Asymmetric diisocyanates

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). 

Rules of symmetry apply to the aromatic isocyanates as well. Polyurethanes based on ethylene glycol (EG) and aromatic isocyanates were also studied by Buist and Gudgeon. Aromatic isocyanates with a plane of symmetry had higher crystalline melting points than did those without a plane of symmetry. Therefore, EG derivatives made with toluene diisocyanate, an asymmetrical diisocyanate, had a lower crystalline melting point than those made with methylene diphenyliso-... 

The HS relaxation temperature was affected by the symmetry of the reactants and by the E value in the rubbery plateau for the urea group concentration and steric hindrance. As shown by Marcos [224], the copolymers were very sensitive to the linkage between HS and SS segments. The use of asymmetric diisocyanates lead to a large domain boundary mixing while symmetric diisocyanates determined a sharper boundary. Higher HS percent crystallinity has been observed in PUs with linear, aliphatic HS compared to those made of aromatic diisocyanates [177,188],... 

Interfdci l Composite Membra.nes, A method of making asymmetric membranes involving interfacial polymerization was developed in the 1960s. This technique was used to produce reverse osmosis membranes with dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Sourirajan process (28). In the interfacial polymerization method, an aqueous solution of a reactive prepolymer, such as polyamine, is first deposited in the pores of a microporous support membrane, typically a polysulfone ultrafUtration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, for example, a diacid chloride in hexane. The amine and acid chloride then react at the interface of the two solutions to form a densely cross-linked, extremely thin membrane layer. This preparation method is shown schematically in Figure 15. The first membrane made was based on polyethylenimine cross-linked with toluene-2,4-diisocyanate (28). The process was later refined at FilmTec Corporation (29,30) and at UOP (31) in the United States, and at Nitto (32) in Japan. 

Above the -relaxation process, the 2,4-TDI/PTMO polymer displayed a short rubbery plateau at a storage modulus of about 5 MPa while 2,6-TDI/PTMO was capable of crystallization, as evidenced by the ac-loss process. This difference in dynamic mechanical properties demonstrates the effect of a symmetric diisocyanate structure upon soft-segment properties. As previously discussed, single urethane links can sometimes be incorporated into the soft-segment phase. The introduction of only one of these diisocyanate molecules between two long PTMO chains inhibits crystallization if the diisocyanate is asymmetric. In the case of a symmetric diisocyanate, soft-segment crystallization above Tg can readily occur. The crystals formed were found to melt about 30°C below the reported melting point for PTMO homopolymer, 37°-43°C (19), possibly because of disruption of the crystal structure by the bulky diisocyanate units. 

As good results for the asymmetric HTR of acetophenone were obtained (conversion 100% and 91% ee) with diurea [19], not only copolymerization of diamine 18 has been performed but 1,2-cyclohexyldiamine 19 was also used. Thus pseudo-C2 polyamide 23, polyureas 24, 24 and 26 or polythioureas 27 were prepared by polycondensation with diacid chloride, diisocyanate [20] or dithioisocyanate respectively (Scheme 12) [21]. With rhodium complexes the conversions varied from 22% to 100% and ee from 0% to 60% for HTR with acetophenone at 70°C, in the presence of [Rh(cod)Cl]2 with diamine polymer imit (23-26)/Rh ratio of 10, 2-propanol and KOH. Polyamide 23 proved to be useless (only 22% conversion and 28% ee) contrary to polymea 25 which presents similar ee to those observed with diamine 18 when (Rh(cod)Cl)2 was employed as the catalytic preciu or for HTR (ie 100% conversion for both of them and 55% and 59% ee respectively). Polyureas 24a, 24b, 25 and crosslinked 26 led to better conversions, 80, 50, 97 and 100% with respectively 0, 13, 39 and 60% ee. Moreover, the chiral crosslinked polyurea 26 presented a slight increase in enantioselectivity over the monomer analog 18 (55% ee and 94% conversion under similar conditions) and the reaction rate appeared to be even higher than in the homogeneous phase. Catalytic system from 25 showed a capacity to recycle (Scheme 13) [20]. 

There is a wide variety of compositional variables which can affect the degree of phase segregation, the organization of the HS and consequently the PUs mechanical properties. The symmetry of the diisocyanate is an important factor. For example, Sung et al. studied a series of polymers based on the diisocyanates 2,4-TDl or 2,6-TDI, extended with BDO, and polyether or polyester SS of molar masses 1 000 or 2 000 g/mol. In the 2,4-TDI polymers, the asymmetric placement of the isocyanate residued with respect to the methyl group resulted in some head-to-tail... 

Lower toxicity than aromatic diisocyanates Reduced hydrolytic degradation vs. HDI-based PUs due to asymmetrical structure [92,133,161,165,166]... 

The principal aim of this research was to obtain new types of linear chains rich in amide bonds and along which asymmetric diketopiperazine chromophores are regularly enchained. The CD curves of polymers compared to that of a model compound tend to exclude any interactions among the chromophores along the polymeric chain. Other polyamides based on diketopiperazines of /-glutamic acid or /-aspartic acid by reaction with a variety of diamines have also been investigated [40]. More recently, Cvescenzi et al. [41] prepared mixed polyamide-polyurethane copolymers by reaction of 3,6-bifunctional 2,5-diketopiper-azine with aliphatic and aromatic diisocyanates... 

A second important type of asymmetric membrane is made by interfacial polymerization. In one example of this method, an asymmetric microporous support is impregnated with a polyamine. This polyamine is then exposed to a diisocyanate to form a cross-linked polymer film. This cross-linked layer can be extremely thin, perhaps around 0.1 pm. It is responsible for the membrane s selectivity. The microporous support, again typically 30 pm, provides mechanical stability. 

To reeognize asymmetric functional groups in diisocyanates, we use the notation AjAj. If the diol is denoted by B2, the reaction with hydroxyl... 

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