Polymerisation reaction processes

The conditions in which polymerisation takes place are very important, concerning for instance the physical form of the polymer that is obtained, the purpose of the form, the quality of the polymeric material, and the costs in economical and environmental terms. Polymers can be prepared  

The system involved in the bulk polymerisation process is the simplest from the point of view of composition and is used for large-scale radical polymerisation. In this process, the initiator is mixed with the monomer, usually under pressure of an inert gas, and the mixture is heated to induce generation of free radicals by thermal decomposition of the initiator. Depending on whether the monomer is a gas or a liquid, the system can be homogenous or heterogeneous 

From an industrial point of view, this process has many advantages. The monomer is converted into polymer without the need to eliminate byproducts. Continuous production of large amounts of polymer is possible. As polymerisation is exothermic, the polymer is usually in a molten state at the end of conversion, and its direct transformation into extruded products is easy. 

In the solution polymerisation process, monomer and initiator are dissolved in a solvent and the solution is stirred. Thus, some of the drawbacks cited above. 

However, new drawbacks are generated by the presence of solvent in this process. Side reactions such as transfer to solvent are possible. The solvent has to be eliminated as much as possible and recycled. Solvent residues can remain in the polymer. From an industrial point of view, this process is used only when the presence of solvent cannot be avoided. 

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A key feature of encapsulation processes (Figs. 4a and 5) is that the reagents for the interfacial polymerisation reaction responsible for shell formation are present in two mutually immiscible Hquids. They must diffuse to the interface in order to react. Once reaction is initiated, the capsule shell that forms becomes a barrier to diffusion and ultimately begins to limit the rate of the interfacial polymerisation reaction. This, in turn, influences morphology and uniformity of thickness of the capsule shell. Kinetic analyses of the process have been pubHshed (12). A drawback to the technology for some apphcations is that aggressive or highly reactive molecules must be dissolved in the core material in order to produce microcapsules. Such molecules can react with sensitive core materials. 

Figure 4c illustrates interfacial polymerisation encapsulation processes in which the reactant(s) that polymerise to form the capsule shell is transported exclusively from the continuous phase of the system to the dispersed phase—continuous phase interface where polymerisation occurs and a capsule shell is produced. This type of encapsulation process has been carried out at Hquid—Hquid and soHd—Hquid interfaces. An example of the Hquid—Hquid case is the spontaneous polymerisation reaction of cyanoacrylate monomers at the water—solvent interface formed by dispersing water in a continuous solvent phase (14). The poly(alkyl cyanoacrylate) produced by this spontaneous reaction encapsulates the dispersed water droplets. An example of the soHd—Hquid process is where a core material is dispersed in aqueous media that contains a water-immiscible surfactant along with a controUed amount of surfactant. A water-immiscible monomer that polymerises by free-radical polymerisation is added to the system and free-radical polymerisation localised at the core material—aqueous phase interface is initiated thereby generating a capsule sheU (15). 

Dow catalysts have a high capabihty to copolymetize linear a-olefias with ethylene. As a result, when these catalysts are used in solution-type polymerisation reactions, they also copolymerise ethylene with polymer molecules containing vinyl double bonds at their ends. This autocopolymerisation reaction is able to produce LLDPE molecules with long-chain branches that exhibit some beneficial processing properties (1,2,38,39). Distinct from other catalyst systems, Dow catalysts can also copolymerise ethylene with styrene and hindered olefins (40). 

Chain transfer to monomer and to other small molecules leads to lower molecular weight products, but when polymerisation occurs ia the relative absence of monomer and other transfer agents, such as solvents, chain transfer to polymer becomes more important. As a result, toward the end of batch-suspension or batch-emulsion polymerisation reactions, branched polymer chains tend to form. In suspension and emulsion processes where monomer is fed continuously, the products tend to be more branched than when polymerisations are carried out ia the presence of a plentiful supply of monomer. 

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

Poly(methyl methacrylate) may be blended with a number of additives. Of these the most important are dyes and pigments and these should be stable to both processing and service conditions. Two particular requirements are, firstly, that when used in castings they should not affect the polymerisation reaction and, secondly, that they should have good weathering resistance. 

The basic RIM process is illustrated in Fig. 4.47. A range of plastics lend themselves to the type of fast polymerisation reaction which is required in this process - polyesters, epoxies, nylons and vinyl monomers. However, by far the most commonly used material is polyurethane. The components A and B are an isocyanate and a polyol and these are kept circulating in their separate systems until an injection shot is required. At this point the two reactants are brought together in the mixing head and injected into the mould. 

As outlined in Chapter 1, polymerisation reactions can be classified as either condensation or addihon processes, the basis of the classification suggested by W. H. Carothers in 1929. More useful, however, is the classification based on reaction kinetics, in which polymerisation reactions are divided into step and chain processes. These latter categories approximate to Carothers condensation and addition reactions but are not completely synonymous with them. 

