Nitroxide-mediated radical polymerization with bisaminooxy compounds as initiators - Controlled biradical polymerization

The bisaminooxy compounds Bis-TEMPO and Bis-TIPNO derived from 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) and 2,2,5-trimethyl-4-phenyl-3-azahexane-3-oxyl (TIPNO) were applied as "biradical initiators" for the nitroxide-mediated radical polymerization (NMRP) of styrene and n-butyl acrylate. It was shown by comparison with analogous alkoxyamines as unimolecular initiators and mixing experiments of mono- and biradical species, that in the case of the biradical initiators chain growth occurs at both sides under NMRP conditions. This enables a two-step synthesis of A-B-A-triblock copolymers. Kinetics and molecular mass development were investigated for the controlled biradical polymerization of styrene at different initiator concentrations, temperatures, and with addition of acetic anhydride as accelerator. For the controlled biradical polymerization of n-butyl acrylate with Bis-TIPNO, the effect of added free nitroxide relative to the initiator concentration was studied. The poly(styrene-block-n-butyl acrylate-block-styrene) copolymers with higher block length prepared by this method show two glass transition temperatures, which indicates microphase separation of the polymer blocks. [GRAPHICS] Structure of poly(styrene-block-n-butyl acrylate-block-styrene), synthesized by nitroxide-mediated radical polymerization with Bis-TIPNO as initiator.