Thermochemically versus photochemically initiated polymerisations

Although the generation of free radicals may be faster by photolytic homolysis the propagation rate should be slower due to the low temperature of polymerisation ( — 20 to -l-15°C as compared to ca. 60°C when using thermochemical initiation). 

Even if this does not reflect the swollen state porosity, it would lead to increased diffusional limitations and a larger specific surface area. The photopolymers probably have a more open pore structure in the swollen state giving the template more rapid access to the sites, which are in this case confined to a smaller surface area. The difference in the conversion of pendant double bonds, and thereby the difference in cross-linking densities between the two types of materials, is probably also a factor that comes into play. An increase in chain flexibility at the sites is likely to cause an increase in the template adsorption-desorption rate coefficients. In this context it is interesting to note that increased rate enhancements were observed upon controlled hydrolysis of the polymer backbone of an imprinted esterase model [73]. 

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Fig. 2.13. Separation factor (a) versus column temperature in the chromatographic resolution of D,L-PA on L-PA imprinted polymers prepared by thermochemical initiation at 60/ 90/120°C (24 h at each temperature) using acetonitrile as porogen and photochemical initiation at 15°C for 24 h using dichloromethane as porogen. For the thermochemically polymerised material the mobile phase was 5% acetic acid in acetonitrile and for the photochemically polymerised material the mobile phase was acetonitrile/water/acetic acid 92.5/2.5/5 (v/v/v). From Sellergren et al. [27] and Sellergren and Shea [13].

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