However, other molecules exist which form free radicals of such high stability that they effectively stop the chain process. These molecules are called retarders or inhibitors the difference is one of degree, retarders merely slowing down the polymerisation reaction while inhibitors stop it completely. In practice vinyl monomers such as styrene and methyl methacrylate are stored with a trace of inhibitor in them to prevent any uncontrolled polymerisation before use. Prior to polymerisation these liquids must be freed from this inhibitor, often by aqueous extraction and/or distillation. 

On the basis of the kinetic characteristics of chain polymerisation reactions, it is possible to predict the final microstructures available by a so-called random process from a simple mixture of two comonomers. Indeed, the global mechanism of copolymerisation can be illustrated as presented in Figure 30. 

Summary Multifunctional (meth)acrylate alkoxysilanes synthesized from commercially available acrylate compounds and mercapto-substituted alkoxysilanes or hydrosilanes are used as novel precursors for inorganic-organic copolymers. The alkoxysilyl groups are available for the formation of an inorganic Si-O-Si backbone by sol-gel processing. The (meth)acrylate groups allow the additional formation of organic polymer units by thermally or photochemically induced polymerisation reactions. 

The salt- and Cu2+-catalysed condensation of peptides provides a very simple polymerisation reaction with remarkable efficiency at 80°C. The proposed mechanism is shown in Figure 8.17 for the dimerisation of glycine. The presence of Cu2+ is important in this process and is unlikely to be present in the geothermal vent environment but it does require only small quantities of O2 to oxidise copper. A better condensation reaction would be autocatalytic and provide a template for future generations - in short, a genetic code. 

As the feed material gets passed the temperature gradient, polymerisation occurs and the fully, polymerised material emerges from the base of the tower. The reaction process gets controlled by a complex array of heating and cooling jackets and coils. 

The subject has been very thoroughly reviewed [1-3], but certain fundamental aspects can profitably be reconsidered in the light of some recent developments [4-6]. Certainly the most startling of these is the discovery that under conventional conditions the polymerisation of styrene by perchloric and other acids, and by the syncatalytic system stannic chloride-water, is not an ionic process. These polymerisations have been named pseudocationic . This finding is in direct contradiction to the beliefs held previously about this and related reactions. Hence a new survey of the whole field has become necessary which must start with an enumeration of those systems for which the nature of the polymerisation reaction has been established with reasonable certainty. From these boreholes one can then try to assess the nature of the intervening territory and to decide where further detailed exploration would be most profitable. 

Figure 7.3 gives an overview of the reactions involved in the hydroformylation of internal alkenes to linear products. It has been suggested that cobalt, once attached to an alkene, runs along the chain until an irreversible insertion of CO occurs. Thus, the alkene does not dissociate from the cobalt hydride during the isomerisation process. There is no experimental support for a clear-cut proof for this mechanism. In alkene polymerisation reactions this type of chain running has been actually observed. 

The mechanistic issues to be discussed are the initiation modes of the reaction, the propagation mechanism, the perfect alternation of the polymerisation reaction, chain termination reactions, and the combined result of initiation and termination as a process of chain transfer. Where appropriate, the regio- and stereoselectivity should be discussed as well. A complete mechanistic picture cannot be given without a detailed study of the kinetics. The material published so far on the kinetics comprises only work carried out at temperatures of -82 to 25 °C, which is well below the temperature of the catalytic process. 

An additional important feature of this class of polymers lies in the fact that their polymerisation and doping processes may be driven by a single electrochemical operation which, starting from the monomer, first forms the polymeric chain and then induces its oxidation and deposition in the doped form as a conductive film on a suitable substrate. The polymerisation reaction may be basically described as an electrophilic substitution which retains the aromatic structure and proceeds via a radical cation intermediate ... 

When products of low molecular weight are obtained from a chemical reaction process, it is often possible to separate these products after they have left the reactor. Thus, the choice of reactor conditions can be taken from a wide range of options. With polymerisation processes, the results of reaction selectivity (i.e. the molecular weight distribution of polymer molecules) cannot be changed easily once the material has left the reactor. Since polymer properties depend on the molecular weight distribution, the relative yields of polymers with particular sizes must be matched to a required specification. Therefore, the choice of reactor type is very important. 

The interaction of chemical and physical rate processes can affect the dynamic behaviour of reactors used for polymerisation or other complex reaction processes. This may lead to variations in the distribution of reaction products. As an example, consider a continuous-flow back-mixed reactor in which an exothermic reaction occurs. A differential material balance may be written for each reaction component... 

If the dilatometer body has one access only, namely through the capillary, the filling process is laborious and slow under atmospheric pressure because of air-locks in the capillary, but it can be swift and easy when done under vacuum. Emptying the body can prove difficult after a polymerisation reaction since the reaction mixture becomes very viscous. Therefore it is common practice to cut open the body after a reaction and to repair it for the next. The best way to break open the body of a dilatometer is to score... 

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