Primary radical termination reversible

Primary radical termination is also of demonstrable significance when very high rates of initiation or very low monomer concentrations are employed. It should be noted that these conditions pertain in all polymerizations at high conversion and in starved feed processes. Some syntheses of telechelics are based on this process (Section 7.5.1). Reversible primary radical termination by combination with a persistent radical is the desired pathway in many forms of living radical polymerization (Section 9.3). 

We first consider the polymerization where each kinetic chain yields one polymer molecule. This is the case for termination of the growth of macroradicals by disproportionation and/or chain transfer (A,c = 0). The situation is completely analogous to that for linear, reversible step-growth polymerization described in Section 5.4.3. If we randomly select an initiator residue at the end of a macromolecule, the probability that the monomer residue which was captured by this primary radical has added another monomer is S and the probability that this end is attached to a macromolecule which contains at least i monomers is S . The probability that this macromolecule contains exactly i monomers equals the product of 5 and the probability of a termination or transfer step. The latter probability must be equal to (I — S) since it is certain that the last monomer under consideration will undergo one of these three reactions. That is, the probability that a randomly selected molecule contains t monomer units is 5 (l — S). Since such probabilities are equal to the corresponding mole fraction of this size molecule, jc,, we have the expression... 

Let us first consider the polymerization where each propagating chain results in the formation of one dead polymer molecule, that is, each polymer molecule consists of one kinetic chain. This happens when the chain radical is terminated by disproportionation and/or chain transfer (i.e., ktc = 0). The probability situation in this case is almost identical to that for linear, reversible step-growth polymerization described in Chapter 5. Thus if we select randomly an initiator fragment at the end of a polymer molecule, the probability that the monomer molecule added to this initiator (primary) radical has added another monomer molecule is P. Continuing in this way the probability that x monomer molecules have been added one after another is (see p. 253). Since the probability that the radical end of a growing chain has terminated is (1 - P), the probability that the polymer molecule under consideration consists of essentially x monomer units is P (1 - P). This probability can be equated to the mole fraction of polymer molecules of this size... 

Vinyl polymerization with initiator-transfer reagent-terminator (iniferter) is interesting as a living polymerization model, in which a reversible combination of polymer radicals with stable primary radicals occurs continuously during the polymerization " . The resulting telechelic polymers initiated the polymerization of other monomers to give block copolymers in good yields... 

Franchi and coworkers reported on the effect of solvents on hydrogen atom abstraction from phenolic antioxidants by primary alkyl radicals , which could be useful when studying oxidations at low oxygen partial pressures because under those conditions the addition of oxygen to substrate alkyl radicals can be reversible (equation 36) and alkyl radicals could play a role in termination reactions (equation 37)" ... 

The bifunctional initiator approach using reversible addition fragmentation chain-transfer polymerization (RAFT) as the free-radical controlling mechanism was soon to follow and block copolymers of styrene and caprolactone ensued [58]. In this case, a trithiocarbonate species having a terminal primary hydroxyl group provided the dual initiation (Figure 13.3). The resultant polymer was terminated with a trithiocarbonate reduction of the trithiocarbonate to a thiol allows synthesis of a-hydroxyl-co-thiol polymers which are of particular interest in biopolymer applications. 

In heterolytic bond cleavage, a bond breaks such that both electrons in the bond stay with one of the atoms in homolytic bond cleavage, a bond breaks such that each of the atoms retains one of the bonding electrons. An alkyl peroxide is a radical initiator because it creates radicals. Radical addition reactions are chain reactions with initiation, propagation, and termination steps. Radicals are stabilized by electron-donating alkyl groups. Thus, a tertiary alkyl radical is more stable than a secondary alkyl radical, which is more stable than a primary alkyl radical. A peroxide reverses the order of addition of H and Br because it causes Br, instead of H, to be the electrophile. The peroxide effect is observed only for the addition of HBr. 

Note that addition of HX by this mechanism gives products violating Markownikov s rule. Thus electrophilic addition of HBr to propene gives isopropyl bromide, CH3CHBrCH3, not n-propyl bromide. The reversal of addition of HBr under conditions where the radical chain mechanism operates is synthetically useful since it allows one to convert terminal olefins into primary alkyl derivatives. 

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