Polymerisation section

The polymerisation reaction is carried out in a batch or in a continuous mode depending on the specific process (details in Section 8.2). The reactor is charged with solvent and catalyst. Depending on the targeted polymer, monomers are added simultaneously or sequentially. Where a random copolymer is required, a structure modifier, usually an ether, is added. These chemicals have the additional benefit of increasing the amount of 1.2 polymerisation of the butadiene, i.e. they increase the vinyl content. 

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Fig. 1. Schematic flow sheet for the polymerisation and isolation of polychloroprene the polymerisation section is fully functional for the manufacture of...

Volume 15 Non-radical Polymerisation Section 6. OXIDATION AND COMBUSTION REACTIONS (2 volumes)... 

Inorganic heterocycles in which other p-block elements are inserted into a phosphazene ring are accessible via a variety of synthetic approaches. The primary interest in these hybrid ring systems is as potential precursors of linear polymers via ring-opening polymerisation (Section 11.4.3). Consequently, recent work has focused on halogenated derivatives. For example, the... 

The 2H- and 3//-pyrrolium cations are essentially iminium ions and as such are electrophilic they play the key role in polymerisation (section 13.1.8) and reduction (section 13.8) of pyrroles in acid. In the reaction of pyrroles with hydroxylamine hydrochloride, which produces ring-opened 1,4-dioximes, it is probably the more reactive 3//-pyrrolium cation which is the starter. Primary amines, RNH2, can thus be protected, by conversion into l-R-2,5-dimethylpyrroles (section 13.18.1.1), the protecting group being removable by this reaction with hydroxylamine. ... 

While for step-growth polymerisations (section 2.2.1) and chain-growth polymerisations without termination (section 2.2.2) an overall distribution of reacting species or one type of reacting species (the monomer) is mechanistically characterising the observed reaction rate, the balance between two distinct and mechanistically different species (the monomer and a macro-molecular radical) is determining the observed rate of chain-growth polymerisations with termination. 

A flow diagram of the traditional polypropylene suspension ( slurry ) process is shown in Figure 3.13. Propylene, diluent (Cg to saturated hydrocarbons), hydrogen, a catalyst and a cocatalyst are continuously fed to the polymerisation section, which normally consists of one or more stirred tank reactors in series. Polymerisation is carried out at 60 - 80 °C and at pressures below 2 MPa. The polymerised polypropylene forms small powder particles suspended in the diluent. A small amount of atactic polypropylene is formed as a by-product in the polymerisation step and is partly dissolved in the diluent. The slurry is continuously withdrawn from the last reactor after which unreacted propylene is removed from the slurry and recycled to the reactor. 

Slow cure can also be overcome by using an activator (or accelerator). There are a mmiber of different activators available to the production engineer and these increase the level of initiators on the surface to negate the stabiliser and thus increase the speed of polymerisation. Section 10.8 discusses the uses and application methods of activators in more detail. 

This means that composition of the chain and chain length is determined in seconds. Terminated chains, in principle, do not take part in further reactions (except when transfer to polymer events occur. Section 2.3). The final chemical composition distribution and molecular mass distribution is determined by the accumulation of rapidly produced dead chains (chains without an active centre). In free radical polymerisation, the active centre is a free radical. In controlled or living radical polymerisation (Section 2.5) the radical is protected against termination and continues to grow during the complete reaction time. 

Separations based upon differences in the chemical properties of the components. Thus a mixture of toluene and anihne may be separated by extraction with dilute hydrochloric acid the aniline passes into the aqueous layer in the form of the salt, anihne hydrochloride, and may be recovered by neutralisation. Similarly, a mixture of phenol and toluene may be separated by treatment with dilute sodium hydroxide. The above examples are, of comse, simple apphcations of the fact that the various components fah into different solubihty groups (compare Section XI,5). Another example is the separation of a mixture of di-n-butyl ether and chlorobenzene concentrated sulphuric acid dissolves only the w-butyl other and it may be recovered from solution by dilution with water. With some classes of compounds, e.g., unsaturated compounds, concentrated sulphuric acid leads to polymerisation, sulphona-tion, etc., so that the original component cannot be recovered unchanged this solvent, therefore, possesses hmited apphcation. Phenols may be separated from acids (for example, o-cresol from benzoic acid) by a dilute solution of sodium bicarbonate the weakly acidic phenols (and also enols) are not converted into salts by this reagent and may be removed by ether extraction or by other means the acids pass into solution as the sodium salts and may be recovered after acidification. Aldehydes, e.g., benzaldehyde, may be separated from liquid hydrocarbons and other neutral, water-insoluble hquid compounds by shaking with a solution of sodium bisulphite the aldehyde forms a sohd bisulphite compound, which may be filtered off and decomposed with dilute acid or with sodium bicarbonate solution in order to recover the aldehyde. 

