Abstraction-fragmentation chain transfer

The cyclohcxadicnc 84 is a good H donor but the cyclohcxadicnyl radical 85 is slow to react and fragments to provide the silyl radical 86 which initiates polymerization. The reported transfer constant for 84 in styrene polymerization at 80 °C is very low (0.00045).  

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Other niultistcp mechanisms for chain transfer arc possible. An example is abstraction-fragmentation chain transfer shown by silylcyclohexadienes (84, Scheme 6.22).130... 

Abstract This chapter summarizes the properties and most representative applications of pH-responsive polymers in the biomedical field.The most common methodologies to synthesize pH-responsive polymers such as emulsion polymerization, group transfer polymerization, atom transfer radical polymerization and reversible addition-fragmentation chain transfer polymerization are described. This chapter also discusses the most important applications of pH-responsive polymers in drug and gene delivery and the use of these systems as biosensors, taking into account the chemical and physical properties of these smart polymer systems. 

For allyl acetate a significant deuterium isotope effect supports the hydrogen abstraction mechanism (Scheme 6,31).183 Allyl compounds with weaker CTT-X bonds (113 X=SR, S02R, Bi etc.) may also give chain transfer by an addition-fragmentation mechanism (Section 6.2.3). 

Chain transfer to polymer is reported as a major complication and is thought to be unavoidable in the polymerization of alkyl acrylates.200 202 The mechanism is believed to involve abstraction of a tertiary backbone hydrogen (Scheme 6.32). It has been proposed that this process and the consequent formation of branches may contribute to the early onset of the gel or Norrish-Trommsdorff effect in the polymerization of these monomers. At high temperatures the radicals formed may undergo fragmentation. 

Due to the high reactivity of the radical fragments, facile chain-transfer may occur (Figure 5.9d i,ii), whereby the radical end of the growing chain abstracts an... 

Hydrogen chain transfer reaction, which may occur as intermolecular or intramolecular processes, leads to the formation of oleflnic species and polymeric fragments. Moreover, secondary radicals can also be formed from hydrogen abstraction through an intermolecular transfer reaction between a primary radical and a polymeric fragment. 

Abstract Macromonomers of structure 4. prepared by the free radical polymerization of MMA using cobalt complexes as chain transfer agents, become incorporated into polymer chains by copolymerization but also act as efficient chain transfer agents by an addition-fragmentation reaction. Macromonomers of general structure 1 have been prepared by utilizing appropriately substituted allylic sulfides as chain transfer agents in free radical polymerizations. 

This experiment demonstrates that tlie eyanoisopropyl radieal is eapable of addition to the double bond of trimer 4 but the radieal 7 so formed is ineapable of sustaining propagation and fragments to olefin 8 (a major produet) and radieal 9. Radieal 9 proceeds to stable products by combination or disproportionation with itself or other radicals (4). The mechanism in Scheme 2, therefore, is likely to be the process responsible for the chain transfer observed in the copolymerization experiments summarized in Table 1. An additional but minor reaction pathway for radical 7 was observed to be termination by hydrogen abstraction. Other radicals in the mixture (e.g. 9 or eyanoisopropyl) would be the most likely hydrogen donors (i.e. disproportionation). 

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