RAFT Equilibrium Constants

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 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. 

There may be other equilibrium constants to consider if penultimate group effects are significant (there is theoretical data and some experimental evidence to indicate that this is the case). There are also a further series of reactions that need to be considered that involve initiator radical-derived RAFT agents. RAFT agents of differing reactivity might be derived from each radical species present. 

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... 

Addition of Os3(CO)io(NCMe)2 to H2 Os(CO)4 ( = 1-3) leads to cluster expansion to give a series of spectroscopically characterized addition products with nuclearities from 4 to 9." The planar raft cluster Os6(CO)2o(NCMe) reacts with P-donor ligands by rapid initial formation of the adducts Os6(CO)2o(L)(NCMe) in a pre-equilibrium step, followed by slow dissociation of NCMe the equilibrium and rate constants vary systematically with the electronic and steric properties of L." ... 

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). ... 

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