Initiation of free radical polymerization by thermal cleavage of carbon-carbon bonds

Free carbon radicals for initiation of polymerization processes can be generated either by light-induced fragmentation or by homolytic thermal dissociation of suitable compounds. For thermal initiation of polymerization processes beside ate, peroxy and some other thermolabile compounds also alkanes with highly substituted carbon-carbon bonds are capable. The reason why this interesting class of initiators, even more than 100 years after their discovery, has found only limited applications in radical polymerization is caused by their special mechanism of chain reaction initiation. The formed and in most cases highly resonance-stabilized substituted alkyl radicals are not very reactive against monomers like methyl methacrylate or styrene but more against growing macroradicals. Therefore, during the initiation step at rather high primary radical concentrations only low-molecular oligomers with two alkane end groups are formed. Under special conditions this process is reversible, whereas in some other cases the reaction can be described as a ''dead end'' polymerization. The oligomers which were formed e.g. in the case of methyl methacrylate polymerization with tetraaryl butane dinitriles are able to re-initiate the polymerization process similar as in a living polymerization which can be used for the synthesis of block copolymers. Mechanism, kinetics and possible applications of such alkane-initiated polymerizations are discussed.