Polymerization Reactions in Alternative Reaction Media

Single and double carbon-carbon bond forming reactions, and metathesis (ROMP) reactions, can be used to produce polymers. However, many other 

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Polymerization Reactions in Alternative Reaction Media SO3K SOgK... 

Apart from polymerization processes with gaseous monomers above their critical points-for example, the synthesis of low-density poly(ethylene) - several SCFs have been tested as inert reaction media, such as ethane, propane, butane, and C02. Among these, scC02 is by far the most widely investigated, because it links positive fluid effects on the polymers with environmental advantages this makes scC02 the main candidate as an alternative to traditional solvents used in polymer syntheses. 

The precise mechanism of these intermolecular reactions is not known. Transient disproportionation processes, although well documented in less acidic media (see below and Section V.A.l), seem unlikely if alkyl cations alone are involved. Two possible explanations for the observed results may be considered. Small amounts of polymeric impurities may be present in the reaction mixture which could serve as a hydride source, catalyzing the intermolecular reaction as indicated in Eq. (16)4°). Alternatively, the intermolecular reactions may result from inefficient mixing during reaction initiation. In this case, unionized alcohols would serve as the hydride source. This latter alternative is consistent with the observation 4°1 that the deuterium in the 1-adamantanol obtained from the rearrangement of 38 is distributed between bridgehead and methylene positions. Unless more than one re-... 

Several alternative polymerization media have been proposed for reducing the amount of unstable end groups in poly(tetrafluoroethylene). These include chlorofluorocarbons, which are detrimental to the environment, perfluoro-carbons, hydrofluorocarbons, and perfluoroalkyl sulfide acids, which are all expensive. Supercritical carbon dioxide has been identified as a viable alternative to aqueous and fluorocarbon reaction media [31]. Further more, mixing... 

Besides alternative reaction media, process and product research has resulted in a number of new classes of polymers, such as dendritic and conductive polymers. At the same time the application field of polymers has been extended, ranging nowadays from bulk materials such as polyethylene and polypropylene all the way to specialty products, for example for biomedical applications. These developments ask for well controlled polymerization processes. In this respect ultrasound has proven to be a clean and safe technology for generating radicals in situ during the polymerization reaction. 

Supercritical fluids are already used as alternative for classical separation processes such as extraction, adsorption, chromatography, and crystallization, as reaction media as in polymerization or depolymerization, as plasticization agents or simply as particles, fibers, or foams formation even some processes becoming commercial. A variety of polymers, including polyolefins. 

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