Dehydrogenated polymerisate

It is possible to prepare a lignin-like polymer, known as dehydrogenated polymerisate (DHP) in the laboratory by treating con-iferyl alcohol in vitro under aerobic conditions with a phenoloxidase... 

VI. Investigations on an Enzymatic Dehydrogenation Polymerisate of Coniferyl Alcohol (Freudenberg s DHP). Svensk Papperstidn. 59 363-371, 1956. 

Chen, C.-L. (1998) Characterization of milled wood lignins and dehydrogenative polymerisates from monohgnols by carbon-13 NMR spectroscopy. In Lewis, N. G., and Sarkanen, S. (eds). Lignin and Lignan Biosynthesis, American Chemical Society, Washington, DC. 

Ca.ta.lysts, A small amount of quinoline promotes the formation of rigid foams (qv) from diols and unsaturated dicarboxyhc acids (100). Acrolein and methacrolein 1,4-addition polymerisation is catalysed by lithium complexes of quinoline (101). Organic bases, including quinoline, promote the dehydrogenation of unbranched alkanes to unbranched alkenes using platinum on sodium mordenite (102). The peracetic acid epoxidation of a wide range of alkenes is catalysed by 8-hydroxyquinoline (103). Hydroformylation catalysts have been improved using 2-quinolone [59-31-4] (104) (see Catalysis). 

The dehydrogenation reaction produces crude styrene which consists of approximately 37.0% styrene, 61% ethylbenzene and about 2% of aromatic hydrocarbon such as benzene and toluene with some tarry matter. The purification of the styrene is made rather difficult by the fact that the boiling point of styrene (145.2°C) is only 9°C higher than that of ethylbenzene and because of the strong tendency of styrene to polymerise at elevated temperatures. To achieve a successful distillation it is therefore necessary to provide suitable inhibitors for the styrene, to distil under a partial vacuum and to make use of specially designed distillation columns. 

Styrene was obtained by catalytic dehydrogenation of ethyl benzene. Styrene gets readily polymerised... 

The polymerisation process repeats itself, and the resulting chain with eight benzene rings, three of which are quinonoid, is dehydrogenated exactly as before to a chain of four quinonoid rings. (Write out the formulae.)... 

In general, a catalytic reaction may be named by adding the adjective catalytic to the standard chemical term for the reaction, for example, catalytic hydrogenation (or, if clarity demands, heterogeneous catalytic hydrogenation), catalytic hydrodesulphurisation, catalytic oxidative dehydrogenation, catalytic stereospecific polymerisation. 

Unlike air, the recycling of water is imperative. Water can be used as reactant, as in hydrolysis reactions. Water can be also a direct heat transfer agent, as steam for supplying heat by some endothermic reactions (dehydrogenation of hydrocarbons), or to remove heat (polymerisation of styrene by suspension process). 

Ethylbenzene is used mainly for the manufacture of styrene. Among impurities, diethylbenzene is very important, because dehydrogenation to divynilbenzene, which is harmful in polymerisation. It is worth to keep in mind that the formation of troublesome impurities should be prevented by the design of the reaction system. In this respect the catalyst plays a determinant role. Zeolite-type catalysts should give less polyalkylbenzene. The use of an excess of reactant can help to shift the product distribution to higher yield in ethylbenzene. Another possibility is to use a separate reactor for transalkylation. The trade-off between of a larger recycle of benzene and the use of a secondary reactor is a matter of optimisation. 

Carbon formation can then proceed in various ways. The di-carbon species shown above may break down to mono-carbon species then polymerise, or polymerisation may occur without this. Processes in which free radicals occur, or acidity in the catalyst, both encourage this, and (as noted above) catalysts for dehydrogenation must be neutral or basic to prevent acid-catalysed reactions of the alkene. The more dehydrogenated the reactant hydrocarbon, the greater the tendency to carbon formation alkadienes and aromatics are notorious in this respect. With alkanes, increase in molar mass (i.e. the number of C—H bonds) also assists degenerate events. 

Disproportionation/dehydrogenation of rosin is also a process that decreases the ability of colophony to oxidize, since the amount of resin acids with conjugated double bonds is diminished in favour of dehydroabietic acid. Polymerization dimerization of colophony decreases its tendency to crystallize. Major products formed are dimers of resin acids, but the degree of polymerisation varies with the conditions (time, temperature) under which the modification is performed. Formaldehyde modification of colophony includes several different reactions performed under various conditions where formaldehyde reacts with resin acids. Colophony can also be modified by reacting metal salts, e.g. sodium, potassium, or barium, calcium, cobalt and zinc, with resin acids to form salts resinates) (McSweeney et al. 1987 Soltes and Zinkel 1989). 

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