Radical chain polymerization activation parameters

The parameter q gives the probability that the active end of the propagating chain adds another monomer to the growing chain. For free-radical vinyl polymerizations, q attains values close to 1 (usual values are larger than 0.99). However, for some nonliving anionic or cationic polymerizations, such as the cationic polymerization of epoxy groups, values of q may be lower. 

Just how many moments are necessary to provide a unique determination of the differential PCLD One example has been shown where the number is finite, but more complex polymerization mechanisms certainly demand more information. Consider, for example, a radical chain group polymerization with a termination reaction as well as a number of transfer reactions. Given all the kinetic parameters (in this case the rate constants and activation energies) it is possible to calculate the distribution. So if kinetic parameters are required for the characterization, measuring one or more moments, enables the rate equation to be used to determine a set of kinetic parameters, provided N > N. Confidence in the accuracy of the moments dictates whether more moments are essential for complete characterization of the distribution. 

In free-radical polymerization, the initiator fragment moves away from the growing chain and therefore can influence chain growth and termination during only the first few monomer insertion steps. Consequently, the values of the polymerization kinetic parameters depend mainly on the monomer type, thus permitting the creation of tables of rate constants and activation energies as a function of monomer type, independently of the type of initiator used during polymerization. 

In chemical kinetics the lifetime of an active intermediate is an important variable which characterizes the time scale of the process in which the intermediate is involved. Different techniques have been suggested for the direct experimental evaluation of this parameter they are used extensively for the analysis of chain reactions [13] such as radical polymerization [14]. These techniques are based on the assumption that the mean lifetime of an active intermediate X, can be evaluated as the ratio between its concentration x, and its rate of disappearance W ... 

Depending on the nature of the active center, chain-growth reactions are subdivided into radicalic, ionic (anionic, cationic), or transition-metal mediated (coordinative, insertion) polymerizations. Accordingly, they can be induced by different initiators or catalysts. Whether a monomer polymerizes via any of these chain-growth reactions - radical, ionic, coordinative - depends on its constitution and substitution pattern. Also, external parameters like solvent, temperature, and pressure may also have an effect. Monomers able to grow in chain-growth polymerizations are listed in Table 2.2 of Sect. 2.1.4. 

The influence of the l,3,5-trimethyl-hexahydro-l,3,5-triazine (TMT) on the radical polymerization of methyl methaciylate (MMA) was studied. The kinetic parameters were obtained (reaction orders and activation energy of polymerization). It is established that the triazine is the slight chain transfer during to polymerization initiated by azobisisobutyionitiile (AIBN) and the component of initiating system if the peroxide initiator is used. Polymers synthesized in the presence of TMT have the higher content of sindio and isotactic sequences in macromolecule. 

The first difference is that free-radical polymerization is monomer-based, which means that the kinetics is (almost) exclusively determined by the monomer M. Once a radical R is formed and added to the monomer molecule to build the growing chain R---M, the reactivity of this growing chain in all reactions is determined by the nature of the monomer irrespective of the nature of the initiating radical, which just forms the tail of that growing chain. So, from a practical point of view, in radical polymerization it is sufficient to determine the kinetic scheme and parameters and their dependences on the system variables temperature and pressure for a monomer system with one kind of radical initiator. Furthermore, all active radical centers from one monomer are identical, and they are hardly in-... 

Before the preparation of systematical (co)polymer libraries to understand the effect of molecular structure on the thermoresponsive properties, the RAFT polymerization conditions were optimized using 2-cyano-2-butyl dithiobenzoate (CBDB) as RAFT agent and azobisi-sobutyronitrile (AIBN) as radical initiator. The RAFT polymerization mechanism and the structures of CBDB and AIBN are depicted in Figure 22.9. The control over the RAFT free-radical polymerization process strongly depends on the ratio of radical initiator to RAFT agent and temperature. Both these parameters control the number of radicals that are generated and, thus, influence the main propagation equilibrium, that is, the number of dormant and active chains in the system that controls the extent of termination reactions. 

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