Free radical chain polymerisation propagation

Free radical chain polymerisation is the method used to prepare the most common polymers. A free radical is generated and reacts with one molecule of monomer (initiation). Then monomer molecules react with this first species, leading to formation of a long chain by successive additions of monomer (propagation). Finally, chains are terminated by reaction of two chains bearing radicals (termination). As radicals are very reactive species, side reactions are likely to occur and modify the simple process (transfer). 

Most emulsion polymerisations are free radical processes (318). There are several steps in the free radical polymerisation mechanism initiation (324), propagation and termination (324, 377, 399). In the first step, an initiator compound generates free radicals by thermal decomposition. The initiator decomposition rate is described by an Arrhenius-type equation containing a decomposition constant ( j) that is the reciprocal of the initiator half-life (Ph). The free radicals initiate polymerisation by reaction with a proximate monomer molecule. This event is the start of a new polymer chain. Because initiator molecules constantly decompose to form radicals, new polymer chains are also constantly formed. The initiated monomeric molecules contain an active free radical end group. 

The photoinduced addition of a thiol (RSH) to an olefinic double bond has been used to produce polymer networks by taking multi-functional monomers [37-44]. The thiol-ene polymerisation proceeds by a step growth addition mechanism which is propagated by a free radical, chain transfer reaction involving the thiyl radical (RS ). The initial thiyl radicals can be readily generated by UV-irradiation of a thiol in the presence of a radical-type photoinitiator. The overall reaction process can be schematically represented as follows ... 

Chain polymerisation typically consists of these three phases, namely initiation, propagation, and termination. Because the free-radical route to chain polymerisation is the most important, both in terms of versatility and in terms of tonnage of commercial polymer produced annually, this is the mechanism that will be considered first and in the most detail. 

Chain polymerisation necessarily involves the three steps of initiation, propagation, and termination, but the reactivity of the free radicals is such that other processes can also occur during polymerisation. The major one is known as chain transfer and occurs when the reactivity of the free radical is transferred to another species which in principle is capable of continuing the chain reaction. This chain transfer reaction thus stops the polymer molecule from growing further without at the same time quenching the radical centre. 

Chain polymerisation involves three major steps (i.e., initiation, propagation and termination). This process of chain polymerisation can be brought about by a free radical, ionic or coordination mechanism. 

Chain polymerisation is characterised by three steps namely initiation (Eq. 5.1), propagation (Eq. 5.2) and termination (Eq. 5.3) where I, R and M refer to the initiator molecule, free radical and monomer respectively and kj, kp and Iq are the respective rate constants for the processes. 

In the hrst step, a redox reaction occurs between Ce(IV) and the -CH2OH end group of PEO, generating a free radical in a-position of the -OH group of PEO. In a consequent step, the radical is transferred from the PEO chain to the vinyl monomer. The radicals formed initiate the actual polymerisation reaction (propagation) ... 

Conversely, when the rate of propagation is faster than chain transfer, products arising from telomerisation and polymerisation are formed in greater concentration. In this section, free-radical addition to fluoroalkenes will be dealt with first, in order to establish... 

Within the framework of this concept, initiation of polymerisation initially leads to accnmnlation of the number of propagating chains and makes the dependence of the nnmber of chains more profound. On the contrary, crosslinking reduces the number of chains, and, at the end of the process, the quantity dN/dt decreases to zero. This approach makes it possible to take into account the unsteady character of the polymerisation. However, becanse the mechanisms involved in the propagation of polylignol chains [3] are markedly different from the classical mechanism of free radical polymerisation, this variant hardly pertains to lignin formation. 

In this process, the initiator (I-I) generates a free radical as the reactive species and the monomers are vinyl or diene. The 7i-bond of the monomers is broken in the propagating steps and generates a new free radical to grow the polymer chain. The whole sequence of the polymerisation reaction is shown in Fig 1.2. 

First-order Markov processes are therefore defined by two independent addition probabilities. Although the propagation steps shown above depict free radical polymerisation, the statistical models are equally applicable to other types of chain growth as found, for example, in ionic and Ziegler-Natta polymers (see section 2.3.4). 

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. 

In the propagation step a monomer molecule adds to the free radical end of a growing chain and in so doing generates another radical. This process is fast by comparison with many chemical reactions, but it is not fast enough to be diffusion controlled under normal circumstances. However, as the polymerisation... 

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