Reactions in extruders

These examples show that it is possible to handle gases and liquids of low molecular weight in reactions in extruders. The presence of a polymer is optional. It is possible to remove volatile by-products through a vent. The catalyst, if any is needed, can be in a coating in the extruder. Viscous mixtures are not a problem. A jacketed extruder can be used to remove heat if necessary. Thus, it should be possible to run many of the usual reactions in extruders. The only deterrents for the chemist in the laboratory might be... 

Shortly after controlled degradation technology was developed, pioneer workers explored polypropylene functionalization reactions in extruders (1). The early work involved grafting of maleic anhydride onto polypropylene to improve adhesion properties. Later work involved grafting other monomers, and even polymers, onto polypropylene in twin-screw extruders. 

For reactive extrusion this curve has some important practical implications, and we can derive some general rules for modification reactions in extruders ... 

The catalyst should be the copper-based United Catalyst T-2370 in 3/16 , reduced and stabilized, in extrudate form. Initially, 26.5 g of this should be charged to the catalyst basket. This catalyst is not for methanol synthesis but for the low temperature shift reaction of converting CO to CO2 with steam. At the given conditions it will make methanol at commercial production rates. Somewhat smaller quantity of catalyst can also be used with proportionally cut feed rates to save feed gas. 

The competition at 200°C between an aliphatic and an aromatic amine toward the formation of an imide is a very selective reaction (Fig. 5.1) for the formation of an aliphatic imide.141 This reaction suggests that the reactive processing in extruder, for example, could be used to transform a melt-processable polyimide with an oligomer end capped with an aliphatic amine. In order to get a perfecdy alternate block polyimide-block siloxane, Rogers et al. used low-temperature transimidization.142 An oligomeric aromatic imide was end capped... 

Elucidation of degradation kinetics for the reactive extrusion of polypropylene is constrained by the lack of kinetic data at times less than the minimum residence time in the extruder. The objectives of this work were to develop an experimental technique which could provide samples for short reaction times and to further develop a previously published kinetic model. Two experimental methods were examined the classical "ampoule technique" used for polymerization kinetics and a new method based upon reaction in a static mixer attached to a single screw extruder. The "ampoule technique was found to have too many practical limitations. The "static mixer method" also has some difficult aspects but did provide samples at a reaction time of 18.6 s and is potentially capable of supplying samples at lower times with high reproducibility. Kinetic model improvements were implemented to remove an artificial high molecular weight tail which appeared at high initiator concentrations and to reduce step size sensitivity. 

W=X is N=N or C=N, never if it is C=C. The ring stability of 3-furyl-carbenes conforms with this rule. Ring opening is again the main reaction in a biradical which is generated by extruding carbon dioxide from a lactone at 675°C and which then collapses to an acetylenic ketone.277... 

Electrochemical reduction of a,a -dibromoketones affords the unstable cyclo-propanone, which is in equlibrium with the dipolar intermediate 22. The cyclopro-panone hemiacetal is isolated in yields of 40 - 85 % from reaction in acetonitrile and methanol at -20 °C [99], The dipolar form can be trapped in a cycloaddition process with furan [100], Reaction with acetic acid leads to the a-acetoxy-ketone.[101]. Unstable three membered heterocyclic rings are intermediate in the reduction of sulphur and phosphorus linked dibromo compounds 23. In these reactions, the heteroatom is extruded leaving ci - and trans-stilbenes as the isolated products [102,103],... 

Cheletropic reactions, in which a single atom is added or extruded, comprise a special case of cycloaddition reactions. Figure 14.4 displays correlation diagrams for two typical cheletropic reactions, the loss of SO2 from a thiirane dioxide (Figure 14.4a) and the loss of CO from a norbornadienone (Figure 14.4b). The addition of a carbene to an olefin is another example which is discussed below (Figure 14.9a). 

Desulfurization reactions in the 1,2-series are encountered among derivatives of both oxathiins and dithiins. 1,2-Oxathiin 2,2-dioxides extrude sulfur dioxide at elevated temperature over zinc oxide, iron or copper oxide to give the corresponding furan (66HC(21 -2)789) [cf. Section 2.26.3.1.2). Copper is a good catalyst for the extrusion of sulfur and sulfur dioxide from dibenzo[c,e3[l,2]dithiin (40) and its dioxide respectively to give dibenzothiophene (66HC(21-2)968). 

Reactions in which the ring is cleaved or completely fragmented are fairly common in these systems. Ring contraction to form five-membered rings is also common especially where sulfur is extruded, and these reactions are considered separately, as are reactions which involve desulfurization or reduction. Reactions which lead to ring opening but are initiated by nucleophilic attack at ring carbons have already been discussed in Section 2.28.3.3.3. 

Fig. 45. Effect of hydrogen partial pressure on vanadium deposition for an Arabian Heavy atmospheric residuum at a reaction temperature of 371°C (700°F) using a -in. extrudate catalyst (Tamm et at., 1981).

The trimers are one of the major oxidative coupling products formed in extruded tocopherol-containing PP and PE. Trimers may either form by direct oxidation of the SPD by alkylperoxyl radicals (Scheme 6, reaction e) or through a reaction of SPD with QM in a Diels-Alder type process (reaction f). Oxidation studies of tocopherol in model compounds provided a clear evidence [44] in support of the latter reaction. It appears reasonable to suggest, therefore, that in the case of PP the major route for the formation of trimers (having much higher stability than SPD at PP processing tempera-... 

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