Backbone unstable

Phosphazene polymers can act as biomaterials in several different ways [401, 402,407]. What is important in the consideration of skeletal properties is that the -P=N- backbone can be considered as an extremely stable substrate when fluorinated alcohols [399,457] or phenoxy [172] substituents are used in the substitution process of the chlorine atoms of (NPCl2)n> but it becomes highly hydrolytically unstable when simple amino acid [464] or imidazole [405-407] derivatives are attached to the phosphorus. In this case, an extraordinary demolition reaction of the polymer chain takes place under mild hydrolytic conditions transforming skeletal nitrogen and phosphorus into ammonium salts and phosphates, respectively [405-407,464]. This opens wide perspectives in biomedical sciences for the utilization of these materials, for instance, as drug delivery systems [213,401,405,406,464] and bioerodible substrates [403,404]. 

Polymers used in medicine fall into two main categories those that are sufficiently inert to fulfill a long-term structural function as biomaterials or membranes, and those that are sufficiently hydrolytically unstable to function as bioeradible materials, either in the form of sutures or as absorbable matrices for the controlled release of drugs. For the synthetic organic polymers widely used in biomedicine this often translates to a distinction between polymers that have a completely hydrocarbon backbone and those that have sites in the backbone that are hydrolytically sensitive. Ester, anhydride, amide, or urethane linkages in the backbone usually serve this function. 

Nitrogen and oxygen can be Incorporated Into the backbone such that they are surrounded by different atom types. For example, organic peroxides contain two covalently bonded oxygen atoms that form the peroxide linkage. These molecules are Inherently unstable. Two covalently bonded nitrogen atoms are also similarly unstable. These unstable structures decompose to form smaller unstable molecules that are used to start the polymerization for some types of monomers. Thus, to be incorporated implies that the molecules are found only singularly in the backbone chain. Sulfur and silicon are considered to be chain formers. They can be found in the backbone in multiple units connected covalently to molecules of the same type or with carbon. Complete molecules with a silicon backbone are possible, and molecules with multiple sulfur links incorporated into the system are common, particularly in sulfur-crosslinked rubber. 

U-SNAr[15][CNlE] process was implemented. These para-cyclophanes 97 and 98 are interesting from the stereochemical point of view because they generate atropisomerism. Moreover, the stereogenic centres of the peptide backbone are configurationally unstable under the cyclization conditions, affording, under thermodynamic control, only two diastereoisomers (atropisomers) instead of the expected four. 

Tables 13.4 and 13.5 contain a summary of typical stability values for a number of polymers and elastomers against typical chemical agents. As expected, condensation polymers generally exhibit good stability to nonpolar liquids while they are generally only (relatively) moderately or unstable toward polar agents and acids and bases. This is because of the polarity of the connective condensation linkages within the polymer backbone. By comparison, vinyl type of polymers exhibit moderate to good stability toward both polar and...

The first family is a directly sulfated CD. These molecules are easy to produce chemically and have the anionic sulfate group randomly distributed at the C2, C3, and C6 positions. The substituent is attached to the carbohydrate via a sulfate-ester linkage that may be metabolically unstable in vivo. One feature regarding this family is the negative charge of the substituent in close proximity to the carbohydrate backbone. 

Firstly, Aharoni et al. [128] described a process of ozonization of polymers or copolymers containing unsaturations in a mixture of two solvents. One of them is inert to ozone and the other one is less reactive than polymer double bonds but more reactive than the single C-C bonds of this polymer. A typical solvent mixture is composed of toluene and 1,1,2,2,-tetra-chloroethane, or xylene and decaline. This mixture permits one to control the attack of polymer only onto unsaturations and not to produce unstable sites in the polymer backbone (as peroxides or hydroperoxides coming from single bond C-C attack) which could decompose in a second step producing undesirable by-products. In this way, only unsaturations are reacted and... 

Several studies have shown that the amide bonds that comprise the PAM AM dendrimer backbone are relatively unstable and begin decomposing at temperatures as low as 75 °C [45,50,52,56-58]. The low onset temperature of dendrimer decomposition is not surprising given that PAMAM den-drimers can undergo retro-Michael addition reactions at temperatures above 100 °C [16]. Far more forcing conditions are required to fully activate the catalysts, which suggests that the dendrimer decomposes into various surface species that continue to poison the nanoparticle surfaces. 

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