Polymer chains: initiation, growth termination

The second mechanism of polymerization, chain-growth, involves three distinct steps initiation, propagation, and termination. An initiator reacts with a monomer to produce a new species which can then react with another monomer, and another, until the monoma is depleted or the growing polymer chain undergoes a termination reaction. This procedure is shown schematic y bdow where refers to a species that can add another monomer. This species is oft a radical, cation, anion, or attached transition metal complex that can insert a monomer between itself and the growing polymer chain. 

In mbber production, the thiol acts as a chain transfer agent, in which it functions as a hydrogen atom donor to one mbber chain, effectively finishing chain growth for that polymer chain. The sulfur-based radical then either terminates with another radical species or initiates another chain. The thiol is used up in this process. The length of the mbber polymer chain is a function of the thiol concentration. The higher the concentration, the shorter the mbber chain and the softer the mbber. An array of thiols have subsequendy been utilized in the production of many different polymers. Some of these apphcations are as foUow ... 

In some cases termination may be brought about by transfer reactions . In this type of reactions though the growth of one Polymer chain is stopped due to formation of dead polymer as in coupling or disproportionation reaction, yet there is a simultaneous generation of a new free radical that is capable of initiating a fresh Polymer chain growth. 

Let us for the moment disregard chain transfer reactions. Radical polymerization then consists of three component reactions initiation, propagation of the polymer chains, and termination of chain growth. The rate of primary radical formation, v, by decomposition of the initiator I, may be written ... 

Kawabata and Tsuruta (5) studied the reaction mode of butyl-lithitm (BuLi) with MMA in the initiation step. Glusker and his coworkers postulated the pseudo-cyclization of chain end as a cause of oligomer formation (6). However, it is still unresolved how the butyl group of the initiator is incorporated into the polymer chain, and how the oligomer terminates its further growth. 

These photoinitiation processes which depend on the formation of free radicals in some photochemical reaction lead to chain reactions, since each molecule of initiator can promote the addition of many monomer units to a polymer chain. The quantum yield of monomer addition can therefore be much larger than unity, but it cannot be controlled since the growth of a polymer chain is then limited by termination reactions in which two free radicals react to produce closed-shell molecules. 

Chain-growth, or addition, polymers are made by adding one monomer unit at a time to the growing polymer chain. The reaction requires initiation to produce some sort of reactive intermediate, which may be a free radical, a cation, or an anion. The intermediate adds to the monomer, giving a new intermediate, and the process continues until the chain is terminated in some way. Polystyrene is a typical free-radical chain-growth polymer. 

Figure 5.9. Reactions involved in free-radical addition polymerization. Shown are (a) (i)-(iii) generation of free radicals from a variety of initiators, (b) initiation of polymer chain growth through the combination of a free radical and unsaturated monomer, (c) propagation of the polymer chain through the combination of growing radical chains, (d) chain-transfer of free radicals between the primary and neighboring chains, and (e) termination of the polymer growth through either combination (i) or disproportionation (ii) routes.

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