Polymerisation reaction

Thus these oxidation reactions present qnite substantial induction periods, periods of time needed for free radicals such as RO and HO to reach some concentration values. 

Gas-phase polymerisations occur by composite mechanisms that may be of a molecular type or of a radical type. In liquid solutions and emulsions, again both ionic and radical polymerisation mechanisms are important, depending upon whether one is dealing with condensation or addition polymerisation. In this section we will consider the formation of macromolecules M made by a repetition of a certain number of identical monomers M by 

The termination step considers the removal of radical Rj by reaction with Rj, Rj,. .., R and this leads to 

The sum of all such equations simply states that the rate of initiation is equal to the sum of the rates of all the termination steps 

This is a set of general equations that can be made explicit for different kinds of mechanisms. For example, if the initiation step is a second-order process. 

Most polymerisation reactions occur via a complex reaction scheme. Relatively few reactant species are involved (sometimes only one) and these are usually well-defined. However, the reaction products can be described in a number of ways. The polymer molecules produced in these reactions vary in size in some cases the size distribution can be very wide. In effect, a polymerisation reaction produces a large number of reaction products and reaction selectivity requires special treatment. For some purposes, the polymer molecules can be treated as a combined group which is referred to generally as polymer . However, the physical properties of any given type of polymer depend on its molecular weight distribution. Therefore, it is often necessary to obtain a quantitative description of the product size distribution. This will depend on both the kinetic scheme for the polymerisation reaction and the mixing conditions in the polymerisation reactor. 

Most polymerisation reactions can be assigned to two catagories usually described as addition and condensation polymerisation (sometimes called chain reaction polymerisation and step reaction polymerisation , respectively). In both types of reaction, some form of initiator or catalyst is usually required. The tranformation which occurs in the addition polymerisation of a single monomer species can be represented as 

Resins produced by polymerisation reactions, known technically as high polymers , are rapidly increasing in number and importance as compared with polycondensation resins. High polymers are usually made by joining together into long chains several molecules with the same type of reactive points or groupings in their structure. These individual molecules are usually olefins or other compounds with double bonds, and are called monomers . The polymer molecule often contains hundreds of monomer units. 

The manufacture of high polymers therefore takes place in two stages (i) production of the monomer, or repeating chemical unit and (ii) polymerisation to a resin. 

If we take preparation of poljwinyl chloride as an example, we have  

CH2 = CHCI+CH2 = CHCI+CH2 = CHC1+. .. n molecules vinyl chloride 

It is possible to form polymers from two or even three monomers that may differ from one another in chemical form and yet be capable of linking end-to-end to form mixed monomer chains. These are known as copolymers , and they form the basis of the most important types of synthetic rubber. 

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. 

The study of reaction mechanisms can be a subtle business but in fact the mechanistic basis of classification into step and chain processes arises from major differences in the two types of process. There is no doubt about the nature of the reaction in almost all cases as can be seen by considering the distinguishing features of the two mechanisms which are summarised below. 

It is well known that most of the polymers are not biodegradable. This problem can be approached in two ways. One way is to recycle the polymer and the other way is to convert it again into the monomers and recycle them again. However, the best way is to make polymers which are biodegradable. 

The monomers (DMT and ethylene glycol) are purified and again polymerised to give PET. 

CpTiCl3 attached to aluminosilicate gels polymerises isoprene homogeneous analogues, e.g. CpTiCl2(0Af), are inactive. 

The Ni or Pd catalysed dehalogenation of dihaloaromatics with zinc is a route to poly(1,4-phenylene) and poly(2,5-thienylene).  

A new catalyst for the cyclotrlmerlsatlon of butadiene to 1,5,9-cyclo-dodecatrlene is the Mn(II) complex The cyclotrlmerlsatlon of butadiene 

Optically active ligands transfer their chlrallly to vinylcyclohexene In up to 62% enantiomeric excess with (42) (eqn.34). 

Benjyrl chloride Is dehydrochiorlnated by M(C0)5C1 catalysts (M = Mn,Re) branched oligomers (n = 40-50) are formed.  

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Acetaldehyde is a highly reactive compound exhibiting the general reactivity of aldehydes (qv). Acetaldehyde undergoes numerous condensation, addition, and polymerisation reactions under suitable conditions, the oxygen or any of the hydrogens can be replaced. 

G. H. Kalb and co-woikeis, in N. A. J. Platzei ed.. Polymerisation Reactions and New Polymers, Advances in Chemistry Series no. 129, American... 

