Chain growth polymerisation

Chain-growth polymerisations ate characterised by chains that propagate by adding one monomer molecule at a time, ie, a -mer + monomer — (a + l)-mer. There ate, however, several mechanisms by which this occurs. 

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

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. 

When carbon monoxide is bubbled through a methanol solution of (dppp)Pd(triflate)2 a carbomethoxy-palladium species is formed, which can undergo insertion of alkenes and hence this is a feasible alternative initiation route to chain-growth polymerisation (Figure 12.4) [13], To ensure a clean formation of the carbomethoxy species, however, exclusion of water is a prerequisite. If during the preparation water was present the formation of a palladium hydride complex (dppp)PdFT was observed (reaction (1), Figure 12.2). 

IPolymerisation chain growth polymerisation and condensation or step growth... 

Copolymerisation is a polymerisation reaction in which a mixture of more than one monomeric species is allowed to polymerise and form a copolymer. The copolymer can be made not only by chain growth polymerisation but by step growth polymerisation also. It contains multiple units of each monomer used in the same polymeric chain. For example, a mixture of 1, 3 - butadiene and styrene can form a copolymer. 

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... 

Polymerisa- tn chain length or chain growth can take place through the formation of either free radicals or ionic species. However, the free radical governed addition or chain growth polymerisation is the most common mode. 

Heck-type step-growth condensation polymerisation involves mainly palladium-based catalysts, although nickel-based catalysts are also effective. It is worth noting that this polycondensation requires a change in the oxidation state of the metal (e.g. Pd) [schemes (30) and (31)] [71], which is in contrast to chain growth polymerisation, such as ethylene/carbon monoxide alternating copolymerisation promoted by Pd-based catalysts [schemes (82) and (83) in Chapter 3], for which the preservation of the oxidation state of palladium, Pd(II), is typical [83-85] ... 

The first step in catalytic reactions of the Heck type is the oxidative addition of the organic halide to Pd(0) species to form an intermediate organopalladium halide constituting the Pd(II) species. This is followed by insertion of the olefinic bond and subsequent /khydrogen elimination [scheme (30)]. The catalyst is recycled by the reaction of the Pd(II)-hydride species with a base [scheme (31)]. It is worth noting here that palladium species, L2(X)Pd—ArCH=CH2, do not propagate the chain growth polymerisation of the CH2=CHArX monomer via its olefinic bond in the discussed process. 

Scheme 6.6 (a) Step-growth polymerisation. Short chains can link to give a variety of sequences, (h) Chain-growth polymerisation involves the addition of new molecules to one end of a growing chain. A represents a free radical or similar active centre where addition of new material can occur... 

Finally, polyesters can also be synthesised via enzymatic processes. This type of polymerisations possesses some advantages over the earlier described chemical synthesis routes, including mild reaction conditions, a higher selectivity, a high tolerance of functional groups and the synthesis of pure reaction products which are metal free [79, 80]. However, reaction times are longer and yields lower compared to chain growth polymerisations. 

The process by which polymers are formed from their respective monomers by the sequential addition of monomers, without loss of any by-product, is known as addition or chain growth polymerisation. The polymerisation proceeds through three distinct steps ... 

Chain-Growth Polymerisation with Termination A major exponent of this class of polymerisations are free radical polymerisations in the presence of a radical initiator. A classic example is the crosslinking (co)polymerisation of unsaturated polyester resins with styrene, initiated by the decomposition of a peroxide initiator. Some important reaction steps involved in free radical polymerisations are sketched in Reaction scheme 4. 

While for step-growth polymerisations (section 2.2.1) and chain-growth polymerisations without termination (section 2.2.2) an overall distribution of reacting species or one type of reacting species (the monomer) is mechanistically characterising the observed reaction rate, the balance between two distinct and mechanistically different species (the monomer and a macro-molecular radical) is determining the observed rate of chain-growth polymerisations with termination. 

The different conversion-dependence of rj is related to the molecular weight evolution and network development. For addition step-growth polymerisation systems, the molecular weight of the polymer chains gradually increases, while for (linear) free radical chain-growth polymerisations the... 

Vinyl, Acrylic and Other Monomers for Linear Chain-growth Polymerisation... 

The other family of fatty acid-based monomers for chain-growth polymerisations that has attracted considerable attention is oxazolines. These heterocyclic structures are polymerised readily by cationic ring-opening processes [118]. The ensuing polyoxazolines have found numerous applications [119] as in biomedicines [120, 121] and as non-ionic surfactants or emulsifiers, among others. 

ROMP is a type of olefin metathesis chain-growth polymerisation (Scheme 5.14). 

Later in the 1950s, Mark classified polymerisation without considering the loss of a small molecule or type of inter unit linkage. To avoid confusion, he based his classification on the basis of mechanism and used step reaction and chain reaction where step-growth and chain-growth polymerisation are also very commonly used. Table 6.1 summarises the main differences between chain and step polymerisation mechanisms [2-4]. 

In this type of polymerisation an initiating molecule is required so that it can attack a monomer molecule to start the polymerisation. This initiating molecule may be a radical, anion or cation. Chain growth polymerisation is initiated by free-radical, anion or cation proceeded by three steps initiation, propagation and termination. The chemical nature of the substituent group determines the mechanism. 

This method can be used for chain-growth polymerisation, but only on a small scale, preferably at low temperature. Heat and bubble transfer may give problems, since the degree of polymerisation (and hence, also the viscosity of the reaction mixture) increases very rapidly from the beginning of the reaction. 

Solution pol5mierisation comprises monomer + initiator + solvent. This is the preferred method to use for chain-growth polymerisation. The solvent helps heat dispersal and reduces the rapid build-up in viscosity in the reaction mixture. 

Kiss et al. [18] studied the multiplicity behaviour of isothermal CSTR / Separation / Recycle systems involving six reaction systems of increasing complexity, including chain-growth polymerisation. Below a critical value of the plant Damkohler number Da < Dd the... 

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