Termination constants dienes

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]. 

The NMR spectrum shows clearly that the , -diene is formed. The two coupling constants for by this method was very useful in the simple doublets must be for the terminal hydrogens and 14.7 Hz is definitely a trans coupling, cycloadditions (B. M. Trost and... 

In many synthetically useful radical chain reactions, hydrogen donors are used to trap adduct radicals. Absolute rate constants for the reaction of the resulting hydrogen donor radicals with alkenes have been measured by laser flash photolysis techniques and time-resolved optical absorption spectroscopy for detection of reactant and adduct radicals Addition rates to acrylonitrile and 1,3-pentadienes differ by no more than one order of magnitude, the difference being most sizable for the most nucleophilic radical (Table 8). The reaction is much slower, however, if substituents are present at the terminal diene carbon atoms. This is a general phenomenon known from addition reactions to alkenes, with rate reductions of ca lOO observed at ambient temperature for the introduction of methyl groups at the attacked alkene carbon atom . This steric retardation of the addition process either completely inhibits the chain reaction or leads to the formation of rmwanted products. 

The presence of electron-withdrawing substituents such as halogen on dienes substantially increases their propagation rate constants and reduces termination rate constants. As shown in Table 11, kc0 does not increase from isoprene to chloroprene so that Cc decreases. The trend continues for 2,3-dichloroduta-diene. Although the propagation rate constant for 1,2-dichlorobutadiene is not known, its Cc is less than... 

The data in Table 2.4 provide evidence that the slow rates and low molecular weights obtained in homogeneous free radical polymerization of these dienes are not due to a low rate constant for propagation but rather must be caused by a high rate constant for termination (as indicated in Table 2.1) (Matheson et al., 1949,1951 Morton and Gibbs, 1963). Hence, under the special conditions of emulsion polymerizations, where the termination rate is controlled by the rate of entry of radicals into particles, it becomes possible to attain both faster rates and higher molecular weights. It is this phenomenon which led to the rise of the emulsion polymerization system for the production of diene-based synthetic rubbers. 

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" ... 

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