Free radical photopolymerization chain termination

Free-radical photopolymerizations (see Chap. 10) of multifunctional acrylic monomers result in cross-linked polymeric networks. The kinetic picture of such polymerizations varies from ordinary linear polymerization because the diffusion of free radicals and functional groups becomes severely restricted. This causes growing polymer chains to rapidly cyclize and cross-link into clusters (microgels). The clusters become linked up into networks. Many free radicals become trapped, but terminations take place by combinations and by chain transferring. The cumulative chain length in such polymerizations can be calculated from the following equation [125] ... 

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. 

Photoinitiated free radical polymerization is a typical chain reaction. Oster and Nang (8) and Ledwith (9) have described the kinetics and the mechanisms for such photopolymerization reactions. The rate of polymerization depends on the intensity of incident light (/ ), the quantum yield for production of radicals ( ), the molar extinction coefficient of the initiator at the wavelength employed ( ), the initiator concentration [5], and the path length (/) of the light through the sample. Assuming the usual radical termination processes at steady state, the rate of photopolymerization is often approximated by... 

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. 

The photopolymerization of methyl methacrylate using a quinoline-chlorine charge-transfer complex has been investigated. Bulk polymerization was found to follow normal free-radical kinetics, whereas in solution variable monomer exponents were observed depending on the nature of the solvent. The kinetic nonideality in solution was attributed to retardation and initiator termination via degradative chain-transfer involving solvent-modified initiating complexes and chain radicals. 

Autoacceleration, where the rate of polymerization increases with conversion in isothermal conditions, is observed in both thermal- and photoinitiated free-radical polymerizations because the termination mechanisms are the same for both. As the chains grow longer, it becomes more difficult for the active centers to diffuse and imdergo bimolecular termination thus, termination frequency decreases and active centers at the chain ends can become trapped. In cases where termination is controlled by diffusion, the pseudo-steady-state assumption is no longer valid and chain length dependent termination (CLDT) may occur (67). As is discussed for chain cross-linking photopolymerizations below, more complicated kinetic treatments must then be considered, including unsteady-state kinetics. 

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 ... 

A PET in intramolecular CPs between pyridinium ions and bromide, chloride or thiocyanate ions for polymerization initiation is described, too [137-139]. As expected, an equilibrium exists among free ions, ion pairs, and CT, which is shifted to the free ions in polar solvents and to the complex in a less polar solvent That complex serves as the photosensitive species for the polymerization (see Scheme 10). The photodecomposition of the CT yields radicals of the former anion and N-alkylpyridinyl radicals. Probably, the photopolymerization is initiated only by X- radicals, whereas latter radicals terminate the chain reaction. By addition of tetrachloromethane, the polymerization rate is increased owing to an electron transfer between the nucleophilic pyridinyl radical and CC14 (indirect PET). As a result, the terminating radicals are scavenged and electrophilic -CQ3 radicals are produced. 

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