Isocyanate end group

Step-Growth Gopolymerization. A sample of a block copolymer prepared by condensation polymerisation is shown in equation 30 (37). In this process, a prepolymer diol (HO—Z—OH) is capped with isocyanate end groups and chain extended with a low molecular-weight diol (HO—E—OH) to give a so-called segmented block copolymer, containing polyurethane hard blocks and O—Z—O soft blocks. 

The molar excess of di-isocyanate is about 30% so that the number of polyesters joined together is only about 2-3 and the resulting unit has isocyanate end groups. A typical structure, with P for polyester groups, U for urethane and I for isocyanates would be... 

The same dibutyltin compounds are used in the industrial manufacture of poly(urethane) foams, the first step in which involves the addition of a polyether glycol to 2,4-diisocyanotoluene, to produce the urethane prepolymer having isocyanate end-groups. 

C in air showed the above absorption. Thus in all cases, the prepolymer possessed reactive isocyanate end groups. 

Crosslinking and branching can be promoted either by the use of a triol as a chain extender, or by using less chain extender than is theoretically required the unreacted isocyanate end groups then react with urethane groups in the main chain to form allophonate or biuret crosslinks. 

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. 

In crosslinking with water, pairs of isocyanate end groups in the chain-extended polymer OCN-X-NCO first react with a molecule of water this results in a linear coupling through urea groupings, with simultaneous elimination of CO2 ... 

Notice that the stoichiometry requires that if the polyol is kept constant in the experiment, the excess isocyanate end groups must be balanced by another polyol. In these experiments, butanediol (BTO) was used. BTO is considered a hard segment. 

Polyurethanes offer a convenient method by which iimnobilization of enzymes can be affected. Prepolymers are polymers with active end groups. While the primary purpose of the isocyanate end groups is serving as chain-extending agents, they also react with the amines that characterize an enzyme backbone. Thus, as many of the studies cited will show, the reaction sequence is (1) preparation of an aqueous solution of the enzyme and (2) emulsification of the solution with the prepolymer. The reaction time is on the order of 0.5 hours compared to the 24 hours required by some methods. 

A foam is formed by addition of the proper amount of water. The water reacts with the isocyanate end groups to form carbamic acids which decarboxylate to give amine groups ... 

Polyhexamethylenecarbodiimide, an insoluble condensation reagent for the synthesis of peptides, is obtained in the reaction of hexamethylene diisocyanate with a phospholene oxide catalyst in NMP. The isocyanate end groups are reacted with ethanol. Linear polycarbodiimides, upon reaction with adipic acid, form polyureides. For example, reaction of the 2,4-TDI derived carbodiimide with adipic acid in DMF produces the polymer, mp295°C.222... 

The polycarbodiimides, having isocyanate end groups, are living polymers, because chain scission can occur by a reversible cycloaddition process. 

Some oligomers contain isocyanate end groups, while others are virtually free of isocyanate end groups, and crosslinking via cycloaddition to the carbodiimide end groups is prevented by steric hindrance. From 3,3, 5,5 -tetraethyldiphenylmethane-4,4 -diisocyanate 8, in the presence of 2-ethyl-1,3-dimethyl-l,3,2-diazaphospholine-2-oxide, a low yield of the diisocyanate 9, mp 88-90 °C is obtained. ... 

Finally, Okano et al. [84] have recently prepared an aromatic disulfide with isocyanate end group according to the reaction ... 

The chemical structure of polyurethanes can be further complicated. The presence of water, for example, can lead to the formation of urea linkages between two chains that have isocyanate end groups. The reaction may take place as follows ... 

Nylon block copolymers were previously synthesized from the anionic polymerization of caprolactam in the presence of polyurethane prepolymers. (11) The prepolymers, prepared from the reaction of diisocyanates with polyether glycols, contained Isocyanate end groups which initiated caprolactam polymerization. Sodium caprolactam was used to catalyze the reaction. This copolymer system is the basis for some current areas of nylon 6 RIM research. (12) NYRIM nylon block copolymers are formed from stoichiometric mixtures of polymeric polyols and caprolactam using poly acyllactam initiation which was described previously. The reactions are as follows ... 

Yoshida and Nakamura reacted poly(ethylene adipate) (EAD) containing isocyanate end groups with HTEMPO to produce a counter radical (Scheme 23) al-... 

Yet another polymer synthesis strategy is to react the difunctional dimer molecules with prepolymers. Equation 7.5 shows an example of this technique.5 In this instance, the prepolymer is one of the Hypo polymers sold by W.R. Grace. Analysis of the prepolymer sample showed it contained, on average, three tolyl isocyanate end groups Mn was about 2000. 

Polyurethanes (PU) and PEG-containing PU with and without free isocyanate end group-coated silica particles can be fabricated and show good potential for use in practical antibacterial applications. 

In the one-shot process, all of the ingredients—isocyanate, polyol, blowing agent, catalyst, additives, etc.—are mixed simultaneously, and the mixture is allowed to foam. In the prepolymer method (Figure 1.29), a portion of the polyol is reacted with an excess of isocyanate to yield a prepolymer having isocyanate end groups. The prepolymer is then mixed with additional polyol, catalyst, and other additives to cause foaming. The quasi-prepolymer process is intermediate between the prepolymer and one-shot processes. 

Segmented block copolymers have been demonstrated by degrading cellulose triacetate and coupling it to low-molecular-weight polyesters or polyethers capped with isocyanate end groups and spinning from methylene chloride to make Spandex type elastomeric fibers [111,112]. 

The other favored reactions are the cleavage of the CO-NH bond to form o-aminophenyl end groups, the cis elimination to form amide and vinyl end groups, and the cleavage of the CH2-CO bond to form isocyanate end groups ... 

Polyurethane-Based Prepolymers Terminated with Silane-Compounds These prepolymers were based on the same chemistry and the same preparation procedure as all prepolymers mentioned above. But the isocyanate end group was reacted with a silane compound. These prepolymers are isocyanate free and have a different cross-linking mechanism. Adhesives based on these prepolymers are commonly used as sealants. They have good elongation properties, good adhesion to different kinds of materials and are usually more resistant against UV rays. They are not mixable with PU prepolymers of any kind. 

The liquid polyurethane rubbers consist of polyurethanes with isocyanate end groups. They are generally cross-linked with weakly basic amines, for example, methylene bis(2-chloroaniline) ... 

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