Termination rate constant, radical structures

Kinetic results were consistent with a bimolecular termination reaction whereas reaction products and mechanisms were something of a mystery. At that time it was known that the termination rate constant for autoxidation of cumene ( ) is about three orders of magnitude smaller than the termination rate constant for autoxidation of tetralin (7.). It was, however, generally accepted that the tennination rate constants for tertiary ( ) and secondary (9 ) alkylperoxy radicals are insensitive to the structure of the hydrocarbon residue in the radical. 

One of the problems of radical polymerization is high-termination-rate constants by combination ( ) or by disproportionation ( ). In view of this, polymer chains of controlled chain length cannot be formed and this technique is ill-suited for precise control of molecular structure (e.g., in star, comb, dendri-mers, etc.) required for newer apphcations like microelectronics. The major breakthrough occurred when nonterminatmg initiators (which are also stable radicals) were used. Because of its nonterminating nature, this is sometimes called living radical polymerization and the first initiator that was utilized for this purpose was TEMPO (2,2,6,6-tetramethylpiperidinyl-l-oxo) [36,37]. A variation of this is atom-transfer radical polymerization (ATRP) in which, say for styrene, a mixture of 1 mol% of 1-phenyl ether chloride (R—X) and 1 mol% CuCl with two equivalents of bipyridine (bpy) is used for initiation of polymerization. Upon heating at 130°C in a sealed tube, bpy forms a complex with CuCl (bpy/CuCl),... 

Free radical addition of HBr to buta-1,2-diene (lb) affords dibromides exo-6b, (E)-6b and (Z)-6b, which consistently originate from Br addition to the central allene carbon atom [37]. The fact that the internal olefins (E)-6b and (Z)-6b dominate among the reaction products points to a thermodynamic control of the termination step (see below). The geometry of the major product (Z)-(6b) has been correlated with that of the preferred structure of intermediate 7b. The latter, in turn, has been deduced from an investigation of the configurational stability of the (Z)-methylallyl radical (Z)-8, which isomerizes with a rate constant of kiso=102s 1 (-130 °C) to the less strained E-stereoisomer (fc)-8 (Scheme 11.4) [38]. 

Rates of radical additions to alkenes are controlled mainly by the enthalpy of the reaction, which is the origin of regioselectivity in additions to unsymmetrical systems, with polar effects superimposed when there is a favorable match between the electrophilic or nucleophilic character of the radical and that of the radico-phile. For example, in the addition of an alkyl radical to methyl acrylate (2), the nucleophilic alkyl radical interacts favorably with the resonance structure 3. Polar effects are apparent in the representative rate constants shown in Figure 4.14 for additions of carbon radicals to terminal alkenes. Addition of the electron-deficient or electrophilic rert-butoxycarbonylmethyl radical to the electron-deficient molecule methyl acrylate is 10 times as fast as addition of... 

He polymerized methacrylonitrile by means of 2,2 -azobisisobutyronitrile. The isobutyronitrile radical generated by initiator decomposition is structurally almost identical with the macroradical end group. Under these conditions, the rate constant of primary radical termination should be very near to the rate constant of termination between oligomeric and polymeric radicals. The studied polymerizations were carried out in dimethylformamide, which is a poor solvent for polymethacrylonitrile. In poor solvents, the change in termination rate with the length of the growing chain should not depend on the excluded volume [10],... 

For conjugated dienes, one must take into account the resonance stabilisation for some of the possible adduct radicals. This is the case for OH addition to one of the terminal C-atoms of a C=C--C=C structure, because of the resulting allyl-like resonance between the "original" p-hydroxy radical and the resonant 6-hydroxy structure. As an example, when the former is a secondary radical and the latter a tertiary one, the corresponding partial site-specific rate constant is denoted as / sec/tert- Generally, for conjugated dienes there are six such additional partial rate constants (beside k, ksec and / tert) They were evaluated from the known total kon for six conjugated dienes they are listed below, in units of 10 " cm s" ... 

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. 

Scheme 29 shows the rate constants that are involved in CRP. The first reaction is the dissociation (activation) of the shortest chain (initiator of the same structure as RiX). The rate constants of propagation (kp) and of termination [kt] are the same as in the traditional radical polymerization (the new method of kp determination based on the pulse laser polymerization-size exclusion chromatography (PLP-SEC) is discussed in Section 3.02.14.4). Thus, k and feda are of major interest. These are discussed in detail in Chapter 3.05 by Fukuda et al. where it is separately described for the CRP with nitroxides (e.g., 2,2,6,6-Tetramethylpiperidinooxy (TEMPO) as the PR) and for the ATRP systems (Scheme 30). 

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