Limitations of cyclodextrin-mediated RAFT homopolymerization and block copolymer formation

The design and synthesis of a new hydrophobic monomer, that is, 4-(tert-butyl)phenyl 6-acrylamidohexanoate (TBP-AA-HO) and its ability to form supramolecular host/guest complexes with β-cyclodextrin (CD) is described. The aqueous CD-mediated reversible addition fragmentation chain transfer (RAFT) polymerization affords molecular masses up to 8600 g mol-1 with polydispersities between 1.2 and 1.4. The surprisingly low molecular weights for higher monomer/chain transfer agent (CTA) ratios are investigated by comparing results obtained from free radical and RAFT radical polymerization in aqueous and organic media. The results indicate a steric hindrance caused by attached CD molecules on the growing polymer chain leading to stagnation of the polymerization process due to a restricted accessibility of the reactive chain end. This hypothesis is supported by matrix-assisted laser desorption/ionization time of flight mass spectrometry. Furthermore, the CD-mediated synthesis of amphiphilic diblock copolymers in variable aqueous media is described. Hydrophilic poly(N,N-dimethylacrylamide) macro-CTAs with different molecular weights are used to polymerize TBP-AA-HO at 50 °C. The diblock copolymers are analyzed by 1H-nuclear magnetic resonance spectroscopy and size exclusion chromatography. The results confirm the polymer structure and reveal similar limitations of chain growth as observed for the CD-mediated homopolymerization with a limit of 7000 g mol-1 for efficient chain extension. © 2013 Wiley Periodicals, Inc