Radical transfer reactions to polymer

Chain transfer to polymer leads to branched polymer chains and thus greatly affects the physical and mechanical properties of a polymer, such as the ability to crystallise. Transfer to polymer does not necessarily lead to a decrease of molecular mass, but the chain length distribution becomes broader. If we consider transfer from a growing polymer chain to another dead chain, this is intermolecular transfer to polymer. If the radical attacks a proton in the same chain we call this intramolecular transfer to polymer, also called backbiting. 

The M mber of side-chains can be expressed in terms of branching density BD = number of side-chains per reacted monomer. The rate of formation of chain radicals generated by transfer = the rate of formation of side-chains, which is  

Independent of Q,(= ) the extent of branching greatly increases with increasing conversion. To minimise the number of side-chains the conversion must be limited. Dilution of the system with solvent does not help according to Equation 2.36. 

A dead chain is reinitiated and will grow longer than without the transfer process. 

The EPR experiments of Yamada on methyl acrylate also support this reaction scheme. He was able to show the occurrence of the midchain radical and also the occurrence of j8-scission (Azukizawa et al., 2000 Tanaka et aL, 2000) in the temperature range 40-85°C. 

---

Another aspect49 is the initial presence of persistent species in nonzero concentrations [Y]o, and it will be discussed more closely in section IV. In the absence of any additional initiation, the excess [Y]o at first levels the transient radical concentration to an equilibrium value [R]s = A[I]o/[Y]o. This is smaller than that found without the initial excess and lowers both the initial conversion rate and the initially large PDI. Further, it provides a linear time dependence of ln-([M]o/[M]), which is directly proportional to the equilibrium constant. Later in the reaction course, [Y] may exceed [Y]0 because of the self-termination, then [R] is given by eq 18. If there is additional radical generation, the first stages will eventually be replaced by a second stationary state that was described above. Further effects are expected from a decay or an artificial removal of the persistent species that increases the concentration of the transient radicals and the polymerization rate (see section IV). Radical transfer reactions to polymer, monomer, or initiator have not yet been incorporated in the analytical treatments. 

---