Diisocyanate molecules

The first step in formulating a urethane sealant is to prepare what is commonly called the prepolymer, typically by reaction of a hydroxy-terrninated polyether with a stoichiometric amount of a diisocyanate. Each hydroxy group reacts with one end of every diisocyanate molecule. 

As the reaction proceeds, the chain length will increase as the hydroxyl groups react with the terminal NCO groups of the already formed prepolymer (Figure 3.2). Further extension of the chain can be carried out by reaction of the terminal hydroxyl group with either another diisocyanate molecule or an NCO from another prepolymer chain. 

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. 

To produce the millable polyurethane elastomers, one can bring diisocyanate molecules into reaction with oligomeric hydroxyl-terminated polyesters or polyethers and lower-molecular-weight diols (e.g., 1,4-butanediol). The ester-derived elastomers are designated AU, while the ether-derived materials are designated EU. An illustrative chemical structure is given here, where toluene diisocyanate was used ... 

This capacity for crystallization is characteristic not only for the [EG-DBDI] structure, but also for similar DBDl based materials produced by changing the glycol as for example in the case of a [BDO-DBDl] structure Fig. 2.19 (a). As shown, an extender with an even number of methylene units should allow the diisocyanate molecule to assume an almost linear anti conformation, so these materials should exhibit an odd-even variation in their ability to crystallize as observed in similar cases [60]. 

The result is a network polymer with structure as shown schematically in Fig. 6.1 [63]. Each three-arm star is surrounded by three diisocyanate molecules, and these are linked together partly by the macrodiol. However, given the molar compositions of these polymers, there must also be some connectivity of junction points through just the diisocyanate, producing localities within the network with much reduced mobility — see dotted lines in Fig. 6.1. 

Haward et al.t have reported some research in which a copolymer of styrene and hydroxyethylmethacrylate was cross-linked by hexamethylene diisocyanate. Draw the structural formula for a portion of this cross-linked polymer and indicate what part of the molecule is the result of a condensation reaction and what part results from addition polymerization. These authors indicate that the crosslinking reaction is carried out in sufficiently dilute solutions of copolymer that the crosslinking is primarily intramolecular rather than intermolecular. Explain the distinction between these two terms and why concentration affects the relative amounts of each. 

Copolymers. There are two forms of copolymers, block and random. A nylon block copolymer can be made by combining two or more homopolymers in the melt, by reaction of a preformed polymer with diacid or diamine monomer by reaction of a complex molecule, eg, a bisoxazolone, with a diamine to produce a wide range of multiple amide sequences along the chain and by reaction of a diisocyanate and a dicarboxybc acid (193). In all routes, the composition of the melt is a function of temperature and more so of time. Two homopolyamides in a moisture-equiUbrated molten state undergo amide interchange where amine ends react with the amide groups. 

Step-growth polymerization is characterized by the fact that chains always maintain their terminal reactivity and continue to react together to form longer chains as the reaction proceeds, ie, a -mer + -mer — (a + )-mer. Because there are reactions that foUow this mechanism but do not produce a molecule of condensation, eg, the formation of polyurethanes from diols and diisocyanates (eq. 6), the terms step-growth and polycondensation are not exactly synonymous (6,18,19). 

Liquid organic rubbers with reactive functionality can be prepared by several methods. End-functional oligomers are preferred. Chains attached to the network at only one end do not contribute as much strength to the network as those attached at both ends [34], Urethane chemistry is a handy route to such molecules. A hydroxy-terminated oligomer (commonly a polyester or a polyether) can be reacted with excess diisocyanate, and then with a hydroxy methacrylate to form a reactive toughener [35]. The methacrylate ends undergo copolymerization with the rest of the acrylic monomers. The resulting adhesive is especially effective on poIy(vinyl chloride) shown in Scheme 2. 

Step-growth polymers, the second major class of polymers, are prepared by reactions between difunctional molecules, with the individual bonds in the polymer formed independently of one another. Polycarbonates are formed from a diester and a diol, and polyurethanes are formed from a diisocyanate and a diol. 

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. 

Polyurethanes are thermoset polymers formed from di-isocyanates and poly functional compounds containing numerous hydroxy-groups. Typically the starting materials are themselves polymeric, but comprise relatively few monomer units in the molecule. Low relative molar mass species of this kind are known generally as oligomers. Typical oligomers for the preparation of polyurethanes are polyesters and poly ethers. These are usually prepared to include a small proportion of monomeric trifunctional hydroxy compounds, such as trimethylolpropane, in the backbone, so that they contain pendant hydroxyls which act as the sites of crosslinking. A number of different diisocyanates are used commercially typical examples are shown in Table 1.2. 

NR can be cross-linked by a blocked diphenyl methanes diisocyanate to produce urethane crosslinks. The cross-linking agent dissociates into two quinonedioxime molecules and one diphenyl methane diisocyanate. The quinone reacts with the rubber via a nitroso group and forms cross-links via diisocyanato group. The performance of this system in NR is characterized by excellent age resistance and outstanding reversion resistance. 

Step growth polymerization can also take place without splitting out a small molecule. Ring-opening polymerization, such as caprolactam polymerization to nylon 6, is an example. Polyurethane formation from a diol and a diisocyanate is another step growth polymerization in which no small molecule is eliminated. 

Bifunctional molecules such as diepoxides, diisocyanates, dianhydrides or bis(oxazoline)s have been shown to increase the molecular weight of PET [1,2] by reacting with its terminal groups. Triphenyl phosphite [3, 4], as well as diimidodiepoxides [5], have also proved to react efficiently with PET while promoting molecular weight enhancement. 

PCL-diol and diphenylmethane-i -diisocyanate (MDI), by R. delemar lipase were examined. These polyurethanes have both the hydrogen bonds among polymer chains and aromatic rings in the polymer molecules. R. delemar lipase could hydrolyze the polyurethanes though the rate of hydrolysis toward polyurethanes decreased as compared to that ward PCL-diol. The rate of hydrolysis decreased with decreasing the Mn of PCL-moiety of polyurethanes (Figure T). 

TGA analysis shows that polymer degradation starts at about 235°C which corresponds to the temperature of decomposition of the cellobiose monomer (m.p. 239°C with decom.). Torsion Braid analysis and differential scanning calorimetry measurements show that this polymer is very rigid and does not exhibit any transition in the range of -100 to +250 C, e.g. the polymer decomposition occurs below any transition temperature. This result is expected since both of the monomers, cellobiose and MDI, have rigid molecules and because cellobiose units of the polymer form intermolecular hydrogen bondings. Cellobiose polyurethanes based on aliphatic diisocyanates, e.g. HMDI, are expected to be more flexible. 

The development of polymer science with the study of new polymerization processes and polymers showed that the original classification by Carothers was not entirely adequate and left much to be desired. Thus, for example, consider the polyurethanes, which are formed by the reaction of diols with diisocyanates without the elimination of any small molecule ... 

According to O. Bayer, the latter procedure, which is used especially for the preparation of elastomeric polyurethanes, is carried out in two separate stages. First, a carefully dried, relatively low-molecular-weight, aliphatic polyester or polyether with hydroxy end groups is reacted with an excess of diisocyanate. A chain extension reaction occurs in which two to three linear diol molecules are coupled with diisocyanate, so as to yield a linear polymer with some in-chain urethane groups and with isocyanate end groups. 

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