Intramolecular cross-links

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. 

Bis-Pyndoxal Tetraphosphate. A second class of bifunctional reagents, described in 1988, involves two pyridoxal groups linked by phosphates of different lengths (89). As shown in Table 4, the yield of intramolecularly cross-linked hemoglobin increases dramatically with increasing length of the phosphate backbone. It is beheved that the site of reaction of (bis-PL) is between the amino-terminal amino group of one P-chain and the... 

Aminopropionitrile (present in sweet peas) covalently inactivates lysyl oxidase, preventing intramolecular cross-linking of collagen and causing abnormalities in joints, bones, and blood vessels. 

An intramolecular palladium(o)-catalyzed cross-coupling of an aryl iodide with a trans vinylstannane is the penultimate maneuver in the Stille-Hegedus total synthesis of (S)-zearalenone (142) (see Scheme 38).59 In the event, exposure of compound 140 to Pd(PPh3)4 catalyst on a 20% cross-linked polystyrene support in refluxing toluene brings about the desired macrocyclization, affording the 14-membered macrolide 141 in 54% yield. Acid-induced hydrolysis of the two methoxyethoxymethyl (MEM) ethers completes the total synthesis of 142. 

The rate of 7-radiation-induced cross-linking in the crystalline and amorphous regions of a crystallizable polychloroprene has been measured by Makhlis et al. [75] who have found a considerably lower cross-link density and less degradation in the crystalline portion of the rubber. The cross-links have been posmlated to be mainly intramolecular in crystalline regions and intermolecular in the amorphous phases. 

In the case of crystaUizable polychloroprene, the cross-link density has been found to be lower in the crystalline region as compared to the amorphous portion of the mbber due to the lower radical mobility in the former. The cross-links are found to be intramolecular in the crystalline zone [387]. 

The core of reversed micelles can be transformed to a highly viscous domain (nanogel) by entrapping appropriate species, such as viscous solvents and hydrophilic macromolecules, or by performing in situ appropriate polymerization reactions or intramolecular cross-linking of water-soluble polymer chains [232-234]. 

Nanogels made up of various intramolecularly cross-linked macromolecules have been prepared simply by performing the polymerization of hydrophilic monomers solubilized in the micellar core of reversed micelles, and they represent distinct macromolecular species from those obtained in bulk [191,240]. 

The total number of molecules in the sol fraction, neglecting intramolecular cross-linking, will equal the number of primary molecules minus the number of cross-linkages in the sol. Expressing these as numbers of moles per equivalent of structural units, we have N = 1/% primary molecules and p /2 cross-linkages in the sol. The number average degree of polymerization in the sol is then... 

Interestingly, the nucleophilic addition of water in the sequence of events giving rise to 41 represents a relevant model system for investigating the mechanism of the generation of DNA-protein cross-links under radical-mediated oxidative conditions [80, 81]. Thus, it was shown that lysine tethered to dGuo via the 5 -hydroxyl group is able to participate in an intramolecular cyclization reaction with the purine base at C-8, subsequent to one electron oxidation [81]. 

Murthy, S.N.P., Wilson, J., Guy, S.L. and Lorand, L. (1991) Intramolecular cross-linking of monomeric fibrinogen by tissue transglutaminase. Proceedings of the National Academy of Sciences USA 88, 10601-10604. 

Junction Functionality. Some results bearing on the possible effect of cross-link functionality on the upturn in modulus are shown in Figure 5(20). The magnitude of the increase in [f ] does not show any obvious correlation with , which again suggests the predominant importance of the intramolecular characteristics of the short chains. At least from the evidence at hand (20), the functionality of the junction points seems relatively unimportant in this regard. 

An effect of network morphology is illustrated by the work of Shen and Tobolsky (ISO). They cross-linked rubbers in the presence of inert diluents such polymerizations tend to promote intramolecular chain loops rather than interchain cross-links. Their polymers had very low stress-relaxation... 

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