Chapter 9: Reaction Mechanisms, Pathways, Bioreactions and Bioreactors

Example 9.6: Reasons for "f" less than 1.0

1. Chain transfer to initiator

\( R_n \bullet + \phi COO - OOC \phi \rightarrow R_n - OOC \phi + \phi CO_2 \bullet \)

However, even though a radical is generated, this reaction does result in a wastage of initiator, there is no increase in the number of radicals or the amount of monomer being converted to polymer.

2. Side reactions

Cage effects

\( \phi CO - O - OOC \phi \rightleftharpoons [ 2 \phi COO \bullet ] \)       [     ]

Most significant reaction in the cage is

\( \rightarrow [ 2 \phi COO \bullet ] \xrightarrow{k_r} [ \phi COO \phi + CO_2 ] \)

[        ] = cage where radicals are held for some time before they diffuse out. Once outside the cage the radicals can react with monomer \( [ 2 f COO \bullet ] \rightarrow 2 f COO \bullet \)

Other reactions

\( \phi COO \bullet \rightarrow \phi \bullet + CO_2 \)

\( \phi \bullet + \phi \bullet \xrightarrow{k_r} \phi - \phi \)

\( \phi \bullet + \phi COO \bullet \xrightarrow{k_r} \phi COO \phi \)

of secondary importance

Average lifetime of neighboring radicals is 10^–10 to 10^–9 seconds with k_f = 10⁷ dm³/mol/s

In the cage:

The concentration of radicals in the solvent cage ~ 10 mol/dm³

Out of cage:

C_Monomer = 0.1 to 10 mol/dm³

C_R1 = 10^–9 to 10^–7 mol/dm³

3. Recombination of primary radicals has no effect on initiator efficiency

Back to Chapter 9