RAFT polymerization equilibrium constants

In a given RAFT polymerization, there are at least four equilibrium constants that need to be considered. Kp (=feaddp/ fe-addp) associated with the chain equilibration process (Scheme 7). This step is sometimes called the main equilibrium. K (=feadd/fe-add) and Kp (=fe p/fep) associated with the initial reversible chain transfer step sometimes known as the pre-equilibrium. Kr (=feaddK/fe-addR) associated with the reaction of the expelled radical with the initial RAFT agent (Scheme 8). This process only becomes significant if the intermediate formed has a significant lifetime. 

In a given RAFT polymerization, there are at least four equilibrium constants that need to be considered. 

The properties of the intermediate radicals in RAFT polymerization may lead to retardation. If fragmentation of the intermediate radical is very slow this can cause retardation directly. The estimation of RAFT equilibrium constants using ab initio molecular orbital calculations provides some credibility for this hypothesis. However, direct measurement of intermediate radical concentrations in RAFT polymerization as measured by EPR, indicates that these species do not have sufficient life-time for slow fragmentation, by itself, to be the cause of retardation. 

In the presence of the RTCP catalysts, Rp is somewhat smaller than in their absence (IMP), as shown in Figure 7.10a (and Figure 7.3a) for the St/PSt I/BPO with and without Gel4 at 80 °C. This is because A undergoes irreversible crosstermination with Polymer (rate constant This mechanism is analogous to the one for the rate retardation in the RAFT polymerization, where the intermediate radical (Polymer-(X )-Polymer) undergoes the relevant crosstermination. In theory, when the quasi-equilibrium of RT holds and the radical concentrations [Polymer ] and [A ] are stationary, Rp is given by eqn (7.3). ... 

In RAFT block copolymerizations, the main requirement to produce AB diblocks with a low polydispersity index PDl is that the dithioester macroinitiator has a high chain transfer constant in the polymerization reaction producing the second block. In other words, in the equilibrium step... 

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