Propagation Straight chains

Reaction Mechanism. The reaction mechanism of the anionic-solution polymerization of styrene monomer using n-butyllithium initiator has been the subject of considerable experimental and theoretical investigation (1-8). The polymerization process occurs as the alkyllithium attacks monomeric styrene to initiate active species, which, in turn, grow by a stepwise propagation reaction. This polymerization reaction is characterized by the production of straight chain active polymer molecules ("living" polymer) without termination, branching, or transfer reactions. 

Now consider that a particular straight-chain propagating reaction ensues, that the initial chain particle concentration is simply 1, and that lmol or 1019 molecules/cm3 exist in the system. Thus all the molecules will be consumed in a straight-chain propagation mechanism in a time given by... 

In straight chain propagation one radical is destroyed while another is produced, so that the net change in the number of radicals is zero. 

Different temperature conditions along a, b, c, d at constant pressure and composition. At temperatures between a and b reaction occurs non-explosively and is in the steady state, proceeding basically by straight chain propagation. The rate increases with increasing temperature in this region. 

For initial temperatures between r and s reaction is again in the steady state with straight chain propagation dominant, but for temperatures greater than s explosion occurs. 

The chain reactions treated in Section 3.4 consist of straight chains. There is an initiation step taking place at a rate r,-. The propagation steps do not produce nor do they destroy active centers. This is so because in each step of the closed sequence, the active centers change only in kind, not in number. Therefore, there exists a steady state defined by the condition that the rate of initiation is equal to the rate of termination. 

In the case of the hydrogen-bromine reaction, each of the elementary propagation reactions led to the formation of a single chain carrier. This type of reaction is said to be a straight or linear chain reaction. Some mechanisms involve elementary propagation reactions in which more than a single chain carrier is formed by the reaction. This type of reaction is known as a branching reaction. Examples of such reactions are... 

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