For both suspension and mass polymerisations at less than 2% conversion, PVC precipitates from its monomer as stable primary particles, slightly below 1-p.m dia (4,10—12). These primary particles are stabilised by a negative chloride charge (4,13). Above 2% conversion, these primary particles agglomerate. Sectioning the PVC grains of either suspension or mass resins readily shows the skins primary particles at 1-p.m dia, and agglomerates of primary particles at 3—10-pm dia (4,7,8,14). 

The narrow molecular weight distribution means that the melts are more Newtonian (see Section 8.2.5) and therefore have a higher melt viscosity at high shear rates than a more pseudoplastic material of similar molecular dimensions. In turn this may require more powerful extruders. They are also more subject to melt irregularities such as sharkskin and melt fracture. This is one of the factors that has led to current interest in metallocene-polymerised polypropylenes with a bimodal molecular weight distribution. 

The styrene-diene triblocks, the main subject of this section, are made by sequential anionic polymerisation (see Chapter 2). In a typical system cc-butyl-lithium is used to initiate styrene polymerisation in a solvent such as cyclohexane. This is a specific reaction of the type... 

Such lenses may be made by machining from rod. More recently processes have been developed where the monomers are cast polymerised in tiny plastics moulds whose cavity corresponds to the dimensions of the lens and using procedures very reminiscent of those described for the manufacture of acrylic sheet (see Section 15.2.2). 

Molecular films are of intense current concern in electronics. For instance, diacetylenes and other polymerisable monomer molecules have been incorporated into L-B films and then illuminated through a mask in such a way that the illuminated areas become polymerised, while the rest of the molecules can be dissolved away. This is one way of making a resistance for microcircuitry. L-B films have also found a major role in the making of gas-sensors (Section 11.3.3). 

Volume 8 Volume 9 Volume 10 Volume 12 Volume 13 Proton Transfer Addition and Elimination Reactions of Aliphatic Compounds Ester Formation and Hydrolysis and Related Reactions Electrophilic Substitution at a Saturated Carbon Atom Reactions of Aromatic Compounds Section 5. POLYMERISATION REACTIONS (3 volumes)... 

Chain reactions are used to prepare a variety of high molar mass polymers of commericial importance and in practice may take one of four forms, namely bulk, solution, suspension, and emulsion methods. These four methods are described in the sections that follow, together with the loop modification which has become of commercial importance recently in producing latexes by emulsion polymerisation for the paint industry. 

Section 3 deals with reactions in which at least one of the reactants is an inorganic compound. Many of the processes considered also involve organic compounds, but autocatalytic oxidations and flames, polymerisation and reactions of metals themselves and of certain unstable ionic species, e.g. the solvated electron, are discussed in later sections. Where appropriate, the effects of low and high energy radiation are considered, as are gas and condensed phase systems but not fully heterogeneous processes or solid reactions. Rate parameters of individual elementary steps, as well as of overall reactions, are given if available. 

Abstract Over the past decade significant advances have been made in the fields of polymerisation, oligomerisation and telomerisation with metal-NHC catalysts. Complexes from across the transition series, as well as lanthanide examples, have been employed as catalysts for these reactions. Recent developments in the use of metal-NHC complexes in a-olefin polymerisation and oligomerisation, CO/olefm copolymerisation, atom-transfer radical polymerisation (ATRP) and diene telomerisation are discnssed in subsequent sections. 

This section deals with the most important control experiments to be considered when molecular complexes or NPs want to be proved as true catalysts. But in some cases both types of catalysts can be present in the same reaction. For example, in the ring opening polymerisation of l,l,3,3-tetramethyl-l,3-disilacyclobutane catalysed photo-chemically by Pt(acac)2, the co-existence of both homogeneous and colloidal catalytic species has been proved, giving each of them different type of polymers [10]. 

Flow-sheets drawn up for batch processes normally show the quantities required to produce one batch. If a batch process forms part of an otherwise continuous process, it can be shown on the same flow-sheet, providing a clear break is made when tabulating the data between the continuous and batch sections the change from kg/h to kg/batch. A continuous process may include batch make-up of minor reagents, such as the catalyst for a polymerisation process. 

In general the mechanism of polymerisation for thiophene appears to be similar to that of pyrrole (Section 4.11.2), occurring via a radical coupling mechanism [423] giving mainly a-a linkages [293,400,405], and involves oligomer as well as monomer radicals, with evidence to suggest that the polymerisation reaction occurs at a lower... 

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