Urethanes addition reactions

The addition—reaction product of bisphenol A [80-05-07] and glycidyl methacrylate [106-91-2] is a compromise between epoxy and methacrylate resins (245). This BSI—GMA resin polymerizes through a free-radical induced covalent bonding of methacrylate rather than the epoxide reaction of epoxy resins (246). Mineral fillers coated with a silane coupling agent, which bond the powdered inorganic fillers chemically to the resin matrix, are incorporated into BSI—GMA monomer diluted with other methacrylate monomers to make it less viscous (245). A second monomer commonly used to make composites is urethane dimethacrylate [69766-88-7]. 

A urethane is typically prepared by nucleophilic addition reaction between an alcohol and an isocyanate (R—N = C=0), so a polyurethane is prepared by reaction between a cliol and a diisocyanate. The diol is usually a low-molecular-weight polymer (MW 1000 amu) with hydroxyl end-groups the diisocyanate is often toluene-2,4-diisocyanate. 

Problem 31.9 Show the mechanism of the nucleophilic addition reaction of an alcohol with an isocyanate to yield a urethane. 

The presence of the methyl group at a double bond therefore involves two new effects in the reaction with carbethoxynitrene—namely the predominance of substitution reactions over addition reactions and the formation of a product in which the double bond is displaced (about 20% of the total mixture). In addition to the products of Structures D, Ei, E2, and E3 expected as the result of Lwowski s work, another product, probably of Structure E4, is formed (Figure 6). This-is now being checked and necessitates the separation of the products either at the urethane stage or more effectively at the amine stage. Once the structure of this product is established definitely, the conditions affecting its formation remain to be studied, and this may yield an explanation of the reaction mechanism involved. 

The isocyanate, SF5NCO, is easily hydrolyzed to SF5NH2 and C02, whereas SF5NCS is hydrolytically very stable. Both compounds undergo addition reactions with substrates containing easily replaceable hydrogen atoms, i.e., alcohols, thiols, and amines. With alcohols (thiols) the isocyanate and isothiocyanate give urethanes and thiourethanes, respectively. 

The polysiloxane network is formed during part fabrication. In injection molding, the acetylenic alcohol, which acts as a fugitive inhibitor of the vinyl-addition reaction, is volatilized at low temperature as the pellets enter the feed throat. The platinum complex is activated at the process temperature of the urethane (170-185 °C). The vinyl-addition reaction is initiated by the melt state, and the parts generated demonstrate mechanical properties consistent with the formation of a silicone IPN. The fabricated parts are translucent. The physical properties of this formulation (PTUE 205) are given in Table 1. 

In the majority of cases the addition product is stable, but in some special cases it is only moderately stable and may either dissociate to form the initial reactants again or decompose to other products. Secondary reactions of isocyanates that are important in the formation of urethane polymers are those with urea and urethanes. These reactions result in the formation of biuret and allophanate, respectively (Figure 2.19). The relative reaction rates of active-hydrogen-bearing compounds with isocyanate are given in Table 2.7. 

Polyurethane is a term used to represent the group of polymers that contain the urethane linkage, sometimes called an ester of carbamic acid (Fig. 1). Polyurethanes are block copolymers of the form (AB) that is formed by a polyisocyanate addition reaction. Some PUs are thermoplastics, while others are thermosets. The properties of the PU can be changed substantially by changing the chemical composition of the polymer s constituents, and thus... 

The formation of a single urethane bond by the addition reaction of hydroxyl and isocyanate functional groups is represented simply by ... 

If such addition reactions can be carried out cleanly in high yields, di- and polyisocyanates might be made from ethylenediacrylate, A(A -methylenebis(acrylamide), an unsaturated polyester made from maleic anhydride. However, it would be desirable to find a noncarcinogenic analogue of the ethyl urethane, preferably one for which the adduct... 

These compounds are highly reactive due to the high unsaturation in the isocyanate functional group. They undergo addition reactions to the carbon-nitrogen double bond. Isocyanates react readily with alcohols to form urethanes ... 

Polyurethane is the generic name of polymers with the urethane (-N-C-0-) interunit linkage in the chain. There are two main synthetic routes for the preparation of linear urethane homopolymer. These are the condensation reaction between a bischloroformate and a diamine and the addition reaction of a... 

A typical polyurethane adhesive may contain, in addition to the urethane linkages, aliphatic and aromatic hydrocarbons, esters, ethers, amides, urea, and allophanate groups. Polyurethanes are formed by the addition reaction of diisocyanates or polyisocyanates with polyols (Figure 3.12) through a step-growth polymerization... 

Even though the area of isocyanates chemistry becomes more and more wide, the largest majority of applications still remains based on the classic addition reactions leading in general to the appearance of urethane or urea groups. Numerous works were dedicated to the elucidation of the mechanism of these fundamental reactions. However, many fundamental problems are yet unclear so far. 

Linear products result if the reactants are bifunctional but higher functionality leads to the formation of branched chain or crosslinked materials. Again, additional reaction of the isocyanate with the urea, urethane and amide groups already introduced during the initial polymer formation is also possible. Chain branching or crosslinking then occurs. 

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