Macroradical species, concentration

Typically, the chain length distribution is characterized by the moments of the distribution. It is also possible to gain an understanding of the distribution by focusing on the microscopic distribution. By knowing the concentration of every macroradical species, one can build a picture of the entire distribution. 

Therefore, concentrations of the polyvinyl monomer, the branching and network formation proceeds as shown in Figure 12. Mlcrogel-llke species are formed first which are highly internally crossllnked. The pendant double bonds in the interior are very Immobile so that their reactivity is strongly diffusion controlled and they almost cannot react at all, even with the monomers. Only the more mobile pendant double bonds in the periphery of these species can enter into reactions with macroradicals and participate in Interbinding of the species together. 

The simultaneous Increase of these two ratios C and f, Is due to the Increase of the macroradical concentration. Consequently this phenomenon Implies that the electronic transfer to the OTtygen (reaction I) Is easier with Ionic species, than non Ionic species. 

With low concentrations of Nujol the resulting Increased macroradical lifetimes should result In a larger percentage of high molecular weight species. 

ATRC, is based on the application of Cu(0) to an ATRP system, leading to the reduction of any Cu(ll) in the reaction medium to form Cu(I). The increase of Cu(l) in the reaction medium dramatically shifts the equilibrium between active and dormant species to the side of the active radical species. So the high concentration of radicals presents in the system favor the recombination reactions between two macroradicals. This method was performed to obtain telechelic polymer from monotelechelic polystyrene and results showed that it is very effective way with high efficiency in a short reaction time. Figure 1 shows a,co-telechelic polymer with double molecular weights as compared with the mono functional polystyrene obtained by ATRP using aldehyde functional initiator. 

Attempts to quantitatively determine the extent of ionic dissociation of all relevant species including macroradicals and polymer molecules and to correlate such speciation with the variations observed for kp is difficult, if not impossible, in view of the complex acid-base properties and polyelectrolyte behavior as well as the coupled electrochemical equilibria. Studies into polyelectrolyte behavior in aqueous solution carried out so far, have been performed at conditions precisely defined with respect to solvent composition, ionic strength, concentration regime, and molecular weight. These conditions differ from the ones met in the actual free-radical polymerization experiments presented in Figure 3 and in Reference Despite this complexity, it has been reaUzed that with... 

Chain mobility has important implications for bimolecular reaction efficiency. Bimolecular reactions depend on (i) the frequency of encounter and (ii) the concentrations of the reactive species. Upon bond cleavage, macroradicals are formed in pairs and can initiate degradative reactions only if they are sufficiently flexible to move apart. If the macroradicals are held rigid, as in thermosets or below the Tg, they may recombine by the cage effect without any detectable molecular change. 

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. 

Hjertberg et al. [114] attributed the increase of Vp (with decreasing monomer concentration) to the stronger decrease of k( compared to that of kdes- This is reasonable, as terminatian involves macroradicals, while only the fraction of small radical species is desorbed from the particles. Termination in particles (k,) completely dominates when the monomer concentration has decreased to about one third of the saturation value. At higher concentrations, the influence of the term involving k., is larger, but this term never completely dominates polymerization. The reaction order of Rp with respect to Vp was found to decrease with... 

The scenario is at the basis of the major CRPs, namely, stable free radical polymerizations (SFRPs) and ATRP. " In these procedures, control is achieved through a dynamic equilibration between a predominant fraction of dormant species and maaoradicals. This allows us to complete initiation before propagation starts or is advanced. Some authors state that an additional requirement is the low concentration of propagating radicals. This is not a necessary condition, particularly when lower is a target. The low concentration, however, may mean that there is a much higher total number of growing chains in comparison with the concentration of macroradicals (i.e., fraction of chains fitted at a given moment with radicals). 

The main equilibrium between dormant and active species is the key step in the NMP process. If K is too high, the nitroxide acts only as a spectator and the macroradical concentration is close to the classical steady-state value. In contrast, if K is too low, the... 

The oxygenated species incorporated by flame treatment are similar to those created during corona discharge treatment. This suggests a common series of reactions following the initial creation of macroradicals by interaction with the ions, radicals, free atoms, excited species, and electrons that compose the flame. The profuse concentration of activation events is such that antioxidants have little impact on the oxidation reactions. X-ray photoelectron spectroscopy reveals that flame treatment modifies the surface to a depth of 40-90 A [53], In addition to oxygenated species there is also an increase in the level of nitrogen within the surface layers of the sample. 

Most acetophenone-type initiators decompose into more than one kind of radical upon irradiation. Typically, two different radical species i ] andi 2 are formed, showing very different reactivities towards the monomer. While the carbonyl radical is efficiently forming macroradicals, the additional radical does not add much to the initiator efficiency. On the contrary, for DMPA the methoxy radical is thought to be involved exclusively in termination steps. In such a case, it is common to say that the carbonyl radical is the effective and the methoxy radical the ineffective primary radical. Their initial concentrations are equal and denoted as p, while the overall initial radical concentration is 2p. Polymerization kinetics can strongly be influenced by the formation of two sorts of primary radicals with markedly different reactivities. 

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