Free radical polymerization chain length dependent termination

Computational Aspects of Free Radical Polymerization Kinetics with Chain Length Dependent Termination... 

A, Theis, M. H. Stenzel, T. P. Davis, C. Barner-Kowollik, Obtaining Chain Length Dependent Termination Rate Coefficients via Thermally Initiated RAFT Experiments Current Status and Future Chal-langes, in Acs Symposium Series on LIving/Controlled Free Radical Polymerization, K. Matyjaszewski, Ed. ACS Press Washington, D.C., 2006 944, 486-500. 

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 In cases where termination is controlled by diffusion, the pseudo-steady-state assumption is no longer valid and chain length dependent termination (CLDT) may occur (67).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. 

Buback M, Muller E, Russell GT. SP-PLP-EPR smdy of chain-length-dependent termination in free-radical polymerization of n-dodecyl methacrylate, cyclohexyl methacrylate, and benzyl methacrylate evidence of composite behavior. J Phys Chem A 2006 110 3222-3230. 

Zhu S, Hamielec AE. Chain-length-dependent termination for free-radical polymerization. Macromolecules 1989 22, 3093-3098. 

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... 

Free-Radical Polymerization in Emulsion. In suspension polymerization, the particle size is fixed by the size of the monomer droplet which contains the initiator. Emulsion polymerization differs from suspension polymerization in that the initiator is dissolved in the aqueous phase and the polymer particle grows during polymerization. Free radicals are generated in the water and diffuse to the monomer-water interface. The length of the polymer chain formed, or equivalently the molecular weight, depends on the rate of free radical arrival and termination. S. Katz,... 

The bimolecular termination reaction in free-radical polymerization is a typical example of a diffusion controlled reaction, and is chain-length-depen-dent [282-288]. When pseudobulk kinetics appUes, the MWD formed can be approximated by that resulting from bulk polymerization, and it can be solved numerically [289-291]. As in the other extreme case where no polymer particle contains more than one radical, the so-caUed zero-one system, the bimolecular termination reactions occur immediately after the entrance of second radical, so unique features of chain-length-dependence cannot be found. Assuming that the average time interval between radical entries is the same for all particles and that the weight contribution from ohgomeric chains formed... 

Scope for Accessing the Chain Length Dependence of the Termination Rate Coefficient for Disparate Length Radicals in Acrylate Free Radical Polymerization... 

The discussion of the rate of bimolecular termination has, up to now, been mainly of a qualitative nature. The scaling of average or macroscopic kt values with viscosity, solvent effects and coil dimensions were discussed without much attention for the chain-length dependence of this process. This dependence originates from the simple fact that free-radical termination is a diffusion-controlled process. Consequently, the overall mobility of polymer chains and/or polymer chain ends determine the overall rate of radical loss in a polymerizing system. As small chains are known to be much more mobile than large ones, the chain length of radicals can be expected to have a profound effect on the termination kinetics. 

In the older literature one can encounter quite a number of papers suggesting that bimolecular termination is not chain-length dependent e.g. 36, 66, 68, 139-143] Experimental evidence, however, has disproved these suggestions and numerous papers have proven termination to be chain-length dependent. The evidence takes several forms and has been obtained employing different experimental approaches, which can roughly be divided in three classes (i) termination studies of free-radical polymerizations, (ii) termination studies of non-propagating species and (iii) diffusion-controlled mimic reactions. 

In this thesis the chain-length dependence of termination reactions during free-radical polymerization has been investigated. Interest in the kinetic parameters describing this process, primarily arises from the profound effect that termination reactions have on the overall kinetics of polymerization processes, on the MWD of the final product and therefore also on the final product properties. 

Chain-Length Dependent Bimolecular Termination in Free-Radical Polymerization Theory, Validation and Experimental Application of Novel Model-Independent Methods... 

Chain-Length Dependent Bimolecular Termination in Free-Radical Polymerization... 

Chain-length dependent bimolecular termination in free-radical polymerization ... 

Smith GB, Russell GT, Yin M, Heuts JPA. The effects of chain length dependent propagation and termination on the kinetics of free-radical polymerization at low chain lengths. Eur Polym J 2005 41 225-230. 

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