Polymerisation techniques

Four techniques are used in most industrial processes for the polymerisation of monomers to obtain corresponding polymers. These include (i) bulk or mass, (ii) solution, (iii) suspension and (iv) emulsion polymerisation techniques. However, other techniques such as interfadal, electrochemical and plasma polymerisation are also used to obtain different polymers, particularly in laboratory or low scale production. 

This technique is less frequently used in the condensation polymerisation process as the removal of by-products is difficult, as is controlling the reaction. The problems mentioned previously in condensation polymerisation produce polymers with a low molecular weight. The process is therefore only used for the preparation of some resins and polycarbonates by an ester exchange process. 

To overcome the major drawbacks of the mass polymerisation technique, monomers or reactants are dissolved in a suitable inert solvent or mixture of solvents, where the polymers are also expected to be soluble. The localised heat accumulation or gel effect is not observed in this case so heat dissipation is not a problem. However, the choice of solvent is critical in obtaining a high molecular weight with a controlled polymer structure. Desirable characteristics of solvents include non-interaction with the components of the medium, moderate volatility, non-toxicity and good solvating power, both for monomer and polymer. 

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. 

The four techniques described above are the most commonly used polymerisation processes. However, other techniques such as interfacial, electrochemical and plasma techniques may sometimes be used, particularly on the laboratory scale. 

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ESBR and SSBR are made from two different addition polymerisation techniques one radical and one ionic. ESBR polymerisation is based on free radicals that attack the unsaturation of the monomers, causing addition of monomer units to the end of the polymer chain, whereas the basis for SSBR is by use of ionic initiators (qv). 

Anionic polymerisation techniques aie one of many ways to synthesise a special class of block copolymers, lefeiied to as star block copolymers (eq. 25) (33). Specifically, a "living" SB block is coupled with a silyl haUde coupling agent. The term living polymerisation refers to a chain polymerisation that proceeds in the absence of termination or transfer reactions. 

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). 

Polybutadiene was first prepared in the early years of the 20th century by such methods as sodium-catalysed polymerisation of butadiene. However, the polymers produced by these methods and also by the later free-radical emulsion polymerisation techniques did not possess the properties which made them desirable rubbers. With the development of the Ziegler-Natta catalyst systems in the 1950s, it was possible to produce polymers with a controlled stereo regularity, some of which had useful properties as elastomers. 

Free-radical polymerisation techniques involving peroxides or azodi-isobutyronitrile at temperatures up to about 100°C are employed commercially. The presence of oxygen in the system will affect the rate of reaction and the nature of the products, owing to the formation of methacrylate peroxides in a side reaction. It is therefore common practice to polymerise in the absence of oxygen, either by bulk polymerisation in a full cell or chamber or by blanketing the monomer with an inert gas. 

Polystyrene produced by free-radical polymerisation techniques is part syndio-tactic and part atactic in structure and therefore amorphous. In 1955 Natta and his co-workers reported the preparation of substantially isotactic polystyrene using aluminium alkyl-titanium halide catalyst complexes. Similar systems were also patented by Ziegler at about the same time. The use of n-butyl-lithium as a catalyst has been described. Whereas at room temperature atactic polymers are produced, polymerisation at -30°C leads to isotactic polymer, with a narrow molecular weight distribution. 

Bead Processes. These processes have generally replaced the above techniques. The styrene is polymerised by bead (suspension) polymerisation techniques. The blowing agent, typically 6% of low boiling petroleum ether fraction such as n-pentane, may be incorporated before polymerisation or used to impregnate the bead under heat and pressure in a post-polymerisation operation. 

Synthesis of strictly alternating copolymers can be achieved via various polymerisation techniques including poly-condensation or Ziegler-Natta polymerisations [123, 124]. 

When the USA and Germany were cut off from the supplies of natural rubber during the Second World War both countries sought to produce a synthetic alternative SBR was the result, and at one stage it was the most commonly used synthetic rubber. It can be produced by both emulsion and solution polymerisation techniques, with the emulsion grades being the most widely used. Emulsion polymerisation yields a random copolymer, but the temperature of the polymerisation reaction also controls the resultant properties obtained. Cold polymerisation yields polymers with superior properties to the hot polymerised types. 

The first phase of polymer chemistry started with unlimited future prospects which encouraged over the years for the synthesis of Synthetic polymers from available monomers making use of simple polymerisation techniques. 

As polystyrene obtained by free radical polymerisation technique is atactic it is therefore non-crystalline. The isotactic polystyrene is obtained by the use of Ziegler-Natta catalysts and n-butyl lithium. Isotactic polystyrene is having a high crystalline Melting point of 250°C. It is transparent. It is more brittle than the atactic polymer. 

On a large scale it is prepared from acrylonitrile by the radical polymerisation technique using peroxide initiators. 

The living polymerisation technique is useful for many applications. Block copolymers, for example, are prepared by using this technique. 

The polymerisation of VAM (reaction 4 in Fig. 2) can be carried out using different kinds of standard polymerisation techniques, the technically most important of... 

Compared to other living radical polymerisation techniques, ATRP offers two important advantages. Firstly, the synthesis of well-defined macro-initiators is facile and allows the preparation of a range of new diblock copolymers. Secondly, the presence of water (or methanol) has a remarkable accelerating effect on the... 

As it will be apparent in the following, both these issues quite severely limit the possible application of standard polymerisation techniques to MIPs. On the other hand, they provide guidelines for critically evaluating the application of the various beaded polymer preparation methods to MIP synthesis. 

Independently of each other, Lambert et al. [69] and Suzuki et al. [70] both gained access to low-generation silane dendrimers (Gl, G2) in 1995. The latter prepared a first-generation polysilane dendrimer 3 by a stepped growth polymerisation technique. Coupling of methyl[tris(chlorodimethylsilyl)]silane (1) to tris-(trimethylsilyl)silyllithiurn (2) led to the first-generation branched dendrimer 3 (Fig. 4.38). 

The effect of reactive plasma and its distance form the PE film surface has also been studied in detail [138]. The surface of polyethylene films was modified with various water-soluble polymers [(poly[2-(methacryloy-loxy)ethyl phosphorylcholine] (PMPC), poly[2-(glucosyloxy)ethyl methacrylate] (PGEMA), poly(N-isopropylacrylamide) (PNIPAAm) and poly[N-(2-hy-droxypropyl) methacrylamide] (PHPMA)] using Ar plasma-post polymerisation technique [139]. Here, the reactive sites were generated on the PE surface under the influence of argon plasma. These reactive sites on the surface were then utilised to covalently anchor the functional monomers as shown in Scheme 11. 

Introduction of ring-opening metathesis as a versatile polymerisation technique (ROMP) by Chauvin and Herisson Nobel Prize Chemistry to Paul J. Flory for his fundamental achievements, both theoretical and experimental, in the physical chemistry of the macromolecules Fully aromatic polyamides developed Aramids, being lyotropic liquid crystalline polymers of high strength, due to extended molecular chains (Morgan and Kwolek)... 

Preparation by Sequential Polymerisation. Two-polymer composite latex particles may be prepared using either emulsion or dispersion polymerisation techniques. A dispersion (latex) of particles of a first polymer may be prepared in the usual manner after complete conversion of monomer to polymer, a different monomer or monomer mixture is added and polymerised to provide the second polymer. 

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