Coupling reaction macroradicals

Silane radical atom transfer (SRAA) was demonstrated as an efficient, metal-free method to generate polystyrene of controllable molecular weight and low polydispersity index values. (TMSlsSi radicals were generated in situ by reaction of (TMSlsSiH with thermally generated f-BuO radicals as depicted in Scheme 14. (TMSlsSi radicals in the presence of polystyrene bromide (PS -Br), effectively abstract the bromine from the chain terminus and generate macroradicals that undergo coupling reactions (Reaction 70). 

Influence of the Living End Concentration When the living end concentration increases, the probability of collision between two macroradicals increases and then the coupling reaction is favoured (Table III). 

Oxidation of Anionic Polymers In the Solid State The ability of the macroradical and of the macroions to diffuse In the mixture, and to interreact Is responsible for the secondary products formation coupling reaction and alcoholate synthesis. To prevent the diffusion phenomenon, we have carried out the deactivation In the solid state. The living polymers have been prepared In benzene, with or without a solvating agent (THF or TMEDA) and the solution has been freeze dried before the oxygen introduction. The experimental results are collected in Table VII. 

Finally, Is the solvating reagent Is added In the vapor phase to the freeze dried polymer, It Is possible to prevent the coupling reaction completely. We think that the THF molecules are preferentially located around the living ends and this hindrance prevents the coupling when the macroradicals are formed. 

Preparation of telechelic polymers by ATRCC was also demonstrated. The concept is based on the activation of the dormant species at the chain ends of polystyrene (PS-Br) prepared by ATRP and also functional ATRP initiator (F-R-Br) in the absence of a monomer. Depending on the number of functionality of the polymer used in the system, ATRCC yields co-polystyrene and a,co-polystyrene telechelics. However, at least in principle, not only the desired functional polymers but also various side products may be formed due to the self coupling reactions of macroradicals generated from PS-Br, and low-molecular weight radicals from F-R-Br. The possible reactions of the model system are depicted in Scheme 3. 

Another crosslinking mechanism, leading to the formation of H-crosslinks, is shown in Scheme 6. Secondary and allyl macroradicals, decay via disproportion (Scheme 6, Reaction 11) or coupling (Reaction 12). Both reactions are exo ermal (AH = -260 kJ/mol and - 313 kJ/mol for Reactions 11 and 12, respectively). 

The first method has serious limitations because well-defined end groups can be observed only if not more than one type of primary radical is formed that does not cause side reactions such as transfer. Moreover, a propagating radical will readily react with another radical, primary or macroradical, either through disproportionation or through coupling reactions (termination) (Fig. 2). The former will produce monofunctional telechehcs with both a saturated and an imsaturated chain end, while only the latter will yield bifunctional telechelics. 

The mode of termination varies with monomer and reaction conditions. While styrene macroradicals typically terminate by coupling, methyl methacrylate macroradicals terminate by coupling at temperatures below 60°C, but by disproportionation at higher temperatures. 

Termination process can take place via either (a) coupling of two macroradicals or (b) disproportionation reaction, thus destroying the active center. Coupling of two growing chains would lead to a single linear polymer chain both with initiator fragment and with the other chain end, as shown in Scheme 3.6. Chain termination can also occm through chain transfer mechanisms by which the radical electron is transferred to other chain or molecule in the reaction medium. 

Introduction. We have, so far, considered ionic propagation, coordination catalysis, and the step reactions of a pol37mer terminus as techniques for the preparation of block copol3nners. Free radical polymerization may also be utilized by application of one of several chemical manipulations. For example, block copolymers may be prepared by coupling macroradicals, or by generating new radicals in the presence of a second monomer by photolytic or mechanical degradation. As an alternate, difunctional initiators may be employed. 

Combination vs Disproportionation. There are two modes of termination one is the direct coupling (combination) of two free macroradicals to give a dead polymer chain of chain length i +j, with the rate coefficient At,c- The other mode is the so-called disproportionation, where a hydrogen atom is transferred from one of the radical chain ends to another radical, yielding two stabilized polymer chains, of which one carries a double bond. This reaction is associated with the rate coefficient t,d. The process is illustrated in equation 44 using polyethylene macroradicals as the example. It is important to notice that—in the case of macroradicals derived from other monomers—in principle any -hydrogen may be abstracted. 

Of special practical interest in spinning from solution of some polymers may become the macroradicals reactions with aromatic diamines. In this way, chemically modified polymers -containing free aromatic aminic-groups may be obtained. In their turn, they may be either diazotized or coupled with phenols or tertiary mixtures of amines, azoic structures being thus formed at the ends of the macromolecular chains. Depending on the chemical nature of the coupling agent employed, different colorations may results ... 

Further investigations in the mechanistic details of the RAFT process, particularly the potential side reactions of the intermediate radical species, were made using the coupled SEC/ESI-MS to map the product spectrum of a series of acrylate free-radical polymerizations mediated via the RAFT [61]. The mass spectroscopic results were compared to modeling estimations made to predict the concentrations of termination products of the intermediate species in comparison to polymeric material generated by recombination of two propagating macroradicals. 

The termination of the growing free-radical chains usually takes place by coupling of two macroradicals. Thus, the kinetic chain length (v) is equal to the half of degree of polymerization, DP/2. The reaction for the bimolec-ular termination is presented in Eq. (8). The kinetic equation for termination by coupling is... 

Scheme 4.11). The macroradicals are generated either through conventional polymerization initiated by TRI or from dormant chains pre-made by ATRP. The process is called ESCP (enhanced spin capturing polymerization) in the first case and NMRC (nitrone-mediated radical coupling) in the second. The formation of 21 is irreversible at the reaction temperature (<80°C), however,... 

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