Urethans 2-oxazolidones

Recently, oxazolidone-urethane-modified isocyanurate foams made by the one-shot process have been reported by Fuzesi et al (128, 209, 210). This process consists of a three-component system. Unmodified oxazolidone foams will be described in the following section. 

Poly(oxazolidone-urethane-isocyanurates) were prepared from an excess of isocyanate-terminated oxazolidone prepolymers and poly(oxypropylene) glycols of different chain lengths via the formation of NCO-terrainated poly(oxazolidone-urethanes) followed by trimerization of the terminal isocyanate groups as shown in the following reaction scheme ... 

Modified polyisocyanates are prepared by incorporating at least one linkage into monomeric polyisocyanates. Such linkages include urethane, carbodiimide, allophanate, biuret, amide, imide, isocyanurate, and oxazolidone. These modifications provide some advantages, e.g., lower vapor pressure, increased viscosity, and controlled reactivity. 

The synthesis and properties of 2-oxazolidones and 2-oxazolidone-containing polymers have been reviewed by Pankroatov et al (115). It is interesting to note that some isocyanate cyclotrimerization catalysts also act as catalysts for urethane, oxazolidone and/or carbodiimide linkages. 

As opposed to flexible urethane foams, rigid urethane foams have a highly cross-linked chemical structure and a high percent of closed cells, e.g., over 90%. Rigid urethane foams can be classified as follows unmodified (or pure) rigid urethane foams and modified rigid urethane foams, which include isocyanurate-modified, epoxy-modified, amide-modified and oxazolidone-modified rigid urethane foams. 

These foams include isocyanurate-modified rigid urethane foams, isocyanurate-modified flexible urethane foams, urethane-oxazolidone foams, urethane-based IPN foams, and urethane-based hybrid foams. 

The modification linkages include urethane, amide, imide, carbodiimide and oxazolidone linkages. A urethane-modified isocyanurate foam (trade name Airlite Foam SNB, Nisshinbo, Ind. Inc.) was first applied to the petrochemical industry as a seamless fire-resistant insulant in 1965 (39). 

Oxazolidone-Modified Isor anurate Foams. The 2-oxazolidone, or 2-oxazolidinone, linkage is considered to be a cyclic urethane linkage, but its thermal stability is much higher than that of a urethane linkage. Kordomenos et al (207) compared the thermal stabilities of urethane, oxazolidone and isocyanurate linkages in terms of activation energy by using model compounds. The results obtained were as follows. 

Therefore, the preparation of foams with a high percentage of closed cells or high-density foams is difficult. These foams are quite different from other isocyanate-based foams, e.g., urethane foams, isocyanurate foams and oxazolidone foams. 

Oxazolidones are cyclic urethanes that are normally very difficult to hydrolyze when compared to esters. Hydrolysis is facilitated if the nitrogen atom hears an electron-withdrawing substituent such as an ester or carbonate. Oxazolidones are stable to a large variety of reagents but terminal oxazolidones in the presence of nucleophilic amines have been shown to react. ... 

Ho et al (1996) examined polyol or polysiloxane thermoplastic polyurethanes (TPUs) as modifiers in cresol-formaldehye novolac epoxy resins cured with phenolic novolac resin for computer-chip encapsulation. A stable sea-island dispersion of TPU particles was achieved by the epoxy ring-opening with isocyanate groups of the urethane prepolymer to form an oxazolidone. The flexural modulus was reduced by addition of TPU and also the Tg was increased due to the rigid oxazolidone structure. Mayadunne et al (1999) extended this work to a series of phenol- and naphthol-based aralkyl epoxy resins. 

Spiro-oxazolidones, which are stereoisomeric (at C-17) with other known oxazolidones, are obtained when 17a-urethanes of pregnan-20-ones are treated with base. 

The reaction of -NCO isocyanate groups with epoxidic rings, in the presence of special catalysts, leads to the formation of cyclic urethanes (oxazolidones) [1, 3, 12, 13, 23-25] ... 

The disadvantage of hydroxyalkyl urea formation is an intramolecular reaction which leads to an oxazolidone, a cyclic urethane (reaction 20.10) [33]. 

The oxazolidone formed is not an interesting component in PU chemistry because it does not have reactive groups such as hydroxyl or amino groups to enter into the PU structure (just the low reactivity -NH-COO- urethane group which leads to an allophanate). 

The reaction between the oxazolidone and monoethanolamine (similar to the reaction of any urethane group with an amine) is very favourable, with the formation of a bis (hydroxyethyl) urea (a diol), which is an ideal chain extender in PU chemistry (reaction 20.12) [32, 33]. 

The scission of urethane groups is explained by the following two consecutive reactions (20.13 and 20.14). The first reaction is the addition of alkylene oxide to the active hydrogen of urethane groups (20.13). The resulting hydroxyalkyl urethane, by an intramolecular transesterification, leads to the splitting of the urethane bonds of PU, the PU network is destroyed with the formation of an oxazolidone and a polyether polyol. 

The alkoxylation process is easy to apply to PU foams having a low concentration of urethane and urea groups such as flexible and semiflexible foams, integral skin foams, PU elastomers and so on. Urea groups react in a similar way with urethane groups, with the formation of oxazolidones and amines by an intramolecular alcoholysis of urea groups (reaction 20.15). 

PIR foams are produced by using standard PU foaming equipment. Unmodified PIR foams have a highly crosslinked structure, and therefore are extremely brittle. What did prove successful was to lower the crosslinking density of the foams by adding modifiers, which led to, modified polyisocyanurate foams such as [40] urethane-modified PIR foam, amide-modified PIR foam, imide-modified PIR foam, carbodiimide-modified PIR foam and oxazolidone-modified PIR foam. 

Tetrahydropyridine derivatives can be obtained by diene synthesis with N-sulfonylimines A facile substitution of quinolines in the 2-position starting with the respective N-oxides and a new efficient synthesis of acridones from 2-amino-2 -fluorobenzophen-ones have been reported. 2-Oxazolidones can be easily obtained from oxido compounds and urethans . 

Oxazolidones result from the reaction of epoxides with isocyanates. Oxazolidones have a high thermal stability. A one-component foam system was developed, based on the formation of urethane and oxazolidone from an isocyanate. Mixing an excess of cmde 4,4 -MDl with an aliphatic polyester polyol and an... 

Tetrahy dro- l,3-oxazin-2-ones from oxido and halogeno-urethans 2-Oxazolidones from halogenourethans Thermal rearrangement... 

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