Polyisocyanates Poly

In Chapter 3, the chemistry and technology of the most important oligo-polyols used for elastic polyurethanes fabrication, in fact high MW oligomers (2000-12000 daltons) with terminal hydroxyl groups and low functionality (2-4 hydroxyl groups/mol) were discussed. Polyalkylene oxide polyols (homopolymers of PO or copolymers PO - EO, random or block copolymers), polytetrahydrofuran polyols, filled polyols (graft poly ether polyols, poly Harnstoff dispersion - polyurea dispersions (PHD) and polyisocyanate poly addition (PIPA) polyols), polybutadiene polyols and polysiloxane polyols were all discussed. The elastic polyurethanes represent around 72% of the total polyurethanes produced worldwide. 

Shore D hardness Polyisocyanate/poly ether amine/DETDA 75 (2)... 

Tetraethylene glycol may be used direcdy as a plasticizer or modified by esterification with fatty acids to produce plasticizers (qv). Tetraethylene glycol is used directly to plasticize separation membranes, such as siHcone mbber, poly(vinyl acetate), and ceUulose triacetate. Ceramic materials utilize tetraethylene glycol as plasticizing agents in resistant refractory plastics and molded ceramics. It is also employed to improve the physical properties of cyanoacrylate and polyacrylonitrile adhesives, and is chemically modified to form polyisocyanate, polymethacrylate, and to contain siHcone compounds used for adhesives. 

PUR are a broad class of highly cross-linked plastics prepared by multiple additions of poly-functional hydroxyl or amino compounds. Typical reactants are polyisocyanates [toluene diisocyanate (TDI)] and polyhydroxyl molecules such as polyols, glycols, polyesters, and polyethers. The cyanate group can also combine with water this reaction is the basis for hardening of the one-part foam formulations. 

It is considered that, if ideal, optically active poly(alkyl(aryl)silane) homopolymer and copolymer systems could be obtained which had stiffer main-chain structures with longer persistence lengths, it should be possible to clarify the relationship between the gabs value and the chiral molar composition. The magnitude of the chirality of the polyisocyanates allowed precise correlations with the cooperativity models.18q In the theory of the cooperative helical order in polyisocyanates, the polymers are characterized by the chiral order parameter M, which is the fraction of the main chain twisting in one helical sense minus the fraction of the main chain twisting in the opposing sense. This order parameter is equal to the optical activity normalized by the value for an entirely one-handed helical polymer. The theory predicts... 

Another significant helical amplification in optically active copolymers with preferential screw-sense helicity is known as the majority rule phenomenon [ 17,18]. In this case, the screw sense of a helical main chain bearing nonracemic chiral side groups is controlled by the ee only and a population of preferential screw-sense helicity and optical activity were nonlinearly amplified by ee of chiral side groups. Since Pino et al. first reported this phenomenon in poly-a-olefins made of vinyl co-monomers bearing nonracemic chiral moieties [21], this majority rule has already been established in stiff polyisocyanates bearing a nonracemic chiral side chain [17,18]. 

In the early stage of helical polymer stereochemistry, a few polymers were known to retain a helical main chain with a predominantly single screw sense in solution at room temperature. For example, in cases of poly( f-bulyl isocyanides) [22], poly(triphenylmethyl methacrylate) [23], polyisocyanate [24], and poly-a-olefins [19], helical structures are kept through side group interactions. Since these pioneering works, many synthetic optically active polymers with a chromophoric main chain bearing chiral and/or bulky side... 

The underlying principle for these phenomena is considered to be the fact that the poly(phenylacetylene)s are a dynamic helical polymer of which right- and left-handed helical conformations are interconvertibly separated by helical reversals like polyisocyanates as mentioned above. Therefore, the remarkable CD induction arises from a drastic change in the population of the right- and left-handed helices of the polymers. 

A more remarkable and dramatic change in the conformation of helical polymers upon photoirradiation has been observed for the azobenzene-modified poly isocyanates. Polyisocyanates are typical dynamic helical polymers [59,60]. Even the optically inactive poly(n-hexyl isocyanate) (20), which is devoid of stereogenic centers, exists as an equal mixture of right- and left-handed helical conformations. Equilibrium exists in solution between both helices separated by... 

The major poly isocyanates used (2) are toluene diisocyanate (TDI) and the less volatile 4,4/-methylene diphenylene diisocyanate (MDI), which, because it is a crystalline solid in the pure form, has to be used in a relatively crude form. The crude polyisocyanate is a mixture of MDI variants that is conveniently a liquid product with a mean functionality greater than 2. The use of a pure, liquid diisocyanate, however, would enable polyurethanes to be formed having relatively enhanced physical properties (2) and would also greatly simplify processing by removing the need to use elevated temperatures, solvents, or isocyanate prepolymers as with MDI. 

Many papers and patents deal with incorporation of additives in the poly-(urethane-seq-butadiene)s to increase the condensation efficiency, to improve the processing, or to improve the properties of solid propellants. A processing aid, trialkylamine 2201 and also tetracycline 221), was shown to reduce the viscosity and increase the pot life of hydroxytelechelic polybutadiene/polyisocyanate systems. It has been shown that the cure of isophorone diisocyanate and hydroxytelechelic polybutadienes was not affected by the addition of the ammonium perchlorate propellant 222). Special modification of HTPBs have been referred to in Refs. 223 236>. 

Preparation of Microcellular Foams. The major polyols for microcellular elastomers include aliphatic polyester diols having a molecular weight of about 1,000 to 3,000, and poly-epsilon-caprolactones. Poly(oxytetramethylene) glycols (PTMEG) can also be used. The polyisocyanates to be used for microcellular elastomers are TDI-prepolymers and liquid MDI, i.e., carbodiimide-modified MDI or urethane-modified MDI. Low-molecular-weight, active-hydrogen compounds such as chain extenders (difunctional compounds) and... 

A research program to have benzylic ether-type resin react with polyisocyanate for producing polyurethane or urethane-modified poly-isocyanurate foam is being promoted. A foam which has intermediate properties between phenolic foam and polyurethane foam can be produced. 

In contrast to polypeptides that have many possible conformations, poly(hexyl isocynate) is known to have a stiff rodlike helical conformation in the solid state and in a wide range of solvents, which is responsible for the formation of a nematic liquid crystalline phase.45-47 The inherent chain stiffness of this polymer is primarily determined by chemical structure rather than by intramolecular hydrogen bonding. This results in a greater stability in the stiff rodlike characteristics in the solution as compared to polypeptides. The lyotropic liquid crystalline behavior in a number of different solvents was extensively studied by Aharoni et al.48-50 In contrast to homopolymers, interesting new supramolecular structures can be expected if a flexible block is connected to the rigid polyisocyanate block (rod—coil copolymers) because the molecule imparts both microphase separation characteristics of the blocks and a tendency of rod segments to form anisotropic order. 

The sensitivity of the helical preference of polyisocyanates to small chiral influences is also observed in chiral solvents. Poly(n-hexyl isocyanate) was found experimentally to have a persistence length of 20— 40 nm depending on the solvent in which the measurements were carried out.66 It was hypothesized that in more polar solvents a local interaction of the solvent would give rise to larger torsional oscillations around the backbone bonds. It was indeed observed that dissolution of poly(n-hexyl isocyanate) in non-racemic chiral solvents, e.g., (5)-l-chloro-2-methyl-butane, changed the persistence length and in addition also resulted in an excess of one helical sense.67 The chiral bias favoring one helical sense by itself is miniscule, but due to the cooperativity, a chiral preference is observed. More recently it was noted that the circular dichroism of these polymers decreases upon the addition of an achiral or racemic... 

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