Symmetrical diisocyanates

Reactivities of isocyanates depend on their structure. Table 2.6 gives the main isocyanates used for polymer network synthesis. Conjugation with aromatic nuclei makes ArNCO particularly reactive. The reactivity of diisocyanates is well documented in the literature. For symmetric diisocyanates such as diphenylmethane 4,4 -diisocyanate (MDI) or para-phenylene 4,4 -diisocyanate (PPDI), both NCO groups have initially the same reactivity. But as the NCO group itself exhibits an activating effect on isocyanate reactivity, the fact that one NCO group has reacted introduces a substitution effect that usually decreases the reactivity of the second NCO group. 

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. 

The use of a symmetrical diisocyanate (HMDl), in which the two NCO groups possess the same reactivity, will produce an oligomer with a higher viscosity than one that contains an unsymmetrical diisocyanate (TDI or IPDI). This is due to the presence of more extended urethane acrylate (Figure 2) in HMDI based oligomers. ... 

A systematic DMA study was carried out by Marcos et al on a series of aromatic and aliphatic PUs chain extended with diamine functionalised poly(oxypropylene)-tipped poly(oxyethylene) of various molar masses [224]. Four diisocyanates, MDI, 2,4-toluene (TDI), hexamethylene (HMDI) and isophorone (IPDI) were employed. The symmetry/asymmetry of the diisocyanate strongly influenced the dynamic mechanical properties. The symmetric diisocyanates (MDI, HMDI) had more phase... 

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],... 

However to keep the properties of polyol chain-extended urethane elastomers in perspective it should be noted that with symmetrical diisocyanates (described later), such as CHDI and PPDI, chain extension... 

When symmetrical diisocyanates are chain-extended with diols, elastomers of good strength properties are obtained. Examples of these diisocyanates are CHDI and PPDT. To illustrate their effect on properties, the following case history of synthesizing a polycaprolactone/CHDI/diol elastomer is given. This also serves to record the quasi prepolymer synthesis route and demonstrate the influence of chain-extender aromaticity on polyurethane elastomer properties. 

MATERIALS USED IN THE SYNTHESIS OF POLYURETHANE ELASTOMERS BASED ON SYMMETRICAL DIISOCYANATE ... 

The contribution which the different types of polar groups make to the properties of polyurethane elastomers depends in part upon their ability to enter into strong hydrogen-bonded associations. The substituted urea linkage, formed by the reaction of an isocyanate with an amine, is regarded as exceptional in its contribution to the physical properties, and it is therefore a preferred component in many bulk elastomers where the structural irregularity introduced by the use of a non-symmetrical diisocyanate or polyol is compensated by the strong urea interactions. 

Klinedinst DB, et al. The effect of varying soft and hard segment length on the structure-property relationships of segmented polyurethanes based on a linear symmetric diisocyanate, 1,4-butanediol and PTMO soft segments. Polymer 2012 53(23) 5358-66. 

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