Synthesis of Cyclic Poly(methyl methacrylate) Directly from Dihalogenated Linear Precursors

Cyclic poly(methyl methacrylate) (PMMA) is prepared by intramolecular radical trap-assisted atom transfer radical coupling (RTA-ATRC) of dihalogenated PMMA precursors. The inclusion of the radical trap nitrosobenzene (NBz) in the coupling sequence affords high yields of cyclic polymers, as observed by gel permeation chromatography and confirmed by H-1 NMR and electrospray ionization mass spectra, which show the presence of the aromatic group from the NBz incorporated into the cycle. Analogous coupling reactions in the absence of the radical trap do not lead to appreciable cyclization or even intermolecular elongation, consistent with chain-end sterics preventing radical-radical coupling as the predominant termination pathway. Thermolysis of the cyclic PMMA, possible because of the labile C-O bond in the alkoxyamine linkage contained in the macrocycle, causes a reversion back to the linear form and is consistent with the role of the radical trap in the coupling sequence. Differential scanning calorimetry is also used to compare cyclic PMMA with its linear analog, with a marked increase in glass transition temperatures found after cyclization.