Photoinitl&tion of Free-radical Polymerization by Transition Metal Derivatives

Two types of photoinitiator are considered: (1) transition metal complexes (mainly carbonyls), (2) chelates of transition metals. In systems of type (1) the metal is in a low oxidation state and an organic halide is generally, but not always, present; on thc other hand, in (2) the metal is initially in a high oxidation state. Comparison between photoand thermal-initiation is made for each type. Carbonyls of Group VIA metals undergo scission of CO on photolysis; the resulting species coordinates with monomer (or solvent) to form a complex which reacts with the halide (e.g. CC14) to generate initiating free radicals (e.g. CC13), Arguments are presented in favour of unsymmetrical photolysis of Group VIIA carbonyls [Mn2(CO),0, Re2(CO)10]. It is concluded that photoinitiation of free-radical polymerization by transition metal carbonyls may proceed by at least three routes: (i) electron-transfer to organic halide with rupture of a C-----Cl or C---Br bond, (ii) reaction with a strongly electron attracting monomer such as C2F4 without rupture of C—F to form an initiating radical, (iii) hydrogen atom-transfer from monomer (or solvent) to a photoexcited metal carbonyl species. A study of photoinitiation by acetylacetonate Mn(acac)3 and Mn" 1,1,1-trifluoroacetylacetonate Mn(facac)3 shows that the monomer selectivity encountered in thermal initiation is absent. The quantum yield for initiation is low (10 2 to 10) in both systems. With Mn(acac)3 the initiating process is scission of a ligand as a radical acac, with formation of a Mn" chelate; this reaction is not subject to significant solvent effects. On the other hand, photoinitiation by Mn(facac)3 shows strong dependence on the solvent and the data are interpreted in terms of exciplex formation between excited chelate and monomer (methyl methacrylate) or solvent (ethyl acetate). No exciplex is formed with benzene as diluent. A brief discussion of the natures of the photoexcited state and the state leading to decomposition is included. We shall be concerned in this lecture with initiation of free-radical polymerization by systems of two types: (1) transition metal complexes with ligands such as carbon monoxide, aromatic isonitriles, triphenyl phosphite, generally, but not invariably, in the presence of organic halides and (2) chelates of transition metals. Systems of type (1) contain the metal in a low, often the zeroth, oxidation state, while in those of type (2) the metal is in a high oxidation state. It will later become apparent that the first group may be further subdivided depending on the precise mechanism of radical formation.