Addition polymerisation/polymers

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). 

Polymer Structure. Isopiene can undergo 1,4-, 1,2-, oi 3,4-addition polymerisation depending on the catalyst type and conditions, lesultiag ia several structures ... 

An alternative route to cross-linking is to start with a linear polymer and then cross-link the molecules by tying the molecule through some reactive group. For example, it is possible to cross-link unsaturated polyesters by an addition polymerisation across the double bond as shown schematically in Figure 2.12. 

It may be noted here that it is frequently possible to polymerise two monomers together so that residues from both monomers occur together in the same polymer chain. In addition polymerisation this normally occurs in a somewhat random fashion and the product is known as a binary copolymer. It is possible to copolymerise more than two monomers together and in the case of three monomers the product is referred to as a ternary copolymer or terpolymer. The term homopolymer is sometimes used to refer to a polymer made from a single monomer. 

Although polyethylene is virtually defined by its very name as a polymer of ethylene produced by addition polymerisation, linear polymers with the formula (CH2), have also been prepared by condensation reactions. For example in 1898 von Pechmann produced a white substance from an ethereal solution of... 

Polymers can be formed from compounds containing a c=c double bond. Alkenes, such as ethene, can undergo addition polymerisation to form a polymer. A polymer is a compound consisting of very long chain molecules built up from smaller molecular units, called monomers. The polymerisation of ethene, to form poly(ethene), is a free radical addition reaction. 

Most of these addition polymerisation are chain growth polymerisations as a particular Polymer chain is formed in a single chain reaction. 

The most important commercial polymers prepared by this method are obtained from monomers containing a carbon-carbon double bond. In such cases the reaction is called vinyl polymerisation. The addition polymerisation is also important for carbonyl compounds and 1, 2-epoxides. 

This type of polymers are obtained by addition polymerisation. Such a polymerisation process involving addition of monomer units to the growing chain is known as chain growth polymerisation. 

The second common method of synthesising polymers (Fig. 5.3) is chain (addition) polymerisation. The most common type of addition polymer is based on ethene CH2 = CHj in which the monomer contains at least one double (tt) bond which on being activated, by free radical attack say, opens up to produce two single sigma bonds and the homopolymer poly(ethene). (Note in Fig. 5.3 the resultant polymer backbone is joined together by carbon-carbon bonds, unlike the condensation polymer systems (Fig. 5.1).)... 

The polymers made by addition polymerisation from two different... 

In addition polymerisation, the molecules of the same or different monomers add together to form a large polymer molecule. Condensation polymerisation is a process in which two or more bi-functional molecules undergo a series of condensation reactions with the elimination of some simple molecules and leading to the formation of polymers. 

Emulsion polymerisation is a special case of heterogeneous addition polymerisation in which the reaction kinetics are modified because the A are compartmentalised in small polymer particles [48, 49]. These particles are usually dispersed in water and reaction (78) occurs in the aqueous phase. Initiating radicals diffuse to the particles which are stabilised by surfactant material. Chain termination becomes retarded physically and a relatively high polymerisation rate is obtained. If chain transfer is not prominent, a high molecular weight polymer is produced. The polymerisation rate is given by the expression... 

The range of monomers which can be employed is largely dictated by the physical chemistry of the emulsion system. For instance, monomers must be sufficiently hydrophobic to allow the formation of stable w/o HIPEs. In addition, most systems which have been studied have used polymerisation methods which require either an initiation step, or addition of a catalyst. This is due to the fact that the first step in the preparation of the polymer is the preparation of HIPE this can only proceed satisfactorily in the absence of any significant degree of polymerisation. Thus, it can be seen that radical addition polymerisation is suitable for the synthesis of PolyHIPE polymers, whereas condensation polymerisation can be more problematical. Also, the latter reactions often generate water as the by-product, hence the aqueous component of the HIPE is inhibiting to the polycondensation. 

When small molecules like ethene join together to form long chains of atoms, called polymers, the process is called polymerisation. The small molecules, like ethene, which join together in this way are called monomers. A polymer chain, a very large molecule or a macromolecule, often consists of many thousands of monomer units and in any piece of plastic there will be many millions of polymer chains. Since in this polymerisation process the monomer units add together to form only one product, the polymer, the process is called addition polymerisation. 

In Chapter 14 (p. 226) you studied the different addition polymers produced from alkenes. Not all polymers are formed by addition reactions, though. Some are produced as a result of a different type of reaction. In 1935 Wallace Carothers discovered a different sort of plastic when he developed the thermoplastic, nylon. Nylon is made by reacting two different chemicals together, unlike poly(ethene) which is made only from monomer units of ethene. Poly(ethene), formed by addition polymerisation, can be represented by ... 

These are examples of addition polymerisation , which can only occur where compounds have double bonds in them. For further reading about polymerisation you might like to find out what is meant by isotactic, syndiotactic and atactic polymers. 

Initiation reactions are usually started by an active free radical such as peroxide (-0-0-), e.g. benzoyl peroxide is a good inititator for the free radical addition polymerisation of styrene to produce polystyrene AICI3 is an initiator for the cationic addition polymerisation of isobutylene to form isobutyl synthetic rubber or azobisiso-butyronitrile compounds (-N=N-) (abbreviated to AIBN). Propagation reactions are the continuing process and, eventually, lead to the termination stage that occurs by combination or disproportionation. This usually occurs when the free radicals combine with themselves and signals the end of the polymerisation process. All polymers formed by this process are thermoplastics. Table 4.1 is a list of common polymers prepared by the addition process. 

The most important industrial applications of radical reaction to date are used for the manufacture of polymers. Around 108 tonnes (or 75%) of all polymers are prepared using radical processes. These are chain reactions in which an initial radical adds to the double bond of an alkene monomer and the resulting radical adds to another alkene monomer and so on. This addition polymerisation is used to make a number of important polymers, including poly(vinyl chloride) (PVC), polystyrene, polyethylene and poly(methyl methacrylate). Copolymers can also be easily prepared starting from a mixture of two or more monomers. These polymers have found widespread use as they possess a range of chemical and mechanical properties (such as strength and toughness). 

Polyacrylonitile (PAN) is a long chain polymer containing acrylonitrile [-CHj-CH(CN)] as repeating unit in the polymeric chain and are formed by addition polymerisation. Acrylic fibre contains 15% or less copolymer. The modacrylic fibres are comprised of less than 85% but at least 35% by weight of acrylonitrile. The comonomers are added to increase the polymer thermoplasticity, solubility, dyeability, moisture regain, etc. 

The most common way of creating polymers is through addition polymerisation - a process which involves three steps, namely polymer initiation, addition and termination. 

---