Some studies in step-growth photopolymerization

Abstract

This work reviews some reported investigations and describes some further studies in step-growth photopolymerization. Benzophenone-type aromatic diketones were prepared by Friedel-Crafts synthesis. The diketones were photopolymerized directly with tetramethylallene and with 2,5-dimethylfuran in benzene solution, yielding low to medium molecular weight polymers containing oxetane rings in the main polymer chains. Irradiation of diketones in a large excess of furan resulted in cycloaddition of carbonyl groups to furan C=C double bonds and the formation of furan-diketone 2:1 adducts. The 2:1 adducts were photopolymerized with equimolar quantities of diketone to give polymers. Spectroscopic evidence indicated that all polymers contained OH groups, arising presumably from hydrogen abstraction reactions. The physical properties of polymers from tetramethylallene and 2,5- dimethylfuran were consistent with the formation of linear polymers, hydrogen abstraction from methyl groups still allowing propagation of the polymer chain. The physical properties of polymers containing furan residues were indicative of crosslinks in the polymers, one possible source of such crosslinks being abstraction of hydrogens a to furan residue oxygens. Pyrolysis of polymers resulted in cleavage of oxetane rings, and polymers and 2:1 adducts from furan were unstable at room temperature. Polybenzopinacols were prepared by irradiation of the diketones in benzene-isopropanol mixtures. Solvent mixtures containing greater than 507% by volume of benzene were found to be most satisfactory. The photochemical oxidative-reductive dimerization of acetophenones, and the photoenolization of 2-alkylbenzophenones followed by Diels-Alder addition of the photienols to dienophiles have been examined as possible model reactions extendable to photopolymerization. Preliminary results indicated that the production oS: high polymers from the diketones studied was unlikely.