Living radical polymerization kinetics

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

Harrison et c /.146,147 have used PLP (Section 4.5.2) to examine the kinetics of MMA polymerization in the ionic liquid 18 (bmimPFfi). They report a large (ca 2-fold) enhancement in Ay and a reduction in At. This property makes them interesting solvents for use in living radical polymerization (Chapter 9). Ionic liquids have been shown to be compatible with ATRP14 "1 and RAFT.I57,15S However, there are mixed reports on compatibility with NMP.1 Widespread use of ionic liquids in the context of polymerization is limited by the poor solubility of some polymers (including polystyrene) in ionic liquids. 

The kinetics and mechanism of living radical polymerization have been reviewed by Fischer,21 Fukuda et at.,22 and Goto and Fuktida.23 In conventional radical polymerization, new chains are continually formed through initiation w hile existing chains are destroyed by radical-radical termination. The steady state concentration of propagating radicals is 10"7 M and an individual chain will have a lifetime of only 1-10 s before termination within a total reaction lime that is... 

The literature on Nitroxide-Mediated Polymerization (NMP) through 2001 was reviewed by Hawker el al. vu 7 More recently the subject has been reviewed by Sluder and Schulte10 and Solomon.109 NMP is also discussed by Fischer110 and Goto and Fukuda" in their reviews of the kinetics of living radical polymerization and is mentioned in most reviews on living radical polymerization. A simplified mechanism of NMP is shown in Scheme 9.17. 

Living radical polymerization currently dominates patents, publications and conferences on radical polymerization. The most popular systems, NMP, ATRP and RAFT, while offering unprecedented versatility are not without drawbacks and still have some limitations. Thus, while the progress in this field since the first edition of this book is substantial by any standard, there remains significant scope for new and improved processes. Further studies of the detailed kinetics and... 

The polymerization kinetics have been intensively discussed for the living radical polymerization of St with the nitroxides,but some confusion on the interpretation and understanding of the reaction mechanism and the rate analysis were present [223,225-229]. Recently, Fukuda et al. [230-232] provided a clear answer to the questions of kinetic analysis during the polymerization of St with the poly(St)-TEMPO adduct (Mn=2.5X 103,MW/Mn=1.13) at 125 °C. They determined the TEMPO concentration during the polymerization and estimated the equilibrium constant of the dissociation of the dormant chain end to the radicals. The adduct P-N is in equilibrium to the propagating radical P and the nitroxyl radical N (Eqs. 60 and 61), and their concentrations are represented by Eqs. (62) and (63) in the derivative form. With the steady-state equations with regard to P and N , Eqs. (64) and (65) are introduced, respectively ... 

While there have been several studies on the synthesis of block copolymers and on the molecular weight evolution during solution as well as bulk polymerizations (initiated by iniferters), there have been only a few studies of the rate behavior and kinetic parameters of bulk polymerizations initiated by iniferters. In this paper, the kinetics and rate behavior of a two-component initiation system that produces an in situ living radical polymerization are discussed. Also, a model that incorporates the effect of diffusion limitations on the kinetic constants is proposed and used to enhance understanding of the living radical polymerization mechanism. 

The synthesis of mixed peroxides formed from /-butyl hydroperoxide and carbon-centred radicals has been studied. The reactions were strongly effected by solvents as well as catalytic amounts of Cun/Fem. The kinetic data suggest that the conditions for the Ingold-Fischer persistent radical effect are fulfilled in these cases.191 The use of Cu /Cu" redox couples in mediating living radical polymerization continues to be of interest. The kinetics of atom-transfer radical polymerization (ATRP) of styrene with CuBr and bipyridine have been investigated. The polymer reactions were found to be first order with respect to monomer, initiator and CuBr concentration, with the optimum CuBr Bipy ratio found to be 2 1.192 In related work using CuBr-A-pentyl-2-... 

Today, the subject is far from being mature. There are probably many more experimental manifestations of the effect than we are aware of, and there may be more kinetic peculiarities and variants. This is the reason we neither present a completely comprehensive description of all pertinent experimental findings nor a complete theory. Moreover, the particular subfield of living radical polymerizations involving the effect has become of practical importance. Hence, in the past few years, a large number of publications and patents have been published on this topic. These are covered in more detail in other parts of this issue, and here, we discuss only those aspects that are important for the operation of the persistent radical effect in living polymerizations. 

For more complete and practically oriented surveys of living radical polymerizations, we refer to other reviews in this issue95 and cover in the following section only some kinetic and mechanistic aspects that are relevant for quantitative treatments. 

A kinetic study of living radical polymerizations of acrylates initiated by the (tetramesitylporphyronato)-cobalt(III) organo complexes (TMP)Co—CH(CH3)C02-Me and (Br8TMP)Co—CH(CH3)C02Me has been reported by Wayland et al.122 They applied an initial excess of the free cobalt complex and obtained the equilibrium constant for the reversible dissociation of the complex—poly(methyl acrylate) bond as K = 4.2 x 10 10 M for (TMP)Co and K= 1.3 x 10 8 M for (BrgTMP)Co from the rate of monomer consumption at 50 °C. The temperature dependence led to a bond... 

This contribution reports on the use of NMR spectroscopy to follow copper(I) mediated living radical polymerization. Carrying out the polymerizations within the cavity of the NMR spectrometer allows the reaction to be closely monitored. This gives extensive information on both the polymerization kinetics and on the nature of the catalyst. 

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