Some aspects of the role of amines in the photoinitiated polymerisation of acrylates in the presence and absence of oxygen

Triethanolamine (1), N-methyldiethanolamine (2) and N,N-dimethylethanolamine (3) have been used in conjunction with the 2-(2-chlorophenyl)-4,5-diphenylimidazolyl radical and triplet benzophenone to initiate the polymerisation of lauryl acrylate. The radical abstracts a hydrogen atom from the amine to generate an alpha-aminoalkyl radical. From the order of reactivity of the amines towards the lophyl (triarylimidazolyl) radical i.p. 2 >> 1 > 3 it is concluded that hydrogen-atom abstraction occurs rather than electron followed by proton transfer. The imidazolyl radicals abstract hydrogen from thiols in a similar fashion more effectively to give thiyl radicals which initiate polymerisation of the acrylate. By following the progress of the polymerisation reactions by RTIR spectroscopy, it was concluded that the effectiveness of the amines to reduce oxygen inhibition is 1 < 3 < 2. The ability of the amines to act as synergists in the polymerisation of thin films of lauryl acrylate initiated by lophyl radicals and tripler benzophenone was assessed. The efficiency of the amines was found to be 2 similar to 1 > 3 for reaction with lophyl radicals whereas with triplet benzophenone the three amines gave similar amounts of polymer. For amines 2 and 3. two types of amino alkyl radical can be generated. It is concluded that radicals of type > NCHCH2OH are more effective initiators than CH2N <.