Question

In normal free radical processes with added initiator, the polymerization rate and kinetic chain length are proportional to the first power of monomer concentration. What would be the effect of monomer concentration on rate and chain length in the thermal polymerization of styrene (which involves no initiator)? Derive a kinetics expression to show this.Assume that initiation is based on the following equation: M + M -> 2 M. (radical in the product).

Answer 1

The thermal polymerization of styrene proceeds through the following steps:

Chain Initiation: QM- 2 M. 2

where k1 = rate constant for chain initiation reaction.

Chain growth: M+M>M M TM

where k2 = rate constant for chain growth reaction.

Chain termination: 2m' → 2P 2 ar

Now we can find the overall rate of the reaction by applying steady state principle to all of the above reactions. Hence the overall rate expression is:

For the above overall rate law expression it is clear that the rate is reaction is proportional to square of the concentration of styrene (M²). Hence this ia a 2nd order reaction.