Simplified Reaction Scheme and Kinetics of Photopolymerization

In the classical picture of radical polymerization which, apart from termination, also holds for cationic chain polymerization, the following steps are usually distinguished  

In is the initiator, M the monomer, R a primary radical and RMj a growing radical consisting of a primary radical and i monomeric units. RM, and RMjR are dead polymers. The rate constants k refer to decomposition, initiation, propagation and termination, respectively. Termination may either occur through combination (c) or through disproportionation (d). Under the usual assumptions of a steady-state total free-radical concentration and of size-independent reactivities of the growing radicals, the rate equation becomes  

However, in many systems of practical use the variation of I with thickness cannot be neglected and the rate equation becomes  

In strongly absorbing systems, the rate of polymerization may vary considerably across the sample thickness in the limit no polymerization at all will occur at the bottom of the sample. The situations which may be encountered in stirred and unstirred solutions of various absorbance levels have been discussed by Shultz a simpler and less widely applicable treatment has been given by Lissi and Zanocco . 

In the photopolymerization of thin films, advantage is often taken of using a reflecting substrate. This will level the intensity gradient across the sample considerably. 

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