Polymerisation reaction processes systems

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). 

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. 

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. 

The MIP is usually prepared as a highly cross-linked, rigid bulk polymer and the polymerisation reaction is initiated by photo- or thermo-labile free radical initiators such as 2,2 -azobis(isobutyronitrile). For molecular imprint-based CEC systems, the introduction of the imprinted polymer into the capillary column has been focused on and several approaches have been developed (see below). The polymerisation process can be performed in between 1 and 24 h. It has been shown that the temperature during the polymerisation process is important. A lower temperature leads to imprinted polymers with higher selectivity [47] or better chromatographic performance [39]. 

But, 1,2- or vinyl BR can be polymerised in the atactic, the syndiotactic or the isotactic form. Hence, five different configurations can be obtained by polymerisation reactions with butadiene (CH2=CH-CH=CH2) as monomer. The product obtained depends on the catalyst system used but is usually a mixture of 1,4 cis-, 1,4 trans- and atactic 1,2-BR. The commercial processes using Co-, Ni- or Ti-based catalyst systems, for instance, produce BR with a 1,4 cis-BR content higher than 90 %wt. But butyllithium initiated homopolymerisation of butadiene results in a product with 1,4 trans-BR contents up to 60 %wt. 

Taking into consideration the fact that fast polymerisation processes are characterised by inequality of chemical reaction time and transfer time ( chem < it is clear that an increase of facilitates the decrease of and both these processes are comparable in duration. The increase in linear flow rate V, i.e., the intensification of heat and mass exchange in the system, is equivalent to a slow dovm of the polymerisation reaction itself, compared with the transfer process. Therefore, the conventional approaches to external heat removal, which normally have such a restrictive effect on conventionally designed fast polymerisation processes implemented in stirred tank reactors, play an essential role at both high V and values when quasi-plug flow tubular turbulent reactors are used. In this case, control of the external temperature can be significantly enchanced due to zone-type catalyst loading. 

I is the portion of heat removed in the cooling zone from the total thermal energy accumulated by the system at the inlet of this zone (taking into account both the polymerisation reaction and cooling process in all previous zones). 

It should be noted that even though heat dissipation is minimised by this technique, the solvent causes other problems. The problems associated with solvents, with the exception of water, are chain transfer to the propagating chain, flammability and toxicity of the solvent, removal of solvent from viscous polymer, cost of solvent, and so on. However, this process remains useful for surface coatings, paints and thin films. In solution condensation polymerisation, the by-product may be insoluble in the medium. This facilitates the polymerisation reaction for higher conversion than in a system where it is soluble in the medium. 

However, these systems offer some important advantages when compared with traditional free-radical polymerisation processes simpler polymerisation kinetics milder reaction conditions insensitivity to oxygen inhibition [2, 3]. These characteristics have made the thiol-ene photo-polymerisation reaction the focus of extensive research, and its application in oleochemistry is of considerable importance in terms of polymer science and technology. 

This process exploits an imusual effect of the difference in solubility of acrylic acid monomer and polyacrylic acid in specific solvents. When products based on the process were first developed [4] and made commercially available, benzene was used as the polymerisation medium. The polymerisation reaction is initiated in a system containing a mixture of acrylic acid monomer and a cross-linking monomer (typically a multi-allyl ether derivative of sucrose or pentaeryrthritol) and, as the polymer network grows, the solubility in the solvent decreases until precipitation of the polymer network occurs in the form of a small particle size powder. The use of a cross-linking monomer results in a 3D network of... 

If gaseous monomers such as ethylene or vinyl chloride are used, the production processes involve polymerisation at high pressures and the polymers formed are commonly referred to as pressure polymers , e.g. copolymers of VA/E or terpolymers of VA/E/VC or VA/E/2-EHA (2-ethylhexyl acrylate). Other non-gaseous monomers may be polymerised together to form polymers in low pressure systems and these polymers are commonly referred to as conventional polymers or atmospheric polymers , e.g. VA homopolymers, Ac polymers of methylmethacrylate. Furthermore, the use of other functional monomeric units to give the polymer specific application properties, such as cross linking in cured textile fabric applications (e.g. V-methylol acrylamide, acrylamide and many others), are often employed and these polymers are commonly referred to as speciality polymers . Common to all polymerisation reactions the processes are usually carried out in a batch-wise system, but continuous processes can also be employed. 

Polymerisation refers to processes in which the overall composition of a compound does not substantially alter, but the molecular weight increases by a multiple of the weight of the monomer. Polymers usually arise when a system can create free radicals. Therefore, polymerisation reactions usually accompany isomerisation reactions and the formation of cyclic fatty acids under extreme heating. In the fully refined (and thus also deodorised) edible oils, polymers represent several tenths of a percent, but their content increases during heating. Oils with a polymer content of more than 10% are not recommended for use. 

The temperature at which decarboxylation occurs is of particular interest in manufacturing processes based on polymerisation in the molten state where reaction temperatures may be near the point at which decomposition of the diacid occurs. Decarboxylation temperatures are tabulated in Table 2 along with molar heats of combustion. The diacids become more heat stable at carbon number four with even-numbered acids always more stable. Thermal decomposition is strongly influenced by trace constituents, surface effects, and other environmental factors actual stabiUties in reaction systems may therefore be lower. 

EPM and EPDM mbbers are produced in continuous processes. Most widely used are solution processes, in which the polymer produced is in the dissolved state in a hydrocarbon solvent (eg, hexane). These processes can be grouped into those in which the reactor is completely filled with the Hquid phase, and those in which the reactor contents consist pardy of gas and pardy of a Hquid phase. In the first case the heat of reaction, ca 2500 kJ (598 kcal)/kg EPDM, is removed by means of cooling systems, either external cooling of the reactor wall or deep-cooling of the reactor feed. In the second case the evaporation heat from unreacted monomers also removes most of the heat of reaction. In other processes using Hquid propylene as a dispersing agent, the polymer is present in the reactor as a suspension. In this case the heat of polymerisation is removed mainly by monomer evaporation. 

The reaction is considerably modified if the so-called emulsion polymerisation technique is used. In this process the reaction mixture contains about 5% soap and a water-soluble initiator system. The monomer, water, initiator, soap and other ingredients are stirred in the reaction vessel. The monomer forms into droplets which are emulsified by some of the soap molecules. Excess soap aggregates into micelles, of about 100 molecules, in which the polar ends of the soap molecules are turned outwards towards the water whilst the non-polar hydrocarbon ends are turned inwards (Figure 2.17). 

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