Anionic polymerization reagents

Unlike the PEG molecules formed from anionic polymerization techniques, there now exist highly discrete forms of the polymer made by controlled addition of small PEG units to create chains of exacting molecular size. These discrete PEGs have a single molecular weight and do not display the polydispersity of the traditional PEG polymers. See Chapter 18 for a complete discussion of discrete PEG-based reagents and their applications. 

The reaction of C—Li reagents with carbon-carbon double bonds has great technological relevance because it is the basis of the anionic polymerization processes. This reaction also affords convenient synthetic routes to cyclic compounds, when internal addition to an olefinic double bond present in the metallated molecule takes place (e.g. equations 68, 83 and 84). 

It still represents a great challenge to conduct anionic polymerizations in an automated parallel synthesizer. Above all, the technique requires an intensive purification of the reagents and the polymerization medium in order to obtain well-defined polymers. Therefore, a special procedure has been described for the inertization of the reactors [55]. It is called chemical cleaning, which is essentially rinsing all the reactors with. yec-butyllithium (.y-BuLi) prior to the reaction in order to eliminate all chemical impurities. This process can be performed in an automated manner. Due to the extreme sensitivity of the polymerization technique to oxygen, moisture, and impurities, detailed investigations on the inertization procedure and the reproducibility of the experiments need to be conducted. 

Polymeric phosphonium salt-bound carboxylate, benzenesulphinate and phenoxide anions have been used in nucleophilic substitution reactions for the synthesis of carboxylic acid esters, sulphones and C/O alkylation of phenols from alkyl halides. The polymeric reagent seems to increase the nucleophilicity of the anions376 and the yields are higher than those for corresponding polymer phase-transfer catalysis (reaction 273). 

A second method, using also anionic polymerization techniques was developed to achieve a better knowledge of the functionality of the branch points. In this method the bifunctional living polymer species is prepared under the same conditions as above. It is then reacted with a stoichiometric amount of a plurifunctional electrophilic reagent14,1S. The chief difficulty is to find a reagent the electrophilic functions of which are isoreactive, to ensure that the reaction will go to completion. 

Anionic Polymerization. Anionic polymerization is limited to nonpolar monomers with carbon-to-carbon double bonds. It takes place in the presence of catalysts capable of generating carbanions—e.g., alkali metals, metal hydrides, metal alkyls, amides, and Grignard reagents. The... 

Methyl methacrylate, a, can be readily converted to the polymer shown, but the stereochemistry of the product depends on the reagents used radical polymerization gives stereoisomer b, while anionic polymerization gives stereoisomer c. Deduce the stereochemistry of each isomer from the H NMR properties of their CDCI3 solutions. 

The initiation of methyl methacrylate polymerization by alkyl- and aryl-lithium compounds is a very complex process which has not been completely elucidated so far. Even less information is available on the efects of or-ganometals not containing lithium. Recently, the interest of some authors has centred on the application of Grignard reagents for the initiation of anionic polymerizations, especially of MMA [171, 172],... 

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