Diols cross-linkers

Comparison of Diol Cross-Linkers in Castable Urethane Elastomers... 

The use of a p-vinylphenyl boronate as functional monomer to be covalently linked with a diol-template [2] is demonstrated in Fig. 2. Following polymerization in the presence of a cross-linker, the template has to be extracted from the polymer network. This requires breaking the covalent bond. During the application of covalently imprinted materials, the target molecules have to reform such bonds in order to be retained. Both making and breaking the bonds is at best a time-consuming process. 

The synthesis of the module is provided in Scheme 10.5 (Kushner et al. 2007). Double alkylation of ethyl acetoacetate followed by guanidine condensation afforded alkenyl-pyrimidone intermediate 24 (Kushner et al. 2007). Isocyanate 25 was coupled to pyrimidone 24 to yield 26. Upon dimerization in DCM, RCM effectively cyclized the two UPy units (Mohr et al. 1997 Week et al. 1999). A one-pot reduction and deprotection through hydrogenation using Pearlman s catalyst gave diol module 27. Finally, capping 27 with 2-isocyanatoethyl methacrylate at both ends provided the UPy sacrificial cross-linker 28, which was thoroughly characterized by H- and C-NMR, Fourier transform IR (FTIR), and mass spectrometry. 

The use of periodate as a cleavage agent does have advantages, however. Unlike the use of cleavable cross-linkers that contain disulfide bonds which require a reductant to break the conjugate, cleavage of diol-containing cross-links with periodate typically preserves the indigenous disulfide bonds and tertiary structure of proteins and other molecules. As a result, with most proteins bioactivity usually remains unaffected after periodate treatment. 

Figure 195 Cross-linkers containing a diol group in their cross-bridge design may be cleaved by oxidation with sodium periodate.

Chain Extenders and Cross-linkers. In addition to the two principal components of most urethane coatings, isocyanate and polyol components, a number of di- or polyfunctional, active hydrogen components may be used as chain extenders or cross-linkers. The most important classes of compounds for this use are diols or polyols (monomers or oligomers), diamines, and alkanolamines. Typical examples of diols are ethylene, dlethylene, dlpropylene glycol, 1,4-butanedio1, 1,5-hexanediol, neopentyl glycol,... 

This listing of chain extenders or cross-linkers is obviously incomplete since many other types of chain extenders may be used where special end use requirements may have to be met, for example, for flame retardance. In this case, halogenated diols, for example, 2,3-dibromo-2-butane-l,4-diol (GAF Corp.), or phosphate or phosphonate group-containing diols, for example, Fyrol 6 (Stauffer Chemical Co.), may be used. 

Estrogens induce tumors in animals, a fact that probably depends on the formation of catechol-type A rings. Arene oxides and/or semiquinones may then be formed, which react as cross-linkers in the same way as polycyclic hydrocarbons, such as benzopyrene, which is oxidized in the body to a diol-epoxide. Electroreduction of the corresponding ortho-quimm to the semiquinone radical, the same product that would be formed upon oxygen oxidation of the catechol, in the presence of adenine, does indeed produce a covalent adduct in 14% yield (Scheme 3.4.5)... 

Diols such as 1,4-butanediol (19) have been reported as cross-linkers for lEM/methyl methacrylate/ethy1 acrylate terpolymers to make glossy baked enamels with excellent impact strength. 

The group of Nagata incorporated UV-curable moieties directly into the main chain of the related polymers [41-43]. Thus, photocurable, biodegradable multiblock SMPs were prepared by polycondensation from PCL diol, PEG, and 5-cinnamoyloxyisophthalic acid [41]. The latter compounds acted as UV cross-linker without any photoinitiator. The semicrystalline photosets exhibited Tg----60 C, and in the range 35-47°C. Both and R values were... 

Chain extenders and cross-linkers (encountered especially in thermoplastic urethanes) are usually short diamines or diols, such as 1,4-butane-diol or te/rafe/s(2-hydroxypropyl)ethylenediamine. Alkanolamines and triols like glycerol are also used. Most of these can be easily determined by GC. 

The versatility of polyacrylamide gels is also shown by the large number of cross-linkers, besides Bis, that can be used to cast gels with particular properties for different fractionation purposes. N.N - 1,2-dihydroxyethylene)bisacrylamide (DHEBA) can be used for ca.sting reversible gels (i.e., gels that can be liquefied), since the 1,2-diol structure of DHEBA renders them susceptible to cleavage by oxidation with periodic acid [48], The same principle should also apply to yV,A -dial-... 

PUs can be both thermoplastic and thermosetting materials composed by HS and SS. Thermoplastic polyurethanes (TPU) are formed by linear polymer chains with functionality equal to two. This means that the SS, the polyol, is a diol and the HS, the isocyanate, is a diisocyanate. The thermodynamic incompatibility between the hard and the SSs as well as the NCO OH ratio, the synthesis procedure, etc. can generate a phase separation into the TPU. On the other hand, the thermosetting PUs are formed by polyols and/or isocyanates with functionalities higher than 2, with a NCO OH ratio higher than 1 or are obtained by using cross-linker agents. 

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