on moisture-curable hydrolysable urethane prepolymers have been proposed [43]. Silane polyurethane hybrid sealants are systems that maximise the beneficial features of each of the urethane-based and silicone-based technologies, whilst minimising the undesirable characteristics. 

A series of compounded flame retardants, based on finely divided insoluble ammonium polyphosphate together with char-forming nitrogenous resins, has been developed for thermoplastics (52—58). These compounds are particularly useful as iatumescent flame-retardant additives for polyolefins, ethylene—vinyl acetate, and urethane elastomers (qv). The char-forming resin can be, for example, an ethyleneurea—formaldehyde condensation polymer, a hydroxyethylisocyanurate, or a piperazine—triazine resin. 

Another fluorescent pigment class (23) is based on a urethane-type resin the primary raw materials are isocyanates, amines, and hydroxy compounds. 

Polybutadiene (PB), 9 558 14 246 24 703 commercial block copolymers, 7 648t oxygen permeability at 25°C, 3 400 physical properties of, 4 376t in rubber compounding, 21 764-765 synthesis, 4 375-377 in tire compounding, 21 807 nel-Polybutadiene, 7 610t Polybutadiene-based urethane sealants, 22 36... 

The instability of primary nitramines in acidic solution means that the nitration of the parent amine with nitric acid or its mixtures is not a feasible route to these compounds. The hydrolysis of secondary nitramides is probably the single most important route to primary nitramines. Accordingly, primary nitramines are often prepared by an indirect four step route (1) acylation of a primary amine to an amide, (2) A-nitration to a secondary nitramide, (3) hydrolysis or ammonolysis with aqueous base and (4) subsequent acidification to release the free nitramine (Equation 5.17). Substrates used in these reactions include sulfonamides, carbamates (urethanes), ureas and carboxylic acid amides like acetamides and formamides etc. The nitration of amides and related compounds has been discussed in Section 5.5. 

The instability of the 3-hydroxymethylindoles over a wide pH range results in the lack of success in acetylation of the hydroxy compound and also in the failure to hydrolyze the acetoxymethylindole without conversion into the bis(3-indolyl)methane (79HC(25-3)l). In contrast with the 3-isomer, 2-hydroxymethylindoles are stable to bases, but are polymerized by acids (79HC(25-3)l). Similarly, it is possible to convert 3-hydroxymethylpyrroles into their acetates and methyl ethers under basic conditions, and reaction with isocyanates yields the expected urethanes (79JMC977). Under acidic conditions, however, they produce the bis(3-pyrrolyl)methanes (B-77MI30504). 

The mechanistic studies of the epimerization reaction still cause confusion. For the first time, direct evidence for Mechanism 1 has been presented based on the incapability of the C-12b methyl substituted vinylogous urethanes to epimerize. Further evidence for Mechanism 1 was provided by deuterium incorporation at the epimeric centre of various compounds (see above), a process most likely due to a mechanism analogous to Mechanism 1. The difference in epimerization rate and deuterium incorporation states merely that Mechanism 1 is not primarily responsible for the acid-catalysed epimerization reaction and hence does not completely discredit it. Evidence for all three mechanisms therefore now exists, revealing the complexity of the epimerization process. The results with p-carbolines and the trapping of 3,4-secoreserpine (27) and secolactam 38 provide strong evidence for Mechanism 3. Mechanism 2, which was earlier considered to be responsible for the epimerization reaction, has since been discredited. Nevertheless, the presence of 2,3-secoreserpine (26) in the trapping reaction remains undisputed and indicates that Mechanism 2 is active under the conditions employed. Thus, several mechanisms may be active simultaneously in the epimerization reaction, so further complicating the matter. 

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