Energetics of the polymerization process

More direct information on the energetics of the photopolymerization process is afforded by DSC measurement with UV-initiation of the reaction. Under single pulse excitation the integral heat evolved per light pulse is a measure for the product nAH and from the ratio of the signal amplitudes at t = 0 and t -+ co, respectively, 

The essence of the energetic studies on TS and 4-BCMU is contained in Fig. 9. In TS formation of the chain initiating species -- a dimer — requires an energy of 1.0 eV. It can be supplied thermally or optically via monomer excitation. In the former case it is this chain initiation reaction that controls the thermal reactivity and its temperature-dependence. Chain initiation can also be produced optically at a yield of order 10 per absorbed UV-quantum. In this case it is chain propagation that determines the temperature dependence of the polymerization yield. However, the activation energy E need not be and in general is not identical with the energy 

There has been an argument in the literature whether the intermediate acting as a chain initiator is a dicarbene of the type  

The argument in favour of the latter was that the diradical should be lower in energy, because its formation requires disruption of only one carbon-carbon rc-bond instead of two in case of carbene formation. Both ESR work and optical spectroscopy have meanwhile confirmed the diradical mechanism for growth of oligomeric chains up to length of 5 repeat units. Upon further addition of monomers, the acetylenic structure becomes energetically more stable causing a cross-over to the carbene mechanism. For further discussion of this topic the reader is referred to the article by H. Sixl in this volume. 

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