CARBODIIMIDE, POLYMERIC

The remaining chapters are carbodiimides with unsaturated substituents, halogenated carbodiimides, acyl-, thioacyl- and imidoylcarbodiimides, silicon substituted carbodiimides, nitrogen substituted carbodiimides, phosphorous substituted carbodiimides, sulfur substituted carbodiimides, metal substituted carbodiimides, cyclic carbodiimides, polymeric carbodiimides and application of carbodiimides. 

POLYMERIC CARBODIIMIDE. II. MOFFAT OXIDATION 4-tert-BUTYLCYCLOHEXANONE... 

Oxalamidinate anions represent the most simple type of bis(amidinate) ligands in which two amidinate units are directly connected via a central C-C bond. Oxalamidinate complexes of d-transition metals have recently received increasing attention for their efficient catalytic activity in olefin polymerization reactions. Almost all the oxalamidinate ligands have been synthesized by deprotonation of the corresponding oxalic amidines [pathway (a) in Scheme 190]. More recently, it was found that carbodiimides, RN = C=NR, can be reductively coupled with metallic lithium into the oxalamidinate dianions [(RN)2C-C(NR)2] [route (c)J which are clearly useful for the preparation of dinuclear oxalamidinate complexes. The lithium complex obtained this way from N,N -di(p-tolyl)carbodiimide was crystallized from pyridine/pentane and... 

Amidinate complexes of copper(I) and copper(II) have been found to catalyze the polymerization of carbodiimides. The copper catalysts are highly active even under air and oxygen. It was shown that the catalytic activity of an air-stable copper(II) amidinato complex is almost equal to that of reported air-sensitive titanium(IV) amidinate initiators. Scheme 225 illustrates the proposed reaction mechanism. ... 

Most references to the use of EDC describe the optimal reaction medium to be at a pH from 4.7 to 6.0. However, the carbodiimide reaction occurs effectively up to at least pH 7.5 without significant loss of yield. Conjugations done under mildly alkaline pH conditions (e.g., pH 8.5) also can be done to limit the polymerization of proteins, while still facilitating the coupling of a carboxylate-containing molecule at a low substitution level per protein. See Chapter 19,... 

Carbodiimide coupling to carboxylate-containing QDs usually involves the use of EDC in a single-step or two-step process to form an amide bond. If a one-step reaction is done, the QD is activated with EDC in the presence of an amine-containing molecule, such as a protein. Many protocols use this method, but it can result in protein polymerization in addition to coupling, because proteins contain both carboxylates and amines. A two-step protocol results in better control of the reaction (Figure 9.61). In the first step, EDC is used in the presence of sulfo-NHS to activate the carboxylates on the particles to intermediate sulfo-NHS esters. After a quick separation step to remove excess reactants, the activated QDs are added to the protein solution to be coupled. This then results in amide bond formation without polymerization of the protein in solution. See Chapter 3, Section 1 and Chapter 14, Section 1 for additional information on this process. 

Biocytin should not be used in a carbodiimide reaction to modify proteins or other molecules, since it contains both a carboxylate and an amine group. A carbodiimide-mediated reaction, as suggested for D-biotin previously, would cause self-conjugation and polymerization of this reagent. 

Fig. 5.8 Examples of polymers grafted from nanocarbons, (a) An ATRP initiator covalently attached to RGO via nitrene and carbodiimide chemistry was used for the growth of poly(2-(ethyl (phenyl)amino)ethyl-methacrylate). (b) A RAFT chain transfer agent is covalently attached to GO prior to polymerization of vinylcarbozole.

At room temperature aliphatic and aromatic carbodiimides are liquid or solid. They usually can be purified by distillation under reduced pressure or by recrystallization to yield neutral products. On prolonged standing they polymerize to yield basic products. The liquid carbodiimides are less stable than the solid ones. The stability of carbodiimides increases with the degree of branching of the alkyl substituents attached to the nitrogen atom RCH2 < R2CH < R3C. For example, diethylcarbodiimide [14, 21] polymerizes in a few days, whereas diisopropyl- and dicyclohexylcarbodiimides [14] are stable for several months. Unsaturated substituents also cause a marked decrease in stability... 

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