Disproportionation from PMMA

A side reaction in NMP is loss of nilroxide functionality by thermal elimination. This may occur by disproportionation of the propagating radical with nitroxide or direct elimination of hydroxy lam ine as discussed in Section 9.3.6.3. In the case of methacrylate polymerization this leaves an unsaturated end group.1" The chemistry has also been used to prepare macromonomers from PMMA prepared by ATRP (Section 9.7.2.1),... 

The reaction between the PMMA and PS model radicals (4 and 5, generated from the unsymmetrical azo-compound 3) has been studied as a model for crosstermination in MMA-S copolymerization (Scheme 7.13).178,179 The value for tcross reaction was 0.56. In disproportionation, transfer of hydrogen from the PS model 5 to the PMMA radical 4 was ca 5.1 times more prevalent than transfer in the reverse direction (from 4 to 5). The value of kJklc(90°C) is between those of Atd/ tc(90oC) for the self-reaction of these radicals... 

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]. 

To quantify the hydroxy-telechelic PMMA, LC was used to show that this two-step reaction yielded about 67% of telechelic oligomers [192], This unexpected low yield was explained by several factors during RAFT some disproportionation may occur leading to dead chains without any terminal thioesters also some side reactions occurred from aminolysis of the terminal thioester, leading to a hydrogen-terminated chain unable to be functionalized into a hydroxyl group. 

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