Chain process disproportionation

Inhibitors slow or stop polymerization by reacting with the initiator or the growing polymer chain. The free radical formed from an inhibitor must be sufficiently unreactive that it does not function as a chain-transfer agent and begin another growing chain. Benzoquinone is a typical free-radical chain inhibitor. The resonance-stabilized free radical usually dimerizes or disproportionates to produce inert products and end the chain process. 

Another method for producing petoxycatboxyhc acids is by autoxidation of aldehydes (168). The reaction is a free-radical chain process, initiated by organic peroxides, uv irradiation, o2one, and various metal salts. It is terrninated by free-radical inhibitors (181,183). In certain cases, the petoxycatboxyhc acid forms an adduct with the aldehyde from which the petoxycatboxyhc acid can be hberated by heating or by acid hydrolysis. If the petoxycatboxyhc acid remains in contact with excess aldehyde, a redox disproportionation reaction occurs that forms a catboxyhc acid ... 

A very interesting recent study describes the intramolecular disproportionation of homodinuclear and heterodinuclear fulvalene complexes in the presence of PMe3.88 Equation (14) shows one of the six reactions reported. In this case, initiation of the radical chain process was accomplished with a catalytic amount of the 19-electron reservoir complex [CpFe(C6Me6)], which reduces 13 to break the Ru—W bond and generate a 17-electron radical center (presumably at Ru). Addition of PMe3 to the Ru is followed by electron transfer to the reactant (13) to afford the zwitterionic product 14 and regenerate the radical intermediate. 

The symbols used are I for initiator, R for the radical derived from the initiator, S for styrene, and R for growing polystyrene radicals, XH for a source of hydrogen radical, and PS for polystyrene. Thus, polystyrene can be formed in the termination step by chain transfer, disproportionation, and combination. Temperature and chain transfer agents can be used to control molecular weight and molecular weight distribution. Polystyrene resulting from free-radical processes is amorphous. 

This process is repeated over and over. Hundreds or even thousands of alkene monomers can add one at a time to the growing chain. Eventually, the chain reaction stops because the propagating sites are destroyed. Propagating sites can be destroyed when two chains combine at their propagating sites when two chains undergo disproportionation, with one chain being oxidized to an alkene and the other being reduced to an alkane or when a chain reacts with an impurity that consumes the radical. 

Two gas-phase reactions of PHj have been studied in some detail (1) The reaction with molecular O2, which shows a variety of product channels and which is of interest for the oxidation of phosphane (a branched chain process see p. 236). (2) The reaction with another PHj radical, which occurs mainly by recombination to form P2H4 rather than by disproportionation to form PHj and a PH radical and which is important for the photolysis of PHj (see p. 206). Second-order rate constants (k) are valid for 298 K (if not otherwise stated) and given in cm molecule" s" , reaction enthalpies AH in kcal/mol or kJ/mol. See also under Chemical Reactions of PH3 (p. 199) for further reactions. 

The majority of radical reactions of interest to synthetic chemists are chain processes . Scheme 1 represents the simple reduction of an organic halide by silicon hydride as an example of a chain process. Thus, R sSi radicals, generated by some initiation processes, undergo a series of propagation steps generating fresh radicals. The chain reactions are terminated by radical combination or disproportionation. In order to have an efficient chain process, the rate of chain-transfer steps must be higher than that of chain-termination steps. The following observations (i) the termination rate constants in liquid phase are controlled by diffusion (i.e. 10 ° s ), (ii) radical concentrations in chain reactions are about... 

If high-purity monomers were used, the molecular weight of the polymers showed an almost linear dependence on the polymerization temperature. This dependence followed the direct proportionality up to ->210°C and reflected the specific feature of SCBs. When the temperature was increased to above 210°C, this dependence deviated from linearity. This is probably associated with an increase in the contribution of inter- and intramolecular chain degradation processes (disproportionation reactions) as a consequence, the molecular weight of the polymer should decrease somewhat and its molecular weight distribution (MWD) should become wider. 

Radical polymerization requires an initiator such as AiBN and proceeds by a radicai chain process to give a largely regioregular, but generally atactic or somewhat syndiotactic polymer. Termination involves recombination or disproportionation. The process is tolerant of impurities, but molecular weight distributions are broad. 

Chain termination can occur by either combination or disproportionation, depending on the conditions of the process (78,79). 

The free-radical polymerization of methacrylic monomers follows a classical chain mechanism in which the chain-propagation step entails the head-to-taH growth of the polymeric free radical by attack on the double bond of the monomer. Chain termination can occur by either combination or disproportionation, depending on the conditions of the process (36). 

The most important mechanism for the decay of propagating species in radical polymerization is radical-radical reaction by combination or disproportionation as shown in Scheme 5.1. This process is sometimes simply referred to as bimolecular termination. However, this term is misleading since most chain termination processes are bimolecular reactions. 

GPC-derived weight average molecular weights are often less prone to error than number average molecular weights. When termination is wholly hy disproportionation or chain transfer and chains are long (>10 units), classical kinetics predicts Xn = XJ2 (Section 5.2.1.3). It follows that Cit can be obtained from the slope of a plot of 21 Xw vs [T]0/[M]t>."4 "5 The errors introduced even when the dominant process for radical-radical termination is combination (e.g. S polymerization) are small as long as X n is small in relation to... 

It is important to realize that, even if the rate of termination is determined by the rates of chain diffusion, the chain end composition and the ratio of combination to disproportionation are not. Knowledge or prediction of the overall rate of termination offers little insight into the detailed chemistry of the termination processes not involved in the rale-determining step. 

Minor (by amount) functionality is introduced into polymers as a consequence of the initiation, termination and chain transfer processes (Chapters 3, 5 and 6 respectively). These groups may either be at the chain ends (as a result of initiation, disproportionation, or chain transfer,) or they may be part of the backbone (as a consequence of termination by combination or the copolymerization of byproducts or impurities). In Section 8.2 wc consider three polymers (PS, PMMA and PVC) and discuss the types of defect structure that may be present, their origin and influence on polymer properties, and the prospects for controlling these properties through appropriate selection of polymerization conditions. 

Note, however, that chain ends 4 and 5 may give different chemistry to those formed in termination by disproportionation (2, see Scheme 8.5) or the processes under (a) above. Chain scission (3 to the double bond will not lead to a MMA propagating species. It is not established whether the presence of these ends will give impaired thermal stability. 

The number of reported reactions in which the radical derived from the decomposition of AIBN plays a role in the termination process has increased considerably. Often these reactions are not radical chain reactions, since the initiator is used in stoichiometric amounts. A few examples of rearomatization of cyclohexadienyl radicals by disproportionation have been reported herein. Below are some other examples, where the phenyl selenide 61 reacts with (TMSfsSiH (3 equiv), AIBN (1.2 equiv) in refluxing benzene for 24 h to give the coupling product of radicals 63 and 64 in good yields (Scheme 9).i24,i25 these cases,... 

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