Alkenes addition polymerisation

The -f-H2C—CHCl section of the polymer chain is the repeat unit of poly(chloroethene) (Figure 15.18). In poly(alkenes) made of one type of monomer, the repeat unit is the same as the monomer except that the C=C double bond is changed to a C—C single bond. Notice that, as in any other addition reaction of the alkenes, addition polymerisation yields only one product. 

Ca.ta.lysts, A small amount of quinoline promotes the formation of rigid foams (qv) from diols and unsaturated dicarboxyhc acids (100). Acrolein and methacrolein 1,4-addition polymerisation is catalysed by lithium complexes of quinoline (101). Organic bases, including quinoline, promote the dehydrogenation of unbranched alkanes to unbranched alkenes using platinum on sodium mordenite (102). The peracetic acid epoxidation of a wide range of alkenes is catalysed by 8-hydroxyquinoline (103). Hydroformylation catalysts have been improved using 2-quinolone [59-31-4] (104) (see Catalysis). 

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. 

Isobutene - In contrast to the complicated picture presented by the polymerisations of most other alkenes, the polymerisation of isobutene at low temperatures is a clean reaction with apparently few complications [10, 16, 17, 18]. The propagation step seems to be a simple addition to the monomer of the tertiary carbonium ion at the growing end of the chain. This difference between the behaviour of isobutene and of most other olefins is so striking that isobutene could usefully be regarded as a standard of reference it would thus be possible to enquire into the behaviour of other olefins by comparing them and their polymers with isobutene and polyisobutene. 

In the presence of an organic peroxide Initiator, the alkenes and their derivatives undergo addition polymerisation or chain growth polymerisation through a free radical mechanism. Polythene, teflon, orlon, etc. are formed by addition polymerisation of an appropriate alkene or Its derivative. Condensation poiymerisation reactions are... 

Simvastatin, a conjugated alkene, can polymerise as a result of peroxyl radical addition. The peroxide-linked oligomers can be subsequently cleaved to produce epoxides, which in turn degrade to form ketones and alcohols [69]. Inclusion of vitamin E (a-tocopherol) into formulations was found to inhibit chain-oxidation of simvastatin, lovastatin and other structurally related statins. 

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 ... 

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). 

Alkenes produce many useful polymers by addition polymerisation. For example, poly(ethene) is made from CH2=CH2 and poly(chloroethene) is made from CH2=CHCL... 

Rememberfrom Chapter 15 that addition polymerisation occurs when an unsaturated monomer, such as an alkene, bonds to itself in an addition reaction. Poly(ethene), poly(chloroethene) and poly(phenylethene) are all addition polymers. 

If one wants to use only a metal halide as the initiator for an alkene polymerisation, an analysis similar to ours can be made for the metal halide alone. In addition, there is a 1 2 equilibrium for the organic halide [35, 36] and a molecular aggregate <-> single molecule equilibrium associated with the metal halide. Thus, a solution of a carbocation salt is more exactly described by the series of linked equilibria summarised in Scheme 5. 

Pseudo-cationic polymerisations are reactions in which an alkene is inserted between the positive carbon atom and the negative heteroatom of a polar bond at the growing end of a polymer chain, without the formation of a carbenium ion they do not differ essentially from the well-known additions of esters to alkenes. The theory of these reactions was devised by Gandini and Plesch [2] and has been brought up to date by Plesch [3]. 

The well-known addition of tert-butyl chloride to ethylene by means of A1C13 without skeletal rearrangement is simply another example of the initiation step of a /-cat polymerisation, involving the insertion of the alkene into an ester, namely the C-Cl bond, which is activated by the A1C13, via a six-centred transition state (I), as shown in equation (ii) ... 

