Hexamethylene diisocyanate, polycondensation with

Numerous polymers require such a precursor for instance, by polycondensation with hexamethylene diisocyanate, Keller [113] developed interesting polyurethanes which are amorphous, insoluble and transparent when they are prepared at temperatures higher than 75 °C, whereas they are brittle when the reaction is performed at a temperature lower than 75 °C. 

Microencapsulation by interfacial polycondensation is a usefiil method to microencapsulate a liquid core material. Especially, polyurea and polyurethane microcapsules have been extensively investigated in various industries [209]. For example, aliphatic hexamethylene diisocyanate (HMDI) and aliphatic ethylene diamine (EDA) have been used to prepare polyurea microcapsules containing insecticide called diazinon [210]. A urea linkage is formed immediately by the reaction between an amine and an isocyanate group (see Figure 4.31), and a polyurea is synthesized by the reaction between an amine with two or more amine groups and an isocyanate with two or more isocyanate groups. 

Polyurethanes (PUs), one of the most commonly used polymers for various blood-contacting biomedical applications, are generally prepared by the polycondensation reactions of diisocyanates with diols or amines [35, 36]. Reactions of diisocyanates with diols result in the formation of urethane linkages while diisocyanates reactions with amines result in urea linkages. Both aliphatic, as well as aromatic diisocyanate monomers, are commonly used for preparing polyurethane biomaterials. Examples include 1,4-butane diisocyanate (BDI), 1,6-hexamethylene diisocyanate (HDI), 4,4-dicyclohexylmethane diisocyanate (HMDI), and 4,4-diphenylmethane diisocyanate (MDl) [37]. Commonly used diols (or termed as polyols) for preparing polyurethanes includes poly ethers, polycaprolactone, and polyesters with molecular weights up to 5000 Da. 

Urethane cross-linker Urethane CD-based polymers are obtained by reaction of the CD hydroxyl groups with diisocyanates via polycondensation reaction. The most widely used diisocyanate cross-Unkers are tolylene-2,4-diisocyanate (TDl) and hexamethylene diisocyanate (HDI) [4]. It is also possible to synthesize polymers with specific properties with exotic diisocyanates such as isophorone diisocyanate (IPDI) (Scheme 2.1) [5]. 

Another approach to increase the molecular weight of polyesters obtained in direct polycondensation is to use chain-extenders, for example through the reaction of oc,(jo-functionalized polyesters with diisocyanates or oxazo-lines " Using hexamethylene diisocyanate as a chain extender, an increase in the number average molecular weight from 33 000 to 72 000 g moU for copolymers with butylene succinate and butylene adipate units was reported. ... 

These polymers are prepared by polycondensation reactions. Polyurethanes containing DCZB moieties in the main chain (26a) were prepared by treating trans-l,2-bis(3-hydroxymethyl-9-carbazolyl)cyclobutane with the corresponding diisocyanate in the presence of dibutyltin dilaurate [204]. The relative molecular mass of the polymer synthesized using hexamethylene diisocyanate as a linking agent was 2700, and that of the polymer prepared with toluylene diisocyanate was 16000. Polymers (26a) exhibit almost exclusively monomer fluorescence in dilute solution, i.e. they practically have no intramolecular excimer-forming sites. Their complexes with TNF show better photoconductivity than PVK-TNF. 

Polycondensation reactions of some urazole derivatives like 4-(4 -N-l,8-naphtha]imidophenyl)-l,2,4-tiiazolidine-3,5-dione, 4-(4 -acetamidophenyl)-l,2,4 triazolidine-3,5-dione and 4-(4 -t-butylphenyl)-l,2,4-triazolidine-3,5-dione with hexamethylene diisocyanate, isophorone diisocyanate and toluene-2,4-diisocyanate in DMAc or NMP solution in the presence of pyridine, triethylamine or dibutyltin dilaurate as a catalyst under microwave activation were earried out. Polyureas with inherent viscosities of 0.06 to 0.30 dL/g were obtained after irradiation for 8-18 min. (Mallakpour and Rafiee, 2003). 

For BTA/DMG polycondensation t = 3.11 is found using a Weissenberg rheogoniometer (lowest frequency 0.1 s ), and for the polycondensation of polyurethanes prepared from hexamethylene diisocyanate and a polyoxypropylenetriol (with and without solvent and using a magnetic sphere rheometer with a frequency near 2 X 10 s ) the exponent t = 3.3 0.3 was obtained Therefore, it seems that the exponent t for the shear modulus of polycondensation samples is very close to the classical exponent value of 3. 

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