PHOTOCHEMISTRY 31 



yield to the yield of the primary process (which is commonly close to 

 unity) is a direct measure of the average chain length. Furthermore, 

 knowledge of the nature of the primary product is frecjuently very helpful 

 in the prediction of the secondary steps. 



The primar}^ act in association polymerization is the formation of two 

 radicals or a diradical. Each radical or diradical can then add to a 

 monomer molecule, forming a new radical of greater molecular weight. 

 Large polymers are built up by the successive addition of monomer mole- 

 cules to the growing radical. In most cases studied, the addition of 

 monomer to the radical rec^uires a heat of activation of a few kilocalories. 

 The specific rate of addition is only slightly influenced by the size of the 

 radical. In the absence of inhibitors the chain is, in the great majority 

 of cases, terminated by a reaction between two radicals. This chain- 

 stopping step eliminates two radicals either by their disproportionation or 

 by their addition. 



Free radicals may be formed by the photochemical dissociation of the 

 monomer or of an added sensitizer such as acetone (Jones and Melville, 

 1946). Since the chains are broken by bimolecular reactions between 

 growing radicals, the rate of polymerization is proportional to the square 

 root of the intensity of the absorbed light. 



A determination of the ratio of the rate constants for the chain-propa- 

 gating and chain-terminating steps may be made by analyzing the 

 kinetics of a polymerization reaction. This analysis is made by the usual 

 steady-state approximate method. Special methods are required to 

 evaluate either of these individual constants. Melville (1947) has shown 

 that these individual constants can be obtained if the polymerization 

 occurs under intermittent illumination. This technic^ue, which has 

 proved very useful in the study of polymerization kinetics, is a relatively 

 old one in photochemistry, having been used by Berthoud and Bellenot in 

 1924 and subjected to a thorough theoretical analysis by Dickinson 

 (Noyes and Leighton, 1941, pp. 202-209). 



INTRAMOLECULAR CHANGES 



Relatively few photochemical isomerizations have been studied ciuanti- 

 tatively. One group of reactions which has received some attention is 

 the cis-trans isomerizations. For reasons of experimental convenience, 

 most of the kinetic measurements have been made with substituted 

 ethylenes. However, knowledge of their spectroscopic properties is 

 limited to the simpler compounds. Figure 1-3, which is taken from the 

 work of Mulliken (1942), is a schematic representation of the electronic 

 energy levels of ethylene. In addition to the ground level A'^, two excited 

 singlet levels, V and R, are shown. Absorption bands, corresponding to 

 transition from N to either V or R, are strong. According to the Franck- 

 Condon prijiciple, the angle between the hydrogens cannot change appre- 



