Polymeric chains hydrogen fluoride

Hydrogen fluoride swells collagen, but probably does not break the peptide bonds. N-carboxy anhydrides are polymerized in hydrogen fluoride to give poly-a-amino acids i with chain length of about 30 units. 

Ethene does not polymerize by the cationic mechanism because it does not have sufficiently effective electron-donating groups to permit easy formation of the intermediate growing-chain cation. 2-Methylpropene has electron-donating alkyl groups and polymerizes much more easily than ethene by this type of mechanism. The usual catalysts for cationic polymerization of 2-methylpropene are sulfuric acid, hydrogen fluoride, or a complex of boron... 

Alkyl esters are formed by addition of the hydrogen halide promoter used with aluminum halide or of hydrogen fluoride or sulfuric acid catalyst to the olefin. Their formation is also inherent in the reaction of the second step of the chain mechanism. If conditions are not favorable for the reaction of these esters with the isoparaffin (by the reaction of step 1 or step 3), the esters are obtained as impurities in the alkylate. In general, they are produced under the same conditions which led to polymerization rather than alkylation. Usually very little ester is presnt in the alkylation product. 

PFSAs contain carboxylic acid groups at the end of polymer main chains as a result of the initiator in the polymerization reaction. The terminal carboxylic acid groups can be attacked by the radical species to produce shortened carboxylic acid groups and hydrogen fluoride. The repetition of this reaction causes unzipping degradation of polymer main chains. 

Stability and performance of AEM prepared using poly(4-vinyl pyridine-costyrene), quaternized with 1-bromooctane, and deposited on fibrous woven paper was unsatisfactory [189]. Sanchez and coauthors [190] discussed problems related to the use of certain AEMs. They pointed out that the so-called Hoffman degradation that involves attack of a hydroxyl on a-hydrogen in p-position to a quaternary ammonium attached to an aliphatic chain may cause its removal, followed by release of a tertiary amine and formation of a double bond at the end of a broken chain. Attachment of DABCO on short leash prevented chain break due to Hoffman degradation, but release of DABCO and generation of a double bond attached to the chain could take place. Perhaps thermal cross-linking by DABCO of poly(vinyl benzyl chloride) may solve this problem. Sulfonated polymers prepared by polymerization or copolymerization of phosphazene, siloxans, styrene, vinylidene fluoride, and various monomers with aromatic backbones, and possibly with aliphatic spacers, have been used. Various imides as well as PPS, PEK, PEEK, PSU, PEEKK, and PPSU can also be used [190,191]. 

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