Kinetics is used to investigate mechanisms of radical additions to alkenes. Outside the solvent cage, the initiator-derived radicals may undergo the desired bimolecular reaction with the substrate, or side reactions. When the substrate is an alkene, the exothermic intermolecular addition of the reactive radical (R ) to the double bond results in the formation of two new single carbon-carbon bonds in place of the double bond. This reaction represents conversion of an initiator into a propagating radical in radical polymerisations, and is becoming increasingly important in a number of synthetically useful intermolecular small molecule reactions. The addition of R to monosubstituted and 1,1-disubstituted alkenes is nearly always at the unsubstituted carbon atom (tail addition), and thus is normally not affected by the individual steric demand of the alkene substituents. Equation 10.4 is the expression for the rate of addition (R ) of R to an alkene where [M] is the monomeric alkene concentration ... 

Organometallic compounds are used widely as homogeneous catalysts in the chemical industry. For example, if the alkene insertion reaction continues with further alkene inserting into the M C bond, it can form the basis for catalytic alkene polymerisation. Other catalytic cycles may include oxidative addition and reductive elimination steps. Figure above shows the steps involved in the Monsanto acetic acid process, which performs the conversion... 

The thermal reactions of l-oxa-l,3-butadienes such as acroleine 2-78 with alkenes such as 2-79 usually need relatively harsh conditions (150°C-250°C) [120]. As a side reaction polymerisation of the a,/l-unsaturated carbonyl compound can take place addition of radical inhibitors such as hydroquinone or 2,6-di-ferf-butyl-4-methylphenol can be helpful in avoiding this unwanted transformation. In the described hetero Diels-Alder reaction the cycloadduct 2-80 was obtained which was then transformed into racemic-/3-santalene 2-81 (Fig. 2-22). 

Addition of hydrogen fluoride to alkenes proceeds, as might be expected, via trans addition in a typical Markovnikov process, with the complicating effect of cationic polymerisation of the alkenes [250] (see also Chapter 7). However, side-products resulting from polymerisation of the alkene may be reduced by performing the reaction in a lower-acidity amine-HF mixture [43] (Figure 3.51). 

Substitution of hydrogen, in an alkene, by fluorine leads to increased reactivity for a number of processes for example, with tetrafluoroethene, heats of addition of chlorine, hydrogenation and polymerisation are 58.5, 66.9 and 71.1kJmol greater, respectively, than for the analogous reactions with ethene [3, 29]. These observations could be attributed either to an increase in the carbon-fluorine bond strength upon changing the hybridisation of the carbon atoms bonded to fluorine [30] or to ir-bond destabilisation by fluorine [31]. 

The most important methodology for the aliphatic C-C bond formation via radical reactions is the addition of the radical to an alkene double bond, both inter -and intramolecularly (with the 5-exo-ring cyclisation mode preferred in the latter case). This reaction leads to adduct radicals that must be converted to non-radical products before polymerisations can take place. For this reason, polymerisation is avoided either by intermolecular trapping of adduct radicals or by intramolecular, homolytic bond cleavage. Hydrogen atom donors X-H, heteroatom donors X-Z or electron donors M"+ are used as trapping agents (Scheme 7.1). 

The high conversion polymerisation of a-decene gave a monomer fraction which contained 60% a-decene, 35% internal alkenes and 5% methylnonenes. The conclusion was that, in addition to methyl group migration, isomerisation of the double bond could also take place, even with the monomer. As polyalphaolefins derived from internal olefins usually give products with inferior temperature/viscosity properties, double bond isomerisation can be a problem. 

Radical polymerisation of alkenes often leads to chain branching and the formation of non-linear polymers. In addition, the stereogenic centres on the backbone of polymers are usually formed randomly to give atactic polymers. 

There is a great deal of information available about the addition of radicals to n bonds, since it is such an important step in radical polymerisation, as we have already seen.969 The regioselectivity in a lot of these reactions is easily explained the more stable products 7.2, 7.6, 7.20, 7.31984 and 7.32,985 with the radical centre adjacent to the substituent are almost always obtained, and the site of attack usually has the higher coefficient in the appropriate frontier orbital. With C- and Z-substituted alkenes, the site of attack will be the same regardless of which frontier orbital is the more important—both have the higher coefficient on the carbon atom remote from the substituent (Figs. 2.2 and 2.5). 

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