Polymers That Photocross-link by Formation of Cyclobutane Rings
Many of the photocross-linkable polymers for photo-imaging in use today react by a 2π +2π type dimerization with the accompanying formation of the cyclobutane rings [108, 109]. The formation of the cyclobutane ring can be simply shown on the photocross-linking reaction of poly (naphthyl vinyl acrylate), a polymer that also undergoes this type of dimerization [110]:
The naphthalenes become bonded to cyclobutane ring in 1,2 and 1,3 positions. Many polymers with other functional groups can also photocross-link by 2p +2p addition. Following is an illustration of some of these groups [111, 112]:
Pendant groups with anthracene moieties, however, are believed to cross-link by a 4 π +4 π type cycloaddition [114]:
Many photo-dimerizations of functional groups, like the ones shown above, require the presence of photosensitizers. These compounds may be selectively excited to their triplet state by sensitizers with the right type of energy levels. For efficient energy transfer to occur, the triplet energy of the donor should be approximately 3 kcal/mole greater than that of the acceptor [113, 114]. This type of sensitization is believed to be diffusion controlled [115]. Trecker [116] lists the steps of the sensitized dimerization reaction as follows:
The quantum yield expression for this scheme of dimerization is shown as follows [115]:
where θ represents the quantum yield of dimer formation, f is the efficiency of sensitizer intersystem crossing, D is the initial concentration of the reactive groups, and k’s are the specific rate constants for the reactions shown in the above scheme.
