ATRP Mechanism and Kinetics

The general meehanism of ATRP has been shown in Fig. 11.16, which indicates a radical pathway. The radical nature of the reaetive or propagating species in eopper-mediated ATRP is proposed on the basis of several experimental observations, which have been sutmnarized by Matyjaszewski and Xia (2001) as (1) The ATRP equilibrium ean be approached from both sides, i.e., either 

In ATRP, perhaps the most important kinetic parameters are the rate constants for the activation ( act) and deactivation ( deact) steps (see Fig. 11.16), which determine the magnitude of the equihbrium constant (Xeq = act / deact)- In the absence of any side reactions other than radical termination by coupUng or disproportionation, K q determines the polymerization rate. While ATRP does not occur or occurs very slowly if Xeq is too small, too large an equilibrium constant, as it has been shown earlier, may actually lead to an apparently slower polymerization. The magnitudes of act and deact depend on the structure of the monomer, on the halogens, and on the transition metal complexes. The measured values of these rate constants for some model systems resembling the structure of the dormant/active species, are shown in Tables 11.3 and 11.4. 

Phenyl ethyl bromide Cu(I)Br/(dNbpy )2 Ethyl acetate 0.016 

Phenyl ethyl Cu(II)Br2/(dNbpy)2 Ethyl acetate 2.4x10  

Problem 11.9 While in a normal ATRR the initiating radicals are generated from an alkyl halide in the presence of a transition metal in its lower oxidation state (e.g., CuBr(dNbpy)2), ATRP can also be initiated by using a thermal free-radical initiator (e.g., AIBN) along with the transition metal compound in its higher oxidation state (e.g., CuBr2(dNbpy)2). Write a general scheme for this latter approach which is named reverse ATRP . 

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