Free radical photopolymerization propagation

Complex Photopolymerization Systems. Kinetic modeling of free-radical photopolymerizations becomes more complicated as comonomers are added to the reaction system and as different polymerization methods are used to tailor the pol5uner properties. Although free-radical reaction mechanisms still hold true, rates of propagation and termination must be reconsidered to account for variables such as differences in double bond reactivities, reaction diffusion, and chain transfer. 

Polymerization of monomer by vinyl addition polymerization is a typical chain process. Three main reaction stages can be identified initiation, propagation and termination. During the initiation event, free radicals are created. In a photopolymerization, the initiating free radicals are formed in a photoprocess. Propagation is the process of addition of monomer to the growing free radical chain. The destruction of the free radical center occurs during termination. 

A polymerization process requiring a photon for the propagation step is called photopolymerization or photoinduced polymerization, when polymerization of a monomer by a free radical or ionic chain reaction is initiated by photoexcitation. 

In an efficient photopolymerization system, the excited photoinitiator L-L must be sufficiently energetic and long-lived to decompose spontaneously or to interact with a second component to produce the active free radicals. Once formed, the free radical R- (here the imidazolyl radical L- reacts with the chain transfer agent, e.g., 2-mercaptobenzoxazole, the monomer M as in thermal polymerization, undergoing propagation, chain transfer, and termination steps ... 

In the case of free radical polymerization with chemical initiation, tenperature exerts an influence at all stages of the process, with the activation energy associated with initiator deconposition generally greater than that of either the propagation or termination steps. In the case of photopolymerization, the initiator dissociation rate is expected to depend primarily on UV light intensity rather than temperature. This is evident in the data for the plateau value of G shown in Figure 2, where an increase with temperature is observed primarily at the lowest UV intensities. 

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