Photoresist polymers photodimerization

Photodimerization of cinnamic acids and its derivatives generally proceeds with high efficiency in the crystal (176), but very inefficiently in fluid phases (177). This low efficiency in the latter phases is apparently due to the rapid deactivation of excited monomers in such phases. However, in systems in which pairs of molecules are constrained so that potentially reactive double bonds are close to one another, the reaction may proceed in reasonable yield even in fluid and disordered states. The major practical application has been for production of photoresists, that is, insoluble photoformed polymers used for image-transfer systems (printed circuits, lithography, etc.) (178). Another application, of more interest here, is the use that has been made of mono- and dicinnamates for asymmetric synthesis (179), in studies of molecular association (180), and in the mapping of the geometry of complex molecules in fluid phases (181). In all of these it is tacitly assumed that there is quasi-topochemical control in other words, that the stereochemistry of the cyclobutane dimer is related to the prereaction geometry of the monomers in the same way as for the solid-state processes. 

Photocyclization is a particularly valuable route to the formation of cyclic compounds. There is a wide variety of photocyclization reactions reported in the literature of organic photochemistry, but relatively few of these have been carried out in solid polymers. The earliest reports concern the photodimerization of cinnamic acid derivatives, leading to crosslinking in solid polymers. These polymers have important applications as commercial photoresists. The chemistry has been reviewed by Delzenne (46) and Williams (47). 

Incorporation of two photodimerization groups in a monomer molecule gives the possibility of preparing a linear polymer, and the binding of photodimerizable groups to the side chain of the polymer produces a crosslinked polymer which is practically important as a negative-type photoresist. 

For instance, polymers containing dimethylmaleimide pendant groups can be cross-linked by UV light with the formation of cyclobutadiene bridges, through the known 2 -h 2 photodimerization mechanism. These polymers have nothing to do with heat-resistant polymers they have been developed and studied because of their potential as photoresists. 

Takemoto, K. and Inaki, Y, Photodimerization of thymine-containing polymers applicability to reversible photoresists, /. Macromol. Sci. Chem., A25, 757,1988. 

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