Mechanism of free radical polymerisation

The mechanism of free radical polymerisations belongs to the class of so-called chain reactions. Chain reactions are characterised by the fast, subsequent addition of monomers to an active centre at the chain end. The activity of the growing chain is transferred to the adding unit. The active centres are present in very low concentrations (10 -10 mol 1 ). The rate of addition is very high (10 -10 units per second) and the time of growth of a chain (time between initiation and termination of a chain) is quite short (0.1-10 s) relative to the total reaction time, which can be in the order of several hours. 

This means that composition of the chain and chain length is determined in seconds. Terminated chains, in principle, do not take part in further reactions (except when transfer to polymer events occur. Section 2.3). The final chemical composition distribution and molecular mass distribution is determined by the accumulation of rapidly produced dead chains (chains without an active centre). In free radical polymerisation, the active centre is a free radical. In controlled or living radical polymerisation (Section 2.5) the radical is protected against termination and continues to grow during the complete reaction time. 

During the formation of a polymer chain a number of subsequent kinetic events take place (1) radical formation, (2) initiation, (3) propagation and (4) termination. Transfer of the radical activity to another molecule is a complication that will be dealt with in Section 2.3. 

Radical formation. The formation of free radicals can take place in a number of ways. Radicals can be produced by photo-initiation, radiation (y-radiation or electron beams), electrochemical reaction and by thermal initiation. Well-known examples of the thermal 

A schematic representation of the decomposition of the initiator (I) into two radicals (R ), with a decomposition rate coefficient (fca) and an expression for the rate of decomposition of the initiator (Rd) is given below  

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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. 

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