Tripotassium hexakiscyanoferrate [13746-66-2] K2[Fe(CN)g], forms anhydrous red crystals. The crystalline material is dimorphic both orthorhombic and monoclinic forms are known. The compound is obtained by chemical or electrolytic oxidation of hexacyanoferrate(4—). K2[Fe(CN)g] is soluble in water and acetone, but insoluble in alcohol. It is used in the manufacture of pigments, photographic papers, leather (qv), and textiles and is used as a catalyst in oxidation and polymerisation reactions. 

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). 

The chemical iadustry manufactures a large variety of semicrystalline ethylene copolymers containing small amounts of a-olefins. These copolymers are produced ia catalytic polymerisation reactions and have densities lower than those of ethylene homopolymers known as high density polyethylene (HDPE). Ethylene copolymers produced ia catalytic polymerisation reactions are usually described as linear ethylene polymers, to distiaguish them from ethylene polymers containing long branches which are produced ia radical polymerisation reactions at high pressures (see Olefin POLYMERS, LOWDENSITY polyethylene). 

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). 

As the length and frequency of branches increase, they may ultimately reach from chain to chain. If all the chains are coimected together, a cross-linked or network polymer is formed. Cross-links may be built in during the polymerisation reaction by incorporation of sufficient tri- or higher functional monomers, or may be created chemically or by radiation between previously formed linear or branched molecules (curing or vulcanisation). Eor example, a Hquid epoxy (Table 1) oligomer (low molecular weight polymer) with a 6-8 is cured to a cross-linked soHd by reaction of the hydroxyl and... 

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). 

A number of olefins may be polymerised using certain metal oxides supported on the surface of an inert solid particle. The mechanism of these polymerisation reactions is little understood but is believed to be ionic in nature. 

There is much evidence that weak links are present in the chains of most polymer species. These weak points may be at a terminal position and arise from the specific mechanism of chain termination or may be non-terminal and arise from a momentary aberration in the modus operandi of the polymerisation reaction. Because of these weak points it is found that polyethylene, polytetrafluoroethylene and poly(vinyl chloride), to take just three well-known examples, have a much lower resistance to thermal degradation than low molecular weight analogues. For similar reasons polyacrylonitrile and natural rubber may degrade whilst being dissolved in suitable solvents. 

Since impurities can affect both the polymerisation reaction and the properties of the finished product (particularly electrical insulation properties and resistance to heat aging) they must be rigorously removed. In particular, carbon monoxide, acetylene, oxygen and moisture must be at a very low level. A number of patents require that the carbon monoxide content be less than 0.02%. 

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. 

Styrene and solvent are blended together and then pumped to the top of the first reactor which is divided into three heating zones. In the first zone the solution is heated to start up the polymerisation reaction but because of the exothermic reaction in the second and third zones of the first reactor and the three zones of the second reactor Dowtherm cooling coils are used to take heat out of the system. By the time the reaction mixture reaches the third reactor the polymerisation reaction has started to slow down and so the reaction mixture is reheated. 

Formaldehyde is a gas with a boiling point of -21 °C. It is usually supplied as a stabilised aqueous solution ( 40% formaldehyde) known as formalin. When formalin is used as the source of the aldehyde, impurities present generally include water, methanol, formic acid, methylal, methyl formate and carbon dioxide. The first three of these impurities interfere with polymerisation reactions and need to be removed as much as possible. In commercial polymerisation the low polymers trioxane and paraformaldehyde are convenient sources of formaldehyde since they can be obtained in a greater state of purity. 

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. 

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)... 

This system was slightly modified by R J. Flory, who placed the emphasis on the mechanisms of the polymerisation reactions. He reclassified polymerisations as step reactions or chain reactions corresponding approximately to condensation or addition in Carother s scheme, but not completely. A notable exception occurs with the synthesis of polyurethanes, which are formed by reaction of isocyanates with hydroxy compounds and follow step kinetics, but without the elimination of a small molecule from the respective units (Reaction 1.3). 

The monomer, styrene, is a derivative of benzene, vinyl benzene (1.2). It is a colourless, mobile liquid that polymerises readily. The first report of the polymerisation reaction came in 1839, when E. Simon described the transformation of what was then called styrof. He believed he had oxidised the material and called the product styrol oxide. Later, when it was realised that it contained no oxygen, the product became known as metastyrene. 

The polymerisation reactions that occur by the chain mechanism are typically those involving unsaturated monomers. The characteristic reaction begins with the chemical generation of reactive centres on selected monomer... 

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. 

The finding that the rates of chain polymerisations are proportional to the square root of the initiator concentration is well established for a large number of polymerisation reactions. An example is shown in Figure 2.1, which also illustrates the method by which such initiator exponents are determined, i.e. by a plot of log R v. log [I]. 

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