Unsaturated chain ends from disproportionation

In certain situations, termination occurs by disproportionation. This termination process involves chain transfer to a hydrogen atom from one chain end to the free radical chain end of another growing chain, resulting in one of the dead polymer chains having an unsaturated chain end (Equations 6.19 and 6.20). 

There has been considerable debate over the reason why TEMPO does not work well for acrylates. CNRS researchers utilized NMR and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) techniques to look at the terminal end-groups on poly(n-butyl acrylate) produced using TEMPO mediation [19]. They found that most of the chains were terminated by co-unsaturation resulting from either elimination or O-H TEMP from the chain-end or disproportionation between TEMPO and the polyradical (Scheme 8.7). 

The termination reactions were believed to result from hydride-ion transfers [262], Later, however, polyethylene, formed with these catalysts, was found to contain approximately equal numbers of saturated and unsaturated chain ends. This contradicts the above concept. It may also mean that the initiation involves 71-complexes that disproportionate to yield coordination of the metal to -CH=CH2 and to -CH2-CH3 groups [262]. 

Evidence for a competing disproportionation mechanism (see Figure 1) for the termination of chain ends is provided by the combined presence of the peaks from 4 and 5 in the MALDI-TOF mass spectrum of this PMMA polymer (see Figure 6) [10]. Confirmation of the presence of the unsaturated and saturated chain ends, arising from disproportionation, was obtained by means of and 13C NMR spectroscopy, respectively [11]. 

Even though the rate of radical-radical reaction is determined by diffusion, this docs not mean there is no selectivity in the termination step. As with small radicals (Section 2.5), self-reaction may occur by combination or disproportionation. In some cases, there are multiple pathways for combination and disproportionation. Combination involves the coupling of two radicals (Scheme 5.1). The resulting polymer chain has a molecular weight equal to the sum of the molecular weights of the reactant species. If all chains are formed from initiator-derived radicals, then the combination product will have two initiator-derived ends. Disproportionation involves the transfer of a P-hydrogen from one propagating radical to the other. This results in the formation of two polymer molecules. Both chains have one initiator-derived end. One chain has an unsaturated end, the other has a saturated end (Scheme 5.1). 

Since the nitroxide and the carbon-centered radical diffuse away from each other, termination by combination or disproportionation of two carbon-centered radicals cannot be excluded. This will lead to the formation of dead polymer chains and an excess of free nitroxide. The build-up of free nitroxide is referred to as the Persistent Radical Effect [207] and slows down the polymerization, since it will favor trapping (radical-radical coupling) over propagation. Besides termination, other side reactions play an important role in nitroxide-mediated CRP. One of the important side reactions is the decomposition of dormant chains [208], yielding polymer chains with an unsaturated end-group and a hydroxyamine, TH (Scheme 3, reaction 6). Another side reaction is thermal self-initiation [209], which is observed in styrene polymerizations at high temperatures. Here two styrene monomers can form a dimer, which, after reaction with another styrene monomer, results in the formation of two radicals (Scheme 3, reaction 7). This additional radical flux can compensate for the loss of radicals due to irreversible termination and allows the poly-... 

Disproportionation, with hydrogen abstraction from one end to give an unsaturated group and two dead polymer chains. 

The reaction is started by the thermal decomposition of an initiator, which in our experiment is tert-butyl peroxybenzoate (18), a compound that produces the free radicals 19 and 20 when heated (Eq. 22.8). If In represents one or both of these free radicals, the course of the polymerization may be illustrated as shown in Equations 22.9-22.12. Equation 22.9 indicates the function of the free radicals in initiating the polymerization. Equations 22.10a and 22.10b represent the propagation of the growing polymer chain. Equations 22.11 and 22.12 show possible termination processes. In Equation 22.11, the free-radical end of one growing polymer chain abstracts a hydrogen atom from the carbon atom next to the end of another polymer radical to produce the unsaturated and saturated polymer molecules 22 and 23, respectively, in a process termed disproportionation. For the termination reaction illustrated by Equation 22.12, Rad may be one of the initiating radicals. In, or another growing polymer chain. 

The double bond formation can arise from both transfer to monomer and termination by disproportionation. These bonds are called terminal double bonds and are located at or close to the ends of polymer chains. The various anomalous and unsaturated structure found in the polymer product may be attributed to the chain transfer processes. 

The double bond content in PVC resins has been found to vary in the range of 1.5 to 4.0 per 1000 VC units. Hildebrand et al. [154] found that the number of double bonds per unit weight of polymer increases as the polymerization temperature increases and is proportional to the number of polymer molecules. Double bond defects can arise from both transfer to monomer and termination by disproportionation. These bonds are called terminal double bonds and are located at, or close to the ends of polymer chains. The various anomalous and unsaturated structures found in PVC resins are considered to the primary cause for the low thermal stability of the polymer. 

---